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Image size and resolution


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#1
vinny_the_hack

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I apologize if this is the wrong forum, but I didn't know where it might better fit.

My personal files are growing in space requirements, and much of the space is occupied by my photos. They are typically in the 4 mb range. Yesterday, I DL'd a resolution changer tool and it worked very well--perhaps too well. I set it to reduce the resolution of all the photos in a particular folder by 50%. Properties suggest that the new images (written to a new folder) are indeed 50% lower. However, the sizes (on the hard drive) are now minuscule, like a couple of hundred kb's. How is this possible? When I compare the new and old images visually, I don't see any difference, which is a good and encouraging thing. I'm reluctant, though, to trash the original, larger-resolution photos for fear I will have forever lost critical clarity I might want for some application.

My questions are:

1. How does reducing resolution by 50% result in reduction of space requirements by 85-90%?

2. If I can't see (visually) any difference in clarity, is there a possibility that a stark difference may appear when using my photos in some application in the future, say when printed or converted to some other format?
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#2
vinny_the_hack

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:(
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#3
starjax

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when messing with photo's, first make a backup before trying. That way you can revert if you don't like the changes.

First is this a windows or a mac system? Next, what program did you use to make the changes?

I'm not a photo expert, but I should be able to get the info you need.
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#4
vinny_the_hack

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Thanks for responding, starjax.

I'm on Vista and the program I downloaded is freeware called EasyImageModifier. it seems to be a nice, little program that does exactly what I want--reduce the amount of hard drive space required based on my specification of a percentage reduction in resolution and being able to specify a whole folder to operate on at one time (I have many folders with many photos in each and it would be impractical to change the images one at a time using a standard photo editor). The program has the good sense to create a new folder in which to place the modified files.

I have noticed before that when using Paint.Net to reduce resolution, the percentage you specify doesn't correspond with the difference in the resultant hard drive usage of the modified file. For instance, Reducing the resolution by 10% may reduce the file size by something close to 10%, but it has been my experience that the higher the percentage of resolution reduction, the greater the differences between original file sizes and new file sizes. In other words, reducing resolution by 40% can result in file size shrinkage of 70% and reducing resolution by 60% may show a file size reduction of 90%. This appears to be somewhat consistent with the use of this program--not that I ever understood that, either.

I just got an idea. I'll use Paint.Net on a couple of these same photos and see if the resultant file is close to or identical to the the ones this program produced, even if it doesn't explain the phenomenon, at least it would indicate that these programs operate in the same manner and the developers seem to know what they're doing. I am encouraged by one thing, though. Viewing the reduced-resolution images at full screen size, they still appear clear, so maybe my concern is unwarranted. In fact, I didn't see any difference visually. It does make one wonder, though, why we are taking photos at 4 mb or larger when 200 kb looks virtually the same.

I'll post the results of my test with Paint.Net as soon as I can.

Edited by vinny_the_hack, 02 May 2013 - 08:49 AM.

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#5
vinny_the_hack

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Editor scrunches everything up (removes all but one space between text). I hope you can make it out.

Results
-------

Original size Reduced 50% by EasyImageModifier Reduced 50% by Paint.Net

4.4 mb 256 kb 392 kb
4.4 mb 807 kb 1280 kb
4.4 mb 623 kb 934 kb
4.4 mb 737 kb 1125 kb


Gives one pause when 50% means three different things when talking about resolution, file size and the program you use.
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#6
starjax

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I haven't had a chance to download the program yet. cursory examination leads me to believe that it the algorithms it uses are more aggressive at compressing the size.

The best image compression is achieved by using the JPG format, but even if the image is already in this format, you can usually still make it take up less space, as the JPG format has an adaptive compression scheme that allows saving in varying levels of compression. The trade-off is that the less space an image takes, the more detail from the original image you lose. You should also be aware that repeated saving in the JPG format causes more and more image degradation.
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