Secret Tool for Smoke Photography (Intermediate)

[What's this got to do with family photos per se?  Nothing. It's just for lighting practice and for fun.]

Not too long ago, I put up a post re smoke photography.  After thinking about it a while, I identified these as the most important factors for a technically good smoke photo:
1. sharpness - this is primarily from the smoke being in focus.
2. clean background - ideally the background is pure black (prior to postprocessing).

Of course there are other factors for a good smoke photo such as manipulating the smoke into an interesting form, but on a purely technical level, those are the most important ones.  If you expected that your camera makes a difference, you'd be right.  But you may be a bit surprised that arguably the best camera for the job, believe it or not, is a point and shoot camera.

SHARPNESS AND FOCUS
Smoke has low contrast and moves constantly, making it difficult to focus on it.  Instead, focus is achieved by using a large depth of field.  A p&s has a far deeper depth of field than a DSLR because of the p&s' small sensor, making it easy to keep much of the smoke in focus, and thus sharp.  In the shots above, I used f/4 or thereabouts.  On a DSLR, the depth of field with such an aperture is quite shallow when shooting as close as we do with smoke photography.  If I had been using a lens with 35mm focal length (52.5mm equivalent in 35mm terms) at a distance of 3 feet, an aperture of f/4 would result in a depth of field of 0.35 feet (see the Depth of Field Calculator).  Even at f/16, the depth of field would be 1.46 feet.

I used a dinosaur of a point and shoot - the Konica Minolta Dimage Z1 (my first digital camera!) from 2003.  Its sensor is 1/2.7, even smaller than the common 1/2.5 sensor size on compact point and shoot cameras.  With the Z1, at a focal length of 7.6mm (50mm equivalent in 35mm terms), also at a distance of 3 feet, an aperture of f/4 has a depth of field of 2.09 feet - deeper than the the depth of field of the hypothetical DSLR at f/16.  If I had stopped down to f/8, the depth of field would have been 6.22 feet.  So although the Z1 has a atrociously slow autofocus system, ironically, it is able to keep the somewhat challenging subject here in focus more easily than a D300 - simply because of its large depth of field.

CLEAN BACKGROUND

The second technical criterion is a clean background.  That is accomplished partly by aiming the flash away from the background (if possible), but also by minimizing ambient light.  In that regard, a point-and-shoot has no mechanical shutter, and thus is theoretically not subject to a sync speed limit.  It is possible therefore to kill the ambient using a very high shutter speed without affecting flash exposure.  In the shots here, I used the Z1's maximum shutter speed of 1/1000 with no problems.

As for the light, one can use a point and shoot's built-in flash (which is weak and therefore should have a fast flash duration, another plus).  However, an external flash with an optical slave can be triggered by the built-in flash of the point-and-shoot camera.  Here I used the YongNuo YN-560, which has a digital optical slave to ignore the TTL preflashes of the point-and-shoot camera.  The YN-560 sync'd optically even at 1/1000 shutter speed.

WHAT ABOUT NOISE?

A point and shoot does have a noiser image than a DSLR, but many point and shoots have very low ISO modes that have acceptable noise in my opinion. The Z1 I used goes as low as 50 ISO.  I have a Casio Exilim EX-V7 that goes to 64 ISO.  By comparison, the D300 has a base ISO of 200 (100 is possible, but with a smaller dynamic range).  Fortunately, with a flash, we can use a low ISO, which will also help kill the ambient light.