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DPChallenge Forums >> Photography Discussion >> Help regarding DPI and resizing
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 05/11/2004 07:31:19 PM · #1
OK here it goes. I think I know what DPI means but... let's say I have an image 3000x2000 at 200DPI and try to resize it in photoshop. I can resize it at 80x533 and then just edit the DPI only and not changing the sizes. So I can end up with a 800x533 image at 200DPI and another the same size but in say 100DPI, does it mean thet the 200DPI even though same size as the 800x533 100DPI will print in better quality?

And what if I encrease the DPI in photoshop without resizing the image does it mean I can increase it's printing quality?
 05/11/2004 07:36:53 PM · #2
No, and no. The detail (quality) available is strictly contorlled by the number of pixels. The DPI only tells how big to print it. you could print a tiny 8px by 10px image at 8x10 inches, (1 DPI) but the pixels would be the size of golf balls. Not good quality.
In order to get decent print quality, you should target printing at >=200 DPI. This means for an 8x10 in ch image, you would need at least 1600x2000 pixels. Make sense?
 05/11/2004 07:42:25 PM · #3
What is your idea of the perfect balance in downsizing for the challenge?
 05/11/2004 08:10:12 PM · #4
Originally posted by Kylie:
What is your idea of the perfect balance in downsizing for the challenge?


What do you mean by perfect balance?
 05/11/2004 08:12:23 PM · #5
I have played with 72, 90, 200 dpi and then the 640 width required here. I am trying to figure out if there is one better recipe than another. 72 or 200 matter when it's that size?, etc. Thanks!
 05/11/2004 08:17:41 PM · #6
The "standard" is 72 dpi for images viewed on a monitor.
 05/11/2004 08:18:02 PM · #7
The dpi you set is actually irrelavent unless you are printing. Then it is used to determine the size of the print.

Images on the screen are displayed usually at 72 dpi, sometimes as high as 96 dpi depending on monitor settings.
 05/11/2004 08:20:18 PM · #8
Thank you both. So it's safe to assume that if I am set to 72 and 640 width with proportion "kept", I will have the best possible to upload? Nothing fancy?
 05/11/2004 08:26:35 PM · #9
Nothing fancy is required. The biggest problem, in my opinion, isn't going to be anything you do to the photo but all the different monitors out there. Your photo is going to look at least a little different on all of them.
 05/11/2004 08:27:56 PM · #10
All that matters is the number of pixels ... and for a DPC entry you can't have more than 640. For a print you want as many as you can get without resampling the image. It is all arithmetic ...

A 640 pixel image will be displayed/printed at:

8.889 inches at 72 dpi

6.400 inches at 100 dpi

3.200 inches at 200 dpi

2.133 inches at 300 dpi

1.600 inches at 400 dpi

etc....
 05/11/2004 08:30:41 PM · #11
Perfect explanation. Thanks again!!!
 05/11/2004 08:31:38 PM · #12
Originally posted by Kylie:
Thank you both. So it's safe to assume that if I am set to 72 and 640 width with proportion "kept", I will have the best possible to upload? Nothing fancy?


Short and sweet: for screen display, totally ignore DPI, it makes no difference at all. Look only at the number of pixels. Use the largest allowable size for submission for best quality, that is, make the longest side 640px, let the short side be whatever it will.
When you save (after resizing), set the compression so that the file size is just below 150k. Best way is use "save for web".
 05/11/2004 08:35:38 PM · #13
heres a good article regarding the whole 72dpi myth :)

essentially you just want to know how many pixels across and how many pixels high your image should be. forget dpi when dealing with computer-viewed images.

dpi should be understood when outputting (printing)

//www.nicholsonprints.com/Articles/dpi.htm
 05/11/2004 08:36:08 PM · #14
That is the part that keeps confusing me . . .I don't think I know how to choosemy compression on the downsizing. I use MS Image Pro 9 usually, but I have a couple others, too. That's where my trouble is and most people here use PS. I guess I'll just keep doing what I have been and keep playing with it. Thank you!!
 05/12/2004 01:01:47 PM · #15
So If I get a 3000x2000 photo and save it as the same resolution (3000x2000) but reduce the DPI in photoshop will this affect the print quality?
 05/12/2004 01:21:30 PM · #16
Originally posted by zeus68:
So If I get a 3000x2000 photo and save it as the same resolution (3000x2000) but reduce the DPI in photoshop will this affect the print quality?


Yes, but it will make the print bigger; and bigger prints are usually seen from a greater distance so it balances out. You could say reducing the DPI increases both the size of the print and the optimal viewing distance.

David
 05/12/2004 04:53:46 PM · #17
I still don't think you are getting to what I am saying. In photoshop you can reduce the DPI or increase it without increasing or decrieasing the size of the photo. Does only the DPI makes a difference?
 05/12/2004 05:09:39 PM · #18
Originally posted by zeus68:
I still don't think you are getting to what I am saying. In photoshop you can reduce the DPI or increase it without increasing or decrieasing the size of the photo. Does only the DPI makes a difference?

When you change the DPI without changing the number of pixels/file size, you affect the physical size the image will display, nothing else.

If you print the same photo at a larger size/lower resolution the photo quality will go down (when viewed from a standard distance) as compared with printing it smaller at a higher resolution.

Think of pixels as a rectangular tiles in a mosaic ... if you have 12 of them in a foot they are each 1 inch square, and 144 of them will make a picture 1' x 1'. If you have six tiles/foot, each of them is 2 inches across, and your 1' x 1' square will only be made up of 36 tiles, while 144 of them will now make a square 2' x 2'.

With 144 pixels you have a certain file size, and the ability to balance quality vs. print size by adjusting the resolution to match your desired use (72 is enough for the screen, 150-300 for printing).
 05/12/2004 05:56:42 PM · #19
The Point Lobos picture in my portfolio, click on my name at left to see, I downsized to 625 pixel width rather than 640 in order that the reduction would allow the software doing the reduction to work with integral numbers of pixels and not introduce artifacts by using fractions of pixels. I was able also to use a higher quality compression as well and still stay under 150K.

I think this helped. I do not want to start an urban myth, however. Does anyone know about resizing and whether integral changes are better or not?
 05/12/2004 06:02:05 PM · #20
Sometimes ... often depends on the image. I'm more likely to take advantage of the better compression quality by using a slightly smaller image, since I usually crop to where I want and then re-size afterwards.
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