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extension tubes for macro shots


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#1 Freya Black

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Posted 01 January 2006 - 06:18 PM

I have some c mount extension tubes and I'm wondering how to use them basically. Does the length of the tube increase the macro magnification? Are some focal lengths better for macro work than others?

I notice that with the tiniest macro tube I have I can get lenses to fit my filmo that wouldn't fit before. How far will the foccusing reach with the extra ring (I think it is 5mm)

I'm open to any information on this subject as I'm preety inexperienced with them.

love

Freya
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#2 Charles MacDonald

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Posted 02 January 2006 - 12:36 AM

I have some c mount extension tubes and I'm wondering how to use them basically.


If you look at a non-zoom lens as you focus it, it gets farther from the film the closer you focus. The Extesnion tube just puts it farther still.

Sugestion, Put your filmo on the tripod, stick the smallest tube behinf the normal lens, Rotate the lens over to the "critical focus side" and squint through the little eye piece on the side of the turet. Bring an object like the label of a film box in front of the lens and move it back and forth - close and far - until you can see the writing in the critical focuser. . you should start with the target fairly close to the lens.

Repeat with the next longer tube.

expect to want to play with the whole set. You will want a bright light to shine on your target.
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#3 Robert Edge

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Posted 02 January 2006 - 01:15 AM

Does the length of the tube increase the macro magnification? Are some focal lengths better for macro work than others?


The formula for magnification is Magnification = Total Extension divided by Focal Length.

The focal length determines the distance between the lens and the subject, aka working distance. This can be important if you are using artificial lighting or are photographing subjects that are sensitive to distance, such as insects.

You will need to increase your exposure to take into account the greater distance that the light travels due to the extension.

If you think in terms of a large format camera with a bellows, an extension tube is just a bellows of fixed length. Consequently, a good book on large format photography, such as Leslie Strobell's standard text, will discuss this in detail. There is also a good discussion, specific to extension tubes, in John Shaw's Nature Photography Field Guide.
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#4 Chris Keth

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Posted 02 January 2006 - 02:05 AM

R. Edge, could you explain your magnification equation? I'm not sure I follow. I know from practice that to achieve a 1:1 rendering on film, a bellows extension of double the focal length of the lens must be used and I can't figure out how that jives with your equation, but perhaps I'm thinking backwards. It's easy to get confused when you start talking about this stuff.

You may need to increase your exposure to take into account the greater distance that the light travels due to the extension.

This is pretty important to good macro work.

Exposure compensation like this follows the inverse square law you're probably familiar with from working with lights. The equation:

(extension/focal length)*2=exposure compensation factor

will tell you what you need. For example a 50mm lens with a 100mm extension:

(100mm/50mm)*2=4

The compensation factor is 4 which means you need 4x as much light, or two stops more.

If you end up with a number that is not a perfect square, just use the 'log' or 'ln' function on a scientific calculator. (log FACTOR/log 2)=stops so for a factor of 3:

(log 3/log2)=

(1.0986/0.6931)=1.58 stops or about 1 2/3 stops more light.




EDIT: I forgot to explain one point. When I say "extension" that means the distance from the optical center of the lens to the film plane, not just the length of the tube. For exposure compensation, the optical center of the lens can usually be estimated if it's not known.

Edited by Christopher D. Keth, 02 January 2006 - 02:14 AM.

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#5 Robert Edge

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Posted 02 January 2006 - 08:53 AM

R. Edge, could you explain your magnification equation?


The reference to total extension in the formula is to the tube or tubes. Assume that you are using a 50mm lens. Adding 25mm of extension will give you 1/2, adding 50mm will give you 1:1. This is the same as saying, if one is using a bellows camera, that you need 100mm of bellows for 1:1. The formula and its application and practical consequences are discussed in some detail in John Shaw's book at p. 120.

Regarding my comment about the relationship between focal length and working distance, I should have added that focal length will also affect angle of view.

There are many discussions about bellows compensation at www.largeformatphotography.info and in the large format and nature sections of www.photo.net. Some people like a very precise approach, others are more rough and ready. I'm in the latter camp, and for large format work I tend to use the method espoused by John Cook at http://www.largeform...ows-factor.html. It works just fine. The same web page discusses several other methods for determining compensation.

This stuff about bellows compensation is irrelevant if you are using certain 35mm SLR cameras with through the lens metering.
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#6 Sam Wells

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Posted 02 January 2006 - 10:03 AM

Are some focal lengths better for macro work than others?


I think the other questions are answered here.

Longer focal lengths are usually preferable in that they give you a more comfortable working distance from the subject. (Put an extension on a 10mm in 16mm format and you've got the lens bumping right on the subject. Altho you can get the dust on the front element sharp if you want :)

I have to say this stuff is much easier with a reflex camera. A virtue of the Bolex rex is it's easy to screw on extensions. Although, I've done it with Arri Std. mount just by pulling the lens slightly away from the film plane in the mount. (Be careful, and maybe bring a big soft pillow with you !)

Actually you'd be surprised how many tabletop macro shots on commercials etc have been done by pulling a lens out on a Std mount Arri 2C. I guess that's "Old School" now.

-Sam
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#7 Stephen Williams

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Posted 02 January 2006 - 10:10 AM

Actually you'd be surprised how many tabletop macro shots on commercials etc have been done by pulling a lens out on a Std mount Arri 2C. I guess that's "Old School" now.

-Sam


Hi,

Using the lens backwards is another good trick. Gaffer tape and a soft pillow come in useful too.

Stephen
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#8 Robert Edge

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Posted 02 January 2006 - 02:38 PM

(Put an extension on a 10mm in 16mm format and you've got the lens bumping right on the subject. Altho you can get the dust on the front element sharp if you want :)


I can't even conceive of this :)

I do tight close-ups with a 4x5 camera using lenses of 150mm and above. That is a walk in the park compared to what you are describing. I once watched a friend do an extreme close-up of a flower outdoors with a 35mm SLR mounted on a light field tripod with a 50mm lens. He got his shot, but only after a lot of frustration. He now owns a 200mm macro lens. Managing a camera and tripod with a 10mm lens while trying to focus on a three-dimensional object at a magnification of something like 1/4x, let alone 1:1, is beyond my comprehension. At the very least, I assume that you'd want to be using a really solid and really precise tripod and head.

Makes one wonder what focal lengths and other gear cinematographers of tiny animals like insects use, or for that matter what Stephen uses when he's photographing things like watch faces.
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#9 Stephen Williams

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Posted 02 January 2006 - 04:00 PM

Makes one wonder what focal lengths and other gear cinematographers of tiny animals like insects use, or for that matter what Stephen uses when he's photographing things like watch faces.


Hi,

For close ups of a watch face I used to use a Nikon 105mm 2.5 lens on a Fries Mitchell AF35R. One with upto 8 inches of bellows extention. I was able to get about 1/4 of a watch face full screen. Recently with an 85mm Zeiss Superspeed I could get in a bit closer, past 1:1. Tracking through 1:1 the focus does weird things like change direction!

Stephen
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