Bit depth = levels of gradient?

Started 2 months ago | Discussions thread
OP filmrescue Contributing Member • Posts: 802
Re: Bit depth = levels of gradient?

D Cox wrote:

filmrescue wrote:

bclaff wrote:

filmrescue wrote:

I've been searching around to find some sort of chart or list that tells me how many levels of gradient there is for different single channel bit depths but I'm not having a lot of luck. Can anyone fill me in or link me to the info I'm looking for. I'd like to see from 2 all the way up to 20 or so.

The answer is not simple and is not the answer you will typically receive.
For example, with most systems there is no visible difference between 14-bits and 12-bits; and certain ly a factor of 4 difference.

You may want to look at what DxOMark call "Tonal Range".

What exactly are you trying to accomplish.

Primarily to be able to talk intelligently about why to be using a high bit depth capture system when capturing extremely low contrast negatives.
Also to be able to calculate for instance, if the negative I'm capturing is occupying 5 percent of the tonal range from absolute black to absolute white, how many levels of gradient do I have to work with if I use a 16 bit scanner or high end camera vs a 12 bit or 14 bit camera and at what point can I expect to see posterization given the capture system.

The other numbers for other bit depths are primarily just because I was curious.

If you are talking about digitizing B&W negatives (silver based), my experience is that the crucial thing is to resolve the grain sharply. Low contrast negatives are not a problem. It's easy enough to adjust the white and black points in the image to give a full range of tones from what was a narrow band of pale greys.

The problem with low contrast negatives is that the signal-to-noise ratio is poor, so every speck of dust shows clearly. This is equally a problem in the darkroom, using traditional printing methods.

Any high resolution camera such as a Sony A7rii or a Sigma sdQH, with a good macro lens at its best aperture, will do the job.

High contrast originals such as most colour slides are a bigger problem. Some cameras have a built in HDR feature which is helpful. The new Panasonic full-frame cameras seem to have a very good dynamic range, but the high resolution model is extremely expensive.

Here's a very thin neg -- it was taken on some kind of ultra-fine-grain film -- and a positive result.

Thanks D Cox...the negatives that i"m dealing with are not like a normal low contrast negative where you actually with the naked eye easily see an image there. These are from recently processed lost and found film sometimes as old as from the 1920s. We just processed a film that expired in 1908 but that one somehow turn out comparatively okay. These are sometimes negatives that with the naked eye appear to have no image at all unless you look very closely over a very even light source. The opposite of the slide you're talking about where HDR can help.  That by the way was interesting info for me. I've never been able to get a digital camera to come even close to a film scanner with a slide because of highlights and shadows blocking up. I'll try that out.

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