Saving JPEG (Quality)

[Old Jim]

I'm a bit confused by the slider/information during the save image process. If I set the slider at the highest number the size of the output image is as much as double the input size. How does that happen? I thought it would always be smaller because of the elimination of the noise. If I set the slider at minimum the output JPEG is too small to be usable. It also seems to me that the location of the descriptive information is opposite where it would make sense. I would think the higher number would result in greater compression and smaller image but it's just the opposite. In any event is there a generally best amount of compression to select?
Old Jim
Beginner with home + version slow learner!

[NITeam]

I'm a bit confused by the slider/information during the save image process. If I set the slider at the highest number the size of the output image is as much as double the input size. How does that happen?

That is one of the properties of JPEG compression method. The original JPEG could be compressed with say, an equivalent of the 80 compression level. If you resave it with a higher compression level then it may become larger (irrespective if noise reduction, which still tends to make file size smaller). If you do the same using any image editor you will get a similar size increase. It is a property of JPEG compression.

I thought it would always be smaller because of the elimination of the noise.

If you take a noisy original image and re-save it as a JPEG with specific compression level N and then take the same original image, clean it with NI and the save the result as another JPEG with the same compression level N, then the second JPEG will be smaller. However, if you increase the compression level N then the file size will grow, for both first and second images in this example.

If I set the slider at minimum the output JPEG is too small to be usable. It also seems to me that the location of the descriptive information is opposite where it would make sense. I would think the higher number would result in greater compression and smaller image but it's just the opposite.

Traditionally for JPEG compression, higher levels mean better quality, lower compression, larger file size. I guess we could make that somewhat clearer in that dialog.

In any event is there a generally best amount of compression to select?

JPEG compression is always a balance between too large file size and too low quality. This balance depends on specific purpose of the image, image content, your personal preferences. You have to try different levels to see what is good for your imaging tasks.

Hope this helps,
Vlad

[Old Jim]

Once again thank you for your prompt reply and through explanation. I guess I'll just have to try different levels on each image. My goal will be to not increase the file size but maintain as much quality as possible. Unfortunately there is no quick feedback as to the consequences of applying the selected amount of compression prior to applying it. It would definitely be helpful to know those consequences before applying but I will live with it. Perhaps you'll consider a change in some future update? I have another question but will post it separately. Thanks again,
Old Jim

[NITeam]

Yes, thank you for the suggestion. I have it in the wish list now.

Vlad