Focal length of macro lenses at 1:1

[george013]

Have bookmarked this page on lenses, some pages in english. Read only a part of them but should answer most questions about different lenses from Nikon and how they work. www.pierretoscani.com Happy exploring the pages are awesome.

Interesting articles. Got me directly and indirectly some further.

Strange enough in the example the focal length is increasing with nearest distance. And the specs of the lens used gives a magnification of 0.5 while the article says 1. I don't know what's wrong.

George

[DanK]

George, I think you mean astronomy, not astrology.

The effective f-stop issue is explained well in one of the tutorials on this site: https://www.cambridgeincolour.com/tu...cro-lenses.htm. The general issue of diffraction from small apertures is explained well in another one of the tutorials, https://www.cambridgeincolour.com/tu...hotography.htm.

[george013]

George, I think you mean astronomy, not astrology.

The effective f-stop issue is explained well in one of the tutorials on this site: https://www.cambridgeincolour.com/tu...cro-lenses.htm. The general issue of diffraction from small apertures is explained well in another one of the tutorials, https://www.cambridgeincolour.com/tu...hotography.htm.

I did read them. But you didn't understand what I'm trying to tell.
There doesn't exist a effective focal length or a effective f-stop.

In order for a camera lens to focus progressively closer, the lens apparatus has to move further from the camera's sensor (called "extension"). For low magnifications, the extension is tiny, so the lens is always at the expected distance of roughly one focal length away from the sensor. However, once one approaches 0.25-0.5X or greater magnifications, the lens becomes so far from the sensor that it actually behaves as if it had a longer focal length. At 1:1 magnification, the lens moves all the way out to twice the focal length from the camera's sensor:

This article denies the existence of 1 of the three variables of the lens-formule: the image-distance. It's redefining basic concepts. And then confusion comes like here. TS wants to know whether the focal length changes with macro and then the different ideas of focal length are jumbled up.
Tell me, what is the behaviour of a lens? And how can that be changed?
George

[Inkanyezi]

I have a semi-serious question - are we over-thinking this?

The focal length changes - but how much does it really matter when one is actually doing photography.

I'm sure it makes a difference, but (in the real world) when I do macro-photography, I don't worry about it - in fact I don't even think about it - I just take a picture of the flower, make sure I have enough DOF, that the important element(s) is/are in focus, and the framing is correct. And if I'm stacking, I curse the wind, but what happens to the FL hasn't been my top priority to date.

Maybe I should be concerned?

G

Getting back to Glenn's take on it. It really doesn't matter a lot, but I tried to convey it before, that IF lenses have their own rules. They do not cling to the simple thin lens formula, and while they inner focus, they change the focal length - to a shorter focal length.

However, visually, in the viewfinder and in the image, the angle of view with the inner focusing lens, may also change, although it wouldn't if the lens would adhere to the simple formula. As in practice it does not, the entrance and exit pupils can change as well, when inner components are moved. So the telecentricity of the inner focusing lens may change, and so will the angle of view when it does.

As we see it in the viewfinder, there is nothing to worry about. It's just the particular behaviour of that lens. The reasons for making macro lenses inner focusing were several:

• The focusing movement of a macro lens may be considerably less, and moved groups weigh far less than the entire system, putting less stress on mount and focusing mechanisms, as well ass less load on the motor.
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• Floating elements may improve correction at varying distances.
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• The distance from lens to subject remains unchanged, which facilitates focusing at very close distances, making the AF system more reliable when approaching 1:1 reproduction.
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• The lens barrel may be rigid, making the design more resistant to wear and tear.
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• The far end of the lens with filter thread will not rotate when focusing.

And no, there's nothing to be concerned about. The question emerges from the observation that even though the lens does not extend, its angle of view might change.

"Effective focal length" is something to disregard completely, an oxymoron actually; a lens has a focal length, and it may change when we focus the lens. The focal length printed on the barrel is its focal length at infinity, except for the rare beast that is used only for macro shots at close distances. So it is quite reasonable that an inner focusing 100 mm macro lens will not have a focal length of 50 mm when at 1:1 reproduction, but some other focal length. As stated before, the IF lenses have their own rules.

Effective f-stop however, is a concept that makes sense, when calculating exposure as a symmetric lens is extended for focusing close. When using extension rings or bellows and setting exposure manually for incident light, this factor must be part of the equation. With TTL measurement however, it is not relevant.