A friend called today and wants to have an image printed at 18 x 24". My first question was "What is the resolution of the image"? It was shot at 2278 x 2828 giving 6,487,744 pixels. My gut feeling is that there are not enough pixels to properly print this image at the given size. I'm also guessing that a print this size would probably be printed at 150-200 dpi.
Can someone a) Correct my assumptions and b) Provide a formula or reference to determine the maximum size for this image to look acceptable.
Thanks for your help.
Ken_K wrote:
A friend called today and wants to have an image printed at 18 x 24". My first question was "What is the resolution of the image"? It was shot at 2278 x 2828 giving 6,487,744 pixels. My gut feeling is that there are not enough pixels to properly print this image at the given size. I'm also guessing that a print this size would probably be printed at 150-200 dpi.
Can someone a) Correct my assumptions and b) Provide a formula or reference to determine the maximum size for this image to look acceptable.
Thanks for your help.
There is no set formula. It isn't just the input resolution that matters, but also the quality of the pixels and the subject matter. So 120 ppi may work, and depending on expectations, it may not.
It is also probably worth noting that 2278 x 2828 is not the same aspect ratio as 18 x 24 inches, so it will have to be cropped.
Depends quite a bit on expected viewing distance and the detail in the image. A portrait can probably get by with a little less than a detailed landscape, for example.
Just to give you an idea, here are the PPIs recommended by the late Bruce Fraser based on viewing distance and a viewer with 20/20 vision:
I calculated 118 PPI for the most restrictive dimension in your case. I suspect it will look OK but not fantastic as long as viewers aren't going to be close to the print. Doing some careful up-rezzing, perhaps with a package like Perfect Resize from OnOne Software may help a little bit, too, but it won't perform miracles.
I recommend doing a test print of a cropped portion of the image at the final resolution. A 4x6 will only cost a few cents and should give you a decent idea.
To obtain an 18" by 24" print at x dots per inch, you would need (18x)*(24x) dots. If you have an image with y pixels, the maximum resolution, in dots per inch, you could obtain (without scaling the image) in an 18*24" print is [y/(18*24)]^0.5, assuming the image has already been cropped to the 3:4 aspect ratio.
If you print the long side (2828) so that it is 24 inches the other dimension will have to be cropped down to 18 inches from about 19.3. That will print at about 118 ppi which seems pretty borderline for a print that size. If you print at 150 ppi then the long dimension will be almost 19 inches (print would be about 15x19). At that size 150 ppi would probably be acceptable if you don't try to view it from too close a distance. At 200 ppi it will be approximately 11x14. I hope this is helpful.
Of course there is also the issue of how will the image will look uprezzed to something closer to, say 200 ppi at 12x18. Again, much depends on subject, quality of image, viewing distance and subjective acceptability. Portraits, architectural images might uprez very well, for example. Landscapes are a different story unless we are talking long exposure water scenes for example.
What is the subject?
Scott
For reference, you can figure out the resolution of an image in Photoshop by:
Image - Image Size
Uncheck the box that says "Resample Image"
Put in your width and height in inches, and the resolution will be calculated for you.
Contrary to the tone of many of these response, I would not hesitate to try a 24" print. I would uprez in photoshop or print out of LR. Both are better at this than we often give them credit. It is important to sharpen after the uprez in photoshop, so that assumes you have access to a file that hasn't been ( over ) sharpened already. I would uprez to a resolution that made my printer happy, 360dpi for epson, 300 for canon.
Most people will be very happy w a large print of an image they like and will not care if it doesn't look perfect at 12", though im not saying this wouldnt produce a perfectly nice print. We get hung up w rules that are overly restrictive. A lot of people believe canvas works well at lower resolutions than paper prints, so that is another possibility.
FWIW, I print large most of the time w 20x30 my normal print. Yes, I prefer large files, but I have lots of older images produced w 6-12 Meg camers, and 16 x24 images are quite good when prepared properly.
I would encourage you to try it before you assume it won't work well. Good luck.
Stuart
