Reviewing Your Historgram

I've been working on an article for a leading UK photography magazine and thought I would share this info-byte that I've written about the histogram - a surprisingly under-utilised and often misunderstood feature of digital cameras.

"The fact that you can review images instantaneously on the back of digital cameras is probably one of the main reasons for their success and revolution of the photographic industry. No longer do you have to wade through hundreds of 6x4s, feeling the crushing sense of disappointment with every blurred and under-exposed image. But, reviewing digital images on a camera is not without its pitfalls.

Firstly, you can’t rely on the screen alone as this can be affected by ambient light and its own brightness level can be altered. Second, the image that is previewed on the small screen is the low-res jpeg embedded in the RAW file and so does not have the full tonal range of the actual RAW file. The only feature that can give you the true exposure level is the histogram.

When I first used a digital SLR, the whole community said that you should photograph slightly darker, with the histogram slightly to the left, in order to preserve the image data and prevent blowing the highlights. That may have had some merit five years ago, but things have changed. Compared to the sensors of the 10D (my first dSLR), the latitude, and tonal range of new digital image sensors are vast. Research has also revealed a fundamental difference in how a sensor captures light data compared to how film develops an image.

Digital sensors actually acquire a significant majority of the image data in the lighter tonal ranges, compared to the dark tones. So, if you under-expose an image and try to correct it, you are trying to generate more detail from less data, hence why the shadowy areas appear to be more prone to noise. The rule is now to shoot accurately or even brighter, capturing more image data in the light tones, and then reducing the exposure in RAW processor (such as Lightroom) where necessary. This generates an image with a greater tonal range and cleaner shadows.

Top: An over exposed image with the highlight alert flashing in red. The histogram is bunched-up to the right-hand edge. Bottom: A reduction in shutter speed by 1 stop to 1/400th sec gives a perfect exposure with whites white and well balanced histogram that does not pass the right-hand edge

If a large block area is flashing and the histogram is bunched up to the right-hand edge, then there is a good chance you have over-exposed and lost vital details. Do not be afraid of specular highlights (tiny flashing highlights on sunlit water, or a dusting of highlights on a swan) as this data will still be there in the RAW file and probably will not even show up in your RAW processor as ‘burnt out highlights’. Besides, highlight recovery tools are now so advanced that you can retrieve an incredible amount of detail from what appears to be blown highlights."