I bought some extension tubes, but am struggling to figure out the best setup

Hardware
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I got a set of extension tubes thanks to some advice I received here a couple weeks ago, but am struggling with the formula to figure how how much extension to add to each lens for workable macro focal distances. I’ve never been much good at math, and I’ve read that having a cropped sensor will/will not affect how the math works. I’ve tried using this online calculator but my results seem to be much closer than it’s telling me. Is this calculator accurate? Does anyone have a good resource to help me understand the math I can use to do this myself? Is my best option just to try each of my lenses on each tube length and try to remember how they do?

Link to the previous thread with my current cameara body and lenses

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I don’t know, but that online calculator seems to give completely wrong results for trivial cases.

When I enter the parameters of my macro lens (and 0mm extension), I should get 1.00 or 1:1, but instead I get:

IMG_20260715_144743
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Which is wrong. Idk if 0.74 + 0.25 being 0.99 (so close to 1:1) is a coincidence or not.

But when I enter 1mm extension tube length, I get:

IMG_20260715_144708
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And that’s noticably different than 0mm and even more wrong.
IMHO, that calculator is really untrustworthy and probably slop, unless I really misunderstood something…

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I was beginning to assume it was slop based on how it “explains” the results further down the page without actually saying anything helpful.

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Yeah, the fact that 0mm extension tube shifts the minimum focus distance back by 3cm is just blatantly wrong. Try this one:

I’ve tried the same basic lens parameters and got 1.00x for the case without extension tubes.

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I like this calculator, mainly because of the image preview:

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I’ve found the near reproduction ratio to be wrong though – 1:2 instead of 1:1 for the same setup without an extension tube. Or specifically, focal length = 105mm, near focus limit = 312mm, extension tube length = 0mm

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Online calculators probably make two assumptions: (1) the lens has only one element that is thin, and (2) focusing is done by moving the entire lens away from the camera. This keeps the maths fairly simple (1/f = 1/u + 1/v, etc).

Assumption (1) is always false. Assumption (2) is probably false.

I suggest you experiment with your camera and lens. With and without tubes, at maximum and minimum focus, point the camera at a ruler. The ruler should be parallel to the sensor plane. You don’t need to actually take photos. How many millimetres of the ruler can you see?

For example, using an old Micro-Nikkor 105mm on my FF camera, I can see 70mm on the ruler, so about 1:2, half life-size. With 180mm extension (bellows and tubes), with the lens at infinity, I can see about 15mm of the ruler. With the lens at closest focus, I can see about 10mm of the ruler, so more than 3:1.

If you keep notes, you can write a handy cheat-sheet, which you can keep with your tubes.

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Photos or it didn’t happen. Ruler with millimeter scale.

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Flower of a Moroccan Mint plant:
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The black framing is due to some component of the extension tubes and bellows. Maybe rearranging them would be better.

I used an old lens because I can’t control the aperture of my newer macro 105mm through the tubes and bellows.

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The link in the OP now says:

This calculator is currently under content review.
The tool is temporarily unavailable while its formula, safety information, and recommendations are being verified.

Ha!

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I’m honestly surprised how it managed to get results wrong in that way. Maybe it quickly queried an LLM each time? :stuck_out_tongue:

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