CHANGE IN FOCAL LENGTH (MAGNIFICATION) WITH FOCUS DISTANCE

[ian moore]

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I have been using an R8 and R9 for some time. I recently bought a 21-35 mm f/3.5-4.0 Leica zoom. Really pleased with it, but i notice that when I change the focus point the focal length changes. I have read that magnification changes when you adjust the focus distance (relating to prime lens).

 

Is this normal? Anyone else experienced this with this lens.

 

Many thanks.

 

 

Ian Moore

[jip]

This is normal for most 'non 'breathing' corrected' lenses. Zooms do it, primes do it.

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[gisling]

It is lens optics.

 

1)Newton lens formula

 

1/D + 1/d = 1/f

 

in which D is the object distance, d is the image distance( from center of lens), f is focal length of lens.

 

2) Magnification is defined by M =D/d

 

From 1) and 2) we have

 

M = (D-f)/f

 

Here you can see, when object distance changes, so the magnification.

 

For example if D =100* f then M =99

 

D=10*f, M=9

 

D=2f, M=1 (this is magnification formular for macro lens, when object distance = twice of focal lens, magnification is unity.

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[mjh]

Put differently, the magnification and the field of view does not actually depend on the focal length, but on the distance between the lens and the image plane. That is why the lenses in our own eyes can constantly change their focal length in order to focus at different distances while their magnification stays constant – our eyes don’t extend or contract when they focus.

 

The reason we think that magnification depends on focal length is this: If we focus on infinity the distance between lens and image plane equals the focal length, so at infinity the magnification does (incidentally) correspond to the focal length. At closer distances the magnification grows and the field of view shrinks, even when the focal length stays the same. In German there is a word for this phenomenon: “Bildfeldschwund”.

[adan]

A lens when focused closer is moved away from the film or sensor.

 

The effect is identical to moving a slide projector away from the screen or wall - the projected image gets bigger as the optical source moves further away.

 

This is why Leica M fixed framelines are often irritatingly "loose" when used at infinity. They are set for the tightest framing (at close distances) to avoid cutting off people's heads accidentally.

 

Consider it a "plus" for your R8/R9s that you actually get to see this happen - us M users have to guess just how much the magnification changed.

 

The Contax G cameras and some fixed lens rangefinders (e.g. the Fuji MF cameras) are able to adjust the size as well as parallax location of the framelines, to account for what you are seeing.

Edited October 16, 2014 by adan

[pgk]

If you go to the website's wiki and look up Leica's pdf for this lens you will find that it has two systems; focusing is carried out with the first system whilst zooming is carried out by simultaneous movement of both systems, towards each other (35mm) or away from each other (21mm). This means that shifting the first system in order to focus will move it relative to the second system and so will change the focal length. So its inherent within the design - and quite normal;).

[tntrue]

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The R 21-35mm is not a parfocal lens. But with the superb image quality it yields I don't mind refocusing whenever I change the focal length.