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]

You don't know at what focal length the 18-55 has its minimum focus distance. it might be at 18mm.

Something does seem out of whack. The Canon EF-S 60mm macro has an MFD of 0.2 m, you would expect it to be the same as the Tamron.

I tried doing a calculation based on field of view, and got totally messed up. I could find no online calculations for magnification ratio.

I think it is more complicated than the specs allow.

[michal]

Well, I'm pretty sure that MFD of a zoom lens is the same for all of its focal lenghts. At least that's how it works for my kit. Regardless it's 18 or 55 mm, MFD will be always 0.25m, so the largest magnification I'll get at 55 mm.

Taking things practically: will I really get triple larger magnification with macro tamron than with my kit canon?

[kvbarkley]

I do with my EFS-60 macro.

[RobertTheFat]

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@michal wrote:

Well, I'm pretty sure that MFD of a zoom lens is the same for all of its focal lenghts. At least that's how it works for my kit. Regardless it's 18 or 55 mm, MFD will be always 0.25m, so the largest magnification I'll get at 55 mm.

Taking things practically: will I really get triple larger magnification with macro tamron than with my kit canon?

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I don't think lens design is that straightforward. You can design a lens for high magnification and short MFD, but then it becomes more difficult to give it, say, zoom capability. Which, I think, is why the better macro lenses are primes.

[Waddizzle]

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@michal wrote:

Well, I'm pretty sure that MFD of a zoom lens is the same for all of its focal lenghts. At least that's how it works for my kit. Regardless it's 18 or 55 mm, MFD will be always 0.25m, so the largest magnification I'll get at 55 mm.

Taking things practically: will I really get triple larger magnification with macro tamron than with my kit canon?

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I am pretty sure that you are wrong.  The MFD is not necessarily a constant across the zoom range.  Some lenses have a special “macro” setting on their zoom rings, which is usually found just beyond the “long” end of the zoom range.

[ebiggs1]

"Something does seem out of whack. The Canon EF-S 60mm macro has an MFD of 0.2 m, you would expect it to be the same as the Tamron."

Not really.  Let's consider what a lens is. Say a 60mm lens.  It is a piece of convex glass. Depending on its diameter and how it is ground will produce a given FL.  However, when Canon or Tamron make a camera lens, the black tube with corrector lenses inside, these things are altered.

[michal]

Yeah, but still, sorry for my stubborness, what else can determine a lens' reproduction ratio, if not MFD + focal length? Assuming MFD = 0.2 to 0.25 m and focal length = 50 or 60 mm, we should always get similar magnification ratios, don't we?

It's true that focal length changes slightly with focusing distance, but still, slightly. Not like 2-3x, I think.

[Waddizzle]

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@michal wrote:

Yeah, but still, sorry for my stubborness, what else can determine a lens' reproduction ratio, if not MFD + focal length? Assuming MFD = 0.2 to 0.25 m and focal length = 50 or 60 mm, we should always get similar magnification ratios, don't we?

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No.  

There is a 25% difference in the MFD and a 20% difference in the focal length.  Also, the focal plane of a macro lens is flatter than that of a stankard lens.  Assuming that the MFD is constant across the entire zoom range of a given lens is a mistake, too.

[kvbarkley]

It must be more complicated than that, my google-foo did not find any discussion about how to calcuate the magnification ratio of a lens from the specs. The only thing I could find was that Bob Atkins had a windows program calculator for DOF that added the magnification.