Understanding Exposure

Light is the key!

One trick which i have written about is understanding the right hand side of the hisotgram in digital photography and exposing to the right theory. A lot of people automatically shy away from this because we all know blowing highlights is the cardinal sin in digital photography!

Learn to understand light, learn to paint with light and learn how your image sensor controls and records light and hopefully you will have a greater understanding of exposure and how to deal with it.

Understanding your image sensor

Your cameras sensor works by recording light reflectance from black (reflectance of 0) to white (reflectance of 255; or 247 shown by my tests) / shades of gray where 18% is average exposure at a reflectance of 119 (in my honest oppion for the Nikon D70). The histogram shows this, Black (shadows) on the left, white on the right (highlights) and midtones in the middle. You'll notice red lines dividing up your histogram on your Nikon D70. The distance between these lines is around 1 f-stop of light; this varies for cameras with different dynamic ranges e.g. fuji s3 could be 1.5 stops. Therefore you know if your photo is peaking on your histogram 1 bar left of center it is around 1stop underexposed.

Now as mentioned, your cameras sensor records light on a linear scale. This is all processed as data levels through the RAW processing engine of your camera. Your camera records the most amount of data levels in the highlights (reflectance of 255/247) on the far right hand side of the histogram. I keep using the reflectance value of 247, simply because 247 is the refletance reached when pure white is recorded and 247-255 is the same value (hence why my custom curves always end at 247). Anyway, the highlights (far right hand side of the histogram) record 2048 data levels. This is halved as you loose 1 stop of light. E.g. as you move from right to left on your histogram, measued by the red dividers. Remember the dividers mark around 1 stop of light. Therefore moving from the highlights and then left to the next red divider (the bright tones), you loose around 1 stop of light and your data levels captured by the sensor are halved to 1024 data levels. This is halved again as you go from the bright tones to the midtones and so forth.

Therefore, if you understand this, you will realise that, if you dont fill the right hand side of your histogram with information you are effectivly throwing away half of your camera sensors's potential.

Working with light

You'll still get good pictures even if you didnt know about this, however, the more tonal spread you can achieve the more impact your images will have. If you can get a full set of tones in your pictures (data captured evenly from the shadows, peaking through the midtones and stretching to the highlights) you will get more contrast, graduation and dynamic range leading to a well rounded image. Filling in those right hand gaps will, as mentioned give you much more data levels to play with if you need to post process. Now to do this you need to understand the laws of photography and effectivly painting with light. Light is the underlying key to all of this. I see all too often people trying to photograph in mid-day or afternoon sun complaining of blown highlights or loss of tones through the mid-section. Its impossible on any camera to get good results of harsh lighting. You have to compromise! And by compromise i mean spot meter your subjects or use center-weighting. If your photography in direct sunlight-midday, theres always going to be a massive difference between shadows and highlights, far too much difference in f-stop's for your camera, or any cameras dynamic range to handle. Therefore your pictures will be dull, your midtones, under-exposed and the highlights blown. The midtones (the middle of your histogram) is where the tones should peak and this is the key. You'll need to get close in to your subject and spot or center-weight the exposure so at least your subject is well exposed. Getting in tight will minimise or even cut out the sky or very bright highlights that can fool your cameras meter.

To get the best tonal distribution you'll need nice soft light which you can find early morning or late evening. Harsh sunlight means a massive exposure difference between the sunlitt areas and shadows, too much for any camera to meter for and record accordingly.

So now hopefully you will now have an idea of how your image sensor deals with light and how stops of light, dynamic range and working with light plays just as much an important part in digital photography as it did with film. Its all too common these days to forget the rules of photographt and working with light by simply blaming very expensive equipment for not performing. Digital is versatile, its easier then ever to take photographs with digital bodies and people are taking more photo's than ever. More people own cameras than ever before now and that means a lot of people who didn't start off with film in which you had to learn how to deal with light and exposure. It also means a lot of trips out with the family and that means mid-day photography. And as we understand from this article mid-day is the hardest time of day to get good results.

This article isn't telling you how and when to take photographs, hopefully what i have written will educate you more into understand the restrictions and factors surrounding exposure and image sensors. Hopefully it has explained how light is the key factor and will hopefully make you think about how the light is going to effect your next frame. This will hopefully give you the scope to learn to manage and work with light and in the meantime, it will help you understand your histogram.


Understanding Exposure | Digital Camera Resource