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Tip of the Month

June 2014
Using Extension Tubes with a Zoom Lens


Although we teach this Tip during our Digital Complete Nature Photo Course, and the Advanced Course, too, not everyone takes our courses and, it seems, almost everyone else do not understand how extension tubes work with zoom lenses.

I can't tell you how often folks we believe are experienced photographers get frustrated and disqusted when they're trying to use a tube and a zoom lens. The comment we most often hear is 'I can't get this damn thing to focus!' Well, there is a reason for this.

As you probably know, extension tubes 'extend' the minimum focus of a lens. Recently, on our South Texas Wildlife Photo Tour I kept a 25mm extension tube mounted on my 800mm lens so that I could focus on birds, since the blinds and the water holes were so close my subjects were within the minimum focusing distance of my lens. This is pretty straightforward, but that's not the case when a zoom lens is involved.

Since an extension tube lets you focus closer, it stands to reason that the image size will get bigger, right? Right! If you think about it, you probably know that any size extension tube gives you greater magnification with a smaller lens than with a larger, but you might not know why.

I try to explain this by citing the formula:

length of extension/length of lens = magnification

To make the math example easy, lets use a 50mm tube (most extension tubes only go to 25 or 33mm, but using two 25mm tubes together would give you the 50mm to satisfy my example).

Using the formula, 50mm extension/100mm lens = 1/2 life-size
If you've used a 100mm macro lens, you KNOW you must be pretty close to your subject to be at 1/2 life size. Now, consider if you were using a 400mm lens and a 50mm tube:

50mm extension/400mm = 1/8th life-size
Again, if you've used an extension tube with a telephoto, you know that you're still some distance away. You can't be as close as you'd be with the 100mm lens, the lens simply won't focus. So let's consider a 70-200mm zoom lens.

Using the same 50mm tube, at 70mm you'd have
50mm extension/70mm lens = 5/7ths life-size -- that is a big image size, and you'll need to be very close to your subject. Now zoom to 200mm, and you'll have
50mm extension/200mm lens = 1/4th life-size, and that's not as big an image size, right? So you'll be further back.

Knowing this, if you're using an extension tube and you're zoomed to 70mm, you'll need to be close. Zoom to 200, and you'll need to be further back. When you are in focus at 200mm and then zoom to 70mm, you will not be in focus until, or unless, you physically move close enough to be back in focus, for that 5/7ths magnification ratio. At 100m, you'll be a bit further back, but if you zoom to 400mm at that distance (where you are in focus at 100mm), you won't achieve focus until, or unless, you move back.

I've found the easiest way to quickly achieve focus is to zoom to 135mm or so, and then step within the minimum distance of the lens (without an extension tube attached) as indicated on the lens. When my subject pops into focus, if I want a larger image size I'll zoom down towards 70mm but I'll also move closer, too, in order to keep the subject in focus. If I need a greater working distance, or if I want a smaller image size, then I'll zoom out towards 200mm and I'll back off, too, to keep in focus.

Actually, if you simply step beyond the minimum focus and then zoom, one way or the other, the subject will eventually pop into focus. If you're really close, you'll be near 70mm, if you're just within the minimum focus range, you'll be near 200mm. But I find most people are too impatient, put on an extension tube, are too far away for the lens focal length they're set at, and get frustrated. The other thing I've found is even being told this, most people won't take the time to actually try this, and discover for themselves that it works! if you've read this, I hope you will, as using an extension tube with a zoom is a really handy alternative to carrying a macro lens, especially when you're traveling.

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Previous Tips, July 2009 onward


Converting RAW to Black and White
Nature Photography Magazine

Photographing Lightning

Bataflae Photo Backpac
The Guide to Tropical Nature Photography

Essential Gear
for Safaris
Take our Digital Nature Photo Course FIRST!
GPS and Home Security

Range IR Camera Remote

Gitzo Monopod 5561T Monopod

Easy Macro with Extension Tubes and Zoom Lenses

FotoSharp Camou Rain Covers

Canon 17mm T/S Lens
Locking Button for the Canon 7D

NIK HDR Program

Silver Efex Pro for Black and White Images

Beware the DELL Software Solution Rip Off
How and What We Pack for Trips

Canon Digital Learning Center

The Movie Mode with the Canon Mark IV
Batch Renaming in Bridge and CS5
Alternate Uses of some Bogen Products

Hoodman Products

Using High ISO and Live View for Focusing in Dim Light

Art Print Scams for Hungry Photographers

Hungry Vultures ruin vehicles in the Everglades

Use a Short Lens for Depth of Field

Get Professional Help!

Mini-Molar Bag
Access America Trip Insurance
Bogen Base for Macro Work

Archived Tips of the Month
prior to July 2009
Most of my original Tips of the Month for the last several
years are available through this link. Warning - some of the links are broken, so some are not available at this time. Also, the 'look' is from my
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