I promised to make sense of using an extension tube and share that information here. So here goes.
I went out to the magnolia tree in my front yard and shot a series of images using different focal lengths with my medium extension tube (21mm). For those who don’t know, an extension tube is simply a tube that goes between your lens and your camera body.
There is no glass in the tube, it’s just empty, but moving the lens away from the body has the effect of magnifying the image and allowing you to get closer to your subject. They are very inexpensive and allow you to get very close to your subject without the expense of macro lenses or magnifying filters.
Here are the images I shot, along with explanations of what I used to shoot them:
First, the baseline shot. It’s from a 24 – 105mm zoom. Shot at 105mm, f8. No extension tube. The shot is reasonably tight, but the background is distracting because there’s so much of it in the shot. A wider aperture would have helped a lot, but I forget to test this. These buds are almost an inch tall.
Now, the same lens, with a 21mm extension tube. First, at 105mm focal length. Here, the magnification is great, and the background is clean and very blurred. I’m probably about six inches to a foot from the subject here. The shot is below.
Now, with the same extension tube, at 24mm focal length. I’m so close to the subject now that the lens hood is touching and shadowing the bud. Depth of field is very shallow, but the background is distracting because there’s more of it in the shot. I’ve used this setup to shoot abstracted extreme closeups inside a flower. The shot is below.
Then I changed to my 70 – 200mm lens, with the same extension tube. Here’s the shot at 70mm. Notice that the background is not blurred entirely, distracting from the shot. Depth of field is shallow, but not so shallow you can’t get the bud in focus. Changing f stops does affect the background, but doesn’t radically increase the depth of field.
Now the shot at 200mm with the same 21mm extension tube. At 200mm focal length I’m about two feet or more from the subject. The shot is below.
Note that there is a little more magnification, but the background is tighter and more blurred, for a cleaner shot. I think this is the best shot of the bunch unless you want to draw in more background for some reason. Also note the extremely shallow depth of field.
Conclusions? At wider focal lengths, you can get extreme magnification and shoot from very close to your subject. Longer focal lengths give only slightly less magnifaction, but you get a tighter background that is very well blurred.
Standing a little farther from the subject is a bit easier, and would be essential if you were shooting an insect, for example. I’ve used up to a 400mm lens on a tripod for this kind of shooting, but you can see that even at 200mm, you can get very tight and get a really clean background.
Depth of field is very critical under every circumstance. I was shooting freehand here, but a tripod and no wind would be ideal. Then you could dial in the focus exactly where you want it. Focus stacking would be good if you need more depth of field.
Wind makes this kind of photography difficult or impossible. The smallest movement of subject or shooter will move composition and focal plane all over the place. Choose a calm early morning for ideal circumstances.