print quality

[MVD]

Hi, I am new to this forum and hopefully you guys may be able to give me some tips. I want to compose and have a photo album printed from my recent holiday, I am using an I-mac and the i-photo program. I am using my own digital photos and some of my friends' photos.
My photos are good enough but if I use any of the others, I get a warning that the picture may not be high enough quality to print. My question is: what is the minimum requirement to get a decent print?? In other words, what size( minimum) KB or MB am I looking for to get a decent picture?
I am a complete beginner at this digital stuff but it would be such a nice memory to a brilliant holiday..if I could get a nice photo album.
Thanks for helping me out....
MVD

[dcclark]

The size in KB or MB is not related to image quality. How are you printing: on your own printer? Is iPhoto warning you about quality? Or are you trying to send your images to an online photo lab for printing? Also, what camera are you using, and what size are you trying to print?

Common problems may be that you are using a "thumbnail" version of the image (the very small preview which some programs make) and trying to print that. You do need to use the original version of the photo to print, if you can.

But really, the short answer is: if you print a photo and you like how it looks, then you have sufficiently good quality. However...

Warning: math content follows!

To figure out a more precise image quality, there are two things you need to know: the size you want to print (for example, 4x6, 5x7, 8x10 inches) and the number of pixels in your image (for example, a 7 megapixel camera has 7,000,000 pixels in an image).

These two will let you calculate the number of "dots per inch" (dpi) or "pixels per inch" (ppi) that your photos will print at. Most places agree that 100 dpi is the minimum acceptable image quality. 300 dpi is considered "very good", but honestly 100 dpi or above all looks great.

Here's how to do the calculation: find the number of square inches in your desired photo size. For example, and 8x10 photo has 8*10 = 80 square inches. Next, we need to figure out how those megapixels get spread around in at 8x10 image. For example, consider a 7 MP camera (= 7,000,000 pixels). Then there will be 7000000/80 = 87500 pixels per square inch in a print. That's a *square* inch, so in a *linear* inch, there are the square root of 87500 pixels, which is about 296 pixels per inch. Thus an 8x10 photo made from a 7 MP camera would have extremely good image quality.

Summary: for a camera with M megapixels, printed at photo size w x d, you will have a print quality of SquareRoot((M*1000000)/(w*d)) dpi.

Even easier summary: 7 - 10 MP will print EXCELLENT quality all the way up to poster-sized.

[BobC]

The original image is not likley the problem. I get this message from online sites when i've done a heavy crop. Then you will need to keep the final image size smaller say 4X6. I've never tried to print an 8X10 of a croped photo, not sure how it would come out. But then my camera is only 6.5 mega pixels. If you shooting a 12 meg camera i'm sure you could get away with more cropping.

DC - thanks for the math behind all this. Is there a way to calculte when you crop or is it mostly an estimate?

[dcclark]

BobC -- the math is easy to do for a cropped photo too. Just use your favorite image editor or viewer to find the dimensions (in pixels) of the image and use that in place of the M*1000000 term above. For example, a 500 x 700 image would have 350000 pixels.

6.5 megapixels is no problem at all. You can easily print 16x20 and still have good quality prints.

[MVD]

Originally Posted by dcclark

BobC -- the math is easy to do for a cropped photo too. Just use your favorite image editor or viewer to find the dimensions (in pixels) of the image and use that in place of the M*1000000 term above. For example, a 500 x 700 image would have 350000 pixels.

6.5 megapixels is no problem at all. You can easily print 16x20 and still have good quality prints.

Thank you Dcclark and Bobc, it all makes a bit more sense now and thank you for the tips. I'll have another go and use your formula. Cheers MVD.

[Sigsauer]

Great info, but here's a question.....
I shoot a 12mp Canon dslr. When I'm just shooting around the house I shoot .jpg, cause it's easier to email to the family across the country. BUT, when I shoot .jpg I get a resolution of around 3 mp. Is this factored in when you're talking about printing poster size from a 7 mp camera? Or are you shooting RAW, processing on the computer, and then getting around 10 mp output? (10 mp is what I get post processing) Just curious because I have some excellent pics of my 10 week old son that I'd love to get large prints of....

[dcclark]

No, shooting jpegs vs raw has nothing to do with image dimensions or resolution. Each will have exactly the same output size. You must be changing some other setting, such as image "size" (i.e. small, medium, or large).