How to determine the length of a needed helicoid

[kds315*]

I have been asked, if there is a way to simply determine the needed length of a helicoid to reach infinity focus.

If you have the register length ("Auflagemass" in german) for a given length, yes, otherwise measurements or an approximation has to be used.

Denote:

RLL = Register Length of a Lens
RLC = Register Length of a camera (Nikon, Canon EOS, 4/3 - you name it)
HLI = Helicoid Length at Infinity
AL = Adaptor Length (usually a few millimeters thickness only)

then:

HLI = RLL - RLC - AL

that also means that HLI cannot be negative and has to be at least some 15mm or so (shortest Helicoid I know of). This also explains why with some lenses infinity focus cannot be reached.

If RLL is not known, the following procedure allows to measure it approximately:

take a white sheet of paper, a ruler and your lens, hold the lens in front of the white paper in a darkened room and point the lens front with fully open aperture to the outside to a bright object (can be the sun also, but be careful not to burn the paper) at least 100x further away then the focal length of that lens until you get a sharp image on that paper. Now measure the distance from that paper to the flange of the lens (end of the thread or mount (could be bajonet etc.) - not just the physical end of the lens!). This is approximately RLL.

If you want to use that lens for macro shooting, say at 1:1 magnification, then point the lens using the same procedure to a bright lamp (desktop tungsten works well) until you see an projected image which has the same size as the original lamp (us the visible lamp filament for instance (stepping down helps if the image is too bright). Keep that position and repeat the measurement at mentioned above). That gives you the approximation for RLL(1x) at magnification 1x.

Now we get

HL(1x) = RLL(1x) - RLC - AL

where HL(1x) denotes the needed maximum extension of your helicoid and

HR = HL(1x) - HLI is the range your helicoid has to cover when turning from infinity to 1x magnification (you can replace 1x with any wished factor of course). If HR is too long, it has to be replaced with a shorter helicoid and a tube of fixed length (say to reach only 0.5x for instance). [Example, if you would need HR = 50mm and the longest helicoid you can find is 25mm, then add a 25mm tube to reach 50mm]

[LucisPictor]

Thanks a lot, Klaus.

[indianadinos]

Thanks Klaus, was just thinking about buying a focusing helicoid ...

[hasan]

Thanks Klaus, great explanation.

I'd like to find a "real" or at least better helicoid for my Leitz Colorplan 2.5/90mm,
(this was my attempt to adapt it for my Pentax K100D: http://forum.mflenses.com/leitz-colorplan-12-5-90mm-t16838.html

I would be interested in this one, as shown here, but don't know where to search?!
http://www.ksmt.com/eos10d/eos_nikki_body22.htm#071212

[indianadinos]

Hi,

[ChrisLilley]

Very useful (if obvious once explained well) and also applies to bellows as well as helicoids.

[motleypixel]

Boy this is quite interesting. Humm...just learning lens construction myself but I'll ask this for starters: When solving for "needed length of helicoid", is this the required length that helicoid must be in order for the optics to work correctly OR simply the required travel of the helicoid for the optics to work? In other words is it a length in which say a lens technician can change or is it a hard manufactured length. Also, how do I calculate RLC or where could I find the value for my Canon 5D Mk II?

I recently tore a Rokkor-PF 58mm f/1.4 down completely...wasn't that bad...removed the rear element mount/aperture ring/focus distance ring and element tube and was able to access the rear side of the aperture for cleaning and then removed all the front elements for cleaning and to clean the front side of the aperture.

Now what I believe to be the helicoid is the AL threaded ring that screws into the inside of the outer main focus ring...I mucked with the factory position of this ring and thus it seems it's now limiting further motion to the rear aka infinity. I also know that there's a main front baffel around the front of the lens with 3 screws that lock the helicoid and I believe this (thanks to web resources) to be the infinity focus adjustment. I need to figure out how far out that AL helicoid piece should be before putting on the distance scale ring and putting the lens back so that I can be close to the full MFD and Infinity Focus of the lens as possible.

[ChrisLilley]

I think this is one of those cases where something disassembled can be re-assembled several ways, and the good advice to you before you started would have been to scratch witness marks so that you could see exactly how they went back together.

I realize that advice is a bit late now.

[RioRico]

Small problem: Calculating the dimensions of a needed helicoid.
Larger problem: FINDING a helicoid of the desired dimensions!

My problem: I use my Schneider Betavaron 50-125/4-5.6 fixed-focus enlarger zoom for both general and macro photography. I can mount it on my M42 bellows which are a minimum of 35mm thick, and easily shoot macros, but cannot reach infinity focus. Or I can mount it on about 30mm of extension tubes to reach infinity, but focusing closer requires an odd dance of moving about and zooming. So I need a helicoid that extends from ~30-32mm out to 60mm or beyond, in order to shoot both macro and non-macro without changing from bellows to tubes.

I have seen M42 helicoids in the US$100 range with throws of ~16-32mm or ~35-70mm. Neither of those would work for me. The short helicoid is too short to be useful; the long helicoid is too long for infinity focus. The obvious solution is to buy TWO short helicoids... but ~US$200 is more than I want to spend on this. I thought to destroy an unwanted M42 lens but 1) I have none I wish to sacrifice and 2) the throw would be too short to be useful.

Can anyone here point to sources of arbitrary helicoids?

[visualopsins]

[Scheimpflug]

[visualopsins]

[RioRico]

[visualopsins]

[RioRico]

[ManuallyYours]

Thanks for these tips. I wonder so many times how to calculate.

[marzipan]

I think the best quick solution is to find two tubes (plastic, cardboard or metal) that slip into each other exactly. The mount is fixed on the end of one tube, the lens block in the other. One can make angled slots in the outer tube, and use screws that fit in the slots to limit the focusing range when you turn the front tube (and so that nothing falls out).
The zooming is actually done in a similar way in normal cheap kit lenses, all plastic and slots and screws. A thought: Put the optical block into a gutted DSLR kit lens?

[16:9]

All tested enlarger, projector and industrial lenses in Delta have the F-FD listed, along with what we've called 'Extension' - the required depth of helicoid (including the adaptor). All Extension figures are quoted at infinity focus.

Because projector lenses never have fixed registration, we sometimes list Extension specified from the rear of the barrel; sometimes from the rear element. Many shorter projector lenses need tube surgery before they can be used.

For an explanation similar to Klaus' above, with a diagram, please see here:
https://deltalenses.com/index.php/2022/02/24/how-to-use-delta/

Note that the correct helicoid for your target lens depends on which camera you're using.

[16:9]

If you have Extension or FFD information about a lens Delta doesn't know, please PM me so we can share it with the community.