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When you shoot an image, the camera records pixels of information. The number of pixels it records is determined by your settings - the maximum number of pixels will be recorded when you are in L mode, or RAW. In L mode, the picture is recorded as a jpeg file, which involves some compression in order to save memory space. In RAW mode, it is recorded 'as is' - for the best quality prints, the first thing to do is put your camera in RAW mode. However, both L and RAW modes will give you the same number of pixels in your recorded image. On the Rebel XTi, that number is 3888 x 2592. Multiplying those numbers together gives you the total number of pixels contained in your image - in this case, slightly over 10 million, which is why the XTi is advertised as a 10.1MP camera - it has the capability to record 10.1 million pixels per image it shoots.
The resolution of the image means how many pixels appear in one inch of it (pixels per inch, or PPI).
Document size means how large your printed picture will be, at the set pixel dimensions and resolution.
The math is fairly simple. 3888 pixels at a resolution of 72 ppi = 3888/72 = 54 inches.
So: if you print at 72ppi, your printed image will be 54 inches on its longest side.
The pixel dimensions of your image CAN be altered, but here's the rub: in order to do so, Photoshop has to do a certain amount of interpolation. It has to do what it calls resampling the image. If you ask it to increase the pixel dimensions, it looks at the existing pixels and decides what colour the pixels next to them should be. It does a pretty good job but, at the end of the day, it's essentially making stuff up, since the camera didn't actually record those additional pixels. Same thing happens if you make the pixel dimensions smaller - it has to decide which pixels to remove.
As far as the resolution goes, 72ppi is fine for viewing on your computer screen, but for printing it sucks ***. The fewer pixels per inch on your printed photo, the larger those pixels will be (stands to reason, right?) - too large, and you can actually see the individual pixels: the photograph looks "pixelated".
In order to see how these three things interact, go into Photoshop, open up a photo and bring up the image size menu.
The last box in the menu is 'Resample Image'. The last thing you want to do, if you can possibly avoid it, is resample the image. If you do so, as I said before, Photoshop will make stuff up.
Uncheck Resample Image and you'll notice that the ability to change the pixel dimensions has gone. Without resampling, you can't add or remove pixels from the image.
Now change the Document size and see how the resolution changes. If you want your printed photo to be 5 inches by 7 inches, the resolution shoots up to 555 pixels per inch, because it's cramming the same number of pixels into a smaller space.
The minimum resolution you really want to print at is 240ppi. If you put that number into the resolution box, you'll see the document size change to show you the largest size photo you can print at that resolution. For the XTi, it's 16.2" x 10.8". You *can* get away with a lower resolution for larger prints if you have to, because they're viewed from further away - if you stand further away, your eyes can't resolve the individual pixels anyway. However, you should always aim for a resolution of 240 to 300 for print purposes.
So, before you print any photo, uncheck the resample image box and change the document size to the size you want to print. If the resolution is too high, check the resample image box and reduce the resolution - Aim to get it to around 300ppi.
Do your sharpening after you have set the print size and resolution in order to get the best results.
I have been struggling with this for quite some time. I just can't grasp it from the articles, I need hands-on learning (this stuff is not that easy to me) so I wanted to meet with a photog. friend in person, but when will I have the time to do that..
Even if I don't crop a photo or edit it at all, I am still never allowed to print a quality 8 X10, as the print company (Walgreens plus others) will give me a warning that it will be a bad print.
I really don't get it. I would like to print 8 by 10s for my baby's room, and then graduate to larger sizes for my house and her room.
I tried raising the ppi one time in GIMP, it still didn't work for me.
Thanks for the article, and I will be elated when I can finally grasp this.