Resolution vs. Image Size ?

Discussion in 'Photography' started by gino_lee, Feb 24, 2007.

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  1. gino_lee

    gino_lee Member

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    I have just become confused about image resolution. Seems like most ppl, myself included are confused about what resolution means. So hopefully somebody can clear it up.

    For example,

    - my old Canon S400 takes photos at a max "resolution" of 2272x1704 (4MP).
    - my new Casio Ex-z60 takes photos at a max "resolution" of 2816x2112(6MP).

    but I think those numbers are actually the size of the image. As a 4MP image can be printed on an A4 paper at full quality. (i've always maintained that most of the users don't need anything more and 4MP)

    I think resolution of a picture is the pixels/inch that the camera can capture. This is probably the most important specification that the manufacturers do NOT advertise.

    When i was in school, we learned that having more points on a graph is just as important as the size of the graph, since it will show more detail of what is actually happening. And i think this is the case here.

    However, now i'm confused. what do the measurements of 2816x2112(6MP) actually stand for? since those are commonly referred to as XXXXpixels x YYYYpixels, and somehow converted to length in inches or cm.

    I think i've confused myself as to how all this works.

    my S400 is 2272x1704, and has a resolution of 180pixel/in.
    my ex60 is 2816x2112, and has a resolution of 74pixel/in.
    which results in the canon taking SMALLER, but CLEARER pictures.

    oh, might I add that changing the quality of the image from fine, normal, economy, etc. on these cameras do not seem to add any pixels/inch. then what does it add?
     
    Last edited: Feb 24, 2007
  2. Ben Stafford

    Ben Stafford Site Admin

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    I believe that most computer monitors can't really display more than 72dpi, so that dpi resolution really just affects print quality. The megapixel numbers are really the dimensions of the image.

    I think your Canon vs Casio sharpness differences are probably caused by too many other factors - sensor differences, image processing differences, different brands. If you could change the dpi setting on one camera, then you could truly compare the images.
     
  3. AKAJohnDoe

    AKAJohnDoe News/Review Writer News/Review Writer

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    A pixel is a single point that can have varying combinations of the three colors RED, GREEN, BLUE in a RGB color space, therefore, a single point can only be one color. DPI is the density of the points. 72 DPI is 72 dot per inch, or, each dot is 1/72th of an inch. PPI (Points Per Inch) for printing is similar. 300 PPI is 300 dots per inch or each dot is 1/300th of an inch.

    At 300 PPI and an image size of 1200x1800 pixels, the actual print size is (1200 / 300) x (1800 * 300), or 4x6 inches.

    Obviously then, a dot size of 72 is a much larger dot than a dot size of 300. Look at an old newspaper image under a magnifying glass (or a painting by Seurat) and you will see that a denser dot cluster can make a more detailed image.

    A 17" monitor can usually actually only display about 13" side-to-side normally, so at 1024x728 resolution that comes to about 78 PPI using the same calculations.

    This actually brings up one of the fallacies of the "more megapixels is better" since the size of the pixel is the other determining factor.
     
    Last edited: Feb 25, 2007
  4. gino_lee

    gino_lee Member

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    yea, if i could. I've tried adjusting the "quality" settings "superfine, fine, etc." but that does not seem to change the dpi setting.

    On the Casio i've also tried to adjust the "sharpness" setting. +2 sharpness does create a larger filesize (MB wise), makes a clearer picture, but still seems to be at 72 dpi.

    but i'm still confused as to what the difference is between MegaPixel and DPI. somehow megapixel translates into print size...

     
  5. AKAJohnDoe

    AKAJohnDoe News/Review Writer News/Review Writer

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    Since the sensor size of a digital camera is not alterable, the only place you can choose are the image capture settings.

    PPI is important for input when scanning and for output when printing.

    So, using the math previously discussed, for any given PPI, the X by Y dimensions of the pixels used for capture will determine the image size.

    The terms PPI and DPI have become somewhat interchangeable in general usage; however, they really are different terms. PPI is Pixels Per Inch, whereas DPI is Dots Per Inch. PPI is used for printing and DPI used for printers. The subtle distinction being that it could take multiple dots to create a single pixel on a printer.

    There are many explanations of this on the web, some probably more eloquently delivered than mine. A Google search on DPI PPI will probably find them.
     
    Last edited: Feb 26, 2007
  6. gino_lee

    gino_lee Member

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    @ AKAJohnDoe,

    you've pretty much summed up what I've found out with some experiments lately. But put it in a much better and more understandable way! now things make much more sense!

    so for example,
    Casio EX-Z60. 2816x2112 @ 72ppi, translates to 39x29inches, thats also what it shows in Paint Shop Pro.
    Canon A700. 2816x2112 @ 180ppi, translates to about 15x11inches, also what PSP shows.
    Nikon D40 SLR. 2816x2112 @ 300ppi, was something like 10x7inches.

    So the Casio has an original picture size that is HUGE, but lower quality, while the Canon has a SMALLER picture size, but higher quality.

    for a first camera the Casio Exilim-Z60 is a great camera, decent picture, VERY FAST, and tiny size, not to mention tiny in price in comparison.

    cool! finally, i can really explain myself when I tell people it is not the MegaPixels that matter, but also the ppi!
     
    Last edited: Feb 28, 2007
  7. AKAJohnDoe

    AKAJohnDoe News/Review Writer News/Review Writer

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    Close. PPI is a function of output on a printer or input on a scanner. In Paint Shop Pro or PhotoShop or most other programs the image attributes can be altered and the image resized, resampled, and varying PPI set for subsequent output. You should be able to have PSP show the same PPI for all three of those cameras, in fact, all three values for all three cameras. However, the hardware within those cameras may be different, in particular the sensor size.
     
  8. ajaynejr

    ajaynejr Active Member

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    When the camera has so many megapixels it also has a horizontal and vertical pixel count for example 4 megapixels or 2272x1704.

    You get the full megapixels and pixel count when you choose super fine resolution.

    You can choose medium or other resolutions (quality settings) to obtain pictures with fewer megapixels and more pictures will fit on the memory card. For example medium resolution on a 4 MP camera might be 1 MP or 1135x852.

    After you take the picture, regardless of the number of megapixels it has you can print it just about any size you want subject to the limitations of your editing software and your printer (the latter's page size).

    PPI as a number you work with is not something the camera has but rather is something you might choose for the picture when you print it. Given the horizontal and vertical pixel count (largest for super fine that you chose earlier), let's say you choose so many PPI such as 180. Then the dimensions in inches are automatically determined for you with no further choice. Or you might choose the dimensions in inches such as 8 by 10 and the PPI is automatically determined for you with no further choice. Some editing software lets you choose PPI but not dimensions in inches and some editing software lets you choose dimensions but not PPI.

    As mentioned previously, PPI does not equate to dots per inch on your printer. PPI should simply be the number of pixels (horizontally or vertically) for the picture (or cropped portion) divided by the desired size (width of height) of the final print.

    Digital camera hints: http://members.aol.com/ajaynejr/digicam.htm
     
    Last edited: Mar 2, 2007
  9. gino_lee

    gino_lee Member

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    I realize that it is possible to resize a picture in a program like PSP, and to subsequently change the ppi.

    but is it possible to change it on the camera itself? I've tried with the Canon S400, A700, and the Casio. Taking pictures in normal, fine and super fine. but everytime, PSP reports no changes in ppi under "original dimensions".
     
  10. ajaynejr

    ajaynejr Active Member

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    It is meaningless to talk about changing the PPI in the camera, unless you are discussing the image sensor which for most point and shoot cameras is a rectangle ranging from a quarter of an inch to a half of an inch across. By selecting a higher resolution (super fine, etc.) the pixels per inch of sensor as processed and then stored on the memory card is greater. If you are taking a 3 megapixel picture 2000 x 1500 pixels and the camera's sensor is half an inch wide, then the PPI relative to the sensor is 4000.

    Then when you upload the pictures to your computer and load them into PSP (or any other program) the picture has a horizontal and vertical pixel count and that locks in the PPI for the currently selected dimensions in inches you will get when you do File, Print from the menu bar. If you select different dimensions for printing, the PPI as reported by PSP will change. Let's say PSP was last used for 7x5" prints. If the picture you just loaded was 2000 x 1500, the PPI will be about 285. If you cropped the picture leaving a 1400 x 1000 portion and printed that as 7x5 inches, the PPI would be 200.
     
    Last edited: Mar 4, 2007
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