I noticed on the picture comparison page at http://www.imaging-resource.com/CINTRO1.HTM that the Toshiba PDR-M1 has good looking images but a huge file size compared to other images from competetors. Is that the price you have to pay for the higher quality? I noticed some images only about 10% lower quality but about 60% smaller file size.

Is there some way to control the file size before you shoot the picture?

-- Lee Beck (lbeck@intrex.net), December 19, 1998

Answers

Lee-

We really need to put up a tutorial piece on image compression - that's what causes the file-size differences you see. A 1.5 megapixel image would translate to about a 4.5 megabyte file in its "raw" form - One byte each for the Red, Green, and Blue colors assigned to each pixel.

Essentially all digital cameras make use of a clever technique called "image compression" to selectively throw away image data in ways that our eyes tend not to notice. The method used is referred to as "JPEG" compression, for reasons not worth going into. If two cameras (or one camera, set to different "quality" modes) with the same image size in pixels produce image *files* of different sizes, it's because the one with the smaller file size is using more compression on the image.

Too much JPEG produces blocky-looking "artifacts" around the edges of objects, and in flat areas of low contrast. Look for boxy "jaggies" along diagonal edges, that sort of thing. (Visit our Tips & FAQs page, click on the "Buyers Guide" link, to see an example of bad JPEG artifacts on a low-resolution image.)

If you're going to be printing out your images, and are concerned about ultimate image quality, you'll usually be better off with a camera that has the capability to produce larger files, and then put some money into a larger memory card or two to store the shots...

At conservative levels, it's hard to see any effect at all from the compression. If you aren't able to see any funny artifacts of the sort described in the Oly 340 images, you'll be perfectly happy with it. Both cameras have higher-compression modes they can store images in, so a camera with a larger file size can almost always be operated in a more storage-conservative mode. (Actually, it isn't documented in our reviews, but the Oly 340 was one of the first cameras we reviewed, and at that time, some of our shooting didn't exercise the very highest quality compression settings! Bottom line, I think both cameras would create similar-sized images if both were operated in their highest-quality/lowest-compression mode.)

Hope this helps!

Good Luck, Dave Etchells

-- Dave Etchells (web@imaging-resource.com), December 19, 1998.

I have a Toshiba PDR-M1 and I've done some very informal testing regarding the file size settings because I wanted to see if the quality trade off wouldn't be worth getting twice as many images on a card(11 vs 23.) Frankly, even at 200% or 300% enlargement onscreen, I have a hard time seeing a difference between the NORMAL & FINE quality settings in most shots. The images seem to get blocky at just about the same magnifications. I've printed a FINE image at about 8X10" with my old Epson Stylus Color printer, that to me, looks as good as or better than a typical 8x10" when held at a distance of 12" or more. I cheat a bit and use cold laminate film over the standard high resolution paper instead of photo papers. I'd guess that a NORMAL image printed that size would look nearly as good. Guess I'll have to try it.

For reference, according to the manual the compression modes are:

FINE: 1/4, NORMAL: 1/8, BASIC: 1/16.

Since the file sizes vary a bit from one image to the next, I'd have to say that the ratio's are approximate, but don't vary by a lot rather than being fixed. My guess is that if image quality would suffer the compression ratio is used only as a target size within a certain range.

-- Gerald M. Payne (gmp@francorp.francomm.com), December 19, 1998.