Here is a copy of an old post I did about jpg's. Maybe it will be useful as background. Good luck. - Jean
Some low-tech notes on JPG and quality:
A jpg file is compressed--if you look in Photoshop you can see that a jpg file about 2 MB is actually 9 MB when it is uncompressed as you see it on-screen.
The jpg creators have done a nifty thing by being able to group similar pixels and identify them with a mathematical algorithm that will take less space than it would take to list every pixel. Some call it an average. You can think of the color pixels represented as if they were graphed like a sine wave, with many points on the curve but some off ("outliers"), though close. Every time you Save a jpg, the mathematical algorithm is recalculated, eliminating some of the "outliers", the points not right on the curve. You get more of an average, and less pixel-by-pixel representation.
Your eye cannot differentiate over about 300 dpi, but printers can print higher resolutions. More pixels help if you are going to spread them over a wider area. If you are going to enlarge your picture, the more actual pixels you have, the larger good-looking image you can create. If you are using jpg averaging, you will start to see pixelation at a smaller size image than if you had all the pixels from your original--but not a lot and possibly not enough to detect with your eye.
With a "chemical solution" standard camera, I enlarged a mountain scene until I could see a hiker in the trees; if that shot had been a digital, I would have seen a pixel-looking hiker and not a smooth image. Digital cameras with more and more pixels are approaching the point where there will be enough pixels to close in on details like that. But if you are not going to make posters or print full-page ads, and if you don't save the same jpg file over and over, losing a miniscule number of pixels every time, your jpgs will be quite adequate.
So try out the size photograph you want to make at a resolution that you would prefer, and see if you can see the detail you want and not pixelation. If it looks good at your size, go for it. One compromise in work flow is to take jpgs and save those as TIFs after any changes. Good luck, Jean
on the 7i, I figured the higher resolution the better the
picture...am I wrong? I have been shooting all in the tiff
format.....I think most of you do it in the jpg....for quality
which is best? and what is the difference mainly, quality wise?
Also what is the diff between the auto and prog mode?
I have been shooting at iso 100 in the program mode, can i change
the iso and EV in the auto mode? thanks..t
Matt