| Making the Exposure | |
Most of the data in an image is at the brightest end. Also, the brightest areas are the least noisy. (For details of why this is, look at the Technical Background section.) So it would make sense to get as much image data into this bright region as possible and bring it back to the appropriate brightness level later during processing on the computer. This is the essence of the ‘Expose to the Right’ technique. In order to use this technique effectively you need to understand the histogram that the camera produces. The histogram shows the distribution of tones in the image. At the right hand end are the brighter tones and at the left, the darker. The height at any point shows the number of pixels in the image with that tone – the higher the bar the more pixels with that tone. If we are to expose to the right we need to move the histogram so that there is something at the right hand end of the histogram, not forgetting the golden rule from the first part of this series – don’t blow the highlights. If the histogram shows a ‘skyscraper’ at the right hand end then the highlights are blown. Less exposure will be needed to bring them back into the dynamic range. If you are working in aperture or shutter priority you will need to dial in negative exposure compensation. In manual you would need to use a smaller aperture or a faster shutter speed. If there is space at the right hand end then the histogram needs moving as far right as possible. This time you will need positive exposure compensation or a corresponding change to your manual settings. But don’t forget the golden rule. On our Axxx cameras we can check the live histogram before taking the picture. On a DSLR you take the picture first and then adjust the settings before making the next exposure and checking again. DSLRs have the added advantage that on the instant playback screen any blown highlights or blocked shadows will blink at you. So you not only know that you have not captured the full range of tones in the image but you know where the problem is. So that’s it, except there are a couple of things to remember. Although the colour space you select in camera can be changed during RAW conversion, the histograms for sRGB and AdobeRGB are different so it is worth selecting, in camera, the correct colour space you will use later for editing. The same can be said for white balance. If you change the white balance setting, the histogram will change. You will get the most accurate in camera histogram if you set up these parameters in camera. Finally, I believe that the histogram that you see in camera may not show all three channels - red, green and blue. Some cameras only show the green channel . So, even if the in camera histogram shows no blown highlights, it is still possible to blow either the red or blue channel. This means it is worth taking the exposure you have set up to expose to the right and then bring it back by a third of a stop and shooting again – in other words bracketing. |
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