Mine eyes - a saving test

[Hanna Larsson]

I tried saving a jpg in Photoshop. I didn't get a quality question then, only when I chose "save as" a .jpg. And what the question is about is how much compression to do. No change in resolution, just in simplification.

I must do some similar testing as you and see what my results are. I will not have time to do that until tomorrow, but I'll let you know how it turnes out.

The fact that your images are the same size suggests that our eyes are telling the thruth, that they haven't changed...?

[Hanna Larsson]

I did a quick test now saving an image 20 times AS a jpg. I chose maximum quality every time and as far as I can see nothing changed. The image looks the same and is the same size.

The first image is the one saved 20 times.

[Mike McCarty]

Hanna,

Would photoshop allow you to save the image without changes? My program would not. I made an indistinguishable change for each save.

The only question left in my mind would be how the abused image would hold up to printing. If it too seems to have no effect I would be inclined to call this a busted myth.

Thanks Hanna

[Hanna Larsson]

I didn't need to make any changes since I every time chose "save as" and overwrote the old file with the same name. I can not just "save" without changes, so that is the same for us both.
I am going to have classes in "digital images" in a couple of weeks in school. I must remember to ask my teacher about this, maybe he knows more than we do, maybe not...

It is however one thing we might have overlooked. Or at least I. The "rule" I've heard is that even if your camera saves images in jpg, you shouldn't do it yourself. Not even to change the name. And to get the images the same size without ruining the original photo I had to save a new file with the supposedly "fresh" image. So maybe the things we don't see on the screen disappeared already then. Anyways, it would be nice to know the answer to this query now that we both spent time thinking about it.

I'll post again if I get new information or insight...

[Holly Snyder]

Mike and Hanna,

When you're comparing the image that's been saved multiple times vs. the original image, enlarge the images to 300+ percent and look at one small area of the images. A drop of water perhaps. You should see a difference. I'm not surprised that you don't see a difference in images saved multiple times as a maximum quality jpg, and then posted on the forum, as they're only displaying 72 dpi.

Just to prove to yourself that jpg is a lossy format, try saving the same image, 20 times, at the lowest quality jpg setting, and there should be a very noticeable difference (without having to magnify the image to see it). Very nice closeups, by the way.

Holly

[Mike McCarty]

Holly,

Here are two images patched together. The one on the right was saved 21 times the other only cropped down from the original. I didn't save the one as anything other than the resolution that it began it's life with.

I would think that if you took two identical images, reduced one to the lowest resolution and then compared it to the other, without any saving abuse at all, that would be enough to make for a stark comparison.

[Michele Rushworth]

Looks pretty clear: when choosing the highest quality (ie not much compression) the jpeg file format seems to be more than adequate for our needs in storing reference photos to paint from.

[Holly Snyder]

The thing is that you're displaying them at only 72 dpi on the forum, so any loss in image quality will probably not be viewable. If you look at those same images in your graphics program, zoomed in significantly (300+ percent) so that you can see the enlarged individual pixels, can you see a difference?

While jpg is lossy, it is an excellent algorithm, so that when an image is saved at the lowest compression (highest image quality), the difference vs. the original isn't that noticeable. But try the same test with the highest jpg compression (lowest image quality), and the difference will be much more noticeable.

Holly