Help with Exporting and Cropping from LR

Discussion in 'Digital Darkroom' started by klt, Apr 25, 2017.

  1. klt


    I am trying to export from LR and I have read so much regarding file sizes, ppi, etc . I am thoroughly confused now.
    I understand (or at least I think I do) that a high resolution digital image file should be at least 300ppi and anything above that I heard was overkill. Now, for the confusion to begin....

    What happens if I crop during developing mode of LR? Does this change the whole resolution, file size, etc.?

    I was using the 8x10 crop tool thinking that I would only need to then export it with a 300ppi and no other dimensions and I was good to go. However, I was told not to crop in developing. Why? How do I determine what file size I need along with the dimensions in order to send it to someone for multiple prints at various sizes of 8x10 or less?

    I will also need settings for websites, Facebook, Linked-In, etc. I have the FB one at 2048 pixels on the long edge as FB has specified and I use 300ppi (again, is it true I shouldn't crop the photo in developing?). However, I'm confused with all of the other file size recommendations/requirements.

    Anyone willing to clarify and help me?

    Thank you,
  2. digitaldog

    digitaldog Andrew Rodney

    The old “always use 300 PPI (DPI) for output is an urban legend. Depends on the output device (and size).
    And you’re confusing this resolution tag as an actual ‘size’. You are referring to what is a resolution tag and it's rather meaningless. It could be 72PPI or 180PPI but it doesn't have an inherent meaning, only what you could produce with the number of pixels you have at your disposal. Work with pixels! For example, let us say you have 1000x1000 pixels to keep the math simple. And to simplify this further, let's only consider the horizontal axis. If you have 1000 pixels and divide that by 72, that is, you provide 72 pixels per inch, you could end up with 13.8 inches using that division (1000/72=13.8). Let's now say you divide up your 1000 pixels using 180 instead. 1000/180=5.5. In both cases, you had 1000 total pixels. The document itself doesn't have a size, other than what space it takes up on your hard drive. The sizes above are examples of what could be produced if you divided up the total number of pixels you have, with some number of which is just a tag within the document. In Photoshop, if you use the Image Size dialog, turn resample OFF (do not allow it to create more or remove pixels), you can enter any value, 72, 180, 1000 into the resolution field and the resulting size is calculated for you. But you haven’t changed the document or the data at all. You just changed a theoretical 'size' if you output your 1000 pixels using that resolution. So again, it's meaningless until you output the data. At that point, lets say you print the image, you can decide how big you wish it to appear and/or how many pixels you want to devote to the output. You have 1000 pixels and someone tells you that you must use 300DPI (which isn't true but that's a different story). 1000/300 would produce a 3.3 inch print. You want a bigger print? Lower the DPI (within reason). You set the DPI for output to use 180 of your pixels to produce 180DPI? You get a 5.5 inch print (1000/180=5.5).

    Work with pixels. That's a fixed attribute of the data unless of course you resample that data (add or remove pixels).
  3. digitaldog

    digitaldog Andrew Rodney

    It‘s a good idea to find out what the true, native resolution of your display is (they are NOT all 72PPI!).

    Measure the width of your display and divide that by the number of pixels its displaying.

    For example, on my NEC 3090, the width is 25.25 inches. Its resolution is 2560x1990. 2560/25.25=101.4 PPI. On my NEC PA272W, the width is 23.5 inches. Its resolution is 2560x1440. 2560/23.5=109PPI.

    This should give you some idea of ‘size’ output on a display based on actual pixels.
  4. klt


    Digital Dog, Is it true to say that most Macs use 72 ppi and 96 ppi for PCs? I guess what I'm super confused about is the actual cropping inside the developing module of LR. Does this not set your dimensions for your file automatically as far as pixels per long edge? Or, does it take this new sized image and then you must resize in the export according to pixels?

    I'm using a Nikon D7200. So,according to my metadata my RAW dimensions are 4000x6000. I went in and cropped it in the develop module at the 4x5/8x10 apect. If I export as is and just keep the 300ppi, I should be able to get an 8x10 printable image of good quality? The cropped dimensions are 2343x2929. That would make it an image size of 2343/8=293dpi, correct? This should be a good quality print?? Do I need to not crop in the develop module and only use the long edge and height?

    So, how do the pros deliver digital files to their clients without having to do this for each and every individual image????

    Sorry for all of the questions, I'm really trying to understand.....
  5. digitaldog

    digitaldog Andrew Rodney

  6. "What happens if I crop during developing mode of LR? Does this change the whole resolution, file size, etc.?"
    - In short, yes!
    Cropping always throws pixels away and effectively reduces the "resolution" of an image.

    However, there are two (at least) confusing and different definitions of the word "resolution". One usage refers to the overall size of the image in pixels, and the other refers to the number of pixels per inch.

    The overall size - pixels wide by pixels high - pretty much determines the maximum print size you can get from an image. It's this that's reduced by cropping.

    The other definition - ppi - stays the same regardless of overall pixel dimensions unless you tell the image editing software to change it. As digital dog says; this is just a virtual tag applied to the file and is pretty meaningless. Pixels have no real size.

    Screen display size has nothing to do with print quality. Almost no modern display has a ppi as low as 72. That was only true back in the day of 14" VGA CRT displays. As DD says you can work out the number of ppi your monitor shows by dividing its physical size by the number of pixels it displays. However you should be aware that a monitor can be set to display more, or less, pixels than its native resolution (that word again), and that monitor size is misleadingly quoted as the diagonal of the screen.

    In view of the above I'd completely ignore screen ppi as any meaningful figure. The only thing that matters as far as printing is concerned is whether you have enough pixels left after cropping to give decent quality. Personally I'd set the lower limit at 150 pixels per inch, but I've seen mural sized prints made from 6 megapixel cameras that look great from their normal viewing distance of a few feet.

    Don't worry about the ppi tag on an image. This can easily be changed during the print stage. The RIP (Raster Interpretation Processor) engine of the printer and its driver will do all the maths to work out how to print any image size to any possible print dimensions.
    Last edited: Apr 26, 2017
  7. You can enter the screen resolution for Photoshop under "Preferences/Units and Rulers". One of the viewing options is "Print Size". Photoshop will use the screen resolution to closely approximate the print size on the screen. Screen resolution is the width (or height) of the screen divided by the number of pixels. Photoshop defaults this number to 72 ppi.

    Most browsers do not reproduce the nominal size, rather render the image at pixel = pixel.
  8. digitaldog

    digitaldog Andrew Rodney

    That’s more than enough data but much depends on the output device. You can easily get away with as little as 180-200PPI of data going out to a modern ink jet printer like an Epson. More isn’t going to hurt so if you want to export at 300PPI, fine. As long as uploading the data isn’t an issue, you’ll be fine. After you find the ‘sweat spot’, make an export preset! Then all you have to do is select one or more images, the preset and you’re done; all images will be exported to 8x10 (whatever) at the resolution you’ve asked for in the Export dialog.
  9. digitaldog

    digitaldog Andrew Rodney

    The image is already rasterized! A RIP is a product that takes non rasterized (Vector data, a page layout description) data and rasterizes it. A print driver isn’t necessarily a RIP! Most are not.
    Raster image processor - Wikipedia
  10. " can work out the number of ppi your monitor shows by dividing its physical size by the number of pixels it displays."
    - Sorry! Both Ed and I got this the wrong way round I now realise.

    It should of course read:
    "You can work out the number of ppi your monitor shows by dividing the number of pixels it displays by its physical size"
  11. digitaldog

    digitaldog Andrew Rodney

    Or read how to do this in Post #3 :)

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