This article is part of the online digital photography course.
This article is a brief guide that explains the overexposure in digital photography and its advantages.
EVERYBODY TO THE RIGHT!... BUT WHY?
This is obviously a reference to the
histogram, now an integral and essential part for the photographers. If you still don’t use it, you should learn to know it better and get used to control the exposure of your images on instead on the preview! When talking of exposure on the right we mean than that the RGB tones curve has to contain more data and reach its peak on the right of the histogram.
BUT WHY?
People who come from analogic have difficulties managing with this technique. Nevertheless it has its point and IF USED AT BEST can provide astonishing results and not just at high ISO. Shortly, there's a reason, let's see which one it is. We can try to divide the gray levels (from black to white) in 4096 levels.
PICTURE 1
The image shows an "equal" distribution of the gray tones. The medium gray is exactly on the half. On a digital sensor that's not how the tones are distributed: in the picture below you can see the REAL distribution of the tones (in gray, obviously) on a digital sensor.
Without entering into details, they are the 4096 gray levels showed, the most obvious thing is that on 4096 levels just 64 are black tones. 128 are dark grays, 256 medium grays and 3584 are distributed on bright grays.
PICTURE 2
If in digital photography I do a medium reading of a zone with dark areas, the sensor distributes my image on the black and dark tones, and will use a gamma related the dark zone as in picture 3:
In a shot of that kind I’ve got the most of the gamma distributed on the dark tones. It's really a narrow gamma. The attempt to increase it (a normal thing in the raw management: regulating the exposure and searching for black and white "stretch" the dynamic gamma) will cause the presence of a great noise and in some cases also “posterization”. The reason is pretty obvious. Try to take a piece of cloth with a thick knit and pull it... it will be full of holes! Then pull a fine knit made of wafer-thin threads and then many more layers and will hardly shows holes when pulling. Here's the same thing.
Pulling only this 64 + 128 tones to spread them on about 2000 levels (the attempt to distribute the gamma as in picture 1) creates these "holes" in terms of noise and posterization (that is nothing more than a series of tone passages in a too narrow gamma).
If, instead, you overexpose your image it will distribute on the medium gray tones and the bright gray tones, covering a much wider gamma:
In this way, pulling the image doesn't bring any trouble because we are working with a pretty thick gamma made of many thin threads.
How much should I overexpose?
There are infinite arguments, moreover on the boards and photographic circles. I think there isn’t a rule. You have to know how to evaluate a scene, the only important thing is not to burn your whites. We may say that our pictures should be overexposed till the limit of burning a channel. This can be checked only on the histogram and not on the preview! How much to overexpose depends by the scene we want to shoot and the camera that we use. A CMOS generally keeps the light higher than a CCD. A 14 bit camera keeps it much more than a 12 bit camera. I heard about an overexposure of more than 3 stops but it was in fact a CMOS processed at 14 bit. It's up to you how much to overexpose, based on the time to evaluate and measure.
If you want to raise the ISO, it’s better to overexpose even of 1,5/2 stops measuring in SPOT on the brighter part of the image. Photography is beautiful and fun because it's all about creativity; knowing how to use your equipment without having any dogmas is part of the creative process.
Good light!
Article written by Diagaz
Translation by Nina Kozul
