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 p.2 #13 · 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".
Yakim Peled wrote:
Here's a proof that 484 words worth more than one picture. 
Really? I dunno...
Helical Focusing Threads
Just a little something I whipped up right now. Probably not the greatest...
The white creamy looking stuff is dampening grease.
Helical just means spiral-like shape BTW.
Helicoid means spiral shaped object. 
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- compromising on stability. 
