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proper digital photo image size and resolution

Posted on 2011-02-25
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Last Modified: 2012-05-11
Hello,
I've got an other digital rebel 6.3MP camera.  When I import the photos and see the images in a detailed view in windows explorer it tells me the dimension of the photo is 3072 x 2048.  I'm trying to determine what this means for me in Photoshop.
When I do FILE > OPEN and bring in this picture it shows 17.067in x 11.378in at 180dpi at the bottom.  When I bring up the IMAGE > IMAGE SIZE dialog box it tells me the Pixel Dim of 3072 x 2048 like I saw before in windows explorer and in Document Size of 17.067 x 11.378 inches at 180 dpi.
So my questions I'm trying to figure out are:
When I change Resolution to 300, the pix dim jumps up to 5120x3413 and document size stays the same because resample image is checked.  Does this mean I will lose quality because it's adding pixels?

When I go back to original Image Size dialog box dimensions but this time uncheck Resize Image and put in 300 DPI the document size goes to 10.24x6.82 inches but Pix Dimensions stay.  Does this mean that the MAXIMUM the pixel dimensions should ever reach using images from this camera is 3072x2048 to achieve a 300dpi quality photo for print and anything that goes over that is just adding filler pixels?

I know a camera can only output so much quality depending on the camera (in this case my 6.3MP camera) What is the maximum I can enlarge it to attain a 300dpi image.
The other reason I'm confused is because you could come at it from another way and start with a new canvas and make it 19 in x 13 in at 600 dpi and then PLACE my picture from above onto the canvas and it looks great.  Can I go this big?  I open Pixel Dimension and they are at 11400 x 7800 though.  How do I know how much is too much in this Pixel dimension area?
Thanks for your help in understanding.
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Question by:nscit
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Expert Comment

by:BillDL
ID: 34986910
Perhaps this excellent page that explains the relationships between the various values will help you to understand things:
http://www.rideau-info.com/photos/mythdpi.html
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BillDL earned 125 total points
ID: 34987003
Actually, here is a great answer to what is meant by a "300 dpi image":
http://www.rideau-info.com/photos/faq.html#question01

In general you are wondering about qualityof images, but it all depends on exactly what you propose doing with the photos you have taken.

First off, your 6.3 MegaPixel camera rating is a measurement of how "fine" the image will be:
http://www.rideau-info.com/photos/faq.html#question04

It would be a good idea to read the other Questions and Answers on that page, and also the other pages written by the author.  They are very well explained and there is no real sense in typing it all out here when there is such a great source of information readily available.

Bear in mind that picture quality (in terms of blockiness or fuzziness) is affected by the image format used by the camera's inbuilt software that compresses the data into a JPG photo, which is the most common format stored on digital cameras.  As explained here:
http://www.rideau-info.com/photos/filetypes.html
the Normal, Medium, Fine, Super-Fine (or similar) settings on the camera usually refer to the percentage compression that will be applied in storing the photo.  JPG images lose overall definition when compressed, but viewed at the right size you may not notice the difference until you zoomed in.

From a personal viewpoint, I suggest that when thinking about digital images stored for viewing on a computer, to be used in a website, or for emailing to others, you use pixel dimensions.  A pixel is the smallest square you have in an image, and the pixel width and height are the easiest judge of whether you have a large, medium, or small image.  This is especially true of images being shared with others over the internet.

If the image is significantly larger (pixel dimensions) than the pixel height and width of a large monitor, the browser is probably going to reduce it to fit inside the screen area or else the viewer is going to have to do a lot of scrolling to view the image.

Printing a digital image needs to be thought about if you are printing on very large paper.  You aren't seeking to print banners or billboards, are you?  If you are just doing like most people are and printing with an ink-jet printer to 7 x 5 inch photo card or even a single image per standard A4/Letter sized sheet, then having very large images (pixel dimensions again) is usually just a wasted exercise because the image is going to be reduced to the size of the paper being used and the printer can only squirt so many blobs of ink per inch onto a given area, so you won't get the quality benefit of the higher resolution of that large image.

This is the theory you should know about for printing purposes:
http://www.rideau-info.com/photos/math.html

I hope you don't mind me referring you continually to that website, but the author (Ken Watson) has done what I think is a fantastic job without being overly complex.

Bill
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Expert Comment

by:Merete
ID: 34990176
96dpi would do fine,
Here's a guide for photoshop
extract
This information only refers to printed material
If you want to change the DPI of an image, go to Image Size, tick off the ‘Resample Image’ option,
and then type in a new DPI, you will see the image dimensions change to reflect this.
Alternatively, if you know the desired output size, you can type this in
(with Resample Image unticked) and see what DPI your image will output to.
Source
Image resolution and DPI explained
http://photoshopninja.com/photoshop-top-tips/image-resolution-and-dpi-explained/

PhotoShop Image Size and Resolution
http://www.awdsf.com/courseware/photoshop/ps2_ImageSize_Resolution.htm
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Author Closing Comment

by:nscit
ID: 34997966
Would have just liked some more direction specifically talking about the sizes I talked about so I could match it up.
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