Effective aperture with teleconverter + macro lens

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enricosavazzi
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Effective aperture with teleconverter + macro lens

Post by enricosavazzi »

Let's see whether I understand the problem correctly, or I am making a mistake somewhere.

We assume that the pupil ratio is 1 for simplicity, so the effective aperture A' is

A' = A * (m + 1)

where A is the nominal aperture, and m the magnification.

So if we focus a lens at m = 2 by extension alone (no added teleconverter), A' = A * 3, and nominal f/4 is effective f/11 (if we follow the "traditional" full-stop intervals, or f/12 according to the formula).

If instead we mount a 2x teleconverter between camera and lens, and focus the lens at m = 1, the lens' A' = A * 2, and nominal f/4 of the lens is effective f/8. If we then add the 2 stops of effective aperture caused by the teleconverter, at the camera side the effective aperture becomes f/16, and total magnification becomes 2.

So where does the extra increase of one stop of effective aperture with the teleconverter come from, or what did I miss?
--ES

JKT
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Re: Effective aperture with teleconverter + macro lens

Post by JKT »

I suspect that the difference (if there is one - that I didn't check) comes from the different working distance.

rjlittlefield
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Re: Effective aperture with teleconverter + macro lens

Post by rjlittlefield »

enricosavazzi wrote:
Sun Oct 11, 2020 7:28 am
So where does the extra increase of one stop of effective aperture with the teleconverter come from, or what did I miss?
When you reach 2:1 by extension only, the distance from subject to lens will be only FL * (1+1/2) = 1.5 * FL.

But when you reach 2:1 as 1:1 plus teleconverter, the distance from subject to lens will be FL * (1+1) = 2 * FL.

The increased distance when using the teleconverter produces a narrower entrance cone, and the effective F-number is correspondingly different.

f/16 versus f/12 corresponds to the factor of 2 versus 1.5 in distance.

--Rik

Lou Jost
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Re: Effective aperture with teleconverter + macro lens

Post by Lou Jost »

When you reach 2:1 by extension only, the distance from subject to lens will be only FL * (1+1/2) = 1.5 * FL.

But when you reach 2:1 as 1:1 plus teleconverter, the distance from subject to lens will be FL * (1+1) = 2 * FL.

The increased distance when using the teleconverter produces a narrower entrance cone, and the effective F-number is correspondingly different.
I am not sure I understand this. The distance between subject and lens does not change when the teleconverter is added.

rjlittlefield
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Re: Effective aperture with teleconverter + macro lens

Post by rjlittlefield »

Lou Jost wrote:
Sun Oct 11, 2020 2:11 pm
When you reach 2:1 by extension only, the distance from subject to lens will be only FL * (1+1/2) = 1.5 * FL.

But when you reach 2:1 as 1:1 plus teleconverter, the distance from subject to lens will be FL * (1+1) = 2 * FL.

The increased distance when using the teleconverter produces a narrower entrance cone, and the effective F-number is correspondingly different.
I am not sure I understand this. The distance between subject and lens does not change when the teleconverter is added.
Exactly. So that distance remains what it would have been for 1:1 without the teleconverter: 2*FL in front, 2*FL in back. Then the 2X teleconverter turns the magnification into 2X, while leaving the front distance at 2*FL.

Do you get some other value?

--Rik

Lou Jost
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Re: Effective aperture with teleconverter + macro lens

Post by Lou Jost »

Got it. When you said "The increased distance when using the teleconverter..." I read that as an increase when inserting the teleconverter, while you were really comparing the post-teleconverter distance not to the pre-teleconverter 1:1 distance but to the new 2:1 distance achieved by pure extension.

rjlittlefield
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Re: Effective aperture with teleconverter + macro lens

Post by rjlittlefield »

Lou Jost wrote:
Sun Oct 11, 2020 4:35 pm
Got it. When you said "The increased distance when using the teleconverter..." I read that as an increase when inserting the teleconverter, while you were really comparing the post-teleconverter distance not to the pre-teleconverter 1:1 distance but to the new 2:1 distance achieved by pure extension.
Language is tough! If I had it to write over again, I would say "The larger distance..." in hopes of implying just a comparison of two values, not before/after.

--Rik

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