Putting the ‘post’ in processing
Bob Newman on knowing the importance of processing as a key stage in the photographic method
One of the terminological slip-ups that I can get aerated about is misuse of the term ‘post-processing’. This is a term borrowed from the film and video industry and, according to Wikipedia, it ‘is the process of changing the perceived quality of a video on playback (done after the decoding process)’. I think that the clause in parentheses is key – post-processing is clearly what happens after processing.
Why is this distinction important? It is because ‘processing’ is a key stage in the photographic method, and learning to get what you want from it is important. The processing phase is when the latent image is transformed into a visible image, and inevitably involves the loss of some information that was contained in that latent image. In film days, you could vary the chemical composition of the developer, its strength, temperature and the length of development, as well as many other subtle techniques that skilled darkroom technicians mastered. Varying these many different parameters changed the outcome and dictated what you could achieve from that image.
Digital has a great advantage in that the development process is non- destructive, so if you don’t like the outcome of what we would now call a ‘raw conversion’ you can try again, but the aim is the same, to use the development process to provide for the range of outcomes that you want. Once you have discarded information during processing, be it shadow or highlight detail, no amount of post-processing will bring it back.
This is made more difficult because many digital image manipulation tools blur the differences between processing and post-processing. Mostly, the advanced commercial tools do a bit of both. Apparent raw processing tools such as Lightroom perform some functions after they have processed the image, while image editors such as Photoshop include processing ability. Nonetheless, keeping the distinction clear does help the understanding that allows fine-tuning of technique.
One of the oft- quoted advantages of cameras with a large dynamic range is that they allow the photographer to ‘lift the shadows in post’. Unfortunately, ‘lifting’ shadows during postprocessing is likely to achieve poor results unless the image was processed so as to preserve the detail in those shadows in the first place. If it was processed ‘correctly’ according to the chosen ISO (of which more later) then it would be likely that most of the shadow information would have been discarded, and no longer be there to ‘lift’. Moreover, the desired tone curve, that resulted in lighter shadows, could have been applied in processing; so no postprocessing would be required.
ISO and processing
Essentially, the ISO control does two things (it may also do other things, such as changing voltage gain somewhere in the read chain, but that is a matter of manufacturer implementation, rather than the ISO standard). It defines a target exposure, setting what the meter defines as ‘correct’ exposure, and it defines a processing regime that will result in that exposure being rendered with the lightness required by the ISO standard. Thus, any use of extended dynamic range means a departure from the processing dictated by the ISO setting.
This image was underexposed with respect to the ISO setting to avoid highlight clipping, then processed to the desired lightness range. No ‘post-processing’ involved