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Old Oct 11, 2003, 3:55 PM   #1
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Default Resolution and Cropping

Hi All,
I am using a D100 and am shooting in Raw mode. Many of my shots need to be cropped. So my question is.....
1) How do I crop a photo without losing the original picture quality?
2) When setting the crop size, the default resolution is 300 pixels/inch. When and why would I change the crop resolution number?
3) Does Crop resolution impact the post cropped image quality?

Many thanks
Eric
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Old Oct 16, 2003, 12:42 AM   #2
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When you crop you remove pixels. An image with fewer pixels can’t be blown up as much or printed as large without losing quality.

I’m guessing that is a Nikon D100 with 6Mp. So if you take a full resolution shot and crop 50% from the image you have a 3Mp image. The 6Mp image will print a lot better at say 13 X 19 than the 3Mp you have left after cropping. There is no magic way to recover the pixels. If you upsample the image back to 6Mp Photoshop or whatever you are using is just guessing at the missing pixels and putting something in that is half way between the adjacent pixels. And the resample decreases the quality slightly.

The PPI is immaterial to the outcome unless you resample. Then the image is resized with the guessed pixels. Most people try to avoid this until the print size resolution gets below a certain threshold. For me that is about 160 PPI. You lose some quality in the resample and it should be avoided if you can get a good print without the resample. If you aren’t going to print you shouldn’t resample unless you want to make it smaller for the web or e-mail.

The best crop you can get from your camera for a 13 X 19 print is about 155PPI without resample. I would go ahead and print with that. You could confirm that by taking a full image, selecting the crop tool, putting 19 in the width and 13 in the height with nothing in the PPI box and drawing the largest crop you can. Then look under Image>Image size and see what PPI you ended up with for a 13 X 19 print. But any cropping without resample is going to take the PPI down to the point the print will suffer with a good inkjet. So if I cropped enough of the image so the resulting PPI is only say 125PPI I would resample up to 300. If you have to resample you might as well get all the pixels the printer might use.

There are two sizes for an image and you should keep them straight in your mind. The size in pixels is the size it will display onscreen at 100%. The PPI is completely immaterial. The print size is the size it will print – obviously. If you increase the print size with “resample” unchecked you will decrease the PPI. You are spreading the same pixels over a larger piece of paper so they are less dense. Don’t just automatically resample when you set a print size. Resample only if it falls below a threshold that you have determined will decrease quality on your printer.

So to answer your questions:

1) You can’t crop without losing pixels. Those pixels can’t be recovered, you can only put in dummy pixels by resample and those don’t improve the quality. If you are just displaying it screen size or printing 8 X 10 you probably won’t notice any quality loss unless you crop a lot. If you don’t resample you won’t lose further quality other than losing the pixels though.

2) The resolution in the crop is immaterial as long as you don’t resample. You can set the size in the box at the top of the page without setting a PPI and it won’t resample. If you don’t put anything in any of the boxes and just crop your PPI should stay the same – you just have a smaller image at that PPI.

3) Basically if you use Photoshop and put anything in the resolution box on the top of the screen with the crop tool selected you will resample and lose quality by resampling in addition to losing pixels. If you look in the size box and the resolution is 300 PPI and then crop with nothing selected, when you look back in the size box you will have a 300PPI smaller image and will have not caused quality loss by resample.
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Old Oct 16, 2003, 11:05 AM   #3
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Default Enlarging the Picture

Quote:
Originally Posted by slipe
When you crop you remove pixels. An image with fewer pixels can’t be blown up as much or printed as large without losing quality.
.
Thank you this helps, but raises another (novice) question:

It sounds as though If I am going to do any image enlargment, that I should do this BEFORE I crop.

How does one "Blow Up"/Enlarge an image in Photoshop Elements? (based on a Nikon D100 Raw image)
......Are there a set up steps to do enlargement (image size/canvas size)?
......Or is enlargement just a function of printing on a larger piece of photopaper?


Many thanks
Eric
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Old Oct 16, 2003, 1:10 PM   #4
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Quote:
It sounds as though If I am going to do any image enlargment, that I should do this BEFORE I crop.
I don’t think it makes any difference. I don’t understand why you need to enlarge a 3008 X 2000 PPI image. Computer screens show 1600 X 1200 pixels at most so you have to do a lot of scrolling to see a 3008 x 2000.

Quote:
How does one "Blow Up"/Enlarge an image in Photoshop Elements? (based on a Nikon D100 Raw image)
......Are there a set up steps to do enlargement (image size/canvas size)?
......Or is enlargement just a function of printing on a larger piece of photopaper?
If you simply change the print size with “resample image” unchecked you don’t resize the image. You simply spread the image out over more or less paper and increase or decrease the print PPI.

If you don’t put anything in the resolution box at the top of the screen with the crop tool selected the same holds true. You don’t resize the image except to maybe remove some.

Keep in mind that the PPI refers to printing ONLY. The pixel dimensions do not have a resolution.

With an unaltered 3008 X 2000 image, open the image box and uncheck “resample image” and leave “constrain proportions” checked. Notice you can not now change the pixel dimensions. Set the width to 6. You will see the height goes to about 4 and the resolution is a tad over 500 PPI. Now set 14 in the width and the height will go to about 9.3 and the resolution to about 214. The pixels don’t change and you haven’t resized the image at all.

Now cancel out the image size box and select the crop tool. Set the width at 6 and the height at 4 with the pixels/inch box blank. Drag the crop so it fills the screen and go Image>Crop. Open the image size box and you will see that the image was resized to 4 X 6 with 500PPI and you still have all of your pixels. You did not resize the image – you just changed the print size. Go back to open in the history and put 14 in the width box and 11 in the height with nothing in the PPI box. Make the biggest crop you can. Go back to the image size box and you will see you have an 11 X 14 print with a PPI of about 180. Your height is still 2000 but you have lost pixels from the width because you had to crop some off to get an 11 X 14, but the image was not resampled. 180PPI is probably a good print without resample. If you go back to “Open” in the history so you have the full pixels and crop a 13 X 19 you will see that you are down to about 155 PPI for the printer resolution and it might be better to resample back to 300PPI – you would really be resizing the image itself if you did that. Check “resample image” and type 300 in the resolution box. You will see that you changed the pixel dimensions to about 5700 X 3900 so you actually resized the image itself. This is where the program invents extra pixels and you lose a little quality. You haven’t lost image quality with anything else you have done. You should avoid a resample if you can get at least 180PPI for the print IMO. And that is only if you have the very latest high resolution printer. If your photo printer is a couple of years old or you are using a general purpose printer that PPI might be 130PPI.

One more time – you don’t enlarge the image if you print on a larger piece of paper. You only spread it out over a larger area. You only enlarge the image if you check “resample image” or put a resolution in the pixels/inch box with the crop tool selected. You should avoid that unless you end up with too small a PPI to print.
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