Need help with printing

I have a few offers for a few photo's that people are asking to buy for a 20 X 30 size.

I'm not sure of the two things....what resolution should I print 20 X 30.

I know about a little that PPI is involved , if I just bump up the PPI in lightroom 350 on a 1600 X 1200 will that be suffice?

Most shots are from a 40D

I'm concerned because one of the black and white shots was taken with my Iphone so i dont know how that will turn out.

I really have no clue about printing.

help

second question is cost. they are giving me 100 for each print. (I think it will cost me 25 or 30 to get it print that size, is this fair to both of us?)

If they are from a 40D, why are they 1600x1200? 40D images are 5 times that size. To be printed 20x30 the 1200x1600 would need to be cropped to 1067x1600. At that size it would be 53 ppi. Resized x3 would bring the ppi up to a minimal 160 ppi, but that would mean that each pixel is replaced by nine similar pixels. At a distance of a couple yards it might pass but closer it would look pixelated.

I wouldn't pay $100 for it, but as P.T. Barnum said ........

how are you able to calculate 1067 X 1600? for 20 X 30

btw on 40 D raw files are 3888 X 2592

Broncobear wrote in post #9055653
how are you able to calculate 1067 X 1600? for 20 X 30

That is just adjusting the pixel dimension you gave (1200X1600) to have the same aspect ratio (2:3) for the print size (20X30) you wanted.

I've just had a 3888x2592 image printed at 30"x20". I sent the image off as is, without enlarging it first to increase the resulting DPI figure.

The resulting picture is acceptable, but the relatively low resolution is obvious when it is viewed up close - once you get beyond 3 or 4 feet viewing distance, it becomes a lot more acceptable.

Given that your 1600x1200 images are less than half the size of the image I had printed, I would suggest that the acceptable viewing distance will be closer to 6 feet or more. I would venture to suggest that the image taken with your iPhone will be a non-starter for printing at that sort of size, unless you are willing to accept a relatively poor quality print.

Broncobear wrote in post #9055664
btw on 40 D raw files are 3888 X 2592

If you have full-sized 40D images, they will do 20x30 very well (but not that phone-camera junk). Just send the full rez files to a good printer and let him worry about the resizing.

You threw things off when in the OP you mentioned a 1600x1200 resolution -- not just because it is the wrong aspect ratio for a 20x30, but because it is a very low resolution picture that could print a 6x8 image nicely at 200 ppi but not do well on anything larger.

A 40D file, converted with no resizing at 3888x2592 will show about 129 pixels/inch at a print size of 20x30. A print, if the original image quality is great, could look "good" but realize that at close scrutiny won't look "great". I'd do a test print of a close crop at 8x10 (crop so that you get the same ppi resolution). Look at your 8x10 from a reasonable viewing distance and ask yourself if it's acceptable.

One thing that can help is to sharpen for your print size, which may look a bit overdone on screen. You can also try resizing with resampling so you have a 20x30 at 250 or 300 ppi and then sharpening the final image and checking your results by viewing it in View/Print Size (if you are using Photoshop).