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Image size - lack of understanding

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Old 08-06-2008, 12:27 PM
Ambrosie Ambrosie is offline
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Image size - lack of understanding

I have been an infrequent user of Photoshop 3 for over 10 years. From time to time I become aware that I have never fully understood some of the basics. One of these appears to be the matter of image size (height and width).

To do the simple jobs I generally use Photshop for, I don't always need to check information in the image size dialogue box. When I do I have, now and again been surprised at the large dimensions I find in there for photographs I've scanned which certainly didn't start out that big.

Today I've been trying to work on a batch of professionally-taken digital photographs which I uploaded from a CD. The details in Windows Explorer told me that each image had a file size of around 5,000KB. I was, therefore, expecting images of high resolution. I was surprised to find they had only 72 pixels per inch. However, I spotted the fact that the images' dimensions were in the region of 5 feet by 3 feet (60" by 40").

I'm aware that manuals advise against altering resolution if you don't know what you're doing but, as I still have the originals on the CD, I tried and found that of course upping the resolution brought the dimensions down to something feasible. I now notice that all the overlarge scanned images have a resolution of 72 ppi regardless of the resolution they were scanned at.

I've tried to work out what's happening and have wondered whether Photoshop defaults to 72 ppi resolution for all images it opens for some reason and the user is expected to adjust this before working on the image.

Could someone more experienced than I tell me if this is the case and whether huge dimensions and low resolution are in some way desirable and, if so, why. If not, is there something I could do about adjusting the resolution Photoshop selects?

I apologise for being so long-winded and thank you for reading this far at least.
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Old 08-06-2008, 01:13 PM
pixelzombie pixelzombie is offline
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Re: Image size - lack of understanding

most home scanners for whatever reason tend to scan files at your case you can take the 5X3' file and by changing the resolution(with interpolation off) to 200 ppi you'll have a file that is 21.6X14.4"...
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Old 08-06-2008, 07:55 PM
smak smak is offline
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Re: Image size - lack of understanding

Images have a pixel width and pixel height. You assign a ppi for the image when you decide to print it.
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Old 08-06-2008, 09:58 PM
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TommyO TommyO is offline
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Re: Image size - lack of understanding

Let me see if I can explain it simply...
There are two reasons your files may be the way they are. First, many cameras save their images at 72 ppi in their default mode. Most people change it to force the camera to the highest density allowed. Maybe your photographer did not.
Second, it is normal for professionals to distribute their images at 72 pixels per inch in order to prevent anyone from actually having a high res image to play with. It will look good on a screen, but will not print well at larger sizes (like you'd want to put on your wall) or edit well (too many edits in Photoshop and it will start to get distorted hues and posterization/banding). You cannot successfully get around this without loosing some data in the image and causing similar issues. Yes, you can resample the image in PS and force it to a truely higher resolution, but PS must guess at the new data and you usually don't get a sharp looking image.

Your images have enough pixels that you can "shrink" the pixels, squish more into each inch, and still have a descent size image to print. (This is what Pixelzombie was referring to above.) Pixels are not a fixed size themselves. To make an image look great, we like very small pixels and a lot of them. The pixels in your image are "large". That is one reason the dimensions throw you off. You are used to thinking about large files with a lot of "small" pixels, with resolutions in the 300-600 dpi range. The dimensions you see (like 60x40) are not necessarily the only dimensions at which you can print the image.

PS does allow you to "non-destructively" change the print size, as Pixelzombie explained. By turning off the "resample" check box, you are telling PS to simply change the number of pixels printed per inch and by shrinking the dimensions of the image... not adding any pixels. It is like zooming, it does nothing but change the numbers you see, shrink the size of the pixels so they fit better on a new page size. The number of pixels don't change, nor does the color/tonal data they contain.
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Old 08-07-2008, 07:29 AM
Ambrosie Ambrosie is offline
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Re: Image size - lack of understanding

Thank you pixelzombie, smak and especially Tommy for your help.

I believe I have now understood that an image in the first instance has a fixed number of pixels and, if it is presented at, for example, 72 ppi, the image size will be mathematically determined (taking into account the height and width ratio). If the image is a good quality one (as might be indicated by the file size) the image size will be notionally large (like my 5 foot by 3 foot ones). If the quality is poor the size will be significantly smaller. I understand now that if you enter a high ppi in the image size dialogue box, a good quality image will become smaller in dimension and will remain good quality because the pixels themselves become smaller. In the case of the poor quality image the problems indicated in the manuals will occur - there will not be enough pixels to fill the image dimensions and Photoshop will create them by averaging the ones which are there which can never result in the same quality of image as one which had naturally-occurring pixels.

I think my problem was in part not appreciating that pixels change size in the image itself, not just when you zoom in and out, and in part that I continue to relate images-as-files to images-as-negatives and images-as-photographs thus expecting them to be related to modest physical dimensions.

Thank you once again - I shall go back to my wedding photographs (for that is what they were) with greater understanding.
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