I have a deep and abiding love for macro photography. On one level, it's a technique that I kind of consider cheating in the back of my mind. It's kind of like black and white in that respect. Take a picture of most things and switch it into high contrast back and white and it makes anything look moody and interesting. Same with macro. Take a picture close enough of anything and you're likely to capture something new and different. Taking my technique to a new level has been an interesting process and my most recent endeavor has involved Extension Tubes. Fun times.
When I started shooting on my 70D, it took me a while before I really got it into my head that getting even a small subject in focus required stopping down to f/8 or smaller. Moving to a 50mm f/1.4 lens from a Tamron 18-200mm f4.5-5.6 was heaven in terms of blowing out the background but I now faced the problem of a prohibitively narrow depth of field.
A couple weeks ago I started experimenting with focus stacking in order to solve the problems associated with narrow depths in macro. It basically involves taking a series of images of a subject and adjusting the focus point with each picture you take. Photoshop then aligns the series of photos, even compensating for "breathing" in the lens as you move from one end of the focus range to the other, and compiles the photos into a single composite using the most in-focus parts found in each image. Making tweaks to the final composite isn't particularly easy but the software is generally reliable under reasonable conditions. You can see a few of the early results here.
Once this technique became a viable tool, the challenge became getting close enough to get truly interesting shots. I'm currently operating with three lenses: a 70-300mm with a minimum focus distance of 1.5m, a 50mm with a minimum focus distance of 0.45m and a 40mm pancake with a minimum focus distance of 0.3m. In lieu of buying a dedicated macro lens, macro extension tubes presented themselves as a possibility for expanding the macro capabilities of my current lenses. These tubes connect between the camera body and the lens and allow the lenses to focus significantly closer to the end of the lens. But when I considered getting a set of these, I had a few questions to which I couldn't seem to find answers online.
My first question was the difference between getting a 13mm, 21mm or 31mm extension (or all three). To answer this question, the image to the right is the same 50mm lens with six different extension tube set-ups all shot at f/2.8. As a second point of reference, the image below shows the magnification of a quarter with a bare 50mm on the left and all three tubes stacked to 65mm on the left. You can see in the bottom image (if you click and expand it full-screen) just how narrow the depth of field is at f/2.8. In both cases, the first shot is at 0.45m and the closest focus is about an inch from the end of the lens.
As much as I wished I could pick one ideal size and not purchase all three together, there really is a great deal of flexibility when it comes to the different levels of magnification allowed by combining the three sizes. I also debated between getting the auto-focus version (which allows communication between the camera and the lens or sticking with manual focus for half the price.
I did find two notes online about the limitations of the manual version. One, autofocus doesn't work and two, you cannot control the aperture and are limited to the widest setting of the lens. This is not strictly true. One thing you can do is put the lens directly on the camera, adjust the aperture to the setting you want, press the DOF preview button and while holding it down, disconnect the lens. Then when you put the lens and the manual tubes on, your lens will be locked at that aperture. In reality, after testing with both photo and video mode, the autofocus through the extension tubes is marginally useful. When adding extension tubes, you drastically cut down your focus range so autofocus can't fix that problem. My own take on the situation is that if you're taking stills, there's really no need for autofocus whatsoever. Aperture is the real issue. If you don'tmind changing aperture the way I describe above or if you have a lens with a manual aperture ring, groovy, save the money. But unless you're shooting on a camera that has video autofocus on par with the 70D (none come to mind) the autofocus isn't something you'll need in reality.
One context where it worked well was using the 13mm tube with the 50mm lens at f/8 for video. With that set-up you're limited to focusing between 6 and 10 inches from the end of the lens so it's a good set-up for shooting insects. It's close enough to get good magnification of things like bees pollinating flowers but not so close that your bumping the bugs with your lens or blocking the light. The video autofocus has sufficient latitude to track your subject in the four inches of focus range at f/8 and it performs pretty well. When you're going for maximum magnification using all three tubes stacked together, forget autofocus. Go with manual and shoot either in Live View or through the viewfinder using your DOF preview button to get an accurate view of what will be in focus.
The final takeaway for me was that the extension tubes are incredibly versatile but it's by no means a magic bullet to replace a dedicated macro lens. You're gonna have to spend hours playing around with them and figuring out exactly how they behave in order to find out just what you can do with them but if your anything like me, you'll love every second of it. My hope is that this post was at least helpful in illuminating the subject a little more.
Now I'm going to post this and go decide if it wouldn't have been a better idea to make a video explaining all this than trying to write it down.