Re-sizing images for the web

Discussion in 'Graphics Programs and Photo Gallery' started by Jon_Are, Aug 8, 2009.

  1. Jon_Are

    Jon_Are TPF Noob!

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    I thought I had this figured out, now I'm ultra-frustrated.

    It is my understanding that web images should be around 500-800 pixels in width and have a resolution of 72 ppi.

    Yes? No?

    Here's my problem: I have images saved as JPEGs in Lightroom (which were processed from RAW images). If I open up an image to edit in CS4, here are the stats from the Image Size window:

    Pixel Dimensions: 18.0M
    Width: 2098 pixels
    Height: 1499 pixels

    Document Size:
    Width: 8.742 inches
    Height: 6.246 inches
    Resolution: 240 pixels/inch

    So, this is too large to post on the web and requires resizing.

    When I change the resolution to 72 pixels/inch, this changes my width to 629 pixels and my height to 450 pixels.

    So, I click OK and the image looks horribly jagged.

    I'm at a loss as to why this is. Could it be that, as a JPEG, I've opened and re-opened it too many times (although I don't think this is the case)?

    Does it have anything to do with 16 bit vs. 8 bit? I'm certain that I saved it as 8 bit, but now, when I re-open and click Image>Mode, 16 bit is checked.

    Any thoughts?

    Thanks!

    Jon
     
  2. UUilliam

    UUilliam TPF Noob!

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    no need to lower resolution
    resolution is for printing, just change the Width and height...
     
  3. Josh66

    Josh66 Been spending a lot of time on here!

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    I just do mine 800 on the long side, with whatever resolution is the default in Lightroom.
     
  4. Andrew Sun

    Andrew Sun TPF Noob!

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    Yeap this. If it's just for screen viewing, 300 or 72 dpi doesn't matter.
     
  5. Opher

    Opher TPF Noob!

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  6. Jon_Are

    Jon_Are TPF Noob!

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    Thanks, guys; that makes it easier.

    Can you re-size in LR, or must I go into PS?

    Jon
     
  7. Jon_Are

    Jon_Are TPF Noob!

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    Also, what about checking (or unchecking) the following:

    Scale Styles?
    Constrain Proportions?
    Resample Image?

    And leave it on Bicubic??

    Jon
     
  8. musicaleCA

    musicaleCA TPF Noob!

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    Yes. It's part of the export dialogue.

    Edit: Constrain proportions makes sure that whenever you change the height or width, the other is changed simultaneously to maintain the same aspect ratio. Let it resample the image using a bicubic algorithm; it'll do fine for downsizing.

    Whatever people may think about resolution, for images that are going to be displayed on a screen, 72 PPI is standard.
     
  9. Josh66

    Josh66 Been spending a lot of time on here!

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    Very easy in Lightroom.

    Go to the export menu, set it up how you want (tell it the dimensions, or just the dimesion of one side), then save that as a preset.
     
  10. Jon_Are

    Jon_Are TPF Noob!

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    This is what I thought. But why do my 72 ppi's look like crap - all jaggedy - but my 240 ppi's look OK?

    Jon
     
  11. Jon_Are

    Jon_Are TPF Noob!

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    Alright, so I've done a bit more research...

    THIS:

    ...is incorrect.

    AND THIS:

    ...is correct, but is not what I was asking about.

    Here is what I found:

    PPI refers to the number of pixels viewable on a screen. There is no correlation between the resolution of digital data (ppi) and the resolution of a printed image (dpi). A dot is a droplet of ink on paper and a pixel is a ray of light on your monitor.

    So, my question remains...if 72 ppi is plenty for screen viewing, why, when I change the ppi resolution of my images, do they look horrible?

    Thanks,

    Jon
     
  12. Perniciouspoof

    Perniciouspoof TPF Noob!

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    People change the resolution of images for the web only because it makes it a smaller file that is faster to load, and because having larger images is unnecessary when web pages are going to be viewed primarily on computer moniters. The Quality aspect of ppi or dpi is shown primarily in printing.

    as to the question of why your photos degrade in quality when you change resolution-

    When you change a photograph from 300 to 72ppi photoshop is throwing away extra pixel information to match the smaller print resolution. Which explains the degredation of quality.

    Instead of straight up converting from the actual file, try creating a new file that's set at 72 ppi and whatever measurements you want and place the photograph on that file. You should get better results.
     

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