Here is one:
http://digital-photography-school.com/aspect-ratio-what-it-is-and-why-it-matters/

Example, for 3:2 aspect ratio, multiply each side of the colon by the same number to keep the "aspect" the same and to not crop anything from your original image:

3x2 is same as 6x4, 12x8, 18x12, 24x16, 30x20

We reverse the numbers in printing to read 4x6, 8x12, 16x24 even though we say 3:2 aspect ratio...just to keep life interesting, I guess

From the above examples you'll see that if using a 3:2 camera, and you want to print a 20x24, you lose six inches on the long side. You can usually select how you want the image cropped even with online printing services (e.g. Costco), where you upload the file, or crop it yourself in editing prior to upload.

Finally, a simple way to envision the concept of aspect ratio is this: if you have a rectangular image and want to print as square aspect (1:1), something's gotta give

First you need to determine the ppi required for your print size.

This will help:

http://www.photokaboom.com/photography/learn/printing/resolution/1_which_resolution_print_size_viewing_distance.htm

Once you have that you can do the math to figure out what you need, in pixels on each side, to print to the size you want.

Using the chart a 16x20 requires a minimum of 89 ppi. If you multiply 89 times the dimensions of the final print you end up with 1424x1780. If you want to use a higher resolution for better viewing at closer distances you can use a higher ppi number. For argument's sake you decided you need 180 ppi which is actually a lot - you end up needing 2880x3600.

So, your image must conform to this proportion, 4x5 or a multiple thereof. If your image is a different proportion - like 2x3 (same as 4x6) then you will either print the full width and have extra space on the top and bottom, or print full height, and need to crop the edges, to conform to the 2x3 proportion.

A 20x24 is the same proportion as a 4x5 and a 16x24 is the same as a 2x3.

Hi ifurnish,

First, the size you wanted is an odd size for printing for the most part. So, your primary problem is that you requested a size that the print shop you chose just does not print in. But they do, apparently, print on 16 x 24 paper and that is why they offered that size for your print. Bay Photo will give you a print that is 20 x 24 so you might check them out, bayphoto.com. They provide quality prints at a decent price and are very quick to do the work and send it to you. It's all done online. I have attached their list below showing the 20 x 24 print.

As for sizes in general. In the "old days, i.e., film" prints had standard sizes of 5 x 7, 8 x 10, 16 x 20, and many print shops today still operate from that perspective, although it is changing (and I don't think film was even sized exactly to those sizes). Since dSLR cameras shoot in a different format (ratio), 3:2 (an image that is longer than it is tall), than the offered print sizes, any photo you take will need to be cropped in order for it to fit into that "old style" sizing. The cropping will come from either one side or a bit off both sides, which works well in many cases.

If you do not crop the image, you can have it printed at its original digital format sizing of 4 x 6, 6 x 9, 8 x 12, 10 x 15 (which can be fit into a 16 x 20 frame so many people use this size), 12 x 18 (for a larger frame), 14 x 21, 16 x 24, 18 x 27, and the list goes on (hopefully I've got my math right). What these numbers have in common is that you add to the "3" and to the "2" the same number to figure out the sizes. So, if you add 3 + 3 you get 6; and then 2 + 2 you get 4 -- for a 4 x 6 print and so on, each time adding either 2 or 3 to the number, depending on the side, to figure out all of the print sizes. I talk about this on a web site I have for Lightroom/Photography, viathelens.net.

Hope this helps.

The school portrait industry always has had viewfinder masks of some sort. We used to make them as "line positives" on Kodalith film and tape them over the ground glass or viewing lens in the viewfinder of Camerz long roll cameras.

Initially, we had Pro Camera of Parma, Ohio, put tiny versions of these in our Canons. When we got to the 50D, the screen was replaceable, so we had them laser-etched by the folks at viewfindermasks.com. They can do the same with many dSLRs.

It does seem possible for a mirrorless camera company aiming for the pro market to do masking in firmware...

* * * * *


Yeah, why crop to make crap !!!
Simple remedy, stay within your own camera's native proportions. For the full frame DSLR that's 3:2
Having a print made...depends where you have it made...my film used to be printed at 10x8 unless
specifically ordering full frame. No difference with digital camera....
The once-popular 4x5 ratio is based on a once-popular old camera used for portraits.
Your 4x3 ratio will have its own standard crop unless you use a custom printer, and then what you
may WANT you will find distorted when wandering afar from your own camera's native aspect ratio.
Learn to shoot WITHIN that ratio and avoid distortion by considering the subject you shoot! Change
the ratio to fit the subject if it needs it! Then when you send out for prints or enlargements you can
avoid ugly distortion. Send for prints in the standard size for the subject you shot. Multiply both
dimensions by the same number. Check the ppi, (which will become dpi) for any chosen enlargement
to be sure you have enough in an enlargement not to make $$ into crap as warned above. (Whoever
said snapshots could be automatic masterpieces or that planning doesn't pay off?)

I have written a Presidential Decree that we, the USA, only support particular aspect ratios. The automated machines will no longer be allowed to arbitrarily whack off part of your photo with auto crop!! It is time for the United Nations to step in and standardize the world of paper!!!

Moral of the story, if you have a standard size for exhibiting, make life easy and crop to fit that size or change the aspect ration by un-checking the keep aspect ratio box in the "image resize" menu.

* * * * *
Yes, agree, it would be a form of "cropping" to change your aspect ratio for a shot, but
you would need to trade one sort of cropping for the other, that of just "cropping" as it
is generally described or shooting for the subject that it is..
Examples abound as to what happens when we change from our native aspect ratio to
something else when shooting certain subjects. That is exactly why we have choices in
cameras. Cropping after the shot, people may "gain or lose weight" if full figure or in facial
features. A tall vase or bottle shot with a native 2:3 ratio can become unpleasantly stubby if
switching to another aspect ratio for a print, etc.
It's up to the photographer to PLAN for what is wanted, rather than cope with problems after
a thoughtless shot. What's the tradeoff between one form of "cropping" for print and another..
as even sticking to your own aspect ratio you will have some trimming to fit a standard paper
size. Your treasured 3:2 landscape may not suffer but your 2:3 portrait subject may. That
landscape example was also mentioned in some previous response on this thread.