Can I increase the resolution of a six-megapixel photo to print on a large canvas?

Question

I took a photograph and the information tells me it is 3008x2000 and 2.2 MB. It's a darker image of two people and their reflections in a window. I'm trying to get it printed on a canvas by Shutterfly or Walgreens, but it's telling me the resolution is too low and that I'd have to choose a smaller size canvas. I really want a bigger one. Is there any way to change the resolution of this photograph now or is it too late?

Photo canvas, 20"×16" 3000×2400 pixels

Photo canvas, 24"×20" 3600×3000 pixels

Those are the recommendations given.

Answer 1

So, in this particular case, I think you have some options not necessarily always available. That's because your photo is really almost the required resolution — just the wrong shape.

It looks like the rule behind the recommended sizes you've given is simply 150 pixels per inch. That is, 20" × 150 pixels = 3000 pixels per inch, and in the other dimension, 16" × 150 pixels = 2400 pixels per inch.

Your image is sufficient in the long dimension (3008 — we'll ignore the slightly odd extra 8, which is typical for camera output; it's such a tiny bit that it's really negligible), but doesn't measure up in the other dimension. That's because it uses the 3:2 aspect ration typical for DSLRs. The canvas sizes, however, are in a 5:4 ratio — a classic photographic shape, matching the venerable 8×10".

So, in order to make your image fit, you need to do one of two things. First, you can lop off the ends of your image, taking a 5:4-ratio square from the middle. In order to make that work, you'd take 254 pixels off each edge, leaving you with 2500×2000. But now, clearly, you're under the limit. Now, personally, I think that you probably can get away with upscaling by 20% to make the web site happy. With a canvas print, this'll probably look just fine unless someone has their nose all up in it.

Alternately, you can "letterbox" — just as with a wide-screen aspect ratio movie on an older 4:3 TV, you put 200-pixel black (or white, or whatever color you want, really) bars on the top and bottom, so the whole image overall is 3000×2400. Problem solved — except that might look funny.

So, the final possibility (and I think the best option, unless your image is really amenable to the 5:4 crop) is to find a print service that will do canvas prints in dimensions that fit the 3:2 aspect ratio. Bay Photo is one such well-regarded print service, but there's others as well. Then, you could get something like a 18"×12" canvas. (Bay Photo actually requests 300dpi sources, but they'll let you use lower ones if you promise not to complain about the results.)

Answer 2

Imagine you have some grid paper and instead of filling in each square with a different color to make the image you want, you get four pieces of paper and use four squares where before you used one. You double the height and width, but the picture is now kind of blocky. This is essentially what happens when you attempt to increase resolution. There are some ways you can smooth out the edges, but fundamentally it is the same thing. If you can live with that, there is a lot of software out there to help (essentially you want to scale your image), but it will never be as good as having a higher resolution picture to begin with. Photoshop is the obvious answer, but lots of programs can do it.

Answer 3

Their objection is invalid.
You meet their recommendation exactly if you do not want to crop the photo, and if you do crop them, while the difference is significant, it is not enough to override considerations of artistic merit and personal appeal.

• At 20 x 16 size fitted to width you match their recommendations at 150 dpi.

• At 20 x 16 size cropped to fit the frame you achieve 125 dpi = 80% of their dpi.

• At 24 x 20 fitted to width you get 125 dpi

• At 24 x 20 cropped to fit you get 100 dpi

You need to be sure BUT 125 dpi should be about as good as 125 dpi.
100 dpi may be getting too low but maybe not.
Try printing the scene on 2 x A3 or 4 a A4 sheets = 20 x 16 and see how it looks.

Their resolutions work out at 150 dpi.

20" x 16":
If you do not crop your image and fit it to width at 3000 x 2000 you acheive 150 dpi - just as they do.
If you crop the image to fit the height you end up with 2500 x 2000.
If you print this full size the resolution is 2500 / 20" = 125 dpi . This is 80% of their minimum recommended resolution. The difference is noticeable but bearable if artistic or aesthetic reasons apply.

24 x 20:

This is still acceptable at 125 dpi if you fit to width and do not crop, but is getting "a bit big" if you crop to fit their aspect ratio. .
24 x 20 = 6:5 If you do not crop your image and fit it to width it will be
3000 x 2000 = 24 x 12 with 3" top and bottom.
resolution will be 3000/24 = 125 dpi.

If you crop to fit, height = 2000 and width = 2000 x 6/5 = 2400.
Resolution = 2400/24 = 100 dpi.
This MAY be OK for your picture but you would have to decide.

---

Detail:

They are applying their limits unintelligently without considering the true situation. Your photo width:height ration does not match either of their canvases which complicates the comparison, but this is easily allowed for.
Consider your print to be 3000 x 2000 for now (rather than 3008 x 2000) to make comparisons easier. The difference is completely irrelevant.

For the 16 x 20 print your image width and theirs are now identical at 3000 pixels wide.
If you fit their width and do not crop it then your picture will be 2000 high on a 2400 high canvas so there will be an unused 200 pixel band top and bottom. This is an artistic decision on your part and does not affect the calculations.
Your image now EXACTLY meets their resolution spec.
(Or exceeds it by 3008/3000 = +0.3% if you use your true 3008 width).

If you decide to crop your image to fit their 3000:2400 canvas you retain the height of 2000 and crop so your image is 5:4 as theirs is - the width is now 2500 and you crop 500 off the width in whatever way seems best.
Your image is 2500 x 2000 against their recommended 3000 x 2500.
The difference is the same in each case 3000/2500 = 2400/2000 = 5:4
If you now blow your cropped image up t fit their full size you expand it by 5/4 = 25% in each direction. ie it is 4/5 or 80% the resolution in each direction or (4/5)^2 = 64% the area.

3008 x 2008 6 megapixel.
The 2.2 MB files size suggests that the compression used is probably not too 'ferocious' so the end quality MAY be bearable at larger print sizes.

If your dimensions had been MUCH smaller than their recommended minimums then upscaling may be very unwise but as it is you are not vastly under their size limits.
If they refuse to print it the may be trying to protect themselves against a future claim if you are dissatisfied.