Saturday, November 1, 2003

Megapixels and digital storage media


By David Gewirtz

This week, we're going to finish up our discussion of megapixels and your first digital camera with a short discussion of those little memory cards upon which you store your images. But before I do, I want to share a comment I got from Tracey Capen, Executive Editor Reviews at PC World:

I was suprised by the information you gave your readers about image compression. All consumer cameras use JPEG by default. TIFF is typically only found on the more advanced models. TIFF files are so big that you'll only get a few shots on the camera's media (unless you buy are really large capacity card), and most consumers will not be able to tell the difference between a TIFF file and JPEG. I recommend photographers use TIFF only when saving an edited file on their PC -- that way you do not get compression on top of compression.

Obviously, Tracey knows a lot about this stuff. I have seen quite a few digital cameras that support TIFF images, but they are huge by comparison to JPEG. Again, test the results before you buy.

OK, now on to storage media, shown in Figure A.


Here are three different types of memory cards. From left to right: Secure Digital, Compact Flash, and Smart Media. (click for larger image)

In my discussions with folks new to digital cameras, I've found some elements of confusion about storage media and digital cameras. In part, that's because the digital cameras work differently than film cameras.

In a standard film camera, the film itself serves two purposes. Film, of course, is the imaging element itself (i.e., light hitting the film exposes it, creating the picture). Film is also a storage medium, holding typically 12, 24, or 36 pictures per roll.

On a digital camera, the storage medium is typically a memory card. The imaging element is a completely different part of the hardware, something called a CCD (Charged Coupled Device) or CMOS (Complementary Metal Oxide Semiconductor). In this article, we're only going to be talking about the storage medium.

Reusable picture storage

If you've used film before, you know it's pretty well a write-once, read-many medium. Let's assume, for example, you have a 24-exposure roll. After you take your 24 pictures, that roll is used up. You can get it developed, and then you can make a whole bunch of prints from the negatives (assuming it's negative film), but you can't take any more pictures using the same roll.

With a digital camera, that's not the case. To some of you, this might be obvious, but many people I've talked to don't get it on the first try. Memory cards for a digital camera are reusable. After you take your pictures, you can upload your pictures to a computer, erase the memory cards and reuse them.