New Ideas about InCamera Histogram

When I moved to a digital SLR in 2003 I thought it was very easy to determine the exposure accuracy of my 'in-camera' photographs and judge whether they were correctly exposed or not. I simply evaluated the histogram and highlights feature and made the necessary adjustments. Even in my books, articles and workshops I encouraged other underwater photographers to do the same as me. But in recent years I have come to realise that the histogram/highlights exposure indications are not what I thought they were. For some time now I recognise that the histogram/ highlight exposure accuracy of the LCD thumbnail is different to the RAW file histogram once downloaded to an imaging program like Lightroom or Photoshop.

Both my topside and underwater shots tend to indicate overexposure on the in-camera histogram when in fact they are not overexposed at all!

My thanks to Seth Resnick and Jamie Spritzer, authors of The Photoshop Lightroom Workbook, for helping me to understand the science behind this concept and allowing me to use their words.

Digital cameras have a dynamic range of about six stops (6 EV) and offer capture in 12 bits, meaning that technically they can capture (2 to power of 12) or 4096 tonal values. If you divided the 4096 tonal values (f-stops) among the potential six stops and if each stop is looked at as a zone, it would be logical to assume that each stop has an equal amount of data!

This assumption is wrong and accounts for one of the biggest misunderstandings when it comes to exposing for digital.

THE BRIGHTEST TONES 2048

LEVELS AVAILABLE

fig. 3.34 One half of the data of a 12-bit capture (4096 levels) or 2048 levels is in the brightest stop (highlights), 1024 in the next stop, 512 in the next stop and so on until one gets to the shadows that contain only 64 levels.

AVAILABLE

AVAILABLE

fig. 3.34 One half of the data of a 12-bit capture (4096 levels) or 2048 levels is in the brightest stop (highlights), 1024 in the next stop, 512 in the next stop and so on until one gets to the shadows that contain only 64 levels.

the ught middle tones two stops down 512 levels available

THE BRIGHT TONES ONE STOP DOWN 1024 LEVELS AVAILABLE

A RAW file is a linear file and the capture of data works much the same way as a traditional f-stops works, i.e. f2.8 allows twice the light as f4, and f4 is twice f5.6, and so on. So, what becomes important is the way in which these 4096 levels of tonality are distributed across the tonal range of six stops.

• A massive one-half (2048 levels) are all contained in the brightest stop — the highlights

• 1024 levels are in the next stop

Until one gets to the shadows which contain only 64 levels, think about this: the darkest stop (the shadows) of the digital file has the least amount of data. It is clear to see (no pun intended) why shadows reveal the greatest amount of digital problems, i.e. mainly digital noise at high ISOs.

Since most of the information in a digital file is in the first 'brightest'stop, the act of underexposure reduces the quality of a digital file. From a technical perspective we should expose a digital file so that most of the data is present. As a general rule this means that a properly exposed digital file is slightly overexposed, we then reduce the exposure slightly during processing.

If we underexpose in-camera we loose valuable highlight data in our RAW files. Underwater we need to make a judgement call on the amount of overexposure rather than the underexposure. For these reasons it is during the processing stage that we achieve rich saturated colours, rather than when we take the shot.

These new ideas are not recommending to always slightly overexpose in-camera. Like I mentioned earlier, it's a judgement call.

Digital Cameras For Beginners

Digital Cameras For Beginners

Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.

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