Convert 8-bit to 16-bit?

[sanking]

I have a few scans that were done several years ago on a drum scan. They are very high resolution scans, but in 8-bit. Some of them depend for nice transition in the high values and I am concerned that I may create some posterizatin in correcting the tonal values. Would it be of any use to change the mode to 16 bit before starting to work on them?

Sandy King

[Mike Davis]

If they are B&W, I don't think that you will need to convert them. In all honesty, I'm not sure that converting them would necessarily help. But 256 shades of gray should be ok.

Mike

[Michael Mutmansky]

Only a slight benefit in converting when doing a levels or curves adjustment. It will offer a smaller increment into which the 'before' levels can be shifted, which will result in fewer levels becoming lost onto another level due to compression of the levels. If no levels are lost due to compression, the same number of shades of gray will exist regardless of the bit level. Considering that most B&W images have about 1/2 of the actual unique levels available in a typical 8 bit file, the odds are that unless you make a strong adjustment, you won't crunch any levels.

However, you could convert to 16 bit and then apply a slight amount of noise on a layer to help dither the image into 16 bits of data first, and then you can try the levels/curves. It may help save you from having to scan again.

Most drum scanners aren't 16 bit anyway, so you'd probably get at best 12 bits unless you went out looking for a 16 bit machine. 12 bits is more than enough if you know what you are doing when the scan is made.

---Michael

[Jack Flesher]

The simple answer is yes, converting an 8-bit file to 16-bit BEFORE making significant image adjustments will prevent posterization ASSUMING the image was not posterized to begin with...

The longer answer -- and even better news for you -- is the issue of posterization is inversely propotional to the size of the negative scanned! (Really!)

,

[Bruce Watson]

What Michael said.

[sanking]

However, you could convert to 16 bit and then apply a slight amount of noise on a layer to help dither the image into 16 bits of data first, and then you can try the levels/curves. It may help save you from having to scan again.


---Michael

I definitely don't want to scan these files again because they are 12X20.

What would be your understanding of a "a slight amount of noise"? Could you specify certain parameters and what would determine low or high values?

Sandy









[Jack Flesher]

Seriously, I doubt your large negatives need any dithering help or noise added. In fact, you probably don't even need to worry about going to 16-bit to make adjustments unless you are trying to significantly stretch some small tonal range over a very large area on them...

[Frank Petronio]

in numbers, trying to stretch a smooth sky area with a value of 230 to 255 over 20 inches... may give you some banding. You just have to see when you do it.

But adding 3-4 pts of Noise selctively to the problem area should hide the banding and not really take anything away from the image.

[Jack Flesher]

Let's clarify one thing -- are they in 8-bit grayscale or 8-bit per channel RGB?

If the former, then by all means you want to convert -- and you can convert them to 16-bit grayscale or 8-bit RGB. Either will give you more room for adjustments.

If they're already B&W images in 8-bit RGB, then you likely don't need to do anything...

Cheers,

[sanking]

Let's clarify one thing -- are they in 8-bit grayscale or 8-bit per channel RGB?

If the former, then by all means you want to convert -- and you can convert them to 16-bit grayscale or 8-bit RGB. Either will give you more room for adjustments.

If they're already B&W images in 8-bit RGB, then you likely don't need to do anything...

Cheers,

They are 8-bit grayscale.

Thanks for the advice.

Sandy King