Question about image resolution...

[benny1414]

Hello! I come primarily from the video world, but have recently starting taking stills as well and I have some confusion about resolution.

I took a photo for a client who wants to print a poster and I want to know how to save it. The photo was taken with a Canon 7d. After cropping in Photoshop, the image size is 2683x2402 with a resolution of 72 pix/in. I want to save it with the best possible print resolution for blowing up to a large size. My questions are as follows:

I think that changing the resolution to 300 and re-sampling the image will increase the dimensions, but how well does Photoshop interpolate the new pixels? Will it look better or worse?

If I simply change the resolution to 300 and uncheck 're-sample image', does this actually do anything? If so, will it be better?

Is a .tiff the best quality format to use for high resolution printing?

I understand that trial and error is the best way to get the answers, but as it is a client shelling out the money for the large print, I want to get it as good as possible the first time.

Also, according to the following site, the biggest acceptable print dimensions for good quality at my resolution is 26" x 24". I can't imagine that this is correct. I realize I did some cropping, but the 7d has a fairly high pixel count and I know that people print larger posters than that, so what cameras are they using? Or film the only way to print large?

How big can I print my digital picture?

I would appreciate any insight on this! Thanks!

 

[Benco]

How big do you need to print it? 26" by 24" with a 2683 by 2402 image is only just over 100ppi, it's low but OK ish. To print larger you'll have to interpolate. There's some useful info here:

Abstract Photography -- Part 2

 

[Big Mike]

]

When it comes to printing, we also tend to talk about pixels per inch, but this time we mean per inch of paper, not on the screen. The common standard for 'photo quality printing' is usually 300 (linear) pixels per inch of paper. So for a 4x6 print, you typically want an image that is 1200x1800 pixels. Some people/printers are going as high as 600 pixels per inch. Another common standard is 240....and the low end limit for 'photos' might be something like 100.

So basically, figure out how large you are going to print it, and resize (re-sample if necessary) the image to increase the number of pixels. That is the number that matters above all....the actual size of the image in pixels.
Of course, to do the calculation, you will need to decide how many pixels per inch you are going to want/need. Not always an easy question. If you're starting will a smaller image, you can certainly re-sample it to be larger, but the more you enlarge it, the worse the quality will be. Also, the bigger the print, the lower the resolution you can get away with. The main reason for that, is because of something called 'proper viewing distance'. We are not meant to view posters with our noses pressed up against them. If you are viewing a poster, it should be from at least 4 feet away (give or take)...at which distance, you probably can't see the loss in quality from a lack of resolution.

Often, it just takes experience to know what you can get away with, and still meet your own expectations of quality. Some people might resize the heck out of a digital file to print at 300 PPI. Others might print at 150 PPI, without resizing the image (or doing so, to a lesser degree). It's hard to really say what will work best. Doing test prints is a good way to help figure it out...but you might also call up the lab and ask what they prefer you do. Maybe they will just want you to send the file as it, and they will use their own software to resize it. Photoshop does a pretty good job of it, but there are others that might be better...Genuine Fractals has a good rep.

If and when you do resize (resample) in Photoshop, you will want to use the 'bi-cubic smoother' option. You may then want to apply some sharpening to the enlarged file.

 

[KmH]

Actually the PPI doesn't tell the computer anything about the image size.

An 800x600 pixel image will display on your computer display exactly the same size - at 1 PPI or at 10,000 PPI. (give it a try sometime)

For the computer, or any other electronic display, the PPI is totally meaningless and only the pixel dimensions matter.

The PPI only has meaning for prints of images.

You will also need to consider the aspect ratio of your crop, because it is an oddball aspect ratio (see chart below). (2683 / 2402 = 1.12)

Pixels / PPI = inches

2683 px / 100 PPI = 26.83 inches. 2402 px / 100 PPI = 24.02 inches.
2683 px / 150 PPI = 17.89 inches. 2402 px / 150 PPI = 16.01 inches.

Inches x PPI = Pixels

If you want a 40 x 30 poster @ 300 PPI = 40" x 300 PPI = 12,000 pixels. 30" x 300 PPI = 9,000 pixels.

Your crop is nearly square, having a long side that is only 1.12 times longer than the short side (2683 / 2402 = 1.12).