Does reducing JPG resolution decrease, increase, or have no effect on image quality?

Question

I'm getting ready to download a bunch of images from Dreamstime. I'll eventually want most of the images in the 400x300 pixel range for my website when I'm done working with them. Dreamstime offers various resolutions to download images in- 1000x750, 2000x1500, 4000x3000, etc. I'm used to printing, where you want to get the largest images possible because that translates into higher quality prints.

I've searched around on Google and Stack Exchange and I can't figure out the answer to this question: Is there a loss in quality if I choose the 4000x3000 images and reduce them to 400x300 in Photoshop? Is the quality loss greater or less than if I reduced the 1000x750 image to 400x300?

Thanks Stack Exchange!

Asked by Jack T

Answer

Bear with me for a second here for some background....

When you downsample a 4000×3000 image to 400×300, you are "discarding" 11.9 million of the 12 million pixels. This clearly reduces "image quality", depending on what exactly you mean by that term.

If you go from 1000×750 to 400×300, you're reducing the area by about 6 times. Again, data is discarded, but not as much.

Now, if your original image had a lot of defects — noise, aberrations, and focus error — the resolution may exceed the "effective image quality", and the extra detail is contributing nothing. In that case, downsampling to some degree doesn't "really" reduce image quality. This is very dependent on the actual original, though — and in any case, going all the way down to a basically-thumbnail-sized 400×300 is pretty drastic.

Hopefully that's all pretty obvious. With that out of the way... In this case, since 400×300 images are your target, the question is really "is there any value in having larger images and downsizing them myself rather than having the stock photo agency do it for me."

The answer to that is: maybe.

1. It's possible that the agency uses poor downsampling and sharpening algorithms for the smaller choices. Or they may use perfectly good algorithms that have the wrong parameters for your taste — for example, too much sharpening. By picking a bigger version and downsampling yourself in Adobe Photoshop or dedicated scaling software, you might be able to do a better job, and tailor the results for your artistic needs and desires.

2. Starting from bigger pixel dimensions allows you more flexibility, since you can crop and still have room left over. You could decide that just the corner or the center of an image fits your use better, and crop out a 400×300 section rather than downscaling.

3. Likewise, if you're doing other manipulation on the image, to change color or make a montage from multiple images, having some extra data to work with at that stage won't hurt (although beware of spending too much time getting detail right when it's just going to get scaled out of perception in the end).

Ultimately, the answer will depend on your particular situation and how it relates to these factors, but hopefully this will give you a way to make that decision.

Answered by mattdm

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