topic Re: Magnification ratio in EF & RF Lenses

Magnification ratio

michal — Thu, 27 Sep 2018 15:51:56 GMT

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.

Re: Magnification ratio

kvbarkley — Thu, 27 Sep 2018 17:38:02 GMT

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.

Re: Magnification ratio

michal — Fri, 28 Sep 2018 08:25:16 GMT

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?

Re: Magnification ratio

kvbarkley — Fri, 28 Sep 2018 12:43:21 GMT

I do with my EFS-60 macro.

Re: Magnification ratio

RobertTheFat — Fri, 28 Sep 2018 13:05:56 GMT

@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?

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.

Re: Magnification ratio

michal — Fri, 28 Sep 2018 13:59:05 GMT

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.

Re: Magnification ratio

Waddizzle — Fri, 28 Sep 2018 14:02:09 GMT

@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?

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.

Re: Magnification ratio

Waddizzle — Fri, 28 Sep 2018 14:04:18 GMT

@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?

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.

Re: Magnification ratio

kvbarkley — Fri, 28 Sep 2018 14:22:27 GMT

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.

Re: Magnification ratio

kvbarkley — Fri, 28 Sep 2018 14:58:58 GMT

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

Re: Magnification ratio

ebiggs1 — Fri, 28 Sep 2018 15:18:21 GMT

"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.

Re: Magnification ratio

Waddizzle — Fri, 28 Sep 2018 18:37:11 GMT

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.

Re: Magnification ratio

RobertTheFat — Sat, 29 Sep 2018 04:09:32 GMT

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

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

Re: Magnification ratio

Waddizzle — Sat, 29 Sep 2018 05:20:50 GMT

@RobertTheFat wrote:
@Waddizzle wrote:
...
It almost sounds like they are saying that a true macro lens effectively has an extension tube inside of it.
That analogy goes only so far, though, because most macro lenses are able to focus at infinity.

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.

Re: Magnification ratio

kvbarkley — Sat, 29 Sep 2018 15:06:12 GMT

@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.

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.

Re: Magnification ratio

ebiggs1 — Sat, 29 Sep 2018 20:58:58 GMT

"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.

Re: Magnification ratio

kvbarkley — Sun, 30 Sep 2018 01:17:34 GMT

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.

Re: Magnification ratio

michal — Sun, 30 Sep 2018 11:22:05 GMT

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 ^^