Correct resolution (dpi/lpi) for film output

[klynam]

Greetings Rebels!

I searched the forums for this answer but I'm not sure I have my search terms right. I'm curious what the correct resolution (dpi/lpi) is for output to photography film.

I've just taken some portraits of a friend's daughter with my Digital Rebel. The native JPG images came into Photoshop at 21.333x32 @ 96dpi. (I think my RAW images come in at 180dpi with proportionally smaller hxw dimensions.)

I need to generate 16x20 photo prints, but I don't know what resolution (96dpi, 180dpi, etc.) standard film output devices require at the finished size for the best quality prints.

Can anyone help?

Thanks,

klynam

[HJMinard]

Those considerations only apply if you're going to print the images yourself. For a printing service, just send (or bring) them your full resolution file (after corrections, sharpening, etc.) and they will decide the optimum ppi/dpi for the print size selected.

[slin100]

The context of your question is not clear. You mention photography film. Are you trying to print your image on a 35mm negative film recorder? But, then, you say you want 16x20 prints. Can you clarify?

I'll assume you want 16x20 prints. When it comes to printing, you have to decide whether or not you want to resample/interpolate the image up to the printer's native resolution. If you choose not to, then all you have to do is set the DPI in the image file to the correct value that will result in the desired print size. The printer will automatically resample the image up to its native resolution. A 3072x2048 image from a Rebel won't fit perfectly into a 16x20 print. If you want to maximize the converage, you'll need to set the DPI to approximately 154 dpi. This will give you approximately a 13.3"x20" image. 154 dpi should give you decent quality. It will depend on how good the printer's interpolation algorithms are.

If you want to resample the image yourself, then optimally you should find out the printer's native resolution. Frontier printers have a native resolution of 300 dpi. Noritsu printers are either 320 dpi or 400 dpi, depending on the model. You can use Photoshop to resample the image up to this resolution. Bring up the Image Size dialog, click Resample image and type in 16"x20" at 300/320/400 pixels/inch for the Print Size. When this file is sent to the printer, there will be no further interpolation. Some feel that this gives the user greater control over the output of the image because it bypasses the printer's unknown and usually simplistic resampling algorithm. The Qimage viewing/printing program employs this technique, and you can use that instead of Photoshop.

There are, also, many other ways to interpolate images, but that is a topic for another discussion.

[evilenglishman]

this page might be useful:
http://www.peak-imaging.com/htmls/reprint.htm#peakonline

[ScottE]

How are you having the prints made? I assume you are taking them to a photo lab since few home printers can handle 16x20.

Good photo labs will have RIP programs that will resample your photo to the pixels per inch needed for their printer. Don't do any resampling to change the pixel size of you photos yourself. Their RIP programs will do a better job.

Other photo labs will just print the file you bring them without any further interpolation. For best results you should resample the images yourself to get a resolution of 240, 300 or 360 pixels per inch. Ask the lab which will work better for their printer. For interpolating more pixels per inch you can use Bi-cubic resampling in Photoshop (CS or Elements). Some people recommend a series of small increases instead of going all the way to 16x20 in one step. Other programs such as Qimage use other resampling algorithims such as Vector or Pyramid instead of Bi-cubic. I have found small improvement in resolutions using those algorithims for prints as large as 16x20, but they are more calculation intensive and can take a long time for your computer to complete.

Dots per inch refers to printer performance and is not directly related to the pixels per inch of your file. If the printer is set for 720 dpi it will use the same number of ink drops per inch, regardless of file size. A too small resolution file will give either a blurry or a pixelated looking picture.