Are 16 bit scans = 8 bit scans converted to 16 bit in CS2?

I think there is a theoretical loss to having been in 8 bit then upconverted into 16 bit. The numeric stair steps between tonal or color changes are coarser in 8 bit, and putting it into 16 bit doesn't smooth them out automatically.

However, most of the drastic tonal demands take place in the editing process (curves, levels, etc) so as long as you're editing in the 16 bit world you should be okay.

It's not like a native 8 bit 4x5 scan is likely to show significant banding/posterizing in the raw scan. It's only when you push around an 8 bit file that it would have problems.

Your scanner produces a 14 bit output that is mapped onto 16 bits (two bits of zero.) The software is discarding 6 of those bits. If you convert back to 16 bits, you now have 8 bits of data and 8 zeros. Get different software for the scanning.

The above post is wrong, you are pushing around 8 bits even in 16 bit mode if you threw away the other 6 bits of data.

The above post is wrong, you are pushing around 8 bits even in 16 bit mode if you threw away the other 6 bits of data.

Not so fast, Jeff. There is an advantage to converting an 8 bit file to 16 bits before drastic editing: rounding off errors in the various editing steps will be smaller for the 16 bit file, even if originally it started out as an 8 bit file.

You get an extra bit at most. Rounding errors aren't going out very far. There's really no reason to zero out the last 6 bits in the original file.

Actually, the practical difference between starting with an 8 bit file and editing it in 16 bit mode vs starting with a 16 bit file is not as drastic as you suggest. Try it yourself in Photoshop, use this article as a guideline for how to massively compress then stretch an image in 2 steps using the levels tool.

http://www.photoshopessentials.com/essentials/16-bit/page-2.php

Create 3 new image files in PS and add the same gradient (color or B&W) in each. One file should start out in 16 bit with a full 16 bit gradient. The other two files should start out in 8 bit with an 8 bit gradient. One of the 8 bit files stays in 8 bit, the other gets changed to 16 bits to complete the edits (even though it's still only an 8 bit gradient). If you don't want to use a gradient, then just grab any color image from a RAW file - render one copy in 16 bit and two copies in 8 bit.

The result is that the 8 bit file gets massively posterized, the 16 bit file still is a smooth gradient, and the 8 bit gradient edited in 16 bit mode looks pretty much like the 16 bit gradient. I know that this is somewhat of a contrived test to show the difference between 8 bit and 16 bit, but I can't imagine a scenario where you would be pushing a file more than this in PS.

A couple side notes... an 8 bit file is not a 16 bit file with 8 bits of extra zeros. If it was it would take up the same amount of disk space as a 16 bit file. A 16 bit file takes up more disk space because it uses greater precision to describe the number of tonal transitions between pure white and pure black (65,536 steps between white and black vs 256 for 8 bit). When downconverting an image from 14 or 16 bits to 8 bits all that is happening is the RGB color values are each getting rounded off to the nearest integer between 0 and 255. It's not a question of throwing away the last 6 bits, it's a question of converting to a less precise scale of color measurement/description. However, an 8 bit image can still diplay 16.8 million colors - more than the human eye can see.

Anyhow, not trying to be disagreeable. I agree with everything you have recommended - Scan in the highest bit file possible (16 bits), get new software if your current software limits you to 8 bits. There is no reason to throw away 14 bit data and save the raw scan as an 8 bit file.

I scan my film as 16 bit TIF's in ProPhoto RGB color space.

You want high bit data from the get go (not sampled up which buys you nothing). IF indeed, the device provides more than 8-bits per color, you absolutely want that data! We now have output devices (at least on the Mac OS) that can send 16-bit data to the print driver.

http://www.digitalphotopro.com/gear/imaging-tech/the-bit-depth-decision.html

sampled up buys you nothing

I have to disagree. As Sheldon and I have maintained, converting 8 bit to 16 bit has an advantage. Of course it is best to not throw any data out to begin with.

I have to disagree. As Sheldon and I have maintained, converting 8 bit to 16 bit has an advantage. Of course it is best to not throw any data out to begin with.

Fine. You have something empirical to demonstrate this (and doing so without using dither on) which affects all things in such comparisons?

Fine. You have something empirical to demonstrate this (and doing so without using dither on) which affects all things in such comparisons?

I'm not going to regurgitate what others have demonstrated so effectively. Study the material that Sheldon referenced and the related material in Real World Photoshop and do some tests yourself. You'll be amazed what you can learn if you really try.