why do photos need color correction prior to printing ?

Started Jan 24, 2013 | Questions thread
apaflo
Veteran MemberPosts: 3,854
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Re: To the OP. Will you Please Post the Picture and ALSO a screen copy of the Histogram.
In reply to coder01, Jan 26, 2013

coder01 wrote:

foreground becomes markedly darker on paper.

I downloaded your image and looked at it with an editor.  Nice shot!  Incidentally, if my monitor calibration is switch from sRGB for the web and a profile to match an Epson 7890 printer, the best description I can think of would be the foreground is  "markedly darker"!

It has a full tonal range and a lot of wonderful detail in the darker areas on the water in the foreground.  I can see where that is going to be a real problem to edit for printing if your monitor is not adjusted to be very close to the printer.   First, lets talk about the "why" it is, and then about "what" to do.

Your camera can capture probably 12 or 13 fstops of dynamic range at its lowest ISO if you nail the exposure of the highlights right at the point of clipping.  Wonderful, except that a JPEG formatted image can only retain about 9 fstops of that dynamic range. Which is fine too, except your monitor may or may not be able to display more than about the same 9 stops at best, and your printer will be lucky to get more than 5 or 6 fstops of dynamic range.

That means that either the white end or the black end of the range is probably going to be compressed.  If it is the black end...  those shadows filled with detail on the monitor won't show any detail on a print.  If it is the white end that gets compressed there won't be much detail in things like white clouds.  Or you might center the dynamic range and lose a little of both.  And you might also compress the entire range evenly, which may be fine and may just look wierd.  But the problem you have is that your monitor is not matching your printer.   If the sky and the building look about the same on both the monitor and the print it's an indication that the monitor can display a greater dynamic range than the print, or that they have a differently shaped gamma curve.

Once again, a properly calibrated monitor used to edit the image is precisely the solution.  If you have that, adjusting the gamma curve for that image will show the correct amount when viewing on the monitor, and then printing will be perfect. Lacking a calibrated monitor there are other steps that can taken by the numbers even though you can't see the effects accurately.

When you display an image with an editor using an 8 bit gamma corrected format there are just about 22 brightness levels per fstop.  That isn't exactly correct, as there are probably only about 20 in the brightest fstop and might be perhaps 30 in the darkest fstop.  But rough numbers are good enough for our purposes.  The points of significance are about 245 for brightness and about 35 for the dark end.  Nothing brighter than 245 or so is going to have any detail!  Commonly you want to know that for the brightest areas of a person's face, or for clothing where you'd like to see detail.  The only things that you should allow to be brighter than that are absolutely pure whites that are supposed to be washed out.  Edges, blown highlights, light sources, and so on are fine at 250 to 255.

In your image that truck and the clouds are probably the only area where anything should be brighter than 245 or so.  The truck in fact is 240 and the clouds range from that up to 254 (and will have almost no visible detail in a print).   That should be fine.

The shadows go all the way down to 0.  On the left and the right virtually all detail is below 50 and about half of the detail in the dark area across the center is too.  That is the darkest fstop that will show any detail at all.

The image would probably print better if either a curves tool is used to change gamma linearity to brighten the shadows; or brightness and contrast tools can be used for the same effect.  The darkest shadws could be raised up to perhaps 25 or so, which will still show up as pure black on a print, but that would raise shadows currently at 30 up to 55 and shadows at 50 up to 75.   (I would also reduce the maximum brightness from 253 down to maybe 247 to keep a little detail in the clouds.)

Doing it by the numbers is difficult, and it might take several prints to get it exactly right.  The best way is to calibrate the monitor to provide an accurate preview.

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