View Full Version : Large size prints and interpolation?

21st of July 2006 (Fri), 16:41
I知 looking to have a large print (at least 20x30, probably 24x36) made from a RAW image of a landscape/building, taken with a Canon 20d. I知 trying to learn digital imaging beyond the basic stuff, and my RAW and Photoshop skills are pretty basic. Other than printing 8x10 and smaller images on my Canon 8500, I haven稚 dealt with the issues of large-size prints, such as interpolating, ppi, etc. I知 trying to learn, but I知 a little confused on some points. I haven稚 decided on an online print lab yet (I値l probably go with mpix for a 20x30, but not many seem to do 24x36 I may go with Printroom for that size, but I壇 love to hear about anybody痴 personal experiences with their prints), but suffice it to say, I want to get the best possible print quality I can.

I知 trying to figure out what I need to do on my end (interpolation, etc.) before uploading the file, but I知 confused after reading other posts on these forums and on other sites. I致e processed the RAW image to my liking using Capture One, and now I have an 8-bit tiff at 300 ppi (7.787x11.68 inches). For these large sizes, is it better to send the file to the lab at 300 ppi, or should I upsample the PPI to something larger or smaller? I read one site that recommended a ppi of around 97 for a 24x36 print, but I知 not sure if this is way to low (and the quality would suffer because of it). Regardless of whichever ppi I choose, should I scale the image using Genuine Fractals to 24x36? Or, is any of this actually necessary? Do I really need to change the ppi from 300, or scale the image to 24x36 before I send it to the lab; or is it better to let the lab deal with whatever interpolation is needed automatically? I致e read that having a larger ppi is preferred, but I致e also read that a smaller one is better or that I should just let the lab handle it, so you can probably see why I知 a little confused. What should I do next? Any suggestions you can offer would be greatly appreciated!


22nd of July 2006 (Sat), 14:23
What you are looking at in PS is ppi. Your lab will deal with dpi (dots per inch).

Your image at 7.787x11.68" @ 300 ppi will print at 20x30" @ 45 ppi which is too low. (If this is confusing, just think of final resolution as 7.9x11.7x300 / 20x30).

To increase size (usually not recommended but you may try), go to Image>Image Size, deselect Resample Image button, type 250 in Resolution (or 300 if you want), reselect Resample Image and type your new dimensions in Document Size. Be sure to select Bicubic Smoother in the Resample Image dropdown. It does a better interpolation though I read somewhere that you may use Bicubic Sharper. BTW, these algorithms were developed especifically by Adobe to deal with upsizing and downsizing respectively.

The good thing about doing it yourself (as opposed to letting the lab do it) is that you will see the result of your resampling. So you may choose to sharpen afterwards - probably recommended as interpolation, by its very nature tends to soften the image.

Also, you should choose a lab and stick to it. Reinterpreting the image for printing is random enough and you will eventually find yourself customizing your images (colour, sharpeness, etc) to your lab's result.

Good luck and keep experimenting.


22nd of July 2006 (Sat), 21:16
First, choose your lab. Second, contact the lab and ask them what they recommend. Some labs have thier own RIP programs to do the interpolation and prefer to do it themselves for best results. Other labs will recommend the pixels per inch that work best for their printers. For example, if you interpolate to 300 pixels per inch and their printer uses 720 dpi for printing, the printing program is going to do a second interpolation during the printing process. That might not improve your image. You may get better results if you had interpolated to 360 pixels per inch, which is more accurately interpolated to 720 dpi by the printer program.

There are better interpolation programs than the bicubic procedure used by Photoshop. As you mentioned, Genuine Fractals is one of the better programs. Also consider Qimage which offers several different interpolation methods. I have found their Pyramid method to be one of the best, but it is much slower than bicubic because of the greater number of calculations done.

22nd of July 2006 (Sat), 22:04
In photoshop go to view-> print size. Then if that looks good DO NOT interpolate!!!!

I worked at a lab for 3 years and one of the most common mistake was to upscale to xxxx ppi rather than just preview at a certain size using photoshops built in features. Interpolation blurs an image, and unless it's 100% needed I would never do it.

23rd of July 2006 (Sun), 03:34
Have a look at these linkss:


I use this software for very large print sizes. The results are truly excellent using the S-Spline algorithm:


---- Gavin

23rd of July 2006 (Sun), 23:04
What you are looking at in PS is ppi. Your lab will deal with dpi (dots per inch).

Your image at 7.787x11.68" @ 300 ppi will print at 20x30" @ 45 ppi which is too low. (If this is confusing, just think of final resolution as 7.9x11.7x300 / 20x30).

The lab's normally quite happily deal with PPI :-)

Also Just to correct you an image of 7.787x11.68" @ 300 ppi will print at 20x30" @ 116 ppi.

Still a bit low but ScottE's suggestion of contacting the Lab is a good one.