An online lab wants my large (over 100 MB) TIFF files as 50 MB JPEGs with a maximum of 80 megapixels for making aluminum 16x24 inch prints. What about final resolution? How do I convert from one to the other?
What about final resolution? [80 megapixels max] How to convert from one to the other?
Your print dimensions of 16" x 24" means the aspect ratio of your print is
24/16 = 3/2(i.e., 3:2). So the ratio of vertical pixels to horizontal pixels h/w = 3/2 (assuming portrait orientation). Let's write that as
h = w * 3/2
You know that the product of horizontal pixels times vertical pixels can be no more than 80 MP, so
h * w <= 80 MP
Then, replacing h in the 2nd equation with its equivalent value in the 1st equation,
(w * 3/2) * w <= 80 MP
w² <= (2/3) * 80 MP
∴ w <= 7302 px, h <= 10954 px
As Michael Clark commented in Caleb's answer, your 100 MB TIFF file is probably much smaller than 80 MP, so chances are you have nothing to worry about.
scotbb answer is good. I like he is using math formulas, but I will complement that.
In my opinion that person on the lab is telling you mumble jumble!
Lets put this simple: 16x24 inches
at 300ppi are 4800x7200px. Thoose are 34.5 Megapixels. Period.
Any picture on a jpg format, regardles you are using the maximum quality will most likley under 20Mb.
Remember to include your color profile.
P.S. The minimum recomended will be 200ppi. 3200x4800px; about 16Mpx.
what about final resolution?
The lab should be able to tell you what pixel dimensions they want to print at 16x24". Unless you've got a very unusual camera, your full resolution image should be fine without downsizing. (There's not much you can do if it's too small, but considering that the original file is 100MB that's probably not a problem.) Even at the highest quality setting, a JPEG image will likely be compressed enough to get your 100MB file down under the 80MB limit.
How to convert
Any photo editor should be able to do the conversion for you. Open the image and save or export it as JPEG, specifying the highest quality setting that gets the size down where you need it.