When to crop w/ 300 dpi

Discussion in 'Digital Darkroom' started by dan_connelly, Jun 18, 2007.

  1. Dumb question maybe -- but when is it best to set 300 dpi for printing?

    For example: I pull images from my canon EOS and generally crop to 5x7 @ 300
    dpi before editing. Am I throwing away pixels or is this moot. I'm thinking
    it doesn't matter -- but now I'm 2nd guessing myself.


  2. I would suggest to crop after all your editing is complete. Actually I'd save the edited image unflattened, unsharpened, uncropped (or cropped to the max size you'll ever print at) then whenever i want to print, re-open the image and flatten, resize, sharpen, and then resave it like: (image name) 5x7 300dpi.tiff This way you can have all your print sizes ready to print at any time.
  3. You are throwing away more than half of your pixels.

    A better work flow is to save the original files, before editing of any sort, since they comprise your digital "negatives". Most of the editing should be done on 16-bit copies of the original files, without cropping or resampling.

    After the editing is complete, make copies for printing at the destination size, resampled to 300 ppi at that size. For print files, you can reduce the file to 8 bits/channel, change the working space to sRGB and save the results as a JPEG.
  4. What is all this talk about resampling to 300dpi or so before printing? Decent printers don't need a particular dpi to be happy as long as it is above a certain value which depends on how big an image you want to print and at what distance you are likely to view the print. If your image is at say 500dpi for the desired size, there is no need to downsample to 300dpi or so.

    I'm really interested to understand why this myth took hold to begin with.
  5. Think Frans may be right. If your using Photoshop uncheck resample image.
  6. [original poster]

    Thanks for the replies. Here's a little more information. I always save an orginal file with no changes added. But I thought that DPI was printer dependant only - and didn't actually change the number of pixels available -- only instructs the printer how closely to put those pixels together.

    I know when I crop to 5x7 or 4x6 I lose some pixels due to aspect ratio and used to think that I wasn't losing much in terms of quality. However, I've started to realize that when I run my edits BEFORE cropping they take much longer to run. Obviously, becasue the picture is a larger file. That makes me think my previous logic is wrong and I am losing more than anticipated.

    Sounds like my workflow may need to lengthen (and file size) but my picture quality should improve.


  7. There are two issues here: how you specify the size of the print you want, and how you optimize your image for printing.
    On the first count, there are two ways to specify what size your final prints will be, and you may have one option or the other or both, depending on what software is involved.
    One is to specify what the desired output size is. For instance, the kiosk attached to many a minilab lets you say you want a 4x6" or 5x7" (or whatever) print; some software you could use to print pictures from your own computer to your own printer may also work this way. In this case, the ppi setting in your file is irrelevant; the image will automatically be resized to fit the paper. You could bring in a 1000x1500 file with its ppi set to 1, a 2000x3000 file with its ppi set to 1000, or a 4000x6000 file with its ppi set to 200, and if you specify 4x6", you get a 4x6" print from every one of them because you said you wanted a 4x6" print.
    The other is to specify ppi and have the image printed at whatever size that works out to (e.g. if you have 3000x2000 file and you specify 300 ppi, you'd get 10x6.7", whereas if you take the same file and specify 200 ppi, you'd get 15x10"). In this case, you do want to calculate what the appropriate combination of ppi and pixel dimensions would be, or else you may be surprised at what comes out of the printer. And (assuming your original file is some odd resolution, such as the resolutions produced by most digital camera sensors) you can do this by changing the ppi, resampling the image, or both.
    As for how to optimize the image for printing, there's a school of thought that you're best off to resample the image to the desired size at an optimal resolution for the output device, followed by applying sharpening tailored to the output device. It seems to me that this is logical, but I will freely admit that I have not done any testing to see if it's right (or, if so, how much better this is than letting the output device or its drivers do the resampling for you).
    You should keep a full-resolution copy of your file. After all, you may think you want a 4x6" print of the image, but maybe when you see the 4x6" print, it's so lovely that you want an 8x12" print. If you only saved a 4x6" 300 ppi version of the file, you're screwed; at 8x12", you only have 150 ppi, and the image will suffer. So if you're going to crop the image for printing, you should make this a separate file. From what you wrote, it seems you're already aware of this.
    You mention the time it takes to manipulate your cropped images. Exactly what happens when you crop the image will depend on what software you're using and what you've told it. I use Photoshop Elements 3.0, and I believe its Crop tool works the same way as the Crop tool in most if not all other versions of Photoshop. If you specify the dimensions of the image you want but not the ppi, then it will crop without resampling. Let's say I use a camera that produces 2000x3000-pixel images, and I want a 5x7". I set the Crop tool to 5x7", leave the ppi blank, and crop it so that the resulting image is as large as possible. That will be 2000x2800 pixels. If I like the 5x7" and want (say) a 10x14" print, that file will come out at 200 ppi, which is around the low end of what most people consider photo-quality. That's all the resolution my camera can offer me.
    Now, let's say I set the Crop tool to 5x7" at 300 ppi. Since I've specified both of these, the tool will give me an image that is exactly 5x7" at exactly 300 ppi. That's 1500x2100 pixels, so the Crop tool will downsample the image. It's just following the instructions I gave it. Now, if I like the print and want a larger copy, I'm screwed; I don't have the resolution to do a good job. My camera had enough resolution to do a good job, but I threw a lot of the pixels away when I cropped and resampled.
    This is one of the reasons why you do your editing on the full-resolution file and save the full-resolution fully-edited file, and then produce separate copies that have been cropped and/or resampled for printing.
  8. Frans,

    You are correct, of course. Most print drivers will downsample an image with higher than needed resolution. That said, a 6MP camera will only give a 7x10 inch print at 300 ppi. While there's no need to downsample for a 5x7 print, anything larger than 8x10 could benefit from up-rezing. If I was oversimplifying, it was to avoid the collection of "if-then-else" statement.

    While many common print sizes have the same aspect ratio (4x5, 8x10, 16x20, etc), others don't (notably 5x7 and 11x14). I always crop to size (or aspect ratio) so that the cuts are made where I wish. I generally resample in order to save a step later.
  9. It depends on who is doing the printing. If you are sending it out, the lab will not be happy to receive a 500ppi file for a 254ppi printer. They will send you a note explaining that you are tying up their FTP server and RIP and ask you not to do it again.
  10. Keep in mind that all Epson desktop printers will upsample (or downsample) your file to it's native 720 dpi resolution at the print size before it starts printing it. Fortunately they do this very well. But there is no need to downsample a file to something like 300 dpi when your Epson is gong to upsample it to 720.

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