Please help me with my new flash

Started Jan 28, 2004 | Discussions thread
Mike Fitzgerald Veteran Member • Posts: 9,542
Re: Try this

Hi Amit --

What you're hopefully getting out of this is that there are several variables you need to consider, and your best asset is your obvious willingness to experiment. Here are my thoughts on a few aspects of this:

1) "Compensation" for bounce

PinHole has described perfectly what goes on with any competently designed auto bounce flash in theory; i.e. the metering sensor stays aimed directly at your subject (assuming "normal" mounting of the flash unit in the accessory shoe or flash bracket) while the head moves independently to set your bounce angle.

For semi-direct, diffused flash using the LumiQuest Pocket Bouncer (= 80-20 with an insert in place), exposure should therefore be virtually identical to what you'd get with direct frontal flash (without the LQ). Naturally there will be subtle subjective differences in the "look" of the image because of the softer shadows the LQ produces.

With ceiling bounce there are two additional factors you need to consider: one of them the nature of your subject, the other mathematical:

(a) Strong overhead lighting gives you a considerably different mood, relative to direct frontal flash, especially with standing people as your subject. There’s greater shadowing of facial detail, clothing and accessories and, even though the shadows are soft, the image will have less "punch" than a direct flash shot.

That alone may be why a bit more exposure can often improve a bounce shot. As you'd expect, the closer you are to your subject, the closer to vertical is the downward light coming from the ceiling and the greater the effect of shadows in producing a sombre looking result.

(b) Ceiling-to-subject geometry. Forgetting about shadows for the moment, let's look at the light fall-off from top to bottom of your subject. Imagine a 6 ft tall person standing under an 8 ft ceiling, shot at such close range that the patch of light (effective bounced source) on the ceiling is almost directly overhead. The distance from that bright patch on the ceiling to the top of the subject's head is 2 ft, while the distance to their feet is 8 ft. Subject illuminance decreases with the square of its distance from a given spreading light source, so in theory there will be a 4 EV (!) fall-off in brightness from head to toe. This is obviously an extreme case, which also ignores both the size of the ceiling patch and secondary bounce from walls and other light coloured objects nearby, but it helps to illustrate an important effect.

Now consider the same subject shot from 8 feet away, with the flash aimed vertically at the ceiling, directly above the camera. Again, to simplify things I'll ignore the size of the patch of light on the ceiling, and we'll assume that most of the light is coming from a point on the ceiling at that distance. This time, the distance from the effective light source (ceiling patch) to the subject's head is 8.25 ft, and to their feet it is 11.3 ft. A much smaller difference in light path.

Here's a post containing two bounce shots taken from considerably differing range. (Note that the other person's images I refer to in that post appear to be no longer available.) --
http://forums.dpreview.com/forums/read.asp?forum=1009&message=7230973

Bottom line: Start by matching the flash and camera aperture, but expect that you’ll probably need to increase exposure somewhat (smaller aperture, i.e. higher f/number, on the flash, or the opposite on the camera) with ceiling bounce alone. The amount of exposure bias will change with subject distance because of the factors described above. The 20% frontal contribution from the 80-20 Bouncer will become more important at closer ranges.

2) Shooting mode

(a) For flash only shots (i.e. where you want to basically avoid any effects from ambient light), Manual (camera) gives you the best control, taking irrelevant variables out of the equation. It also makes learning easier when you’re only changing one thing at a time. My own recommendation for shutter speed is that you initially keep it up a bit – say 1/125s to 1/250s – until you are comfortable with the cause/effect relationship in your flash experiments. When you’ve achieved that, you may wish to play with lower shutter speeds so as to allow the ambient light to contribute more to the exposure. If you look at Shay’s current wedding photos thread you’ll see that he quotes a shutter speed of 1/30s for the bulk of his shooting on that occasion. A low speed like that has the potential to provide more balance lighting of the surroundings, beyond the main flash-to-subject range. The mixed colour temperatures can often be used to good artistic effect in such a setting, as sometimes can deliberate background subject movement; but you should initially regard that as an advanced technique and start with higher shutter speeds, even though you believe you’re freezing your subjects adequately at slower ones.

(b) For outdoor fill-in shooting you’re best off using Aperture Priority. That allows the camera to change the shutter speed freely (within available limits) to expose the background correctly, while the fixed aperture leaves the flash to take care of the main subject.

3) Overexposure at close range

I don’t know what to expect from the Promaster in non-dedicated mode, save to say that it should be able to throttle itself back without any trouble for quite close subjects. Naturally, average subject reflectivity will have a significant effect on the result, but from an electronics viewpoint – both componentry and design technique – getting this to work properly is really old hat. The technology has been there for decades and there’s just no excuse these days for the poor design we seem to be seeing so often:
http://forums.dpreview.com/forums/read.asp?forum=1009&message=4476053

Mike

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