whcraft wrote:

 

I am using Photoshop CS.  I have been tasked with creating pictures for our office hallways.  Most of the files I am working

with were taken with an old digital camera.  They are approximately 31" x 24"  at 72 dpi and are in jpeg format.  I need these files

to be print quality at 15' x 12'.  How much image quality will I lose if I resize them to 15" x 12" and up the dpi to say 225 or 300?

Any help / suggestions would be greatly appreciated.

First forget about DPI to begin with.  Start with how many pixels are in the images you are starting with.  Width Pixels and height pixels also note the images Aspect ratios.  Next how big do you need to print these pictures for you office hallways. Do or can these prints have the same aspect ratio of the original images? You do not want to have to crop and throw away pixels to match a particular aspect ratio when printing large. 

If the images need to be 15 feet by 12 feet you do not need to print them at a high resolution like 300 DPI for they will not be viewed up close.   However  Digital Camera do not capture the amount of pixels you will need to print a 15' image well you will need to resample you images up in pixel size.   15' x 12' at 100 DPI would require 18,000 x 14,400 that is 259MP.   Your starting with  31" x 24"  at 72dpi   which is about the same aspect ratio (15'6" x 12')  with 2,232 x 1,728 pixels 3,856,896  about 4MP.  I do not know how well a 4MP image will resample up in size to print 15' images. 

The largest print I ever had made was a 6'x8' and printed at 100dpi and printed on canvas. I up sampled from a 8MP image. I was very happy with the image but the images was of a painting which was a soft subject so the printed image did not need to be that sharp or contain all the much detail. My 8MP was up sampled to 69MP or 8.6 time the number of pixels.

Going from 4MP to 259MP or 64 times the number of pixels seems like a very large jump to me.  If I were you I would resample one of your images to that 15'x12' 100DPI  size then set the marquee tool to a fixed size 8" x10" selection and make a selection then use menu Image>Crop.  The make a test print of that 8"x10" crop to see how it prints look at the print from a distance of 5' to 10' to simulate viewing distance.

Billboards can be printed at around 20 ppi and they will look very good from the distance. Large format prints don't require higher resolution.

The thing to remember is: "You can't make a silk purse out of a sow's ear".

(Unless you are a magician. Unfortunately Photoshop doesn't have magic powers – it just knows lots of mathematics.  )

In that case see Silkrooster's post above.

You're looking at a roughly 2x linear pixel dimensions increase if you stick with 300 ppi. That should be doable, but they will probably look a bit soft.

Try careful sharpening after resizing. You'll have to be careful, because jpegs are often already quite crudely sharpened in-camera. Excessive grain and artifacts/halos should be avoided at all costs.

Since these images are jpegs, make sure to save them as psds or tiffs before you start working on them. Jpeg is a lossy format and the quality deteriorates every time you resave it.

Mm-hmm. An office laser will often do just fine with images in the neighborhood of 150 ppi. It can sometimes depends on the image content. If it is a big group picture with many faces that will be tiny at print size, sometimes you can't win either way. Too few pixels or the interpolated results of resampling--either one can leave fine detail perceptually obliterated. You're always better off not resampling, if you can get away with it, so it's best to try that first.

whcraft wrote:

 

It was conducted by one of the original creators of Photoshop

Ah...

The action being discussed is probably "stairstep upsampling", which introduces artifacts that resemble texture, and thus are touted by some to produce a "better looking" upsampled result.  I experimented heavily with that myself, and I never perceived the results as better than a simple Bicubic upsample with sharpening afterward.

Whcraft, if you're really serious about improving the look and feel of your image when upsampled, please crop out a small piece of it and post it here (or eMail it to me).  I'll be happy to show you what can be accomplished.

-Noel

The number of pixels have more than doubled in both width and height. Whether you see it or not pixelation occurs. Granted it is possible to blur the pixels which gives it a soft look, but none the less pixelation did occur and is not recommended. Generally the end result that you are looking for is a nice clean sharp image. However it does matter on how far away the image will be from the eye.

As emil emil plainly put it, you can't add detail that is not there.

http://en.wikipedia.org/wiki/Pixelation

I been thinking about it, and I was wrong in away. For some reason I had it in my mind the size was going up and ppi did not which would cause pixelization.

I think I was confused as photoshop is adding additional pixels that are not original pixels.

So you are correct in order to see pixelization you have to have a low rez image.

See I had it in my mind that the smaller the image is the sharper and the larger it is the more degraded or pixelized it becomes. But in that case the ppi is a constant.

Sorry for the confusion...