However, my camera, a Digital Rebel EOS XTi, has a (at least in my view) peculiar bracketing function. As far as I can tell, I can bracket Blue/Amber, or Green/Magenta, but, I see no way to bracket both. In addition, fairly extensive web searches on the subject have turned up nothing useful that might tell me (1) if bracketing in bot axes is even possible, and, if not, which axis ought I to bracket.

Cheers, Henry

Well, nothing, but it doesn’t gain you anything.

Think of the histograms. Let’s say normal processing is a 12-bit sensor saved as RAW format and converted to 16-bit (TIFF) for photoshopping around. This corresponds to a contrast-range in the source scene - sky 6 stops brighter than ground, for example. When you do HDR, you build a 32-bit image with greater range and greater detail - more information in shadows and highlights and smoother throughout. If you were to convert a pure HDR image straight to a JPEG, you’d simply think it looked very low-contrast.

If you take an 8-bit JPEG and apply a gamma curve to darken it 1 stop, all that happens is the lower part of the histogram (midtone and darker) get compressed and the higher part expands - so you lose detail. By doing that twice your 32-bit image has merely gained errors in tone-placement - you don’t gain image-data, merely the illusion of it which might look favourable if you’re lucky.

Tonemapping is where you use some algorithm to simulate apparent increased contrast locally within an image - mostly spatially (adjacent pixels) with some consideration for the pixel values. This is where you have the option to make it look whizzy but disgusting, or exercise some restraint and strive for normality

ObExample: hdr panorama.

Cameras: what you want to avoid is any pixel changing value unnecessarily between exposures - so you need a still scene or very long exposure for motion-blur - and fix the aperture and focus distance and vary the speed only. You can get somewhere with a simple point-’n'-shoot and exposure compensation; a dSLR would be better so you’re shooting RAW in the first place and have a nice lower-noise sensor. There’s nothing to stop you doing it with film if you want, as long as you can control the scans to represent the different exposures.

Software: free alternatives include FDRTools and qtpfsgui.

If in doubt, try it. You’ll see that you cannot create depth and detail where there is none.

I just don’t understand how that be classed as “cheating” at HDR, as the whole point of shooting in RAW is that you can change EV value and WB later.