Mine eyes - a saving test
 

Mine eyes have seen the glory of the coming of the Lord. Actually, I don't think they have, or if they did, they didn't know how to interpret it.

I get curious about the oddest things. I took three photographs each produced thusly:

Nikon D70, ISO 200, JPEG-Fine-Large, each 3008 x 2000 pixels. This is the largest JPEG file produced by the D70.

Each began the test as a virgin file. Each was then saved 25 times (not in photo shop but a wannabe). Each was then cropped to 400 x 600 pixels. I then resized the original image to 400 x 600.

The program would not allow me to save the file unless I had actually altered it in some way. So each time I made an indistinguishable mark before saving.

Each of the large images below are only resized to 400 x 600 and otherwise untouched (except for the one save to resize). The first cropped image represents the virgin file after it was saved 25 times and then cropped to what you see. The second cropped image is the virgin file cropped and saved once.

Next image ...

And the third and final ...

Incredible (I mean it's really neat - not unbelievable).

The differences on my screen @72 dpi are nearly if not impossible to see. Is the same true at your higher resolution?

I purchased the camera and 2 lenses from a camera store in NY and won't receive it until next week. I am still expecting to buy an additional card (1G 40x) and a remote. I can hardly wait to start experimenting.

Maybe there is some part of this that I don't grasp. I have always heard that saving JPEG images would eventually sap the life out of them. They continue to be compressed and have data thrown out after each successive save.

So I wanted to see just how far from perfect I could go. You would think that the example above, after being saved twenty five times would show more signs of wear, but I don't see it. Especially after cropping down as far as I did.

I have destroyed many an image, but it has been through the over manipulation of something that started out being poorly exposed. It would seem that only the act of saving a JPEG that was properly exposed does not create the kind of deterioration that I expected to see.

If someone can point out how I have misunderstood this issue I would like to hear it.

It would have been interesting to see a RAW image as part of the control group. I would guess that the quality of the image would be noticably better at the tight crop.

I don't know, Mike. The answer might lie in the higher resolutions when trying to print.

I think what I said above doesn't make good sense. It has to be true that the saving program doesn't no beans about whether the image is properly exposed or not. It just saves whatever it finds and lets mine eyes determine if it is proper. But, the point is still the same.

You might be right John, maybe if I took the harshly saved image to the printer I might see a difference, but I don't see why really. If I get ambitious I might try it.

I also don't understand the "resolution" thing anymore. When I worked in film the only way I could get my image into the computer was to scan the hard copy. At the time of scanning I would select a resolution. This had a real impact on the quality of the image. Now, when I check the resolution of my digital images they all say 200 no matter what. I suppose if I change that number to the down side I would start loosing something, but I can't imagine having a positive impact by increasing that number after the fact. I thought maybe the ISO had something to do with resolution because I shot most of my stuff at 200. But that ain't it, the image below was shot at 800 ISO and it still shows 200 resolution.

Mike-

You are correct about increasing the resolution would have no effect. You would have to lower the size of the image to gain a higher resolution. Example, you have a 24"x18" image @ 200 dpi. To increase the resolution you would decrease the size ( say to 12"x9") and then raise the dpi (say 300). This would give you more pixels per inch and consequently more information per inch.

Wouldn't this begin to give a different, computer created, look? It seems to me that anytime information gets moved, crowded, separated, that that begins to take the image away from it's origin.

What of this example: I have a large file, too large to post on the forum. I need to reduce the file size so I reduce the pixels from 2000 x 3008 to 400 x 600. In doing so I have created a less sharp image. Could this effectively be done by reducing the resolution. Wouldn't this reduce the file size as well? I don't understand the difference between reducing resolution and reducing pixel size. Why would I ever reduce resolution if I can just reduce the pixels?

(English is not my first language, but I think you will get the picture... ;) )

Reducing pixels and reducing resolution can be the same thing. 2000 x 3008 could be the "pixel size" of an image. Resolution tells you how these pixels are displayed/placed. Were you to show this image in all its actual pixels on a screen with 72 ppi it would be 27,8 x 41,8 inches in size. If you change resolution, but keep the "pixel size" the original amount of pixels the image consists of, the parameter that is changed is the size in inches. At res. 300 the size will be 6,7 x 10 inches, and still 2000 x 3008 pixels.

For printing (real printing) an image and get good quality it should be res. 300. Then your total amount of pixels constrain you in how large print you can get, in inches. Increasing the resulution in the computer without porportional reduction in size will not give an image of higher resolution in reality. The information in the exsisting pixels will just be spread out on some more pixels. But likewise, an image isn't any better, regardless of it's massive amount of pixels, than the information saved in these pixels. A digital camera can have all the pixels in the world, but poor optics and not good enough techniqe to fill these pixels with useful information.

I don't know if I've written anything you didn't already know and I would gladly be of further assistance if you think I can help you, or anyone else for that matter...

Hanna,

How can you know so much yet be so far away?

Thanks for that, I think I understand it better now.

Do you have an opinion regarding my saving test above?

Do you think that even though the many savings of the JPEG file didn't seem to reduce the quality of what we see on the screen, would you expect negative consequences to show up when the file is printed?

Your test was very interesting since I've heared the same thing as you through the years, "do not save your images in .jpg, or they will be gradually ruined". It leaves me puzzled...

When saving, were you asked to choose quality level? And if so, what did you choose? I suppose you took the best possible alternative, but anyway? Maybe the fenomenon is only apparent in a specific program?

As I've understood it .jpg takes away information that we don't see when displayed on a screen. Smart for sharing images, if you don't compress them too much, but since the file gets smaller, something must get lost? I think they get simplyfied and that subtle nuances dissapear first. Although my screen is set on displaying "true color" I doubt it really does, so maybe we all should see a diffence if the images vere printed. If the print is good that is...

How did your image files change in size? (kB) That would be very interesting to know...

When I save it does not ask me specifically if I want to select a quality level. I must have made that choice before I select to save. Therefore the images remained at 200 resolution throughout.

The bird photo in it's unaltered state was 1.54 MB
The bird photo after being saved 25 times, full image was 1.54 MB

The boat photo in it's unaltered state was 982 KB
The boat photo after being saved 25 times, full image was 981 KB

I tried something else here:

The first is the unaltered, the second saved 20 times.

I took the leaf that was unaltered other than having been cropped to forum size. I then took this smaller file and saved it 20 times. I thought that if it started with much less information it might make a difference.

The file started and ended at 41.1 KB

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...?

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.

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

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...

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

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.

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.

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

Holly,

In rereading your post I don't think I did exactly what you suggested. The images above are just severely cropped. I can zoom in as far as I want in my program and when I do they still look pretty similar.

This one is cropped about as much as I dare (expecting the ghost of Ansel Adams to appear).

I don't have any particular stake in proving this one way or the other, it's just something I got curious about.

It seems from what you say that if it is your intention to screw up the image you certainly can. Or, it may actually be screwed up but I am unable to see it. However, if you pay attention the information can be maintained sufficiently. Certainly to the extent that mine eyes are able to utilize it. This gets into the realm of "a wink is as good as a nod to a blind man."

You peaked my curiousity Mike, so I decided to spend too much time and do another test.

The first image is a section of your original image enlarged to 1600% and saved as a jpg at maximum quality. In the second, I began from the first image, and drew a T, pixel by pixel, each time saving it as a jpg at maximum quality. There's hardly any image degradation, just a very slight color change of some of the pixels.

The third image began as the first, but saved as low quality -obvious deterioration. The fourth began from the third, and drew a T in the same location as in image two, pixel by pixel, each time saving it as a jpg at low quality. Again more deterioration.

Images one and two are much closer than I would have suspected. It may be hard to see, but if you look at the top row, fifth and sixth pixels from the right, you'll see a color change.

I guess the upshot is that jpgs, at max quality, are certainly fine to paint from!

Ok, back to work! :)

Holly