Digital Outback Photo
- Photography using Digital SLRs

 

Photo Techniques #003

"Flash for Contrast Control"

by Neil Turner

 

When I accepted Uwe's kind invitation to write this series of short photo technique essays I asked for some suggestions about subject matter. The most common response, as well as the first one to come from Outback Photo's editor himself, was about fill-flash. The reason that there has been quite such a long gap between essays is that I find the whole concept of fill flash a bit limiting and I have been racking my brains to find a wider subject matter that allows me to put fill-flash into it's proper place in the scheme of things. I was still floundering around trying to get my ideas organized when someone e-mailed me and asked me to write about applying the zone system (of exposure control) to modern digital cameras. A perfect place to start.

Ansel Adams was a master of photography, a man who learned exactly how to handle light and make it produce exactly perfect negatives for printing in a world where different grades of black and white paper did not exist in the way we have known them for the past twenty years. His idea was to expose the negative and process it in such a way that when you put the negative in the enlarger you would nearly always have the same exposure and you would always get a print that had a range from black to white that exhibited all of the tones in between. We have other ways of controlling contrast now, so the zone system in it's purest form is pretty much redundant (step back and await flames). What we need now is a way of getting our digital images to exhibit exactly the right degree of contrast between the the shadow detail and the highlight as well as having the "micro contrast" in the detail without having to resort to hours using Photoshop.

Going back to fill flash for a few minutes, contrast control is what fill flash accomplishes best. The classic case is where you have a bright background and a dimly lit subject in the foreground. You add enough flash to bring the foreground subject up to the same exposure as the background, "filling-in" the shadows. Otherwise to correctly expose the foreground would have left a badly overexposed background and with digital cameras or poor film this would have meant ill defined edges and a bleed of light onto the subject and possible image degradation. Fill flash can be a lot more subtle than this though.

I have no concrete idea what the correct contrast ratio for a digital image should be, and of course it will vary depending on the end use. I'm sure that an ink jet print can take a few more stops of contrast than an image that is going to be reproduced in a newspaper for example so as photographers we need to be aware of the end use and try to control the contrast in the image accordingly. It's not only a question of controlling the range between highlight and shadow across the picture (image contrast) but also in controlling it between subtle details within the picture too (micro contrast). Flash can be used to increase or decrease the micro contrast surprisingly easily.

I would need to write a book to give every example of how, but put simply contrast can be controlled by the direction of the flash in relation to the direction of the main ambient light. If you have got a strong ambient sunlight coming from directly above the face of a person, they will have heavily shadowed eye sockets, top lip and neck. This is too much contrast, so by adding flash from the same direction as the camera you can reduce the contrast, making the picture far more pleasing. This is the commonest form of fill flash for wedding photographers and is also in common use by a lot of celebrity and paparazzi photographers too. An example of the calculation goes something like this:

  • The ambient correct exposure for the areas of the subject lit by the sun is 1/250th at f11
  • The ambient exposure for the shadowed areas of the face is 1/250th at f4
  • The shadowed areas are three stops underexposed = unpleasant light
  • Flash needs to be added (according to taste) to make up some or all of that deficit
  • The flash exposure needs to be enough to make a substantial difference to the shadows, without blowing out the the highlights.

At this point everyone has their own technique. Some would shoot (if possible) at f16 and set the flash to expose at f11 (by dialing -1 on a TTL unit). Others would leave the lens on f11 and the flash at -1 or f8. My personal preference would be to do the latter in order to retain as much of the natural feel of the picture as possible. By doing this we have basically engaged some rudimentary contrast control. Not only have we controlled the overall contrast between dark and light but we have also done some micro contrast control because the light was all coming from above, it was what we call "cross light" and cross light shows up lumps, bumps and blemishes. Try using a torch to investigate how much more detailed contrast you can get by using cross light and think of late evening sun when every detail of the grass or the road suddenly shows up with all of those thousands of tiny shadows. Adding flash from the same direction as the camera reduces all of those tiny shadows without affecting the tiny highlights - that is what controlling micro contrast is about.

Of course most of this can be done with reflectors, so it's you decision whether to use flash or reflectors and each circumstance will dictate it's own answer but the concept of using an extra light source to reduce contrast is, I hope, now a clearer one to you. So what about increasing contrast? That is just about reversing the thinking. Adding flash from off of the camera's axis will add macro and/or micro contrast to an otherwise flat lighting scenario. Those horrible rooms or outdoor locations where the light is unexciting often give flat and boring pictures. By taking the flash off of the camera and using it cleverly you can increase the amount of image contrast by as much as you want, livening up the whole image considerably. In this scenario bouncing flash off of a wall or ceiling may not be enough and a direct flash or a lightly softened flash will be the favorite. The angle from the camera will need to be judged at the time, but 45 degrees or more is often the way to go when you are trying to boost contrast. Again we can play games with torches to experiment on direction. I always carry a Mini-Maglite for just this purpose.

Ansel Adams was a brilliant photographer as well as a master technician. He designed a system that was right for the way he worked and right for the medium in which he worked. Times have moved on, and if he were around today I'm sure he would come up with a technique to handle image contrast every bit as useful as the Zone system used to be. My solution is to use flash because I don't shoot mountains and can control the smaller spaces that I work in. There has been so much written about fill flash, especially in the instruction manuals for modern SLRs and flash units, that I don't need to regurgitate the basics. I hope that my thoughts on contrast control will get you thinking, will have you getting that old torch out and will make you realize that it's often easier to do things in camera rather than relying on Photoshop.

 
 
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