Pixel aspect ratio setting vs Cropping the photo

[Yan Zhang]

When I process my files in Photoshop, I always want to save the final file in a ratio of 3:2, as this is a standard ratio in many printing shops with relative cheap price.

I can set such ratio in crop tool, but feel not so convenient. What I like to do is to
do some crop without bound with 3:2 ratio, then I adjust the file ratio using Pixel Aspect Ratio tool - which may twist a bit of the image, but not noticeable for most cases.
But I have two difficulties with this tool:

(1) how to set the proper ratio factor (in my case, is 3:2)? I tried many numbers, and could get a precise for this.

(2) once applying the defined custom pixel aspect ratio, the file looks like in the right ratio, but
after I save the file, and open it from Picasa to view, it is not in the right ratio at all. What is the reason?

I remembered that sometime ago Colin mentioned this in a thread, but I could not find my answers.

Thanks in advance.

Yan

[Colin Southern]

Hi Yan,

Easiest way is to simply use IMAGE -> IMAGE SIZE with resample image turned on - then you can just plug in the dimensions and resolution you want and it does the rest

[Dave Humphries]

Hmmmm, not an area I know much about (no change there then), but if I understood correctly, Yan was after being able to crop for composition into something other than 3:2, to suit picture content, but wished to send standard ratio files to the print shop.
Having done the crop, wouldn't expanding the canvas with sufficient pixels in one direction to take the file back to 3:2 be better?
Suggest either fill the blank void with black, or leave transparent?
Might take a bit of maths.

The methods proposed seems fraught with danger of getting odd results, either on screen, in print or both.

Am I talking rubbish? (as usual)

[Yan Zhang]

Hi Colin and Dave,

Thanks very much for your help. I'll have a try.

[Colin Southern]

Hi Colin and Dave,

Thanks very much for your help. I'll have a try.

No worries Yan,

I normally just crop the image to whatever looks best (often I'll shoot a little higher so that I have excess sky to trim off thus biasing the aspect ratio more towards my favoured 2:1 anyway) - crop to suit - then just tell photoshop to make it 44" wide, 22" high, and 180DPI, and see how it looks from there.

Ley us know how you get on.

[McQ]

I can set a 3:2 ratio in the crop tool, but feel not so convenient. What I like to do is to do some crop without bound with 3:2 ratio, then I adjust the file ratio using Pixel Aspect Ratio tool - which may twist a bit of the image, but not noticeable for most cases.

I am usually never one to push absolutes, but in photography I would advise against ever using the "pixel aspect ratio" setting in photoshop. This setting is mainly used for still images from video data, and other files that do not have square pixels. To my knowledge, all digital cameras out today capture their images using a 1:1 pixel aspect ratio. Anything other than 1:1 will therefore skew/distort the image. The image aspect ratio should really be set using the cropping tool only, unless there's very special circumstances...

The reason that your images were not looking correct in picassa is because most programs assume a 1:1 pixel aspect ratio *for all photographs*, or are simply not capable of reading pixel aspect ratio information in the first place. Just stick with 1:1 and crop to 3:2 (or otherwise) and you will have no problems.

[dendrophile]

(2) once applying the defined custom pixel aspect ratio, the file looks like in the right ratio, but
after I save the file, and open it from Picasa to view, it is not in the right ratio at all. What is the reason?

Yan,

I have never used the custom pixel aspect ratio, for the reasons that Colin and Sean have mentioned. Normally PS treats pixels as square. The custom pixel aspect ratio setting (View->Pixel Aspect Ratio->Custom Pixel Aspect Ratio...) I imagine just sets some metadata somewhere that tells applications the ratio between height and width (or width and height, who knows!) I'm then guessing that Picassa doesn't know that the pixels are no longer meant to be square. We could test this by comparing two images, one with a square pixel aspect ratio and one without. I would predict that the image files differed only in a few bytes, and that the majority of these two files would be (identical) pixel data with the same number of rows and columns. I guess I am only saying that Picassa doesn't read or understand the aspect ratio metadata.

(EDIT Note, playing with Photoshop, I get the impression that the aspect ratio is read from the file. However, there is a tick box that allows the ratio to be used or not, View->Pixel Aspect Ratio Correction, so the pixel data is not changed at all in the saved file. PS CS4 seems to warn that this setting is only for preview purposes, so I imagine it's simply not usable to help us achieve what Yan wants.)

I'm with Dave, though. Can't you crop your photos down to whatever you like, and then use the Image->Canvas Size... menu to fill the top/bottom or left/right sides with black or white or transparent, to get the photo back into a 3x2? I feel this would damage the image less than a resize would.

Alternatively, if you happen to be doing crops that keep the width of a landscape shot (so, for example, taking a 3x2 and doing "a Colin" to turn it into a 2x1) one way to add the black/white/transparent top/bottom is to use the selection tool with the fixed ratio style, and width=2 and height=1. Draw the selection that you want. Once you have made your selection, invert the selection (Select->Inverse) and then fill the selection with black/white/whatever using Edit-Fill... This will add the chosen colour to the top and bottom. Note that with this approach the aspect ratio of the image has not changed, all we've done is mask out the top/bottom of the image.

Just my thoughts.

Graham

[Dave Humphries]

Alternatively, if you happen to be doing crops that keep the width of a landscape shot (so, for example, taking a 3x2 and doing "a Colin" to turn it into a 2x1) one way to add the black/white/transparent top/bottom is to use the selection tool with the fixed ratio style, and width=2 and height=1. Draw the selection that you want. Once you have made your selection, invert the selection (Select->Inverse) and then fill the selection with black/white/whatever using Edit-Fill... This will add the chosen colour to the top and bottom. Note that with this approach the aspect ratio of the image has not changed, all we've done is mask out the top/bottom of the image.

Thanks Graham,

Hadn't thought of doing it this way, seems easier, must give it a try sometime.

Cheers,

[Yan Zhang]

Dear all,

Thanks very much for your kind help. I have tried several ways as you suggested, but so far
I nly worked out one approach as Colin suggested. What I did was:

(1) crop the file as I needed, without ratio constraint, but I tried to to get close to 2:3 ratio,
(2) Then I click "Image=>Image size", set "document size" with width=18 inches, height=12 inches, and Resolution =180 pixel/inches, then click OK, the file gets to the ratio I want.

But I still have a question with this:
(a) how do I set the width and height numbers? I noted with different width and height numbers, the document size significant changes though having the same ratio, e.g. setting width=24 inches, height=16 inches will have a much bigger files of setting width=18 inches, and height=12 inches. What are the suitable numbers should I use?

(b) how to set Resolution number? I set 180, get a small size file, but if I set 350, get a very large file. What is a suitable number.

I may ask too many questions.

Again, thanks for your help.

[Colin Southern]

Hi Yan,

Essentially photoshop is just trying to be helpful by presenting you with the same information a couple of different ways, to save you having to work it out yourself.

The number of pixels in an image is what dictates how much information is stored in the image (pretty much "end of story right there" in most cases); a 3000 x 2000 pixel image contains a lot more information that a 300 x 200 pixel image, but the "ideal" number of pixels really depends on what you want to do with the image.

The size of the image (as opposed to it's pixel dimensions) are only of real relevance when you print the image - and then resolution starts to come into it some more. If you print an image 12 inches wide by 8 inches long - at a resolution of only 50 pixels per inch (ie a 600 x 400 pixel image) then it'll look very grainy - whereas if you print an image at 300 pixels per inch (ie a 3600 x 2400 pixel image) then it'll look very sharp, but if it's size is still 12 x 8 then it'll then be a very big file.

So - basically - for a given number of pixels in an image, the bigger you print, the lower the resolution (or in other words the bigger you print the worse it looks close up). But wait - there's more ...

Three more things to consider ...

1. Although resolution drops with increasing size (unless you resample the image) (more about that later), viewing distance increases with size - and the ability of our eyes to resolve minute changes in tone of course decrease with the increase in viewing distance (only photographers look at 10 foot prints from 6 inches away!) (there's an old joke that with photographers, minimum viewing distance is limited only by the length of their noses!) So when people ask "how big an image can I print from an 8mp camera" the real answer is "as big as you like - just don't expect it to be sharp if you're viewing up close" (you could print one 10 miles wide and 16 miles high that would look sharp from space, but here on earth you'd be looking at pixels the size of football fields)

2. Past a certain resolution the eye can't resolve further detail anyway; in my opinion there's little to be gained in going over 180 pixels per inch (which equates to up to FIFTY tone changes in every square millimeter) (don't know if your eyes can resolve better than that, but mine sure can't - at ANY viewing distance) (so it really depends on what the intended purpose of the image is - 50 tone changes per square millimeter is "slight overkill" if it's a billboard that's going to be viewed from 30 feet away).

3. Other factors need to be taken into account - a 300MB image with lots and lots and lots more lots of pixels would be great if you're printing really large and you want lots of details -- but post a 300MB file into a forum like this, and no-one will be thanking you! In contrast, a 300KB file might be fine here, but hopeless if you're going to print it 12 feet wide and 8 feet high - so it's a case of "horses for courses" again.

Resampling ...

When I mentioned before that for a given image size, the bigger you print, the lower the quality, that's true - but there's a way to wring a bit more out of a small image - and it's called "up-sampling" - which in a nutshell is where the likes of photoshop adds additional pixels based on what it thinks should go there (which is worked out by looking at what's already there). It can be mildly useful as a way to get a better print from an image that really doesn't contain enough pixels - but where it really comes into its own is when you're doing what you're doing - changing the aspect ratio.

If you don't add or subtract pixels then the only other way to do it is to change the shape of the pixels - and I think you've discovered already that that doesn't work very well! So resampling is usually the only way to go - thankfully photoshop is pretty darn good at it; all we have to do is tell it how many pixels we want -- OR -- the physical dimensions and resolution (which equates to the same thing which is why when you change one it changes the other) - and it works everything out for you.

So - as a rule - if you're posting online then often making the image 6 inches wide by 4 inches high at 100 ppi works well - for printing I typically use 180 ppi. (although I often break my own rule my making my images 1024 x 512 pixels so that people can look at a more detailed image if they wish - software here automatically downsizes them for display).

However if you want to print an image that you've taken at a fairly small size and it happens to work out at something like 400 ppi then it's pointless to down sample it (there's no benefit to such a high resolution, but no downside either) (in this case I'm talking about resampling being turned off) (just be aware that with it turned off you can't change the aspect ratio unless you crop).

Lot to digest here so I'll shut up for now - let you plough through it all - and come back if you don't understand anything!

[Yan Zhang]

Dear Colin,

Thanks for your very detailed information and help. I have already gained a lot from you. I'll take some time to understand better.

Thanks again!

[Colin Southern]

You're very welcome!

Some books might help to explain it better - I'm not the best at explaining these things.