Quite a lot to think about there. Maybe — in the spirit of Bill Claff's choice of standardized print size and viewing distance and Ray's desire for something similar in the form of DR after downsampling to 8MP — there is an informative curve of "usable DR vs resolution", or "resolution at various levels of scene brightness". At one extreme, even an image file with SNR<=1 per pixel might be salvageable by sufficient downsampling. In fact, that is sort of what we do with traditional black and white film, where the raw signal is all 0's and 1's, because each silver halide grain is either exposed or not, but there are many billions of "chemical pixels".
Photographic Dynamic Range is what
you decide through your own tests in relation to particular circumstances which can vary. The print size will also have some bearing on the amount of shadow noise and detail which is acceptable. Noise can be significantly reduced if one is prepared to sacrifice detail, which is less of a concern on the smaller print.
When I bought the Nikon D7000 to replace my Canon 50D, the main attraction was its high DR, almost 2.5EV better than the 50D at base ISO. Shortly after taking delivery of the D7000, I made a thorough comparison of the DR capability of both cameras by taking a serious of shots of the same target in the same lighting conditions, starting with an ETTR shot with both cameras, then reducing exposure successively, one stop at a time, with each camera until I reached 13 &2/3rds stops of underexposure, the DR limit of the D7000.
At the different DR limits of both cameras, the images were similarly degraded, but some detail was still discernible. At 13 stops underexposure on the 50D, no detail at all was discernible. It was all pure noise.
However, at 11 stops underexposure, the D7000 image was acceptable, but not the 50D image because it was at its DR limit. At 9 stops underexposure, the 50D was acceptable for shadows, but the D7000 was significantly better and more detailed.
In other words, the PDR of the D7000 was about 11.5 EV and the PDR of the 50D about 9 EV, which corresponds with the DXO engineering DR differences of about 2.5 EV. At 8 stops underexposure, the D7000 had an insignificant edge, and at 7 stops underexposure there was no discernible difference.
The issues as I see them, are:
(1) How to determine the PDR of your camera? Do you just accept someone elses view, or do you take the trouble to do your own testing?
(2) How do you determine the DR (or SBR) of the scene you are about to shoot?
I've already described the first process, so what about the second process? If you have plenty of time to organize everything in a studio, then no problem. However, if you are taking numerous shots of various scenes as you travel, there needs to be a quick and reliable process.
The quickest process is to simply automatically bracket all exposure, which is what I used to do when using the older Canon DSLRs. However, bracketing most shots fills up you memory card faster and involves a lot more processing time for those shooting in RAW mode.
With my Nikon cameras, it has become apparent that by far the majority of my shots do not require bracketing if I take the trouble to find the optimum ETTR exposure.
If I do get the impression that a particular scene might exceed the PDR capabilities of my camera, then it takes me a little more time than it takes to get a ETTR shot, because a mathematical calculation is required.
Using a single focusing square, and camera in manual exposure mode, I simply move the focusing square to the brightest part of the image and adjust exposure with my thumb on a wheel untill the meter in the viewfinder is at the far right where the + sign changes into an arrow head. That is the correct exposure for an ETTR shot, which I've determined through my own tests. No calculations are required, except if the shutter speed is too slow for a hand-held shot, in which case I'll raise the ISO and readjust the exposure.
If I think the lighting in the shadows, in another part of the composition, might be inadequate for PDR quality, I'll simply move the focusing square to those shadows and reduce the shutter speed, with thumb on wheel, till the camera's meter moves to the extreme right, as though I'm taking an ETTR shot of the shadows.
I then have to do a mathematical calculation to convert the differences in shutter speeds into numbers of stops or EVs. Alternatively, as I move the exposure adjustment wheel, I could count the number of clicks involved before the exposure meter reaches the far right. If the cameras is set for 1/2 a stop exposure increments, then I could divide the number of clicks, or changes in exposure, by 2 to get the difference in EV range between the highlights and the shadows. However, the D810 allows for 1 stop changes in exposure, which would make it even easier.
If the difference between the two ETTR readings, one for the brightest highlights, and one for the deepest shadows which have meaningful detail, is greater than my calculated PDR for my camera, say 11 stops, then I need to bracket exposure.
However, even if one has gained a general PDR rating for one's camera through one's own testing, this rating can change according to circumstances. A good example would be the photographing of the amazing ruins at Angkor Wat. In a particular scene there might be an amazing and detailed engraving in the stone, in the deepest shadows. After using my method to check the lighting conditions in those shadows, one might find that the shadows are in fact within the PDR range of one's camera. On the other hand, because the content of the shadows is so interesting, one might decide to bracket exposure in order to get the maximum quality and detail in those shadows.
This is the one major reason why I prefer the higher DR of the Nikon cameras.
There are far fewer occasions when these sorts of time-consuming difficulties arise. Most of the time I can simply take an ETTR shot and be confident that the shadows will be okay, at base ISO and/or a couple of ISO stops above.
