What determines image quality?

tcengineer,

As you've discovered, this is a complex issue. I agree that two of the key elements is compression (both file-type and degree of compression applied) and DPI. One of the most common file types used on the web is JPG. The JPG format is a lossy compression format - to compress the image, information is discarded. Each time the file is opened and re-saved, more information is discarded. In addition, the user can set the level of compression applied to decrease file size (which corresponds to an increase in information that is discarded). So, JPG files from an identical original can look different depending on the degree of compression applied, and number of sequential saves made. GIF images also discard data, the data they discard is color information - to "compress" GIF images, the software reduces the number of colors available in the image (to 256 or less).

For print purposes, typically you want to work with formats that do not lose information. True bitmap images, even after compression, have a one-to-one "description" of every pixel in the image. BMP (rarely used), TIF, and now PNG images are good examples. PNG does an excellent job of managing compression for the web, while retaining information.

In addition, monitors display information at different per-inch densities that print. Typically for print, you want to work with an absolute minimum of 200dpi, and 300dpi is better. Monitor resolutions are typically only 96 or 72dpi, so images for the web are usually at those lower dpi settings.

Last, actual image dimensions matter. A smaller image is always going to have less information. We have gotten used to dimensions in pixels, so a 1000 X 800 image won't look good printed at 200dpi at any sizes larger than 5" X 4", and would look even better at 3.3" X 2.6" (300 dpi). A 400 X 300 image (reasonable size for a screen), won't look good at anything larger than 2" X 1.5" (200dpi).

Cheers,
LHerrou

Pay special attention to this part from lherrou: "Last, actual image dimensions matter. A smaller image is always going to have less information. We have gotten used to dimensions in pixels, so a 1000 X 800 image won't look good printed at 200dpi at any sizes larger than 5" X 4", and would look even better at 3.3" X 2.6" (300 dpi). A 400 X 300 image (reasonable size for a screen), won't look good at anything larger than 2" X 1.5" (200dpi)."

Your 96 dpi image probably had smaller pixel dimensions than your 72dpi image.

The images below are all saved as JPEG with a medium compression.
The first is 200 pixels per inch, the second is 72 pixels per inch, and the third is JUST 4 PIXELS per inch!!!

The pixel dimensions of all three are 600x383 though... They are exactly the same image. The DPI is set different, but they all have the exact same pixels.

@statler: sorry to point it out, but all three images that you posted are sized to 383x600@72dpi. They look identical because they are identical.

I managed a large lab operation years before digital was available and on the beginning cusp of the digital revolution. Digital cameras are great and they revolutionized photography. All of this input is correct, but these comments are all about the number of pixels and compression.This part is simple really, more pixels=better image, compress as little as you have to for your medium and make it the very last thing you do. Once compressed, do not work on that file any more. It's done, know when you have cast the last stroke, or open an uncompressed version and use that if you have to do more. Really, DON'T rework a compressed file.

There are other factors that can make or break sharpness and a nice image with "snap".

Shape of the pixel.

Quality of optics. A $100 point and shoot undoubtedly has lower quality optics than a $600 dslr. The actual quality and way the glass is polished is a big part, but the light gathering capability of the lens is an even bigger issue. The less hard the sensor has to work to resolve an image, the less "noisy" an image will look. (This used to be grain and silver halide go figure.) The light gathering capability of the lens will be listed in the specs of the camera. That's what all the f-stop business is, you want the lowest possible number. A maximum opening of f2.8 is far superior to a maximum opening of f4. Needless to say (but I'm gonna say it anyway) clean your optics. I can't tell you how many nice images I've seen ruined from lotion, sunscreen, cheeto gunk. The lens doesn't clean itself :) use a microfiber cloth. Never a paper product.

The actual exposure. Everyone thinks the forgiving exposure latitude of  digital makes proper exposure an outdated concept, but the pro's  know better. Nail the exposure. Don't shoot into the sun or other bright things unless you want an "artsy" looking photo. Keep the sun on your back and the light on your subject.

Proper balance of contrast,density (exposure,lightness, etc.) and saturation. This is so important, it's almost everything. It's amazing how much something with proper contrast looks sharper.

A final note on dpi. Almost all commercial printers (light/chemical process) output at 300dpi, so if you're going to print, work with the actual intended output size in inches@300dpi for best results.

And again, DON'T rework a compressed file. I thought I'd go ahead and beat that into the ground.

I would normally recommend TIF, but I suspect most of your viewers are going to have images in JPG format, so let them upload in that format. Recommend a size of 600 X 900, which will equate to 2" X 3" in 300 dpi, but set a  minimum of 400 X 600 (200 dpi).