If you do not resample, the quality will not change. If you resave as a JPEG, the quality will deteriorate, so don’t do that.
To expand on Scott’s message, JPG is a "lossy" compression type. This means that each time you save the file, it compresses the data and tosses some out. If you save the file repeatedly, you will soon notice deterioration in the picture.
TIFF on the other hand is lossless. You can do multiple saves without affecting picture quality.
Holly…..
The images coming into Photoshop from your camera are just pixels the fact that they have a default document resolution with them is not significant until you get to wanting a physical size.
The trade off is to get the physical size at a good resolution without resampling. Most people doing their own prints go for a resolution of between 200 to 300ppi. (Some even as low as 150-180).
So in the IMAGE > IMAGE SIZE dialog box you un-check the Resample Image check box and then adjust the height and width to what you want and see if the resolution stays between your limits, eg. 200-300. If not then you need to resample because you just don’t have the right number of pixels to start with.
Resampling is an artificial process and can result in quality changes but small amounts are still really good. There is a theory that says if you need large amounts of resampling you should do this in 10% increments rather than one go, but I have no evidence to say this is right or wrong.
Just to comment on the JPEG quality issue. Yes it’s very good practise to use a loss less format such as TIFF or PSD but saving JPEGS many times does not always mean a noticeable loss in quality…it depends a lot on what changes you make between each save.
Colin
Colin,
There is a theory that says if you need large amounts of resampling you should do this in 10% increments rather than one go.
Many of us have tested this theory and found that it may have worked on old versions of Photoshop, but with newer versions it takes more time and makes no difference or is worse.
it depends a lot on what changes you make between each save.
No. Any change (90° rotations might not, but I don’t even think they are excluded from recompression in Photoshop) will result in the entire image being recompressed. Keep your masters in a lossless format and save to JPG only for your final output format if the client\project requires it.
PH…
Using a lossless file format is very good practise and one that I always do but it’s not true that a resave of a JPEG file will automatically degrade it’s quality. I’ve run tests with multiple saves of the same unaltered image and then overlayed the original and have seen not a single pixel difference. Similarly I’ve made changes, e,g levels in restricted area and made multiple saves and only seen pixel changes in the altered areas.
I’m not advocating the use of JPEG for working copies but what I am saying that doing this with JPEG is not necessarily going to lead to a loss of quality.
Colin
I may be mistaken but I think there is a linux based tool that will compare images and output the differences.
A similar result MAY be obtained by using layer blending I hope and maybe someone with far greater experience and knowledge that I have may enlighten us?
The question of comparing different versions of the same image has scientific and technical ramifications as well as creative, perceptive ones.
Deebs….
I just set the original on one layer and the re-saved on another and set the blending mode to difference. Then moved around the image at high zoom looking for anything that wasn’t black.
Not too scientific but enough for me.
Colin
Colin,
True. It is possible to resave without damaging the file, but how often is someone going to open a file in Photoshop and not make any changes. It doesn’t take long for a few minor edits to degrade the quality considerably and then you have to go back to the original (if they still have it) and start over or use the messed up product of bad saving practices.
Now if someone is just opening JPG’s and making a one time color correction and quick cleanup and they are keeping the originals because they just want a CD with simple JPG’s to give to friends and view as a slide show every once in a while. Then they won’t have much need for a lossless master.
As long as they know what is happening each time they save a file. I am amazed by the number of people that keep only JPG’s and overwrite their only copy of a file multiple times. Bad archiving practices are not reserved exclusively to picking a bad file type. Unfortunately it is often too late when someone finds out they should have been doing something different.
True, too true.
Besides that, I believe some of the onscreen slideshows, wonderful as they are, are not to the highest resolution anyway so I guess there may be losses in the program showing JPGs too
