Tuesday, July 10, 2012

The Very Basics of Fill-Flash

My post yesterday introduced the concept of fill-flash for photographing cars. On bright sunny days, fill-flash is absolutely essential for cars. But fill-flash is very useful in many other situations also, such as shooting most any subject in bright sunlight - be it people, animals, or sandcastles; when the sun is really bright overhead, there will be harsh shadows cast over your subject that fill-flash will serve to bring out the detail hidden in the shadows.

Yet, fill-flash can be a difficult concept to grasp. We start off using our cameras by turning the flash on only when there isn't enough light to give a decent exposure, or when the shutter speed is so low that the subject will be blurred by motion or camera shake. But fill-flash is different. It comes into play when there is plenty of light - when the natural ambient light is so bright we have to put on the sunglasses, we would naturally think that using flash would be totally unnecessary. So, in my car photography I hopefully demonstrated how useful flash can be, not to provide enough light for the subject, but only to light up the parts of the subject that are hidden in shadows.

Fill-flash has some theoretical basics I'll introduce you to. 1) First, a camera flash, no matter how big and powerful it might be, is never more powerful than the sun, so when you're using your flash to fill in shadows, don't worry - it is still the sun that is providing the overall exposure of the subject and the scene around it, and the flash, even with it's output purposely cranked up a notch or two (recommended), itself has little influence on the overall exposure. 2) A camera flash has a limited range - the one built into your camera is only capable of lighting a subject that is 10 to 15 Metres away at the maximum - everything else in the picture that is beyond that is not exposed in any way by the flash, and the exposure of your pictures background is set by the natural ambient light. 3) The camera's White Balance is being influence by two factors - the ambient light, and the light from the flash, so it is best to set the Camera's White Balance to "Automatic" (AWB) allowing the camera to set the best overall WB Colour Temperature. 4) Finally, the Camera's shutter speed can never go higher than it's Flash Sync Speed. This means that the speed of the flash's "flicker" dominates the exposure over and above that of the shutter, and if you manually set the shutter to be faster than the Sync Speed, then part of the picture will be cut off by the shutter returning to rest. This means that even though my camera is capable of a high shutter speed of 1/8000 sec., the highest speed I can use is the high Sync Speed of 1/250 sec.

This highest Sync Speed of 1/250 becomes important in the overall exposure, if the flash is to have it's desired effect. So, I need to introduce you to a little math known as the "Sunny-16 Rule". This is a universal rule that was used in the old days when cameras were not equipped with light metres; one could easily calculate the correct shutter speed and aperture to get a well exposed photo using this rule:

"Sunny-16" means that under bright sunlight, with an aperture setting of f16, you would set your shutter speed at the same number as the ISO (or ASA) of your film; therefore if you have ISO 100 film, the shutter would be set at 1/100 sec (or the closest available, say the more typical 1/125) with the lens aperture set to f16. Or if you have ISO 200, you set the shutter at 1/200, or if ISO 400, you set the shutter at 1/400 sec.

Now, most agree that "Sunny-16" means you use f16 and the above shutter speeds with full sunshine reflecting off a bright sandy beach. If the sun is partly blocked by clouds and / or the reflective surfaces are darker than sand, you need to let more light in, either with a lower than f16 aperture number such as f11, f8, or even f5.6, or with a slower shutter, like 1/60 or 1/30 for ISO 100 Film.

This rule still applies today with Digital Cameras, and it's not something you have to worry about normally, but when using fill-flash, it does become useful. You see, normally, when set to Full Automatic Exposure of any kind, your camera is free to select any of your higher shutter speeds, like say 1/2000 in bright conditions, (and at the same time it will auto-select a lower aperture number like f5.6 or even f4). But with your flash turned on, the camera has to limit itself to it's highest Flash Sync of 1/250 (in the case of Canon DSLR's). So, it's best to consider "Sunny-16" to "fool" your camera's exposure system.

A typical example - you're seldom in lighting conditions when ISO 100 combined with f16 and shutter speed of 1/100 is appropriate - it's usually a bit dimmer than that, even on sunny days. Therefore, if you're surrounded by anything other than bright sand, then f8 would be the more appropriate aperture. Remember also that the flash will have no effect on your overall exposure - all it's going to do is brighten some shadows to reveal the detain within them. The only "issue" is that your shutter speed limit is 1/250 or less. So to finalize my example. for this car show, it was partly cloudy, the reflective surface was grass which is darker than sand, the lot was surrounded by high dark trees and I wanted to "err on the side of over-exposure" to make sure I was getting a lot of "brilliance" from the cars' details. So the combination I dialed in for most of these shots was as follows:

  • Mode - Aperture Priority
  • Shutter - 1/250 (which was blinking at me in the viewfinder because it was limited by the flash being turned on, and so the camera "thought" I was going to overexpose with such a "slow" shutter)
  • ISO - 200 
  • Aperture f4 or f5.6
  • Flash Compensation - +1
  • Distance from cars - about 5 to 7 metres
  • Picture Quality setting "RAW" because it's much easier to correct exposure mistakes with RAW
With these settings I succeeded in getting near perfect exposures (some of them I had to brighten even further with my RAW software).

Normally I am not so technical with this Blog, but flash-fill can be a funny beast, and if you left it all up to your camera by using a fully automatic mode (Program = "P" on my Canon), then you would end up severely underexposing every time. I tried a couple of shots in P-Mode first and indeed they were very dark, as the camera "thought" that f11 would be correct because the flash was turned on. 

If you still feel kind of "in the dark" about this whole fill-flash thing, it would be a good idea to take your camera out into your yard and practice before heading to the beach or a car show.

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