jvarszegi
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 p.1 #20 · Lenses for XTi (Portraits and People Photos) | |
alexandre wrote:
why not a medium f-stop and a bounced flash?
The f-stop is only part of the equation that determines how much light the flash will put out. In general, I've found this is an effective way to use a single shoe flash:
- choose the widest aperture possible whilst still getting the shot
- choose the highest ISO you can live with (usually in the 800-1600 range, after that the noise gets awful unless you have a fancy-shmancy 1DIII)
- choose the slowest shutter speed that will both freeze subject motion and counteract camera shake
- use manual mode
- bounce the flash on the ceiling, if possible-- and a fair distance away from the subject (this often means back over your shoulder)
- "gel" the flash to match the color of the ambient light
The first three measures are designed to reduce the amount of light the flash puts out. When you do this, the resulting exposure has much more ambient light than if, say, you shoot at f/11 ISO100 1/500s, in which case the flash would put out a nuclear blast of light, the picture would be very ugly with harsh shadows and a dark background, and the flash would eat up batteries quickly and take a long time to recycle between shots.
The resulting light can look very soft and natural:
http://www.smugmug.com/photos/101036795-L.jpg
The shot above is fairly stopped down for me, and what I would call borderline wasteful-- f/2.5, ISO 640. I could have shot at f/2.2, for instance, and had just slightly less DOF, which might have even improved the shot. Every third of a stop helps. Here's another, a little less harsh on the flash output even though at f/2.8:
http://www.smugmug.com/photos/95787206-L.jpg
Now, at the other end, we have shots like this, at ISO 100, f/16, 1/200s:
http://www.smugmug.com/photos/28806868-L.jpg
And the middle ground, at ISO 100, f/5.6, 1/200s:
http://www.smugmug.com/photos/38134252-L.jpg
Notice here the harsh shadow under the hand, the almost-blown ("hot") shirt, the dark background, etc. etc. etc. What could be done to improve this shot? Well, just going to ISO 200 would have halved the light put out by the flash, giving a huge benefit. But once you go to 200, why not 400? That would cut it to 1/4! This is the sort of learning and experimentation process that people usually go through when they're learning to shoot flash.
The difference is obvious. Once you start realizing you want to lessen the harshness of the flash, it becomes a tuning exercise-- how much softer can you make it? And your flash shots improve. The last shot is better than the one before it, but not as good as the first one I posted. (Or that's my strong opinion, anyway.)
All of this also explains why catchall advice, such as "always shoot at ISO 100 f/5.6" etc. is wrong. It's not necessarily wrong because the advice cannot work well on occasion-- it is wrong (IMHO) because it is wasteful for the situations where you could go with a wider aperture etc., and is doomed to failure when you need more DOF, which happens with groups fairly often.
To answer the question: if you need a middle aperture, so be it-- the shot demands it. But if you can reduce DOF and still get the necessary parts of the frame in focus, why not do it? The light will be less harsh on the subject, there will be more ambient / background light, and people used to seeing P & S pictures will swoon over your dreamy backgrounds. 
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