A lot of digital SLR owners tell me that they rarely change their camera's lens—not because they don't own several, but because they're worried about getting dust or dirt in the camera. That's a reasonable concern: Dirt can collect on the sensor and gunk up your photos. But having interchangeable lenses is the whole point of using a digital SLR, and it's a shame not to get the most from your substantial camera investment. To help ease your concerns about switching lenses—especially outdoors, on the go—let's look at the best technique for swapping lenses.
If you leave your camera's lens port open to the elements for too long, debris can find its way in. And removing dust and other contaminants from a camera isn't a trivial problem, so you should be as careful as possible—without letting a fear of damaging your DSLR prevent you from changing to a more appropriate lens when you need it.
First, prepare for the lens swap. Make sure that both the lens currently on your camera and the lens that you want to switch to have their lens caps firmly affixed to them. Lost the lens cap three years ago while on vacation at the Grand Canyon? You absolutely should replace it. Not only does the cap prevent your lens from getting dirty, smudged, and even scratched, but it lets you stand your lens on its front element in readiness for the swap. Without a lens cap, you can't do that. You can buy a replacement lens cap at any camera shop or online; you just need to know the diameter of the lens (which is marked right on the lens barrel).
Set the lens that you want to attach to the camera on a stable, flat surface, with the front element pointed down—I often set mine of the ground if there's no other surface handy. Don't worry about dirt. Professional photographers subject their equipment to much worse, and their stuff lasts for many years. Loosen the end cap without completely removing it. (With a little practice, you can do this with one hand.) This way, you can pull it off quickly while protecting the rear element in the meantime.
Set the lens that you want to attach to the camera on a stable, flat surface, with the front element pointed down.
Next, make sure that your camera is turned off. This is an important step because it cuts off power to the camera's sensor. When the power is on, the resulting electric charge could attract dust.
Now you're ready to swap lenses. I recommend that you point the camera down. Hold the body with one hand and disengage and remove the lens with the other hand. By keeping the camera pointed downward, you'll prevent anything from falling into the open lens port, and you may also encourage contaminants already in the camera to fall out.
By keeping the camera pointed downward, you prevent anything from falling into the open lens port.
Keeping the camera pointed downward, set the lens down next to the other lens, also on its front element. Quickly remove the end cap from the rear element of the new lens and place it on the rear element of the old lens. Don't spend any time tightening it yet; covering the exposed lens element is enough for now. Then pick up the new lens and attach it to the camera body.
When you've securely affixed the replacement lens, go back to the old lens and screw the end cap onto it. Now you can put that lens in your camera bag and get on with some shooting. That's it!
You don't have to rush through the process like a maniac, but you should make the swap briskly. With practice, you should be able to swap lenses so that the lens port is completely exposed for only a few seconds.
It's inevitable that swapping lenses will lead to gunk landing inside your camera, but that's not the end of the world. Despite what you might have heard, your camera can tolerate a little dust without anything showing up in your photos. Not every speck is going to announce itself like a neon light. Much of the dust that lands in your camera doesn't reach the sensor at all; instead, it lands on the mirror, where you can see it when you preview photos in the viewfinder, but the sensor will never detect it. Dust the mirror with a photo brush occasionally, and you'll be fine. If you do notice some dust collecting and creating a persistent mark in your photos, however, it's time to clean the sensor. You can take the camera to your local shop and have the people there do it for you, or you can do it yourself.