CUSTOMER: This one’s got optical zoom, and this one has digital. What’s the difference?
EMPLOYEE: There is no difference. They’re the same thing.
I’m going to give this employee the benefit of the doubt and say that he wasn’t doing what I’ve seen, and been on the receiving end of, so often at chain stores: salespeople who’ll tell the customer anything, as long as it results in a purchase at the end of the conversation. No, we’re going to assume that this guy honestly didn’t know there’s a big difference between optical and digital zoom, or what that difference is. After this, you’ll know the difference, and won’t get snookered.
First off, let’s see what optical zoom is. The “optics” in question come from the lens on your camera, whether it’s the fixed lens on a compact, or the interchangeable lenses on an SLR. The movement of the optical elements (a fancy-ish term for the bits of ground glass that make up the lens) is what allows the lens to focus, and to zoom. Doing either of those things just involves finagling the right combination of movements of those different pieces. Some lenses can have a dozen or more elements, usually arranged in groups; how they’re arranged will affect not only your zoom and focus, but also sharpness and focal distance.
Now that we’ve covered optical zoom, how does digital zoom work? Well, you’ve still got lens elements to handle focus, but they’re being used only for that. The zoom function has nothing to do with the lens. Instead, you essentially have a prime lens on your camera, and the camera’s image processor is taking that image (let’s say it’s effectively 50mm) and cropping it in camera, making it look closer, as if it was shot by a longer lens. This is the same thing that a photo editor like Photoshop or GIMP is doing when you crop an image. The quality of the crop will depend on how much you’re cropping, and on the quality of the resizing algorithm the camera is using, which generally won’t be up to par with even a decent dedicated editing program
The difference between the two should be pretty clear at this point, but in case it’s not: The advantage of digital zoom is sometimes a design consideration (it’s difficult, though not impossible, to squeeze optical zoom into a camera phone), and just as often a cost consideration (precision ground glass, coupled with precision engineering, isn’t cheap). The advantage of optical zoom is that despite the added expense, you don’t have to trade optical quality for the added “reach.” If you’ve already got your camera, and it offers only digital zoom,* my advice would be not to use it. Take the picture at the camera’s native resolution and default focal length, transfer it to a computer, and crop it then. You’re still starting with the same initial photo that the camera would be before it “zoomed,” but you’ll be using better software to crop, and should get a better result because of it.
*Or if, like many compacts, it offers optical up to a certain point, and then switches to digital past that