Jpeg / Tiff
 

Was talking to someone the other day who's wife was taking a photoshop course at the local college. OK; this is what you'd call second hand information but what his wife had apparently been telling him was that a JPEG file can lose quality each time you open it therefore you should always save as a TIFF file.
I'm pretty new to this so can anyone confirm this to be fact or otherwise & if so offer a brief explanation.

Thanks,
Al.

The JPEG mathematical process, Discreet Cosine Transform (DCT) is lossy so the reverse calculation to reconstruct the image cannot reconstruct the original image but can be very close. I have not studied TIFF in great detail but is known to be lossless therefore any compression applied can be reversed perfectly to reconstruct the original image 100%. There is a trade off between minor image degradation of JPEG (if used correctly) and huge TIFF filesizes for hard disk storage and image transportation.

She is quite correct. Essentially there are two sorts of image file:

LOSSLESS: TIFF, BMP, various others: open a TIFF file, make a small change, save it. The []only[/i] think that is different is the change you made yourself. You can open, edit, save 100 times over and it doesn't matter - the picture quality remains exactly as good as it was in the first place. (Unless you messed something up yourself.)

Problem: all the lossless file formats are huge, slow, clumsy great things that take up a heap of space and are impractical for many purposes.

LOSSY: JPG, PNG, GIF, various others: When you save the file, the saving process throws away as much of the leftover near-useless information as it can, and only saves the most important bits (in the hope that you won't be able to see any difference, because your eye is only human). This saves an enormous amount of space and makes it practical (for example) to have pictures on the internet - if all we had were TIFFS or BMPs, our broadband connections would cost five times as much and go slower than dialup.

Problem: some information is thrown away every time you save the file. So, if you start with a ficture and crop it, then save as a JPG, then adjust the brightness and save, then resize, and so on (and this is exactly what you do when you are preparing an image - go through a whole series of steps, saving as you go so that if you mess up you can go back one or two steps and try again), what happens is that the small, not-really-noticable image quality degredation you started with is repeated over and over, and you wind up with a very small file with hardly any detail.

Solution: Save as you go using a LOSSLESS format such as BMP or TIFF. When you are happy with the final image, then save as JPG. Throw away all your intermediate saved TIFFS to reclaim the wasted space. (Keep the final TIFF if you have the room for it - you might want to make another, different JPG later - in a different resolution, for example - but use a small, fast, almost-as-good JPG copy of it for everyday purposes such as a screensaver of to post here.)

I do not think that a jpeg will lose quality just by opening it. If you edit it in any way and then re-save you will lose quality but not just by opening it.

Sincere thanks everyone; will now experiment saving in lossless files for images to be worked on. Am I right in assuming then, (as I've never done this before), that if I shoot a RAW image it would defeat the object by saving it as a lossy type file & that I would need to save it as a lossless file? (Assuming I'm going to make adjustments to the said image). Have I got this right or have I got completey lost & started barking up the wrong tree so to speak?
Thanks again....Al.

Al a Raw file remains a Raw file always. However when processed another file is created. When that is saved in the format of choice it becomes that choice. If you choose jpeg it will only noticeably deteriorate over time through regular resaving and over enthusiastic compression. If you are going to do a bit of work over time, it is best as others have said to work with a Tiff

Thanks Stephen.
The jigsaw in my mind is coming together now!
By the way; just had a brief look through your gallery; I've been to Tunisia once....I'll always remember those blue doors....fantastic.
Al.

Whatever format you capture your original image in (jpeg, raw, tiff) you should keep that image as your digital negative. To edit your image you should only work on a copy.

The best format in which to store your working copy is the format of your image-editing program (PS, PSP, etc.). That format will store all the extra info (layers, undo steps) associated with editing.

When you've got your final image you can save in whichever format will be best for the final purpose - normally jpeg - with the quality depending on the final display medium.

With the low price of storage nowadays it makes sense to keep all three images (digital negative, editing file, final output).

Sounds like good advice. I think I'll have to re-visit my work habits on this one.