Photo 'Pixel Aspect Ratio' question

This may be fairly obscure, but I wonder if there is some knowledge or experience of this issue out there ....

On the train, flicking (intently & most studiously) through Mr Evening's 680 page book on Photoshop CS2, I noticed a setting within the 'new document' dialogue. This was 'Pixel Aspect Ratio'. The writer explained that this would be required for outputs such as widescreen video displays where the pixels are not square, but wider in fact.

Well I use such a display format - with my Pentax K7 videos, which are re-rendered into a PAL widescreen HD resolution. I use images from the K7 as well, in the introductions, with panning and multiple image effects within the frame.

I am wondering if for this specific output purpose, when resizing shots for the videos, if they should also be converted to a wide 'Pixel Aspect Ratio' at the same time, to match the aspect ratio of the widescreen monitor?

And secondly - the setting I saw was in the 'New Document' dialogue in PS ... this creates a new blank canvas, but there's no content. When re-sizing a photo I would use 'Image Size' - but this has no setting for Pixel Aspect Ratio. So if I am to try this feature, I don't understand how to apply it to an actual photo. How do the New Document settings (a blank canvas) relate to an actual photo, and can I put a K7 image onto a new blank canvas with the adjusted pixel aspect ratio setting ??

Any thoughts, ideas, or solutions gratefully received.

So let me get this straight, for Wide screen displayswith rectangular pixel aspect ratios you'd make the pixels narrower to compensate the perspective to avoid things looking stretched?

Yes, that part of it - although its also about matching the aspect ratio of the source (ie photos) to the output ratio (ie a widescreen TV) - good practice. I admit, I'm also fishing for info and ideas on whether this is actually necessary, or offers any quality benefit. I'm mainly interested in the effect it might have on the quality of pan / zoom motion paths of photos across the video frame - the interlacing here can cause jaggies and ripples on the images as they move around.

The photos are part of the video stream of course - so what I am unsure of is whether during the rendering process (into a new 'widescreen PAL HD .mpg file) the format template actually re-draws all the pixels into the appropriate rectangular shape? If so, because the photos are part of that same video that may mean their pixels are also re-drawn to the new aspect ratio? That seems logical to assume ....

But, being a quality obsessive I can't help wondering if there is some advantage to importing the photos pre-prepared with the rectangular pixels already - the less work the rendering process has to do to change the photos the better I feel? And then I saw the option in photoshop - but its not clear how to do it on an image file, its only there on the New document dialogue.

If you go to the view menu in CS3 and later there is a menu item called pixel aspect ratio. If you click on this it opens an options dialogue. Select the ratio that you want and the image will be displayed accordingly.

Now save the image to a new file name and the display information is I believe save with the file. Close the file and then reopen and the file is displayed with the previously saved aspect ratio.

Photoshop will then suggest that you turn off pixel aspect correction. If you go to the view menu you will see a menu item pixel aspect correction with a checkmark next to it. If you unclick the checkmark the file returns to its original aspect ratio. If you then close the file without saving and then reopen the file the image is again displayed with the previously saved aspect ratio.

I tried viewing the files on a Wide Screen Smart TV and both images displayed with the original aspect ratio. The same occurred when I previewed both files in Window Explorer. So the basic file is not changed and the pixels remain as shot, Photoshop is merely adding information, which allows the image to be displayed with a different aspect ratio in Photoshop and possibly other Adobe CS products.

In effect Photoshop is providing a preview of what the image would look like if it was stretched or shrunk to a different ratio by your display screen.

If you decided that you can live with the stretched rectangular pixels , then to effect a permanent change you would need to go to Image menu , Image Size and untick the constrain proportions box and input pixel numbers in the ratio that you want.


Regards - Steve

Thankyou very much for that explanation - I'll try out those options as you detail, although I only have CS2 & Elements 8 - not sure if the options will be there, but I'll have a good look and report back.

Regarding Image Size - that was where I was getting confused, because there is no 'Pixel Aspect Ratio' setting there - are you saying that if the pixel aspect ratio had been changed previously as you described through the View menu, that unticking 'constrain proportions' would in fact produce a file with pixels in the new ratio?

No, the pixel aspect ratio setting from the view menu should be considered as just a custom view for that image.

My understanding is this

I have an image in a 4:3 ratio
I want to see what it might look like if it is displayed in 16:9
I set the appropriate ratio via the view menu and decide if I like it or not.

There are no specific aspect ratio presets in CS6 image resize dialogue.

By way of an example

A K5 image comes out at 4928 * 3264 pixels.
If I resize the width to 1920 I get a height of 1272, so I can either crop 252 pixels from the height dimension and maintain the original proportionality. I can create a custom 1920*1080 preset for the crop tool, which will help a lot with this or

I can untick the constrain proportions dialogue and enter 1920/1080, which in this case will give me a squashed looking image.

The pixel aspect ratio on the view menu in effect provides a preview of this without actually changing the file itself. So in this case you can open a standard K5 file go to the view menu, select pixel aspect menu and use any of the presets or create your own custom ratios.

This is a useful feature if your image is going to be displayed on a number of fixed aspect displays. It allows the user to quickly toggle through the ratios.

Regards - Steve

Many thanks again Steve, I understand then that the View option is only to offer a preview of a diffeent display ratio, to check the effect say on a widescreen monitor, but without having to actually resize the image.

I haven't been able to advance on this question yet as I have not been able to use the laptop with Photoshop installed on it for a few days. I come back to the 'aspect ratio' setting I saw though originally in the 'new document' dialogue .... still not sure how you would actually introduce an image with the pixels in a recangular format onto a new blank document? Oh well, have to keep ploughing through the Photoshop book!

Open the existing 'square' document
Crop or select all
Copy
Open a new document, with the aspect ratio you want
Paste special into new document.

Once again this provides only a preview. You can save with the aspect ratio settings, but outside of Photoshop the image is viewed in its original form.

Regards - Steve