Monday, April 2, 2012

Full Frame DSLR FAQ

Sorry to disappoint you guys, but I'm still with the dark side :)
With the release of the Canon 5D Mk III, Nikon D4, and Nikon D800, previous generation full-frame DSLRs have become available at a significant discount, putting them within the reach of photographers considering high-end crop sensor DSLRs.  Should you make the move?

This FAQ discusses the differences between full-frame DSLRs and "crop sensor" DSLRs.  It is divided into two sections: Basic and Intermediate.



PART 1: BASIC

1.  What is a full-frame DSLR?
It is a DSLR with a sensor that is about the same size as a 35mm film frame (36mm x 24mm), which became the dominant still photography format for consumers starting in the 1960s.

2.  What is a "crop sensor" DSLR?
It is a DSLR with a sensor that is smaller than a full-frame sensor.  It's sometimes called a "crop sensor" because the resulting image seems like a cropped version of a 35mm sensor image (even though there is no actual cropping involved).  There are many crop sensor formats.  Synonyms: small sensor camera.

3.  What is an APS-C sensor DSLR?
An APS-C sensor is a type of a crop sensor that measures about 24mm x 16mm, with some slight variation between manufacturers.  It is called APS-C because it is similar to the 25.1mm x 16.7mm size of the Advanced Photo System film format introduced by Kodak in 1996 (like Kodak Advantix, if you remember those).  The APS-C sensor size is currently the most common sensor format for DSLRs, especially entry and mid-level DSLRs.

4.  What is a crop factor?
It is the ratio of a full-frame sensor length (or width) to the corresponding length (or width) of a crop sensor.  For a sensor measuring 24mm x 16mm, the ratio is 1.5x.  (Take note that this is not the ratio of the area of a full-frame sensor to a crop sensor.  A full-frame sensor has more than twice the area of an APS-C sensor.)  When a lens is used on a crop sensor body, it has a field of view on a 35mm body when the focal length is multiplied by this ratio.  For example, a 50mm lens used with a Nikon APS-C sensor DSLR results in a similar field of view with a 75mm lens used on a full frame DSLR.  Sometimes referred to as "focal length multiplier," even though the focal length is never actually multiplied and retains the same distance regardless of the format.

Check the image below, it represents different sensor sizes, with the crop factor shown at the left edge.

Sensor Sizes Compared - Credit: Wikipedia

Now that you've seen how much difference there is between sensor sizes, let's relate that to real world cameras, some famous full frame DSLRs are the Nikon D3 and the Canon 5D, and famous crop sensor (APS-C) DSLRs are the Nikon D90/D300 and the Canon 60D/7D. The 1/2.3" sensor is your average point and shoot sensor, just for fun, see how many 1/2.3" sensors you can fit inside a full-frame sensor, the answer is 35.

5. What are the advantages of a full frame sensor DSLR over a crop sensor DSLR?
  • Shallower depth of field (see below).
  • Generally has less high ISO noise.
  • Generally has better dynamic range (can record more shadows and highlights data).
  • Viewfinder appears larger.
  • Lenses with traditional focal lengths can function as designed on a full-frame (where they are often too long for an APS-C camera).  For example, 24-70mm for standard zoom, 70-200 for telephoto zoom, 85mm for portraits.
  • All other factors being equal, a full frame camera has a higher diffraction limit (i.e., a narrower aperture before the effects of diffraction cause image sharpness to deteriorate).

6. What are the advantages of a crop sensor DSLR over a full frame sensor DSLR?
  • Deeper depth of field (see below).
  • The DSLR body is usually smaller, lighter, and less expensive.
  • The lenses are usually smaller, lighter, and less expensive.
  • A crop sensor DSLR can use lenses made for either full-frame DSLRs or crop sensor DSLRs whereas a full-frame DSLR generally cannot use a lens made for a crop sensor DSLR without heavy vignetting. In Canon's case, you can't use crop sensor lenses (EF-S) on a full frame.
  • When a full-frame lens is used on a crop sensor DSLR, you are using mostly only the center part of the lens, which tends to be the sharpest and least susceptible to vignetting, i.e. while your 50mm f/1.4 lens is sharp wide-open on your APS-C camera, it might not be as sharp at the same aperture on a full-frame camera, since now it's using the whole projected image of the lens instead of only using the center area of the projected image (usually sharped than the edges) in the APS-C case.

Part 2: Intermediate

DEPTH OF FIELD

1.  How much shallower is the depth of field of full-frame?
Short answer: for the same field of view, about 1.3 stops (more than twice as shallow). But see below.

2.  If you use the same lens on a full-frame camera and a crop sensor camera, will the depth of field be different?
Answer: There are two possible answers to this question. One way to answer the question is that the depth of field will be the same, given the same actual focal length, aperture and same camera-to-subject distance, regardless of format.

To make it more clear, I made the below illustration quickly using MS Paint, the black circle is the image projected by the lens (remember, we are using the same lens for both sensor sizes), now if we place the full frame sensor in the middle of the projected image, we capture most of the projected image. But if we put the smaller crop sensor in the middle, we would only capture the middle part of the projected image, and as a result it will look as if it is magnified. But will the DoF be different? No, everything is exactly the same in both situations, the only difference is in the magnified look. Simply stated, if you take the full frame shot and crop it to match the crop sensor shot, both images should be identical.


The other possible answer takes into account that the circle of confusion for a crop sensor is actually smaller than that of a full frame sensor.   The APS-C sensor is like cutting the middle of the FF sensor then magnifying it to the same size as a full frame sensor, so any blurriness becomes more noticeable.  It is the same reason why the diffraction limit is lower (wider aperture) for APS-C compared to FF.
You can try this in the DOF calculator.  If you keep all variables constant and the only thing you change is the camera (from APS-C to FF or vice-versa), you'll see that: 1) the circle of confusion is smaller for an APS-C; and 2) the DoF is actually shallower for APS-C compared to FF.  Therefore it could also be said that at the same actual (not equivalent) focal length, aperture and same camera-to-subject distance, the crop sensor camera has a shallower depth of field.

I have tried this out as an experiment, using a 50mm at f/2.0, with a full frame and with a crop sensor, and I don't see much of a difference in depth of field (the text on the far block is just as blurry in both shots).

50mm f/2 on full-frame sensor
50mm f/2 on APS-C sensor

3. If you use the same lens on a full-frame camera and a crop sensor camera, and you crop the full-frame image, will the depth of field be different?
Answer: The depth of field will be the same. Here are a couple of test shots. Both were with a 50mm at f/2.  The top one is with an APS-C camera. The bottom is with a full frame but I forced the camera to crop the image to an APS-C size. The resulting images are essentially identical.

50mm f/2 on APS-C sensor


50mm f/2 on full-frame camera with forced APS-C crop.

4.  If you use a 75mm lens on full-frame and put a 50mm lens on a crop sensor, will the field of view be the same? Will the subject distance be the same (for the same field of view) and will the depth of field be the same?
Answer:  The field of view will be the same and the subject distance will be the same. The depth of field will be different -- the full frame will have a shallower depth of field under these conditions (because of the longer focal length being used on the full frame camera).

Here are test shots that demonstrate this. The top one is with a crop sensor camera at 50mm, f/2.8. The bottom one is with a full frame camera at around 75mm, f/2.8.  Both shots have essentially the same field of view, and I was shooting from an identical distance from camera to the subject. The depth of field is quite different, as you can see from the blurriness of the farther block.

APS-C at 50mm f/2.8

Full frame at around 75mm, f/2.8
5.  What if I want a deeper depth of field?  Is an APS-C camera a better tool than a full frame camera?
I have not tested this, but at least one commentator believes that a full frame camera has the advantage for a deeper depth of field because you can take advantage of the higher diffraction limit and better noise to use a narrower aperture to achieve the same depth of field with a better signal-to-noise ratio.  See   http://www.clarkvision.com/articles/dof_myth/

LENSES

1.  Can you use a crop-sensor lens on a full frame?
Answer: It depends on what camera you have.  Usually, the image circle of a crop-sensor lens will not fill the frame of a full-frame image.  (Note: there are a few crop-sensor zoom lenses that can fill a full-frame at longer focal lengths).
On Nikon full-frame cameras, the camera can automatically detect that a crop-sensor lens ("DX") is being used and can crop it automatically. (However, the automatic cropping can be turned off if you wish).
In Canon's case, crop sensor lenses (with the EF-S designation) cannot be used on full frame cameras, other than the vignetting, they have a different design with the lens' rear element closer to the sensor, and if used on a full frame camera, the camera's mirror will hit that protruding part of the lens when taking a picture. Now you don't want to do that to your multi-thousand dollar camera.

APS-C lens used on a Nikon full-frame; automatic cropping disabled

2.  If you use a crop-sensor lens on a full-frame will you retain the high ISO advantage of the full frame?
Answer: Yes but at a lower resolution. If you down-res a crop sensor photo to the same size, the gap in ISO performance will be reduced. The shot below is with a crop-sensor lens on a full frame body, with automatic cropping activated. The shot was made at 25,600 ISO (with +30 luminance noise reduction applied in Lightroom 4).

APS-C lens on a full-frame; 25600 ISO

3.  Is a full frame camera better for taking wide angle photos?
Answer: A lens will appear wider on a full-frame camera assuming it can fill the entire frame. However, there are now several ultrawide angle lenses for APS-C cameras. For example, the Sigma 8-16 (APS-C only) has an equivalent focal length of 12-24mm, which is just as wide as, for example, the Sigma 12-24 on a full-frame. These crop sensor lenses are usually less expensive than their full frame counterparts, and have similar performance.  Moreover, a crop sensor camera has a deeper depth of field which is often desirable for landscape photos.


NOISE

1.  How much better is the full-frame noise performance?
Answer: This will of course vary with the cameras being compared. In general, for the same generation, the difference is about 1.5 stops. For example, the Nikon D300 (announced August 2007) was measured by DXO to have a signal-to-noise ratio of 30dB at 679 ISO. The Nikon D3 (announced together with the Nikon D300) was measured at 2290 ISO using the same standard, a difference of about 1.7 stops. The Nikon D7000 (announced September 2010) has a score of 1167 ISO, a difference of about 1 stop compared to the Nikon D3.

[mshafik: in my case I don't really believe/care about DXO's measurements, noise is a very subjective topic, and what appears bad for me, might look very good for someone else. I find that shadow noise is my worst enemy, so when I am taking a shot at a high ISO, I make sure that I expose the photo correctly or even slightly to the bright side. But to answer the question, comparing my Canon 60D to my 5D Mark II, I find that 6,400 ISO on the 5DMk2 is as good as 1,600 ISO on the 60D].

2.  Does the full-frame high ISO performance make a real world difference in the kinds of shots you can get?
Answer: Of course, previously I was exclusively shooting at 1,600 ISO, and only used 3,200 ISO for emergencies (with not-so-great results), but now I am using 3,200 and 6,400 ISO without a second thought. The benefit is the ability to shoot in darker situations with available light only, and the ability to use faster shutter speeds to reduce blur and freeze action.

DYNAMIC RANGE & COLORS
1. How does the dynamic range of a full-frame compare with that of a crop sensor?
Answer: All other factors being equal, a full-frame should have greater dynamic range because it has greater capacity for collecting photons.  Source: http://www.cambridgeincolour.com/tutorials/digital-camera-sensor-size.htm and http://www.clarkvision.com/articles/does.pixel.size.matter/
I cannot test this scientifically because I sold my Nikon D300, and my remaining cameras have too many other differences between them (my full frame D3 is one generation newer than my APS-C D70, and my D70 camera is mid-level as opposed to high end).  In addition there is a lot of variation in dynamic range between different manufacturers' cameras.  Plus, there is an element of subjectivity in judging dynamic range because people have different levels of what they consider to be acceptable noise or color shifts.

For what it's worth, I tested my full frame D3's and APS-C D70's ability to recover highlights and shadows.  I did a 10-stop bracketed exposure, from -5 underexposure to +5 overexposure, then reversed the under- or overexposure in post using Lightroom 4, with white balance corrected and zero noise reduction applied.  The slideshows below shows the results.
First, the D70:


Next, the D3:

The full album for the D70 is here, and the D3 is here, both with full resolution JPEGs.

In terms of recovery from overexposure, the D70 has possibly usable results from 3 stops overexposure, but there are several areas that were blown out and have no detail or have no color.  The D3 has a better result at recovery from 3 stops overexposure, with less blown out areas that have no color.

When recovering from underexposure, the D70 has banding noise, yellowish tint in the highlights and a bluish/purplish tint in the shadows when recovering from -5 and -4 stops underexposure.  Meanwhile, the D3's recovery from -5 stops underexposure has slight yellow bands and a slight bluish/purplish tint in the shadows.  It may be usable.  Here is what it looks like with a +40 noise reduction in Lightroom applied (click for full size):


I don't have a real world shot demonstrating the dynamic range of FF, but here's a demo of sorts.  Here's the original shot (the light is a bug zapper with bluish light, and nearby is a yellowish CFL lamp):

And here's the shot after adjustments in Lightroom 4 (click for full-resolution JPEG):