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How much resolution for projectors?

Discussion in 'General Discussion' started by mrdinh, Sep 15, 2005.

  1. mrdinh


    Mar 8, 2005
    North Dakota
    I will be attending a local awards banquet for the end of season racing here. I have another chance to sell my pictures of the racers during the season. The track owner wants to do a slideshow of my pictures. In order for me to do this they want a cd of the images. What dpi is good enough for the slideshow but not high rez enough for them to print my pictures? Help is appreciated

  2. I would think that 72 DPI, same as for web or monitor display, would be standard. If you project an image it doesn't need to be physically large or with a high resolution. As a matter of fact, I assume they will simply be using a laptop with a video out connection to an LCD projector or similar, since they asked for the images on CD. In that case you would treat it exactly as you would for a monitor display. Although you did not mention how far away the screen will be from the projected image. You might want to make it just a little higher res in case the screen will be a good distance from the projector, but it is basically projecting an image from a monitor display.

    We've all seen Word and Excel docs in meeting rooms like this. If it were me, I might bump up the brightness and/or the saturation slightly on images that would be projected, as you will always lose a little color and light levels when anything is projected. If you're web standard is 72 DPI I might go for 100 just to be safe, but I don't think it'll make a big difference. Anyway they only need to get the idea. They know the prints will look much better.
  3. Looking at the specs for digital projectors the best most of them can do is around 1024*768 pixle so your images need not be any bigger than this. As has already been pointed out treat it as for display on a monitor (or even on the web). I remember most people back in the days of slides used to take pictures through super expensive lenses and then project them through the equivalent of coca-cola bottle bottoms - a few of use used Leitz projectors with top quality Leica lenses, there was a difference - with digital projection your images will be as good as most people got with slides but not as good as using slides projected through a Leitz projector.
  4. mrdinh


    Mar 8, 2005
    North Dakota
    i'm not sure how the slideshow will be done or how large is the room...so the distant to the viewing area is not known...

    i want enough rez to view the pictures but prevent them from print direct from my cd...

    i guess i could put a copyright on each image...not sure if that will help

  5. nfoto

    nfoto Guest

    You must think pixels, not dpi. Most projectors do XGA, that is 1024 x 768 pixels. Whether the "dpi" is 72 or 300 or 4000 has no bearing on this at all, because it won't influence the number of pixels projected.

    Unless you are using a really good viewing program, the projected image quality will benefit from the images being scaled down to the targeted size (1024x768). Don't forget to run unsharp masking on the final images.

    The added benefit of this down-scaling is that unwanted use of your images is unlikely because the files are so small.
  6. Pixels for viewing display, correct, but I mentioned DPI because the question of printing was asked. Normally 72 DPI is the highest pixel density you will notice on a monitor (or other projected display), but too low for printing in high res detail. A 72 DPI image wouldn't normally be proper for commercially printed use. The larger the resolution (1024X768, etc.), the less dense those pixels would be at the same DPI. I think if he goes 1024X768 at 72 DPI he will have an image that projects well but without enough pixels for a commercially viable print. It is also good to put a transparent copyright notice as a watermark, either at bottom left or right of image. I wouldn't put it in the center of the image for a presentation.
  7. nfoto

    nfoto Guest

    1024 pixels are just 1024 pixels and the DPI assigned is just a yardstick to tell how big the print will appear on paper, if printed with that DPI setting. It has nothing to do with resolution as such. You can set the DPI to 1024 for example and the long dimension on the print will be 1 inch. Or 512 dpi and the long dimension will be 2 inches. Or 256 dpi and the long dimension will be 4 inches. And so on, ad nauseam.

    On a high-resolution monitor the 1024 pixel image will be small, on a low-resolution monitor it will be large. Again, dpi has nothing to do with how the image is rendered.
  8. MontyDog


    Jan 30, 2005
    #1064 - You have an error in your SQL syntax;
  9. Resolution issues can be very confusing, that is true indeed. But if we are talking about printing, which I think the question was asking about, 72 DPI isn't usually enough pixel density for a high-res image of a reasonable printing size. It is however usually the highest density of pixels our eyes can realize on a monitor or projected display, and looks just fine for digital presentation. I hope I haven't misunderstood the question or added to any confusion.
  10. nfoto

    nfoto Guest

    You certainly have. 72 dpi is just one of the lingering digital myths and doesn't really relate to anything (except for the old 12" Apple b/w monitors, from which the whole thing originated).

    My example intended to show that it is the number of pixels which is of interest, not the dpi. We can set the dpi to any value we wish. It won't change the resolution at all.
  11. Then possibly I am very confused indeed. I think I'm not following something here. Do you mean to say that printing an image at 72 DPI will at arrive at the same resolution to the eye, at the same physical size as printing at say 300 DPI? I don't think the answer is yes, and I don't think that's what youre saying. But I'm the first to admit this conversation has lost me.
  12. nfoto

    nfoto Guest

    I'm not saying anything about the physical size. I said "dpi is a yardstick". amongst other things, it scales the image. People confuse dpi with resolution. These concepts are unrelated.

    Do remember digital images are basically dimension-less. We scale them into a wanted output space. Resolution as such is already given by the captured image. We cannot change that afterwards (except for lowering resolution by throwing away data).

    To give an example from my D2X. I could say the 35.5 MB TIF coming off the camera is a 4008 dpi file with long dimension slightly less than 1 inch (in fact, 23.7 mm). Or, I can say it is a 300 dpi file with long dimension 36 cm (I'm in a metric country). Or it is a 150 dpi file with long dimension 72 cm. All of these are equivalent and have the same resolution.

    What the naked eye perceives as "sufficient sharp" depends on a number of factors, which include print size, viewing distance, image contrast, and more. So saying that "72 dpi is sharp" is ambiguous and not really meaningful.
  13. Debunking the 72 DPI myth: http://www.scantips.com/no72dpi.html
    I found this quite wordy article that explains quite graphically everything Bjorn has said here using much fewer words. It's worth a read to fully understand.

    Bjorn, thanks for pointing us in the right direction. However I still have a question about printing such an image. If I create an image at 72 DPI, my monitor may not know or care about DPI. That's understood. But if I then print that image which was originally created at 72 DPI, I will still have to increase the DPI for printing. Won't that image lose density resolution by the time I get it scaled to a larger print size, if it was originally scanned or created at 72 DPI? Or is the DPI it was created with totally irrelevant and of no real limitation?
  14. nfoto

    nfoto Guest

    Given you have sufficient number of pixels, you can scale the image in Photoshop and change the dpi setting there.

    Many P & S compacts deliver jpgs with "72 dpi" and an output (print) size of gigantic dimensions. Then, you have to go to PS and set the dpi to whatever value you need for printing, but keeping the pixel dimension (pixel count). So in effect you "rescale" the image to the output space, but you don't change pixel number, or resolution.
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