Chuck Norcutt wrote:
> I meant to mention that the round-off and loss of some brightness values
> when editing a JPEG may or may not be a bad thing since modest losses
> will not be visible in a print. Although the dynamic range of a JPEG is
> very limited in comparison to what the camera can record, the dynamic
> range of a print is much less than a JPEG.
>
I think you might have avoided some confusion here if you had said
8-bit, rather than JPEG. The issue applies to any 8-bit image, whatever
the source.
> When you look at the histogram of the image after doing significant
> brightness/color adjustments you may see that it looks pretty "spikey".
> Instead of a smooth distribution you'll see "holes" where certain
> values no longer appear and spikes where round-off errors have caused
> values to accumulate at that spot. If these adjustments are severe
> enough it will show on the image as "posterization". It looks like a
> poster painted with too few colors. But what shows up on the histogram
> and the screen may not show on the print. The print doesn't have the
> ability to show all the color and brightness detail in a JPEG and what's
> missing might not have been visible on a print even if it was perfect.
>
For those who haven't seen it, here is an example. I've taken an image
and processed it, saving an action as I did so, then run the same action
on it again, but at a different bit depth. So both versions have been
identically processed
<http://www.moosemystic.net/Gallery/tech/8vs16Edit/July406.htm>. Down at
the bottom, you can see how different the final, 8-bit, histograms are.
As Chuck points out, it can take quite a bit of this to be visible. I
think I see very subtle difference as I roll the mouse over the image,
but it could be wishful thinking and in any case, isn't significant -
and sure won't show up in a print.
I'm not all that happy with either result, not my best work, although
the lighting is tricky indeed. Impromptu shots can be a technical trial,
but those expressions are great! I guess I was distracted by the demo
nature of the work, but it does show what was intended. I do think I'll
redo it before sending on to the parents and making prints for them - if
I can just remember that other kid's name. :-)
I certainly have had images where I forgot to convert to 8-bit at the
start, that did start to visibly deteriorate as I worked with them. My
basic idea is that I should convert every 8-bit image to 16, then back
to 8. Why take a chance of wasting work?
Moose
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