Magnification ratio

[michal]

Hi!
I have a theoretical question.
I always thought that maximum magnification ratio of a lens can be calculated when given (maximum) focal length and minimum focusing distance. In fact things seem different.
A popular canon kit lens EF-S 18-55 mm f/3.5-5.6 IS II has minimum focusing distance equal to 0.25m and magnification ratio equal to 1:2.94 (according to specs). On the other hand, tamron SP AF 60 mm f/2.0 Di II LD (IF) offers 1:1 magnification and its minimum focusing distance is 0.23m. How is it, that tamron has almost triple magnification, compared to canon, while their specs are so similar? I'm not taking aperture into consideration, as it doesn't matter. Both lenses are designed for APS-C, they won't cover FF.

[kvbarkley]

Sometimes it is easier to do the experiment:

EFS-60mm Macro at MFD (Measured at 20 cm)

:

EF-S 18-55 older model at 55mm MFD 23.2 cm (measured)

Same EFS-18-55 at 18mm MFD 22.5 cm measured

Note the big difference in magnification from the EF-S 60 to the EFS 18-55 at 55mm

[Waddizzle]

Here are a couple of interesting articles that may shed some light on the subject.  The first article explains magnification.

https://www.dpreview.com/articles/6519974919/macro-photography-understanding-magnification

The next article explains “how to calculate magnification” in layman’s terms, but the explanation seems to apply more to a telescope than a camera lens.  However, the artiicle does digress into explaining how focal length applies to a camera lens.

https://www.quora.com/What-is-the-relation-between-magnification-and-focal-length

The explanation of a camera lens almost seems to imply that there is a difference in how the focusing element are positioned inside of the lens barrel.  It claims that a macro lens positions the lens elements at a further distance from the sensor than a conventional lens, which is what causes the high magnification.  

It almost sounds like they are saying that a true macro lens effectively has an extension tube inside of it.

[RobertTheFat]

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@Waddizzle wrote:

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It almost sounds like they are saying that a true macro lens effectively has an extension tube inside of it.

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That analogy goes only so far, though, because most macro lenses are able to focus at infinity.

[Waddizzle]

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@RobertTheFat wrote:

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@Waddizzle wrote:

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It almost sounds like they are saying that a true macro lens effectively has an extension tube inside of it.

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That analogy goes only so far, though, because most macro lenses are able to focus at infinity.

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Pure speculation on my part, but the article makes sense.  I am sure the macro lens designers intentionally design the lens to focus out to infinity.  Maybe the focusing elements only become “extended” at the shorter focusing distances.  

Bear in mind how some zooms have a “macro” position on their zoom rings.  When you turned the ring to “macro” something would happen inside of the lens.  You could feel whatever that something was when you turned the zoom ring.  I’m guessing the focusing elements were being moved away from the camera slightly, just like an extension ring would.

So far, the article offers the best explanation why one lens can have a 1:1 magnification ratio, while another lens with a similar focal length may have a magnification of 1:4.

[kvbarkley]

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@Waddizzle wrote:

@RobertTheFat

So far, the article offers the best explanation why one lens can have a 1:1 magnification ratio, while another lens with a similar focal length may have a magnification of 1:4.

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Be sure to include "approximately the same minimum focus distance" in there, too.

One thing to note is that the MFD is measured *at the film plane", I don't have them in front of me, but I should have measured working distance in my little experiment above.

[ebiggs1]

"One thing to note is that the MFD is measured *at the film plane" .."

I think you have missed the point I was trying to make.  Poorly perhaps. A lens is any convex piece of glass.  Agree? A camera 'lens' we are so familiar with is a black plastic tube with several lenses inside that alter it.  The close focus distance really has nothing to do with it.  The lens is altered by the lens' internals. In order to be a true macro lens it should be able to get to 1:1. That's what the internals do for it. Consider the Canon EF 180mm f/3.5L Macro USM Lens.  Same 1:1 but the close focus is farther away.  Something on the order of a foot and a half.

[kvbarkley]

Which is why you use longer focal lenses. It is true that the longer the focal distance, the greater the working distance from the front of the lens.

[michal]

Thanks for the links! I've actually browsed through the first article before but I missed a point which is the clue, I guess:

"for a fixed focal length, magnification is inversely related to subject distance. This relationship isn't linear, i.e. if I get a 1:4 magnification from a shooting distance of 40 cm, I won't necessarily get a magnification of 1:2 (twice that) from a shooting distance of 20cm."

This may answer to my question ^^