Introduction to Basic Lighting for Family Photographers

(version: 1)

Lighting is critical to producing a quality photograph of any genre, including family photos.  This intro to lighting is intended not as a tutorial in itself but just to let you know the photographic possibilities from lighting.

1. What do you mean by "lighting"?

In photography, lighting is the artistic use of light and shadow in the photo.

2. Why is lighting important?

Here are some ways lighting affects your photo:

• Volume: A photo is flat. Proper lighting helps create the illusion of three-dimensionality.
• Emotion: the lighting in a photo by itself can have a powerful impact on the emotion or feel of the photo, even when other elements (such as composition and color) are unchanged.
• Beauty: some ways of lighting are more flattering for the subject than others. This applies not only to portraits but also to objects.
• Color: knowledge of proper lighting can help you to emphasize or deemphasize colors in the photo.  For example, colors are generally more washed out in bright, direct sunlight.
• Composition: the patterns of light and shadow in a photo can themselves be elements of composition.

Because of the immense impact of lighting on a photo, for many photographers, the lighting of a photograph is at least as important as (and sometimes more important than) its composition or subject matter.  Lighting can make or break your photo.  To summarize: lighting is a BIG DEAL.

3. What are the basics I need to know about lighting?

For family photographers, you should at least know concepts about the characteristics of light (see #4), portrait lighting (see #5) and the overall lighting scheme of a photo (see #6).

4. Characteristics of light itself:

• Intensity - the brightness of the light source.
• Quality (hard vs. soft) - hard light casts high-contrast shadows with clear edges. Soft light is scattered and produces blurry or no shadows.  Example: direct midday sun in summer without clouds produces hard light.  An overcast day produces soft light.   The larger the apparent size of the light source to the illuminated object, the softer the shadows will be.
• Direction - self-explanatory.
• Shape - is the light beam wide or narrow?  Does it cast a pattern (for example, when you shine the light through the leaves of a plant or through a glass of water)?
• Color - the obvious examples are when gels or filters are used.  But different sources of light also have different color temperatures.  Extreme examples are candlelight (warm colored) or old fluorescent lights (greenish).  Less obvious examples are shade (bluish) and flash (slightly bluish).
• Number of lights - actually increasing the number of lights or simulating such an increase (through time controls or reflectors or v-cards).

5.  Portrait lighting

Portrait lighting is distinct from the lighting of other subjects (such as in product photography).  Although family photographers on the run rarely have the opportunity to prepare the lighting setup for proper portrait lighting, family photographers can benefit from awareness of portrait lighting concepts to use more flattering light on subjects.

a. Light positions and functions in portraits.

• Key or Main - the light that produces the dominant highlight on the subject.  The position of the key light varies depending on the lighting scheme (see #6 below).  Note: in a few lighting schemes, there is more than one key light.
• Fill - illuminates some or all parts of the subject that are in shadow from the key light. Fill light may be omnidirectional, from the same direction as the camera (on-axis fill) or an extension of the key light (in which case it's about 90 degrees from the direction of the key light).
• Rim or kicker - a highlight on the contours of the subject generally used for subject-background separation.  Rim light is generally behind the subject, and either above, to the side, or both.
• Background - light(s) illuminating the background.
• Hair - used to add a highlight on the subject's hair.  Generally positioned above the subject.
• Catchlight - the pattern of light (if any) reflected in the subject's eyes. Not really a light position itself but something to be aware of in portrait lighting.

b. Basic portrait lighting schemes:

• Broad vs. short: when the subject's face is on a three-quarter view, broad lighting is where light illuminates the side of the face that is closer to the camera.  Short lighting is where the light is illuminating the side further from the camera.  If the subject's visible ear is on the illuminated side, it's broad. If not, it's short.
• Split: the subject is facing the camera and the key light comes from the side and illuminates only one side of the face.
• Loop: light comes from around 45 degrees from the subject's face. The nose casts a shadow pointing toward the corner of the mouth, in the shape of a loop. Can be broad or short. A fairly common portrait lighting scheme.
• Rembrandt: similar to loop, except the light is coming from a higher angle.  The nose's shadow merges with the shadow below the cheekbone. There is a tell-tale upside down triangle highlight below the eye in the side of the face that is partially shaded.
• butterfly / paramount: subject is facing the camera. Light comes from above and in front of the subject. The resulting shadow of the nose is shaped like a butterfly.

6.  The overall lighting scheme of the photo:

• Shadows and chiaroscuro: In lighting it's important to pay attention not just to the highlights but also the shadows. Chiaroscuro is the artistic technique of using highlights and shadows, often with strong ("dynamic") contrast, to create the illusion of three-dimensionality.  Caravaggio's works exemplify chiaroscuro.
• High key vs. low key: high key photos use tones that are mostly brighter (think of those mac vs. pc ads) while low key photos are darker and have lots of shadows (as in the matrix trilogy).
  
  
  
  
  
  
• Specular - diffuse - shadow - transfer: in the real world, the highlight and shadow areas are not of even brightness and can be further differentiated.  Specular highlight is the bright "shiny" part of the highlight.  Diffuse is the rest of the highlight.  Shadow is self-explanatory. Transfer is each area between each of the foregoing areas (specular-to-diffuse transfer, diffuse-to-shadow transfer).  Check out the late Dean Collins' theory of 3d contrast.
• Motivation: the logic for the direction, intensity, quality and color of artificial light so as to appear as if the artifical light is natural.  For example, if the sun is camera left (as evidenced by shadows in the background), adding light that comes from camera right will look unnatural.  Here is a good explanation of motivation.
• Background vs. subject vs. foreground: the lighting of the background, subject, and foreground are often distinct and can be analyzed as separate elements of the photo's lighting scheme. Generally, to emphasize the subject, a common technique is to underexpose the background. Alternatively, the subject might be underexposed relative to the background for photos with strong backlighting (as in many of Rarindra Prakarsa's amazing photos).
• Light ratios: the difference in intensity between two areas of light in the photo (usually comparing the highlight and fill). If the highlight is 1 stop brighter (i.e., 2x brighter) than the fill and the fill light is omnidirectional and illuminates the entire subject, then the fill is 1 unit of light, while the highlight is 2 units of light (from the highlight) plus 1 unit of light (from the omnidirectional fill) for a total of 3 units, thus the ratio is 3:1.  If the fill is not omnidirectional and illuminates only the shadow (this is less common), then the ratio is 2:1.
• Tonal range: the difference between the brightest tone and the darkest tone in the photo. Qualities to aim for (note: I'm still trying to figure this out): full range of black to white represented in the shot; a smooth transition between tones in the photo; a balanced distribution of tones that is neither dull ("A"-shaped histogram) nor contrasty ("U"-shaped histogram).

7. What can I do to control lighting?

• Observation: in my opinion the most important lighting technique is observation of existing light -- observing its characteristics, imagining ideas for using it, and determining whether and how to enhance it.
  
  
  
  
  
  
  
  
• Choice of time and place for the photo: you can increase the odds of having lighting conditions that match your intended lighting scheme by being mindful of the pattern of light at different times of the day, weather, and seasons.
  
  
  
  
  
  
  
  
• Positioning the subject: sometimes, you can control the position of the subject - in the shade, in the light, or somewhere in between, plus the direction they are facing, thus changing the direction of light relative to the subject.
  
  
  
  
  
  
  
  
• Choosing the shooting angle and direction: you might not be able to move the sun, but you can choose your shooting angle and direction to change the relative direction of sunlight.
  
  
  
  
  
  
  
  
• Light modifiers:  all sorts of gadgets are used to alter the quality of existing light (e.g., a white bedsheet to act as a diffuser for sunlight) or artificial light (e.g., a portable softbox attached to a flash to soften its light). Check out diyphotography.net to get a glimpse of some creative modifiers.
• Flash: by giving you other sources of light besides existing ambient light, flashes can help you gain more control of the lighting in your photo. See TTL Flash FAQ.
• Exposure: exposure controls (shutter, aperture, ISO) can be used to alter the impact of the light on the photo.  For example, a very fast shutter with shallow aperture and low ISO will virtually eliminate ambient light.
• Postprocessing: PP can be used to enhance, mitigate or simulate lighting. For example, dodging and burning techniques can make hard light look soft.

8. Where can I find additional resources on lighting?

• Strobist Lighting 101 and Lighting 102 - lighting concepts and techniques by professional news photographer David Hobby.  David's shots often require set up (thus can't be as easily used by family photogs on the run) but the logic he uses is often applicable to on-location family photos as well.
• Neil van Niekerk's On-Camera Flash and planetneil.com Flash tutorial.  Neil is a wedding photographer.  His lighting techniques are often applicable to family photos.
• Bob Krist's Secrets of Lighting on Location. Shows how renowned photographer Bob Krist lit some of his on-location (as opposed to studio) shots. Many of the techniques and logic he uses are applicable to family photos. Although the book was written before the advent of digital cameras and modern flashes, digital technology has only made the techniques more usable and easier to apply.
• Allison Earnest's Sculpting with Light.  A guide for portrait lighting.
• Light: Science and Magic. Lighting concepts.  Much of the book deals with reflection and controlling how the light appears in the photo.  Most lighting techniques are for product photography but some concepts can be applied to family photos.
• Dean Collins' videos (http://www.software-cinema.com/training). Dean was a master of lighting. Like Bob Krist's book, Dean's videos were produced before digital era. However, the concepts and ideas are still applicable.
• http://www.photoflexlightingschool.com/ - discusses a variety of lighting techniques.
• This blog. Search for the tag "lighting."

Comments on first family's photo

Yahoo news reported that the first family's first official photo was released. Here's the photo, taken by Annie Leibovitz. Of course the photo looks good and I like the expressions on their beautiful faces, but from a purely technical point of view, I'm a bit surprised that the lighting looks so simple and frankly underwhelming to me.  Maybe it's "deceptively simple" and too subtle for me. It seems possible to duplicate the lighting by simply bouncing the flash behind and camera right. Or by a large umbrella or softbox. From the catchlight on the original size photo, I'm guessing Annie used a softbox. I can't tell whether a fill light was used.  Doesn't seem like it. I'm wondering whether Annie used a hair light, because the first lady's hair has a shine, but the shine could have come from the key.

From the background it looks like ambient was killed (intentionally I assume).  Looking at the exif, I'm kinda puzzled too. It says:

• Canon 1Ds MkIII
• f/8, 1/125, ISO 400, 0 EV.

If I wanted to take this same shot, I would have instead gone to max sync speed (1/250 for the 1Ds MkIII)  and used a lower ISO.  Because there's no ambient anyway, the faster shutter and lower ISO wouldn't change the lighting of the shot.  The lower ISO would have less noise (though I concede that the 1Ds probably has negligible noise even at ISO 400) and the fast shutter would minimize blur from subject movement (though I am guessing that with virtually no ambient, the light was all flash, thus shutter speed was effectively the duration of the flash, i.e. 1/1000 or faster).  On the other hand, if Annie bounced the flash and didn't use a softbox, I can understand the use of ISO 400.  I doubt she did though, because bouncing can be more unpredictable for white balance, and there may be unintended reflections.

If I were to take my own version of this shot, I would have allowed more ambient light, because I think the darkness in the rooms in the background seems unnatural and to me off-putting. Or with Annie's unlimited resources, I would have placed another flash in the rooms at the back to light them up. The light from the flash also looks a bit cold. Perhaps it needs a gel. I might have also placed a gold reflector in front of them and bounced a bit of flash into it to add even more sparkle and warmth to their eyes.  In terms of composition, I think I would tried a very slightly lower height to decrease the negative space on top. I also think I would have picked a room with a less somber color (given the current economy).  In terms of choice of equipment, I believe the leader of the free world and his family deserve a shot from a medium format camera (digital or otherwise) for posterity.

Panasonic Lumix GH1 battery in stock

The GH1's battery has been out of stock at some retailers such as Adorama. That's an issue because Panasonic tweaked the GH1 so that it won't accept third-party batteries. The good news for GH1 owners is that as of the time of this writing, the battery is in stock at Amazon at its normal price of a little over $50.

Scan the Frame

Summary: I improved the composition of my shots by taking a moment to scan the frame.

The Issue:
Old habits die hard.  When composing a shot, I usually frame the shot, move the focus point to the point of interest where I want to focus (usually the eyes), half-press the shutter then take the shot.  Although I do think about the background and the rest of the picture, I tend to do so only while thinking of the shot, before I look through the viewfinder.  The problem with that process is that I'm too focused on the point of interest and not sufficiently mindful about the rest of the picture when taking the shot.  The result is that sometimes I inadvertently leave out important elements of the picture.

For example, this shot would have looked better with our toddler's hands visible in the frame.

Sometimes, I can crop to disguise the error. Other times, I can't, or I would have to crop very tightly.

The Solution:
Awareness of the problems created by my old habits led me to take a second to scan the frame just before taking the shot.  It's not easy to resist the temptation to simply take the shot when our toddler is running around and suddenly has a great expression on his face.  But taking a moment to scan the frame improved the composition of my shots and helped me to ensure that all important elements are there.

I'm also considering getting a wider-angle zoom like the Tamron 17-50 VC if I can find a way to fit it into our budget. :)

Bouncing light off the ground

 

The idea of lighting a subject from below sounds unnatural and impractical except to make the subject look scary.  However, in some cases, it works under circumstances where other techniques don't.

This past weekend I experimented with upside down lighting by bouncing light from the ground. Upside down light is not the first lighting scheme I think of unless I want to do a "scary" photo.  But it just so happened that while our toddler was getting a haircut this weekend, he would look downward from time to time to avoid the scissors. I then remembered Joe McNally use upward light as fill in one of the bridal portraits in the Nikon video "A Hands-On Guide to Creative Lighting."

So I tried bouncing the light from the floor and got the shot at the top of this blog entry.  For comparison, here is a shot at a similar angle but with ambient light only:

In my view, the upside down light bounced from the floor was not off-putting in this case and was a reasonable choice for lighting.  Bouncing light from the floor is a technique to consider when there are no walls or ceilings to bounce from - all it takes is a reasonably light colored floor like sand or cement - like this shot at a playground, where I bounced the light from the sand.

Surprisingly, the bounced light showed up even though the shot was at ISO100 (shot was at f/2.8, 1/200, ISO 100, TTL flash triggered via CLS at 0 FEC). In fact in some shots, the bounced light was too prominent and I reduced it by as much as -0.7 FEC.

These are my observations for making floor-bounced light work:
1. Upside down light looks much better on the subject when it's soft (not hard).  In the real world, light rarely comes from below except in specific circumstances like a campfire.  However, upside down light does occur from time to time when there is a strong light from above that reflects from the ground.  When that happens, the upside down light is pretty soft because the floor is acting as the light source for the upside down light.
2. Bounce the light in front of the subject, not directly below the subject.  When the light is bounced directly below the subject, the shadows will be cast upward.  If the light is bounced in front of the subject, the shadows from the upside down light are less noticeable.
3. Floor-bounced light looks best when the subject's face is slightly lowered. This diminishes the upside down shadows.

4. To maximize your flash's power and efficiency, shoot at sync speed (if ambient is bright), and use the longest zoom for your flash.

Evolution of a family photographer

Although my photography skills still need a lot of improvement, I find it nice to look back once in a while to see some progress and chuckle at my naivete.  As I look back, I also see how our baby has grown into a toddler in just a couple of very short years. I'm sure in the blink of an eye he'll be married and have his own kids.

I started out taking snapshots just like everyone else, without any knowledge of photography:

Like most people, I figured out that the deer-in-the-headlights look is not very flattering. So I turned off the flash:

 

Sometimes, though, the shots came out blurry:

 

I thought, there must be a better way to take photos. So I learned about exposure and about how I could get sharper shots in low light without a flash by getting a camera equipped with a wide aperture lens.  After doing some research on cameras, I chose a Pentax K100D because of the shake reduction and excellent high ISO performance, and paired it with a Pentax 50 f/1.4. I got shots like this:

Then one day, my bro (who had more experience in photography) asked me to buy a flash for him. First thing I thought was wow these flashes are really expensive for something so simple! Second thing I thought was, what's so useful about a flash?  So my bro told me about how you can bounce the flash to the ceiling and get more natural looking light. I researched this some more and found Neil van Niekerk's planetneil.com blog. That convinced me to get an external flash with bounce and swivel, the Pentax AF540FGZ.  By bouncing the light from walls (not just ceilings) I got shots like this:

I noticed though that sometimes, the colors in the shot looked kind of strange and figured out that the flash was picking up the color of the surface it was bouncing from:

I learned about color temperature, and tried to use that to fix the white balance issues in my shots with Picasa (back then I didn't know about the neutral picker, or magenta-green tint or RAW).  Playing around with Picasa, I also learned about improving contrast, which helped me get improvements like this:

I plateaued there for a while, not seeking any development of my photo skills.  My shots seemed better than the previous month's not because I became a better photographer but primarily because each month our baby became a little cuter:

and a little cuter still

Armed with the planetneil techniques and just minimal photo skills, I managed to get some decent holiday shots:

Without adequate knowledge of flash exposure though, I got inconsistent results like this flash underexposure:

Playing around with the flash one day, I tried using the flash off-camera for fun:

So I got a cactus trigger and used it to trigger the flash wirelessly, often rather ham-handedly:

I turned to Strobist.com to try to get a better understanding of lighting. I read Light: Science and Magic and learned about reflections (or avoiding them).

I also bought a lightstand and umbrella, and learned from Strobist.com how to do portraits with black backgrounds:

However, I felt that running back and forth between the camera and the manual flash, plus the K100D's slow (albeit usually accurate) focus were too limiting for our baby who has getting more mobile.  So I sold my Pentax gear and got a Nikon D80 (with a Tamron 28-75 f/2.8) and an SB800.

I made all sorts of excuses to take off-camera flash shots with CLS, like this shot at a friend's party:

For a while I experimented with snoots and shaping light:

But eventually, the novelty of using a snoot wore off and I reached another plateau. Sadly, for several months, I took only a few photos of our quickly-growing toddler.

Then one of our friends had a birthday party for their daughter. Some of the shots were ok but some needed improvement.  I was reminded of the limits of my photography skills. I also saw the need to do selective postprocessing adjustments, which I learned by playing around picnik.com.

To help keep me on my toes, I then created this blog and hence the roadmap.  And that's where I've been ever since.

Upping the ante by adding soft light to your rim light

 

So, you've skillfully positioned your subject to have the sun act as the rim light. That's a great way to put some drama in your shot. But there's no reason to stop there.  If you didn't use your flash for the rim light (or if you have an extra flash handy) you can use a handheld umbrella as directional fill to get a cross-lit effect.  The hard rim light (from the sun or a flash) plus the soft key light (in our case, from an umbrella) is a lighting scheme that emulates the look of a studio shot like this shot of a fireman by Joe McNally, and you can do it on the run without setting up.

In the shot above, I waited for our toddler to come into a spot where the sun was angled as rim light above and behind where I wanted to take the shot.  I had a handheld reflective umbrella-softbox (aka softbrella or umbrellabox) ready to be triggered via CLS. I shot in shutter-priority at the maximum sync speed (1/200 for the D80) to diminish the ambient and allow the light from the flash to become dominant as the key light. EXIF: f/2.8, 1/200, ISO 100.  Flash was on TTL at 0 FEC. The focus was not exact (it's a bit behind the eyes), which I tried to remedy in post processing by increasing sharpness, and increasing local contrast ("clarity" in photoshop).

In the shot below, the sun was above and behind the subject to the camera's left.  The handheld softbrella was camera right. For the exposure, I tried Neil van Niekerk's approach of using manual exposure mode, figuring that because I was underexposing the ambient anyway, then I didn't need to worry about getting exposure exactly right.  I chose 1/200 (max sync) and f/4.0 (where the lens has a higher MTF to get a sharper shot, and with a slightly greater depth of field to have more room for correct focusing).

At ISO 100, that was about 1 stop underexposed, which is fine because the flash was going to be the key light anyway.  Flash was on TTL at 0 FEC.  Although the ambient was underexposed by 1 stop, there was no need to adjust FEC.  If I had been on shutter priority, and had dialed in -1 exposure compensation, then I would have had to increase FEC by 1 stop to get the same flash exposure because with Nikon TTL, the exposure compensation setting affects the flash exposure as well.

Next time I have a similar opportunity, I would like to angle the umbrella so that its light is coming even more from the side, making the directionality of the light more apparent.

Inspiration from other photographers: Roy Sabay

I am amazed at Roy Sabay's work.  Where do I start?  Lighting, postprocessing, dodging and burning, composition, tonality...  I find it even more astonishing that he got serious with photography only as recently as October 2007 - that's a few months after I started!  Most of Roy's photos seem to be set up as opposed to candid shots; nonetheless, I hope to learn more from observing Roy's work and apply what I can to my family photos.

Taking Halloween pictures - part 1: the plan

Last year our toddler's Halloween pictures came out mediocre (like in the shot above) because I didn't understand how to control the flash properly.  This year, with better understanding of flash controls, I'm looking forward to the many photographic opportunities available at Halloween.

Here's my plan for this year. We're spending Halloween at Disneyland again, so it will be a good test of whether and how much I can improve my shots over last year's pictures:

1. Balance ambient and flash
a. in TTL, ambient can be controlled not just by shutter but also by ISO and aperture. Last year I controlled ambient only by slowing the shutter, which sometimes led to unintended flash blur like this:

b. I will watch flash exposure carefully and decrease FEC as necessary. TTL may be fooled with predominantly black costumes.

2. Gel the flash with CTO to simulate firelight and candlelight.
Last year I didn't use a gel at all, which made the light from the flash obvious and artificial:

This year, I will be putting two layers of 1/4 CTO (so that I can remove one and get 1/4 CTO if necessary).

3. Use handheld reflective (not shoot through) umbrella:
My plan is to use soft but restricted light, kind of like this photo that I saw on flickr:
http://www.flickr.com/photos/fotokurse-berlin/57502154/
I don't know for sure how the shot above was taken, but I believe that look can be achieved with a gridded softbox, which makes the softbox much more directional.  (Either that or maybe a softbox was moved in very close to the subject to make the light fall off faster.)
Anyhow, a shoot-through umbrella results in more spill.  I want more directionality to add more drama.  I don't have a gridded softbox (or a softbox for that matter), but I do have a reflective umbrella, which is more directional.  Reflective umbrellas (especially the silver ones) can be more specular, but the one I'll be using is a softbox-umbrella hybrid I bought from ebay (like this one).
As an added bonus, Halloween is probably one of the few days in the year when no one will give a second look to an umbrella.

4. Use noise reduction software

Until recently, I never bothered to use noise reduction software.  Now that I understand that I can use ISO to increase ambient exposure (instead of just using a slower shutter speed), I see the need for noise reduction software so I can be more daring with higher ISOs.

The stuff I plan to bring are:

• Nikon D80
• Tamron 28-75 2.8 (I would prefer a 17-50 2.8 if I had one so I could capture more scenery as "establishing shots")
• SB800 w/ two 1/4 CTO gels
• tripod - for long exposures and as a boom for rim light

I have other more ambitious ideas like using the D80's multiple exposure mode to make ghost-like images and composite shots with a "larger" moon, but these are what I plan to do at a minimum.

Taking photos at aquariums and exhibits

Taking photos at aquariums is easy once you know a few basic concepts.  Note: I used aquarium photos here but the concepts are the same as those for museums or other exhibits with glass displays.

Disclaimer: be sure to check if taking photos is allowed, and whether flash is allowed. For museums, be aware of copyright laws.

Goals of a decent aquarium photo:

1. Aquarium looks natural (similar to how you see it with your eyes).

2. Subject (your spouse or kid) is adequately lit.

3. No distracting reflections.

Aquarium photos pose these challenges:
1. aquariums tend to be dimly lit, making it hard to take pictures of your spouse or kids without using a flash.
2. aquarium glass often shows the reflection of the camera flash instead of the fish.
3. the ambience of the aquarium is usually lost in the photo.  Taking a shot in auto mode will usually result in an overexposed, washed out subject with a deer-in-headlights look, and a bright reflection of the flash on the aquarium, with the water appearing black or navy blue and the colorful fish nowhere visible.

There are just three basic concepts you need to take proper aquarium photos:

Concept 1: Balancing ambient vs. flash.
The key to showing the colors of the fish is to meter for the aquarium's existing light (instead of metering for the subject, such as your spouse or kids), and then let the flash adjust accordingly.  To learn how, check out this tutorial.

Concept 2: Understanding reflections.
To avoid unwanted flash reflections on an aquarium or other reflective surface, you need to analyze the range of angles where the flash will be visible as a reflection.  The basic tool for such analysis is a diagram (until you get to the point where you intuitively know which reflections will be visible).
1. Draw the camera and the surface of the aquarium that is within the camera's field of view.
2. Draw the position of the flash (if the flash is on-camera, the flash is of course in the same position as the camera).
3. Draw a line from the flash to the left edge of the aquarium.
4. Draw the reflection from the line in step 3.
5. Draw a line from the flash to the right edge of the aquarium.
6. Draw the reflection from the line in step 5.
If the camera is between the lines in step 4 and step 6, then the reflection will be visible in the shot.  If the camera is not in between those lines, the reflection will not be visible in the shot.
In the diagrams below, the blue bar represents the aquarium surface visible within the camera's field of view. The dotted blue lines are imaginary lines perpendicular to the edges of the visible aquarium surface.  The red lines represent the lines in steps 3 and 5.  The green lines represent the lines in steps 4 and 6.

In the top diagram, a camera with on-camera flash is squarely facing the aquarium.  The camera is in between the green lines, thus the flash will be visible.
In the middle diagram, a camera with on-camera flash is facing the aquarium at an angle.  The camera is not in between the green lines, thus the flash will not be visible.

In the bottom diagram, a camera is squarely facing the aquarium, but an off-camera flash is off to the side.  The camera is not in between the green lines, thus the flash will not be visible.

An alternative way of doing a reflection analysis is to diagram the camera in the same way as you would for the flash.  If the flash (or any other object) is outside of the green lines from the camera, the flash reflection won't be visible.

As an exercise, how does the focal length of the lens affect the visibility of a flash reflection (assuming that the camera distance is adjusted so that the same amount of the aquarium is visible)?  Hint: A tele lens will more easily avoid reflections.

Concept 3: Controlling flash intensity.
With TTL, flash is adjusted by flash exposure compensation.  See the TTL Flash FAQ and this tutorial.

Specific strategies:
1. For cameras with on-camera flashes:  Change the angle of the aquarium to avoid a flash reflection within the cameras field of view.
Sample photo (from a point and shoot camera):

In this shot, I avoided a direct flash reflection by shooting the aquarium at an angle (like the middle diagram above). (As for the reverse letter C-like reflection on the upper right of the image, I think it's a reflection of the retroreflective band on our stroller.  That shouldn't show up in normal situations.)

2. For off-camera flash: Take a photo directly facing the aquarium, but position the off-camera flash to the side and angle it greater than about 45 degrees (the exact minimum angle varies with your field of view and focal length).
Sample photo:

In this shot, the flash was at camera right, aimed at about slightly more than a 45 degree angle.  In retrospect, I should have tried aiming the flash at an even sharper angle so that the reflection on the bottom left would not be visible but at least by aiming it at an angle, it is much less distracting than a direct flash glare.

3. Ambient-only: some aquariums don't permit flash photography.  In addition, sometimes it is feasible to avoid using the flash if the aquarium is large enough and emits enough light to be used as a light source for your subject. Keys to making the shot work:
a. shoot in RAW if you want to correct the white balance.
b. use noise reduction software. For the examples below I used Neat Image.
The photo at the top of this blog entry was taken ambient only.  Here's a similar shot where white balance was adjusted. If I had taken the shot in jpeg, it would have been impossible to get an acceptable output with this extreme white balance adjustment (the color temperature of the bluish light from the aquarium was over 50,000K).

Sample:

Of course, the foregoing strategies can't address every conceivable situation.  However, I hope that by discussing lighting concepts (including samples of applying those concepts), I can help you can generate your own solutions.  For example, here's a different kind of situation: at Legoland SeaLife in California, they have aquariums with glass domes in them from which you can get an immersive view of the fish while people looking at the aquarium can see you.  Taking an ambient-only shot, we got the following picture:

There was room for improvement in that shot.  The aquarium looks natural enough, but the subject was not easy to see through two layers of reflective glass and the water in between.  One solution is to use an off-camera flash, which I placed under the glass dome, aimed upward at the subject, then triggered via CLS:

Other resources:
Here is a related post about how to avoid reflections on eyeglasses.

If you liked the discussion about analyzing reflections, check out: Light: Science and Magic.

If you are interested in reading about balancing existing light with flash, read: On-Camera Flash or the TTL Flash Tutorial.

To learn more about wireless flash, read: Nikon CLS Practical Guide, Strobist.com, and this blog.

Panasonic GH1 back in stock!

Any family photographers out there searching for the GH1 can rejoice. At the time of this writing, it appears to be back in stock at Amazon.com and Adorama at the normal price of $1499.95 (kit lens included).  I ordered one for a family member while it was backordered just a couple of days ago from Adorama and was shocked to see a shipment notification today.  I'm supposing other retailers have received supplies too.