Yay! for Sean ... and a quick question about manual flash
Many thanks for starting this forum, Sean. I hope it will become a great source of help and information on macro photography.
To tag a quick question to what is essentially a lame attempt to be the first person to post on the forum : is there any way to easily measure the amount of light lost to a diffuser or softbox attached to a flash and come up with a revised GN for doing manual flash photography? I can only think of testing as the alternative but am wondering if there is any other way.
...apart from a flash meter, obviously.
They aren't that expensive any more, even new.
Not to be a complete idiot, but why do you want to use a softbox/diffuser when shooting closeup. If the flash isn't too far from the subject it is effectively a large, not a point, source of light and will give fairly soft lighting. If I weren't non-digital I'd show you some scans, but since I'm not you'll have to take my word for it.
If your problem is specular reflections, a light tent -- a diffuser around the subject, not around the flash -- is the classical solution.
Well, there are two controls for light:
Size, relative to the subject, and
Distance from the subject.
Distance from the subject affects the gradation across the field,
so if you want an even light across the field, you move the light back,
but if you want a variation across the field, the light belongs close to the subject.
I suppose if one is making a clinical image,
the light should be further away,
but for an expressive image, you can bring it close.
"One of the painful things about our time is that those who feel certainty are stupid,
and those with any imagination and understanding are filled with doubt and indecision"
Dan, you are right. My problem is indeed specular reflections. But I can't figure out a way to put a light tent around insects in the field, as in this shot, for example.
Originally Posted by Dan Fromm
The other thing I have been trying is cross polarized flash, but so far my results have been miserable - need to experiment a bit more with it to figure out what the problem might be.
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Flashmeter or testing are the options. It's not that big a deal to test, if you don't have a flashmeter. Before I had a flashmeter, I tested all my studio reflectors and diffusers and came up with guide numbers, and would map out studio setups in advance before shooting, so I would have the right f:stop. As long as I didn't change the light-to-subject distance, I had some flexibility in moving the lights during the session. It was absolutely reliable.
So do a test with slide film at a non-macro distance so you don't have to calculate exposure factor. Figure a softbox is going to cost you between 1.5 and 2.5 stops and shoot a series of frames a half stop apart.
Then when you are shooting macro, don't forget to include the exposure factor for high magnification.
In a darkened room with a fairly bright light ie 200 watt hold an incident meter in a fixed position reading the central part of the beam. Take a reading at a particular shutter speed without the diffuser in place, note the fstop. Take the same reading with the diffuser in place at the same shutter speed. How much did the fstop change? The change is your correction.
Claire (Ms Anne Thrope is in the darkroom)
I understand the advantages of using slide film (narrow latitude, no printing compensation), but would using b/w film and viewing the negs themselves be reasonably valid?
Originally Posted by David A. Goldfarb
If you are good at reading negs, you could do that, but slide film usually will give you a more precise test. If you're using B&W neg film, you could print a contact sheet at minimum time for maximum black for this sort of test.
Though small manual flash units are inexpensive, if you are working in the studio you may also want to consider using fiber optic lighting for close-up/macro work particularly as the subject gets smaller and smaller. Metering will be simpler and easier and you can see the actual effect on your subject. Another item that can be useful is to use small mirrors and work with 1 flash or light source.