Hello. My goal is to submit a TIFF file in the highest quality possible to Nations Photo Lab for printing. The website help section for Nations Photo Lab says that the file submitted must have 8 bit color depth. I have read conflicting reports about the color depth of files exported from GIMP. My question is: as a GIMP user, what do I need to do in order to get a TIFF file of a picture which has 8 bit color depth? Thank you.
IIRC, Image>Precision>8 bit; then Image>Mode>indexed; then file>export> tiff
Beware that â8 bit color depthâ might mean either â8 bits per channel per pixel (eg 24 bits total in three channels)â or â8 bits total per pixelâ.
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set the image precision as GIMP is calling it to 8 bit before export. I personally retain it at 16 bit for archival storage.
Thank you for this reply. There is one part of your answer that I am confused about. The help page for Nations Photo Lab says that the file must be in RGB color mode. That you can see here:
If I do the part that is Image > Mode > Indexed, then it would appear that I am leaving RGB color mode, although I am not certain about that. If I do the Image > Mode > Indexed part, then will that make me ineligible for Nations Photo Lab? And, if I do only the part that is Image > Precision > 8 bit, will that be enough by itself to get a TIFF file with 8 bit color depth? I appreciate any replies about this. Thank you.
That web page explains what they mean by â8-bit depthâ:
8-bit color depth= A bit references the amount of tonal variation in an image. With the RGB color mode, the image would contain 256 tonal variations each for red, green, and blue. You are able to save your image files in different color depths; however, we highly recommend submitting your files in the 8-bit color depth as this is the optimal depth for our printers.
So they expect images to have three channels (R, G, B), and each pixel should have 8 bits in each channel, for a total of 24.
âIndexedâ (aka âpalleteâ) is different, and not what they want.
Export it as a jpg. Bay Photos told me they prefer them to tif or png for printing my astrophotos. But I donât know much about printing others can probably give you better help.
Why bother with a 8-bit color depth tiff? It wonât be very different than a good quality jpg, not sure we can see a differenceâŚ
The website for Nations Photo Lab:
says that the file submitted must have 8 bit color depth regardless of the file type, even if it is a JPEG.
I just got an email from the customer service for Nations Photo Lab. The person said that, for Nations Photo Lab, TIFF is the file type that will result in the highest image quality. It might be that different printing companies are not all the same in this regard.
This is CYA. They must have customers that send 60% quality JPEG and wonder why the picture is bad. And in a TIFF you can use JPEG compression, so this doesnât completely remove the problem. A Q95 JPEG is as good as a TIFâŚ
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I canât agree more 
I wonât be surprised either if they receive on a regular basis a JPEG auto colored-filter/cropped on a phone with a pseudo âfreeâ app saving at medium quality JPEG if you donât watch the advertising, but looks good on a tiny phone screen then people complains when receiving the printâŚ
Lossless compression is going to be the highest quality, but if youâre not planning on doing further processing, you wonât be able to tell the difference between lossless 8-bit TIFF and Q99 JPEG under almost any circumstances.
Youâve basically lost too much information with the 8-bit conversion already. It is a bit concerning that a photo lab doesnât support higher bit depth TIFF for better quality.
I think you are right. A few days ago, I did a google search on the keywords âbest photo printing companyâ. Almost all of the search results said that Nations Photo Lab had the best image quality. I am baffled that they require 8 bit color depth, but they were clearly #1 in the reviews of the photo printing companies.