Why is DPI changing?

[Skipper]

Every time I resize a photo in PS th resolution changes too and I don't want it to. It's 300. Then I resize it to, say, 8x10 or 5x7 or whatever, and the resolution changes, usually to some odd higher number like 461 or 529. Rarely will the resolution go lower but sometimes. I'd like it to remain at 300 regardless of what size I make the photo. What I am doing wrong? I always change it back to 300 but that makes the photo increase or decrease in size, although the number/dimenaions stay the same.

[RABaker]

Skipper,

In order to better understand your question I compared what you wrote to the behavior of both PSE9 and PSCS4 (the two PS programs that I have).

From your description, I think you are resizing with the "Image Size" dialog window. If you look at this window you will see that there is an upper box where you can specify the image "Pixel Dimensions" and another box below that where you can specify the "Document Size." If you use the Document Size portion of this dialog to specify the image dimension in inches, and the "Resample Image" checkbox near the bottom of the window is not selected, the image pixels-per-inch (PPI) resolution will automatically adjust itself to keep the original pixel dimensions but fit these pixels into the new "inch" size you have specified. In order to have the PPI remain at the initial value of 300 you have, you must select the Resample Image checkbox before you specify the new size in inches.

[Note that in you want to use the upper "Pixel Dimension" portion of the Image Size dialog window, it is active only after you have selected the Resample Image checkbox.]

Good luck,
Richard Baker

[mrchile]

If you are using the image size function, make sure that you have the resample image box checked. However, if you are changing the aspect ratio of the image, you should be using the crop tool instead so as to avoid distorting your image.
For instance, if your original image has an aspect ratio of 2:3 out of the camera and you want an 8x10 you should crop it with the cropping tool.

[RABaker]

Yep - MrChile is right. I should have thought about that when I was writing my first response - sorry. I guess that is what happens when I'm in a hurry...

Good luck,
Richard Baker

[suci]

I don't see any reason, ever to resample an image to an arbitrary number such as 300 dpi, unless the file is to be shared on the Internet. And then it should be a copy.
If the image is to be printed, the printer doesn't care if your file is 300 dpi or 3000 dpi.
On the other hand if you resize to 4/6 and resample to 300 dpi, you have thrown away up to three fourths of image resolution, depending on the camera you use. And if you resize a 200 dpi file to 300 dpi, you gain nothing, but artficial resolution that exists only in theory.
Cropping should always be done with the resample box unchecked and let the resolution fall where it may.

[Skipper]

Thanks very much everyone.

I use PS CS 4. I have been cropping using the crop too, with the values (i.e. 4x6 or 8x10) plugged in up top.

The resample image box is checked (though I only see that later, same time I see the whacky resolution). So are other two boxes, constrain and scale.

Yes, I am thinking I'm losing data, quality when I switch back to 300 and it gets smaller. But of course I don't want to skew or distort the image by plugging in values for two sides in 'image size'.

I have photo up now. Image size says it's 14.1 wide and 9.4 high. And 300 dpi. If I crop with crop too to 4x6 it's 4 x 6 and then 453 dpi.

[suci]

Skipper, when you crop to any size including anything other than a 2:3 ratio image, your image is not resampled. Unless you monkey with the resolution numbers and like I said earlier I can't see any reason for that..
Nikons give you a 300 ppi resolution out of the camera, which makes your image 9.493X14.293 inches. Canons give you a 72 ppi resolution out of the camera, which for the 20D translates to32.444X48.667 inches. Both of those dimensions translate to a 4/6. The D90 at 4/6 is 712 ppi, and the 20D is584 ppi.
You can after cropping resample, if you wish with the resize function and we routinely do for images to be displayed here, but I never resample for printing. Again if you have a 4/6 image at 712 ppi, then resample to 300 ppi, you threw out 412 ppi resolution. Why would you do that? Your expensive DSLR just becomes a low resolution box camera.
Resampling to a lower resolution for printing is analogous to printing a negative with a piece of matte glass in your enlarger.

[Skipper]

Yikes Suci, the last thing I want to do is degrade the quality of image that is going to be printed. I called lab I use and rep said to drag and drop as is (large and 300) into software and crop to size templates there as it will retain resolution. Doesn't answer the original question but sounds like it's going to work for prints for the time being. I hear what you're saying about leaving the dpi alone after resizing but printers (not this one I'm using perhaps) always say they want 300 or thereabouts, not 500 or 700 or 653.....

[mrchile]

Skipper, when you crop to any size including anything other than a 2:3 ratio image, your image is not resampled. Unless you monkey with the resolution numbers and like I said earlier I can't see any reason for that..
Nikons give you a 300 ppi resolution out of the camera, which makes your image 9.493X14.293 inches. Canons give you a 72 ppi resolution out of the camera, which for the 20D translates to32.444X48.667 inches. Both of those dimensions translate to a 4/6. The D90 at 4/6 is 712 ppi, and the 20D is584 ppi.
You can after cropping resample, if you wish with the resize function and we routinely do for images to be displayed here, but I never resample for printing. Again if you have a 4/6 image at 712 ppi, then resample to 300 ppi, you threw out 412 ppi resolution. Why would you do that? Your expensive DSLR just becomes a low resolution box camera.
Resampling to a lower resolution for printing is analogous to printing a negative with a piece of matte glass in your enlarger.

I'm not sure what you are talking about Janos, but when I crop, I always have the resolution set at 300ppi. I've used the crop tool to enlarge images for printing to as big as 20x30 inches with res at 300 ppi without any appreciable loss of resolution and as small as 4x6 at 300 ppi, and prints look great.

[suci]

. I hear what you're saying about leaving the dpi alone after resizing but printers (not this one I'm using perhaps) always say they want 300 or thereabouts, not 500 or 700 or 653.....

The printer needs a minimum 300 ppi res, for a high quality print, but 200 will do, but that number is as said the minimum, not maximum.
I print all my files at full resolution even if it's a 4/6 at 712 ppi. The printer will only give you inferior results if the resolution is too low [like 72 ppi for a 4/6], but not if the file is 712 ppi. It's possible that some old inkjet printers would slow down at higher resolution, but none of the current printers arefazed by it. Actually you can print directly off the memory card, with most modern printers.