I have an incredibly detailed image made with a 42MP camera which I'd like to get printed at Costco. They do 24x36 luster prints on an Epson 78xx for US$10! My Epson 3880 will do only a maximum of 17" wide.
I have their profile, made the image 8 bit, and resolution of 180 but it's still 80MB. How can I bring it down to the 10MB Costco says is their maximum file size?
Any suggestions would be greatly appreciated.
IIRC, with Costco all you can upload is a regular JPEG, although you can convert it to the specific Costco location's specific printer's profile, as downloaded from Dry Creek. Anyway, at 8 bits per channel, the full (uncompressed) size from your 42 MP camera would only be a little under 121 MB, so the JPEG compression would have to be a little over 12:1. I suspect you'd find that even a relatively detailed subject may look quite good from a JPEG with 12:1 compression. Certainly for $10 I recommend you try it and see.
Anyway at 24x36 inches you only have 221 ppi to start with, so going down to 180 ppi only reduces your pixel count by about 34%, so the compression would still be about 8:1--which again I think may very well look fine. And I think that a good 180 ppi would very probably look fine in a print that large. However, I
suspect you're likely to get overall better results by sending 221 ppi compressed 12:1 than you are sending 180 ppi compressed 8:1.
Now if in fact the limit is 30 MB or 50 MB instead of 10 MB, then you should be fine regardless. Even 4:1 compression (for 30 MB) in a JPEG will usually be pretty close to visually lossless / indistinguishable.