Mikes Photography

pasvorto · Posted: Fri Jan 04, 2008 8:51 pm Post subject: Photoshop question

I have used CS2 to edit RAW files. In the 'image size' box, I have 2 values: the pixels and the pixels per inch. I find the default PPI to be 300. Now, when I change that to 400, the size of the image increases and is much easier to fool with. Also, if I change the print size, the image size also changes. Why should increasing the print resolution change the size of the image on the screen? Also, is it just a waste of time? Should I just use the zoom, rather than what I am doing?
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creators · Posted: Fri Jan 04, 2008 11:43 pm Post subject:

I don't understand when you say, 'when I change that to 400 (PPI), the size of the image increases ', because in print terms the size of the image decreases. PPI (pixels per inch) and DPI (dots per inch) are the same thing, they tell you how many of the pixels in your picture are printed to each inch, so a picture that is 2592 x 3872 pixels at 300 PPI/DPI will print out at 8.64 x 12.9 inches, whereas at 400 PPI/DPI it will print out at 6.48 x 9.68 inches. Changing the resolution does not change the size of the image on screen, it doesn't affect it at all, and if your picture does seem to change size, which I find very odd, then your version of CS2 is doing something that mine doesn't. On screen a pixel is a pixel is a pixel, of unvarying size, the PPI/DPI only affects how it prints out and is information that is sent to your printer and is only of use to your printer, it has no earthly value in terms of what you see on your screen. So yes it is a waste of time so far as viewing the image onscreen is concerned, but it is of immense value in deciding what size you want your prints to be. It is far better to change the PPI/DPI in order to change print size than resampling because resampling discards pixels in order to resize the image, changing the PPI/DPI retains all the pixel information of the original image but compresses it into a smaller area so giving you the best possible print copy. If you go into 'Image Size' once your picture is open in CS2, type in the size you want your print to be but untick the 'Resample' box, you will obtain the best quality print your printer is capable of producing. If you want to know mazimum size pictures you can print from your camera without the picture looking pixelated, the lowest reasonable PPI/DPI to obtain a decent picture is 72 DPI, so my camera which gives me 2592 x 3872 pixels is capable of a maximum print size of 36 x 53.7 inches, though I haven't found a printer that'll do that yet. Very Happy

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pasvorto · Posted: Sat Jan 05, 2008 12:10 am Post subject:

Evidently I haven't explained myself very well. Maybe I will take another shot at the manual.
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"Nothing exceeds like excess"

peteski · Posted: Sat Jan 05, 2008 12:24 am Post subject:

creators · Posted: Sat Jan 05, 2008 1:45 am Post subject:

Yep, and 72 DPI is perfectly adequate for any print, though submitting pictures for publication usually means that will ask for 300 DPI.
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pasvorto · Posted: Sat Jan 05, 2008 1:50 am Post subject:

I think I may have figured it out myself. Maybe not.

I open a file and it gets displayed. I go to the image size options and change the DPI from 300 to 400 and the image on the screen gets bigger. Maybe this is what is happening...

If the image is an 8 x 10 and I increase the DPI from 300 to 400, it is still 8 x 10 but more DPI. If the screen is trying to resolve these additional "pixels" it must get larger to account for them. Initially it displayed, say 300 pixels to convey 1 inch. Now it has to display 400 pixles to convey that same inch. More monitor room is required for the additional pixels.

I realize that pixels applies only to the image, not to DPI.

Maybe I am making a tempest in a teapot.
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"Nothing exceeds like excess"

creators · Posted: Sat Jan 05, 2008 2:03 am Post subject:

Fraid not. If the image is 8 x 10 and you change the DPI from 300 to 400 you have effectively reduced the print size to 6 x 7.5. There are no additional pivels, you would only get additional pixels if you resampled the image to make it bigger, say from 8 x 10 to 10 x 12, in so doing the program would have to generate extra pixels to make the enlargement which it does by examining the surrounding pixels to each pixel and guessing what colour value the extra ones would be. Most modern programs do this pretty well and Photoshop has no peer in how it does this.

Are you confusing DPI and resampling/resizing, because increasing DPI/PPI does not add any more pixels to those already there.

No this isn't a tempest in a tea pot, DPI is one of the most misunderestood areas of picture production.
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creators · Posted: Sat Jan 05, 2008 2:18 am Post subject:

To enlarge on all this - When I do my stereo pictures I take two images both 3872 x 2592 and place them side by side making an image of 7744 x 2592. At a resolution of 72 DPI this gives me a printed image size of 107.5 x 36 inches. If I resize this using resampling at 72 DPI I end up with a jagged ugly printout which is hopeless to view, so what I do is resize it by increasing the DPI to 1290.7 pixels per inch which prints out at 6 inches wide. My computer then tells my printer to print at that resolution and I get pin sharp, superb prints because no information is lost even though effectively no printer can cope with that kind of resolution it still works because all the information is there blended seamlessly together.
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creators · Posted: Sat Jan 05, 2008 2:44 am Post subject:

Just while I am in pedant mode and apologies if this has all been way too much information. I think what's happening is that Photoshop is resizing your images to fit the screen, though I am at a loss to think why it would, Photoshop just doesn't do that generally. It's the only explanation I can think of that fits what you have written here.

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