Although auto fill-flash is a nice feature to look for in a flash unit/camera combination, it's not always necessary if you - and your camera - know what they're doing. Your plain jane TTL flash may already be smart enough.
I spent a sunny afternoon a couple summers ago at a work social function, having fun knocking back a few cold ones, and generally enjoying the company of co-workers I don't get to see that often. (I'm one of these "telecommuters", working with people 3 provinces away...) In the crowd was a fellow camera buff, who I noticed didn't take a shot without using his flash. It was a Canon 10D digital camera, with a flash unit that had a "fill" setting.... something only advanced models have. The photos were amazingly balanced from a tonal perspective... no harsh shadows despite the sun. It was hard to tell a flash was even involved. I was impressed - and disappointed that none of my flash units had this feature. I began suffering equipment-envy!
All Was Not Lost
Jim Hutchison is a part-time photographer who resides in Burnstown, Ontario with his wife Moira.
I happened upon a newsnet article mentioning that my camera model (Pentax ZX-5n) had the innate ability to "do" balanced fill flash, although the feature is not documented. I read and absorbed! Following the writer's instructions, I shot a roll of print film at various settings, noting f-stop, shutter speed, etc. After reviewing the results a few days later, I was quite pleased with the balance between natural light and the flash's output. (Thanks to whoever the writer of that short article was...) This first experiment was indoors, therefore with daylight print film in the camera, it was easy to differentiate between ambient and fill light by the colour temperature. The warmer the light, the less flash there was... and vice versa - but each exposure was perfect! Let me explain in more detail how this is achieved.
Exposure Compensation Dial: the Secret
Most, if not all SLRs have a means to manually over-ride the camera's automatic exposure settings. My particular model has 3 ways of achieving this: auto-bracketing, an OSI dial, and a DX over-ride setting. Auto-bracketting can be set to either 1/2 or full-stop compensation, so every time I take a shot with this option activated, I hold down the shutter release for 3 exposures. One will be normally exposed, and the 2 others over and under-exposed by one-half, or a full stop. The half-stop is suited to slide film, as opposed to the full-stop compensation that is suited more for print film's wide exposure latitude. Great if you suspect the camera may not be exposing the shot quite right. It burns up 3 times the film, but you may never get a chance at that shot again!
The other two means of over-riding the camera's idea of a good exposure are really the same in their results, but the OSI dial is more convenient than going through the gyrations of over-riding the camera's DX reader. The OSI dial can be manually flipped back and forth through a range of plus and minus 3 EV in 1/2 steps, whereas re-setting the DX reading is somewhat different. What the camera reads as 800 ASA for instance, can be over-ridden to 1600 ASA. This takes more playing around with the camera's menu, and is used where one wants to "push" the film for the entire roll.
Why Am I Boring You With All This?
...because TTL flash, and exposure compensation are tightly integrated. Here's the trick: Take note of your camera's auto settings for a particular shot with the flash turned off. Note the f-stop and shutter speed. (f-5.6 at 1/500 for example). Next step: put your camera in full manual mode and dial in those same settings. You should note also the camera's meter will indicate a properly exposed shot. This is called "metered manual" mode. Now here's where the TTL magic comes in.
Turn on your flash. The exposure compensation dial (ISO) is now used to control how much flash is emitted! That simple. So by moving the dial to +3, the flash pumps out more flash, but the TTL/camera combination exposes it correctly. Move the dial to -3, and there's much less flash, and again, the camera knows to expose the shot by allowing the right amount of light hit the film plane. I can't explain the technical details, but I've seen it work. That's enough for me! My best fill-flash on really contrasty sunny days (for people head and shoulder shots) is at the -2 setting, but that's just a general rule I use - if it's a large crowd and I'm further back. I'll dial in -1 or leave it at 0. This is due to the inverse-square law, which is nothing more than a complicated way of saying "The farther you move away, the more flash you'll need."
Not All Cameras Can Do This!
Your camera may not have the option of setting exposure compensation in manual mode... but it's simple to tell if it does. This wastes 2 frames, but it's worth it if you can squeeze even more performance out of your investment!
In a normal-to-dim lit room, set your camera to f-5.6 and 1/250 shutter speed. Turn on your flash, and set the exposure compensation all the way to one extreme (-3 for example). Take a picture, but don't look through the viewfinder... look at the wall or objects the flash will hit. Note its brightness level when you take the shot. Now dial in the other extreme on the OSI dial and do the same, and note any difference in brightness of the flash. My camera - as mentioned before - is a Pentax ZX-5n, and the flash is a Sigma EF-500 ST, which is a decently-powered strobe. The difference in the amount of flash between the 2 settings is huge, so I didn't have to resort to my light meter to compare them.
So... give it a try and let me know. Please send me your flash/camera model numbers and the results... I'll publish them at the end of this article for reference.