Mine eyes - a saving test - Page 2

Hanna Larsson · 01-19-2005, 05:45 AM

(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...

Mike McCarty · 01-19-2005, 10:53 AM

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?

Hanna Larsson · 01-19-2005, 12:25 PM

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...

Mike McCarty · 01-19-2005, 01:06 PM

Quote:

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?

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.

Quote:

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

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

Hanna Larsson · 01-19-2005, 03:23 PM

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 · 01-20-2005, 02:59 PM

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 · 01-20-2005, 03:14 PM

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 · 01-20-2005, 05:12 PM

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 · 01-21-2005, 09:49 AM

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 · 01-21-2005, 10:30 AM

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.