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Archive 2009 · Why the M645 120/4 was designed this way?

Yakim Peled
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p.1 #1 · p.1 #1 · Why the M645 120/4 was designed this way?

The distance from the front element to the front of the lens is 4 cm. This length is constant regardless of the overall length of the lens (which alas changes considerably). I was wondering why it was designed thus. After all, these 4 cm seems to have no function at all and if so, why not design a shorter lens?

Happy shooting,
Yakim.

Jul 22, 2009 at 10:23 AM

mawz
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p.1 #2 · p.1 #2 · Why the M645 120/4 was designed this way?

Room for the focusing helical at minimum extension and something of a built-in hood. You need a fair bit of barrel to contain a helical capable of 120mm of extension (the necessary amount for a 120mm lens to reach 1:1)

This sort of design is common to a lot of macro lenses which use a straight helical for focusing. My Tamron 90/2.5 functions the same way.

Jul 22, 2009 at 10:31 AM

Yakim Peled
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p.1 #3 · p.1 #3 · Why the M645 120/4 was designed this way?

Built in hoods can be designed much better e.g. like the 300/4 IS and 400/5.6. But what exactly is that helical thing?

Happy shooting,
Yakim.

Jul 22, 2009 at 01:35 PM

Daniel Heineck
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p.1 #4 · p.1 #4 · Why the M645 120/4 was designed this way?

The helicoil is what the internal lens element assemblies will ride along as they are moved near and far. Since a macro lens needs a large focus throw in order to reach macro distances (think about adding tubes, something inside the lens has to do the same thing), the helicoid is quite long in comparison to the lens elements.

There isn't a built-in hood like the 300/4 or 400/5.6, but it functions as such, and is a part of the lens. There's no twisting it and sliding out--it's permanent.

Jul 22, 2009 at 02:25 PM

Yakim Peled
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p.1 #5 · p.1 #5 · Why the M645 120/4 was designed this way?

About the helical/helicoil issue I really don't understand. I think this English is a bit too technical for me and to be honest, I'm not sure I'd understand even if you were talking in Hebrew. So, can you post a picture/diagram to illustrate this?

And if it is meant to be a permanent lens hood design then I think it's badly designed.

Happy shooting,
Yakim.

Jul 22, 2009 at 03:05 PM

jcolwell
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p.1 #6 · p.1 #6 · Why the M645 120/4 was designed this way?

The helicoid is the gear system that the lens uses to extend itself when you rotate the focus ring. Imagine that near the bottom of the lens, there are some teeth around the circumference of the lens. The teeth fit into slots on a cylinder that fits inside the lens barrel and is attached to the focus ring on the outside, and to the optical elements on the inside. When you rotate the focus ring, you're sliding the teeth up the slots in this cylinder, and so the lens gets longer. This internal cylinder twists in a "helicoid" pattern, so that the front element does not rotate as you extend the lens. You know what the DNA double-helix looks like, right? Well, that's the same shape that each slot or groove in the cylinder has.

The M645 A 120/4 Macro actually has a "double helicoid" focus mechanism, which you can see by looking at the two "levels" of lens barrel that appear as you turn the focus ring. There are many "do it yourself" repair links that show the process of taking a lens apart, and most of them will show the "helicoid".

At infinity focus, the front element is about 60mm away from the mount at the base of the M645 lens. The M645 "register" is 63.3mm, and so the distance from the front element to the sensor (or film, whatever that is) is about 120mm - no coincidence - that's optics.

At 1:1 focus, the front element is about 150mm from the base. The reason that the front element is recessed so far into the body of the lens is that the internal helicoid cylinders required to move the front element so far away, are so long that at infinity focus, they actually fill the space and length between the outer lens barrel and the inner elements and light path. If you had only one helical focus cylinder (or "twisty gears"), then the body of the lens would be even longer, but the front element still has to be at the same distance from the lens mount at infinity. Many super-zooms (e.g. 24-300) have "triple helicoid" focus mechanisms, which reduces the overall length at infinity focus, but also reduces the smoothness of the focus action, and increases mechanical complexity.

As usual, it's a trade-off. For example, if the A 120/4 Macro had a "quadruple helicoid" (four internal geared cylinders), then it would be considerably shorter and the front element would be at or very near to the end of the lens, but it would have to be much larger diameter fo fit the extra gearing, and it would also be heavier, and require more effort to focus (moving four helicoid cylinders takes more work than moving two of them).

P.S. I don't think that any Mamiya M645 lenses are "badly designed" - there's usually a good reason for something that's not "intuitive".

Jul 22, 2009 at 03:23 PM

sirimiri
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p.1 #7 · p.1 #7 · Why the M645 120/4 was designed this way?

Also serves as an integrated lens hood

Jul 22, 2009 at 05:25 PM

Yakim Peled
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p.1 #8 · p.1 #8 · Why the M645 120/4 was designed this way?

jcolwell wrote:
The helicoid is the gear system that the lens uses to extend itself when you rotate the focus ring. Imagine that near the bottom of the lens, there are some teeth around the circumference of the lens. The teeth fit into slots on a cylinder that fits inside the lens barrel and is attached to the focus ring on the outside, and to the optical elements on the inside. When you rotate the focus ring, you're sliding the teeth up the slots in this cylinder, and so the lens gets longer. This internal cylinder twists in a "helicoid" pattern, so that the front element does not rotate as you extend the lens. You know what the DNA double-helix looks like, right? Well, that's the same shape that each slot or groove in the cylinder has.

The M645 A 120/4 Macro actually has a "double helicoid" focus mechanism, which you can see by looking at the two "levels" of lens barrel that appear as you turn the focus ring. There are many "do it yourself" repair links that show the process of taking a lens apart, and most of them will show the "helicoid".

At infinity focus, the front element is about 60mm away from the mount at the base of the M645 lens. The M645 "register" is 63.3mm, and so the distance from the front element to the sensor (or film, whatever that is) is about 120mm - no coincidence - that's optics.

At 1:1 focus, the front element is about 150mm from the base. The reason that the front element is recessed so far into the body of the lens is that the internal helicoid cylinders required to move the front element so far away, are so long that at infinity focus, they actually fill the space and length between the outer lens barrel and the inner elements and light path. If you had only one helical focus cylinder (or "twisty gears"), then the body of the lens would be even longer, but the front element still has to be at the same distance from the lens mount at infinity. Many super-zooms (e.g. 24-300) have "triple helicoid" focus mechanisms, which reduces the overall length at infinity focus, but also reduces the smoothness of the focus action, and increases mechanical complexity.

As usual, it's a trade-off. For example, if the A 120/4 Macro had a "quadruple helicoid" (four internal geared cylinders), then it would be considerably shorter and the front element would be at or very near to the end of the lens, but it would have to be much larger diameter fo fit the extra gearing, and it would also be heavier, and require more effort to focus (moving four helicoid cylinders takes more work than moving two of them).

P.S. I don't think that any Mamiya M645 lenses are "badly designed" - there's usually a good reason for something that's not "intuitive"....Show more →

Here's a proof that 484 words worth more than one picture.

BTW, considering all the pros and cons I think I'd rather have a quadruple helicoid or even go one further to IF design….

Happy shooting,
Yakim.

Jul 23, 2009 at 01:43 AM

trumpet_guy
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p.1 #9 · p.1 #9 · Why the M645 120/4 was designed this way?

The Leica 60/2.8 Makro-Elmarit has the same kind of design.
I didn't realize it's the helicoid that makes it that way.
Interesting.

Jul 23, 2009 at 02:12 AM

phuang3
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p.1 #10 · p.1 #10 · Why the M645 120/4 was designed this way?

"the distance from the front element to the sensor (or film, whatever that is) is about 120mm "
If I remembered correctly, it's the distance from "optical center" to the sensor defines the focal length, not the front element. By the way, IF macro lenses will reduce its focal length at close up. For a IF 100mm macro, depends on design, it may become 60mm at 1:1 ratio.

Jul 23, 2009 at 05:52 AM

Yakim Peled
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p.1 #11 · p.1 #11 · Why the M645 120/4 was designed this way?

phuang3 wrote:
For a IF 100mm macro, depends on design, it may become 60mm at 1:1 ratio.

75mm IIRC.

Happy shooting,
Yakim.

Jul 23, 2009 at 06:06 AM

Yakim Peled
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p.1 #12 · p.1 #12 · Why the M645 120/4 was designed this way?

Let's talke about the good things. Canada is a place I'd really like to live in. Cold, big and rarely in the news. The exact opposite of Israel.

Happy shooting,
Yakim.

Jul 23, 2009 at 06:54 AM

jcolwell
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p.1 #13 · p.1 #13 · Why the M645 120/4 was designed this way?

phuang3 wrote:
"the distance from the front element to the sensor (or film, whatever that is) is about 120mm "
If I remembered correctly, it's the distance from "optical center" to the sensor defines the focal length, not the front element.

The rear element of the M645 A 120/4 Macro is recessed quite far as well (I'm not at home right now to estimate the distance), so the optical centre isn't all that far away from the front element, at infinity focus.

Too bad this thread is getting out of control, it has lots of good stuff in it.

Jul 23, 2009 at 07:49 AM

jamesdak
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p.1 #14 · p.1 #14 · Why the M645 120/4 was designed this way?

I do have and use the Leica R 60/2.8 macro which is built similar. I'll concur with the others about how well this does serve as a hood. I religiously use a hood with my various lenses but have not seen a need with this design. It was cool to get a detailed explanation of why it's built like it is.

Jul 23, 2009 at 07:56 AM

thrice
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p.1 #15 · p.1 #15 · Why the M645 120/4 was designed this way?

The Leica 100/2.8 APO-Macro-Elmarit-R has a recessed front element in it's standard 1:2 configuration, but with the 1:1 elpro adapter the front element is right at the front. It's a screw in APO magnifier designed specifically for this lens and sits right back in the lens in front of the usual front element. The lens still almost doubles in length at maximum magnification.

Jul 23, 2009 at 08:53 AM

jcolwell
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p.1 #16 · p.1 #16 · Why the M645 120/4 was designed this way?

Daniel, does the bold font that you used for APO have any special significance ?

Jul 23, 2009 at 05:31 PM

Cableaddict
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p.1 #17 · p.1 #17 · Why the M645 120/4 was designed this way?

Another lens kinda' like this is the Zeiss 50/2 makro. Its 1/2" of glass, then about 3" of open plastic.

The front element doesn't move, so I assume all that plastic is solely a built-in hood.
-but why they include an additional, detachable 1" hood is puzzling. - maybe so that if you drop the lens & break the front, you can replace it.

Jul 23, 2009 at 05:40 PM

Yakim Peled
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p.1 #18 · p.1 #18 · Why the M645 120/4 was designed this way?

Long live IF.

Happy shooting,
Yakim.

Jul 24, 2009 at 05:53 AM

panos.v
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p.1 #19 · p.1 #19 · Why the M645 120/4 was designed this way?

IF is generally more useful in AF lenses as it makes them faster to focus. For MF a long helical like in this lens will do for a much much nicer user interface, with a much smoother and longer throw allowing precise focusing.

Jul 24, 2009 at 07:22 AM

Yakim Peled
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p.1 #20 · p.1 #20 · Why the M645 120/4 was designed this way?

What can I do? I don't like the longer throw. It's way too long for me and it becomes a liability instead of an advantage. I also don't like the way the center of gravity changes so even if you are on tripod, the center is not on the head but much further away, making it less stable. If only it had a tripod collar like the MP65…

Happy shooting,
Yakim.

Jul 24, 2009 at 07:34 AM

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