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Contrast ratio explained
The problem
Auto iris and Back light button
Getting it right


people standing in front of bright window in a cathedral.CONTRAST RANGE

by Christina Fox

Take a look at the image on the right. The people are too dark and their backgrounds too bright. Is it due to bad exposure or poor camerawork? In fact, it's neither. The trouble with video cameras is that they cannot capture a wide range of contrast within the same image. This picture was recorded on a bright day inside a dark cathedral. This sort of shot was never going to work.

Man standing in front of bright ice rinkVideo cameras can cope with a contrast range of around 50:1. In other words, if you stand someone in front of a bright background (like the man in front of the ice rink on the left) and that background is more than 50 times brighter than the person - it becomes an impossible shot to get right.

Here, the background is almost right. The man is just too dark in comparison.


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man in coffee shop with bright windowIn this example the man drinking his coffee is almost right but, the window behind him is way too bright.

None of these three shots are usable.

Why didn't the camera operators who recorded these pictures notice this problem when they stood in the room?


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Several million years of evolution has given your eyes (and brain) a contrast ratio in the 1000s:1. Video camera technology has a long way to go before it gets that good.

But, there is a way to check what the camera will record. You squint.

It is simple, cheap and very effective. I learnt this simple technique from a posting by Wayne Orr on the VX2K/PD150 forum (well worth joining if you have a VX2000 or PD150).

To check it works, try turning off the light in the room you are in and putting your hand in front of your computer screen. You can probably clearly see the screen and your hand. Now squint. The screen probably still looks OK but, your hand will now be in silhouette. This is the shot your camera will record.

Of course, contrast range isn't always a problem. If you want to hide someone's identity, for example, you can use it to your advantage to create a silhouette.

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Unfortunately, auto iris isn't going to help. It will usually try and get the brightest areas of the shot right - especially if they make up a large area of the picture, as in the image on the left below.

picture shot on auto iris
picture shot with back light feature
picture shot using manual exposure
Auto iris tries to get the brightest part of the picture right. So, here the person is in dark silhouette. Some cameras (like the PD150) have a backlight button. This is meant to compensate for bright backgrounds. I've never been impressed with this camera feature, and this picture still isn't right.

Setting the iris manually will get the person right. But, it looks like aliens are about to land outside.

Sometimes you have to realise a shot just isn't going to work.

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» Shining extra light on the subject is one solution, as it will reduce the contrast range. The man at the ice rink would certainly have looked better with a little artificial light on him. Even a reflector might have helped. However, if all you've got is a small camera light, it may not be enough on a very bright sunny day. The sun will nearly always beat you when it comes to a contest of who is the brightest.

» Darkening the windows will also help sort out the problem. Shut the curtains or if you've a big budget (and plenty of time) put filters on the windows. Either method will reduce the contrast range below 50:1. Simple maths.

» In the coffee shop you could have shot it in the evening (it was actually shot mid afternoon). Outdoor light levels would have been lower and so the contrast range within the camera's limits.

» How about avoiding the problem altogether. If possible shoot people in front of a wall rather than a window.

[Of course, another solution is to use film, which can cope with a contrast range in the 100s:1. This large contrast range is partly responsible for that elusive film look.]

© 2000 - 2010

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  • Gain showing the effect of 0dB up to 18dB on a night time shot.
  • Getting creative with white balance shows a variety of lighting conditions and white balance selections.
  • No-cost filters shows how to get interesting effects using coloured paper for white balancing.

    Christina Fox