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Old Oct 5, 2012, 9:17 AM   #1
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Default Please help....question about extension tubes

I have a Nikon D5100 DSLR. I would like to be able to shoot some 1 to 1 macro shots. I can't afford the Nikon 105mm F/2.8 IF ED AF-S macro lens. The Nikon 40mm F/2.8 AF-S DX macro lens is affordable but I don't like the fact that you have to be one inch from the subject to get a 1 to 1 macro shot....too close. So, I was thinking about getting the very affordable Nikon 35mm F/1.8 AF-S DX lens and using a set of Vivitar extension tubes to make this lens have the ability to shoot 1 to 1 macro. QUESTION: Would this work ? What would the distance from the front of the lens to the subject be ? Would I be better off just buying the Nikon 40mm F/2.8 AF-S DX macro lens ? Please help, what are your opinions and recommendations ? Thanks.
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Old Oct 5, 2012, 9:34 AM   #2
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There's no way to know what magnification ratio you'll get with a particular lens on extention tubes without trying it. But in general, a shorter focal length lens will give you less working distance than you'll get with a longer one.

If you don't want to spend the money on a new Macro lens, Adorama has a Sigma 90mm f/2.8 1:2 macro lens for $199, which isn't much more than a set of good extension tubes will cost you. KEH.com also has a good selection of macro lenses for the Nikon mount.
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Old Oct 5, 2012, 9:41 AM   #3
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Extension tubes won't change the lens to subject distance. All you are really doing with extension tubes is moving the camera sensor further from the lens, allowing the image circle to be larger. Think of it as like an overhead projector (do they still use those in schools?) where the further back from the screen the projector is, the larger the image on the screen.

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Old Oct 5, 2012, 10:37 AM   #4
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Quote:
Originally Posted by VTphotog View Post
Extension tubes won't change the lens to subject distance.
That's not entirely correct. The purpuse of extension tubes is so that you can focus closer. The effect that prevents focusing at infinity is also what permits focusing closer than the lens' minimum focus distance.

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All you are really doing with extension tubes is moving the camera sensor further from the lens, allowing the image circle to be larger.
That's not entirely correct either. Some lenses are more telecentric than others, so the change in the size of a projected image varies from lens to lens. For lenses that are perfectly telecentric, the size of the image circle never changes, however many extension tubes you use.
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Old Oct 5, 2012, 1:57 PM   #5
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I just took a look at Vivitar's extension tubes.

Steer clear.

There are two classes of third-party extension tube in common distribution. One is made by Kenko, is fully automatic (autofocus and autoexposure) and a set of three sells for from $150 to $200. (Individual tubes are also available.) The other is poorly manufactured, they frequently don't fit very well, and often suffer from light leaks. I've seen a set of three selling for from $100 to $10.

The Kenko ones, from the outside, look just like their teleconverters (except that they don't have any optics.) They have a molded plastic wedge-shaped lens release. The other ones have a stamped sheet metal lens release. These are frequently sold under store-brand labels or Vivitar.

If the Kenko tubes are more than you wanted to spend, check the used market. There's no reason to buy a new extension tube over a used one. There's nothing that can get worn out or scratched. Just steer clear of the stamped sheet metal lens releases.
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Old Oct 7, 2012, 7:50 PM   #6
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An extension tube increases lens magnification by an amount equal to the extension distance divided by the lens focal length. For example, adding a 25 mm extension tube to a 50 mm lens will give a magnification gain of 0.5X. Therefore, if the lens's original magnification was 0.15X, then the new magnification will be 0.15X+0.5X=0.65X. The closest focusing distance will also decrease to ~210 mm.

http://www.cambridgeincolour.com/tut...es-closeup.htm
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Old Oct 8, 2012, 5:27 AM   #7
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The Nikon F-Mount has a flange focal distance of 46.5mm.

The Canon EF-Mount has a flange focal distance of 44mm.

The Leica M-Mount has a flange focal distance of 27.8mm.

I think it's highly unlikely that increasing those distances by the same 25mm will result in the same increase in magnification.

It's neat and convenient, but it's not that simple. At the very least, variations in telecentricity will throw off that calculation.
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Old Oct 10, 2012, 8:58 PM   #8
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I would think that the flange focal distance has little effect on the magnification from extension tubes compared to the effect of the tubes on focal length. I concur that some lenses change focal length when focusing that this can cause some uncertainty as to the magnification.
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Old Oct 10, 2012, 9:56 PM   #9
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So it's your position that a lens that has it's flange focal distance almost doubled would have about the same magnification as a lens that only had it increased by a little over 50%?

And I'm presuming that you'd stipulate that both lenses would be perfectly telecentric, otherwise the degree to which they were not telecentric would certainly throw off that calculation of their magnifications.
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Old Oct 11, 2012, 6:12 AM   #10
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I define flange focal distance as being from the flange to the sensor which would be the same for a 200mm or 50mm lens. Add a 50mm extension tube to each and you should get something like 1/4 life size on the 200mm and life size on the 50mm.
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