Today’s Question: I scanned a 35 mm slide at maximum resolution (11×17 at 300 pixels per inch). I want the print to be 19 inches (on the long side) and to be as sharp as possible. Will I get better quality if I have Photoshop interpolate the (extra) pixels to adjust the image size, or is it better to reduce ppi (to around 270) to keep the same size file?
Tim’s Quick Answer: My recommendation is to first scan your original transparency at the maximum optical resolution of your scanner. After optimizing and saving your master image, create a copy and resize that copy in Photoshop to the intended print dimensions, at a resolution optimal for your print process.
More Detail: The numbers provided in the question suggest a scanner resolution of about 3,300 pixels per inch. This is based on a 35mm slide scanned to produce an image that is 3,300 pixels on the short side (11 inches at 300 pixels per inch). Since the short side of a 35mm slide is about one in in size that translates to about 3,300 pixels per inch. If the scanner actually offers a higher resolution (such as 4,000 pixels per inch) I recommend scanning at that higher resolution.
After applying all of the adjustments you feel are necessary to optimize the image, I would then save that final result (with all layers intact) as a Photoshop PSD image or a TIFF image. This file can now server as your “master” image for all future output.
When you want to make a print, you can create a copy of your master image, flatten that copy, and resize to the intended print size. I recommend performing that resizing step in Photoshop, although in actual fact today’s printer software also does an excellent job of resizing the final image at high quality.
I recommend resizing the image to be printed based on the “native” resolution of your printer in terms of how image data is processed. This varies among different printers, but in general is around 300 or more pixels per inch. For many photo inkjet printers a resolution of 360 pixels per inch is optimal. But again, the specific number varies based on your printer, and can often range from around 300 pixels per inch up to around 720 pixels per inch.
Let’s assume a printer that renders the final output data at 360 pixels per inch. If you leave the image resolution at 270 pixels per inch in order to not apply any interpolation (or to keep the file size smaller), that image data is still going to be interpolated upward. If you give the printer less data than it needs (360 pixels per inch in this case), the printer software will simply interpolate the image data up (from 270 pixels per inch in this example) to the necessary output resolution. In other words, you may as well resize the image to the final output dimensions in Photoshop rather than having the printer software perform that additional work.
This issue used to be more significant than it is today, because printer software didn’t do as good a job of interpolation, especially compared to Photoshop. However, today’s printer software actually does an excellent job with this interpolation. So resizing to the final output dimensions in Photoshop before printing is not quite as important as it used to be, but I do still recommend this approach as a best practice when printing.
Also note that once the image has been resized to the final output dimensions, you should apply sharpening to optimize that final print. And, of course, sharpening is best applied at the final output dimensions, which is one additional reason to resize in full in Photoshop. That way the sharpening truly is the final step, rather than resizing in Photoshop, then sharpening, only to have the printer software apply a further change in the pixel dimensions during the print process.