Unscrew A Lens To Focus Closer?

When you focus a lens closer you move it away from the film. The 15mm

VC Heliar focuses down to 0.3 meters, or about 1 foot. Does anybody

know how close it would be focused if you also unscrewed it one

complete 360 degree turn from the camera?

You are sharp. Aren't you, Al?

 

If you are referring to this:http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00FhNr

 

Yes, in principle, it does. You will get to about 15 cm with a few turns. This is the easy part. Try it.

 

As I said, I will disclose more details after a few rolls.

Interesting and nice link Vivek. I think you might standardize the 'turn of the screw' with a washer placed between the body and the lens.

You could use washers of differing thicknesses, of course. DIY extension rings for screwmount.

 

Cheers.

Al, what's the punchline?

I have Cooke-Perkins gadget with a ground glass at one end and a LTM mount on the other with the exact LTM registry. I will measure everything carefully and post it in the near future.

Just how much will that affect the f/number? ;-)

Hi Ben, Thanks and I take Al's post at its face value.

 

Greg, Yes, the effective aperture does go down (as it should) with every turn.

Okay, forgive my ignorance, why does the effective aperture change with every screw turn ?

 

When you use a large-format rail camera to focus you move the front standard further away from the film plane. At infinity, the distance between the lens centre and film plane is the same as the lens focal length. To focus closer than infinity you move the front standard (the lens) away from the film standard. The aperture stays the same doesn't it ? For exposure it does. You only need to worry about bellows effect when you're really pushing the front standard out there (into the realm of 2x focal length if memory serves well).

 

So if you push the 15mm out there a few screw turns it's not going to make a difference ... correct me if I've missed something ...

 

Do you have to adjust aperture when using extension tubes on helical focusing lenses ??

Racking the bellows out to double the focal length decreases the effective aperture by two stops! Same with a macro lens on 35mm. At 1:1 lifesize you've lost 2 stops!

 

There's only about 2.5 full turns possible with the 15 VC, and I'm just thinking one turn so I won't be chasing a lens rolling down the street.

 

Thanks for your help, folks. I just shot some pix with a "guess" of about 6 inches. Any more solid figures will be welcome!

Vivek, so that's how you did it! Brilliant.

Don't they make extension rings for screwmount?

 

Cheers.

Ben, the mount is M39x1. That's one (1) turn per millimeter. So unscrewing the lens exactly one turn extends it one mm more than its mount will allow. I don't think there are any M39x1 1 mm tubes. IIRC, the thinnest one from ELNY is ~ 7 mm. Could be mistaken, though.

Dan is (as always) right on the money! The smallest extension ring made was 7mm.

 

Al, You have used a 47mm Superangulon camera for many many years, haven't you? How many bubble levels did it have? Pay attention to levelling the camera.

 

As I said, unscrewing/putting an extension is the easy part :-)

Perhaps you could have a special thin extension ring machined, would be cool.

 

Cheers.

Dan,---Disabuse yourself. The thread diameter may be 39mm, yes. But the

thread pitch is 26tpi, not a Metric thread.

 

Don't ask why, live with it!

Al,

 

Neat idea. As best as I can figure from a quick calculation is one turn is a close focal distance of about 13cm. One-and-half turns is about 10cm. And two turns is about 8cm.

 

This is a rough guess assuming that the lens mount is M39x1mm (which it is not--it is M39x26t.p.i.--but close enough). The distance is relative. That is measured from from where you get your .3m measurement.

 

If you want a better calculation contact me offline.

Yes, Leica thread has a different <a href="http://www.a1.nl/phomepag/markerink/mounts.htm">pitch</a>.

<p>

Whose <i>idea</i> is neat, Miles? Using extensions with a lens (any lens) is as old as photography itself

Dan wrote: "Ben, the mount is M39x1. That's one (1) turn per millimeter."

 

Jerry Wrote: "Dan,---Disabuse yourself. The thread diameter may be 39mm, yes. But the thread pitch is 26tpi, not a Metric thread."

 

Okay. 26 threads per inch / 25.4 millimeters per inch = 1.0236 threads per millimeter. Sounds pretty close to me.

Ben, if you'll think a little about how the pieces have to nest -- lens rear thread, 39 mm diameter, has to go inside extension tube; tube's rear thread, 39 mm diameter, has to go inside the body's lens mount -- you'll see that an extension tube shorter than the length of the threaded section at the rear of the lens is impossible. What is possible is a flat washer -- a spacer, really -- 1 mm thick between the body flange and the flange at the rear of the lens.

 

In fact, this approach was used to collimate the AGI F135's 38/4.5 Biogons to the camera body. The spacers were sized in steps of 0.01 mm and the lenses' flange-to-film distances at infinity were measured to 0.01 mm. And there they were.

<so I won't be chasing a lens rolling down the street.>

 

Always take Monkey with you when using this setup. As your assistant, he'll make sure the lens won't get away. Smudges and a few bite marks maybe, but it'll be safe as houses, as they say elsewhere in the empire.

Perhaps a better question is how far you can rack it out before you fog the film...

 

Jerry

Jerry, I wouldn't risk more than one full turn without something to seal it for light, black yarn perhaps. This is one situation where a black finish would actually serve a purpose...LOL

 

As for using Monkey as an assistant, I was more thinking of using him as a model rather than a chaser of lenses rolling down the street. He's the reason I want to be able to get in real close with the camera. His head is only about four inches top to chin. Then there's a lady who comes to Starbucks with this cute little fuzzy grey dog who's smaller than Monkey, and I see some interesting picture possibilities there too.

just get some shim stock and cut it up

"Whose idea is neat, Miles? Using extensions with a lens (any lens) is as old as photography itself"

 

Of course it is. I just like Al's idea of exploiting a screwmount as a close focus device.

i think it's Viveks idea. if the berry photo wasn't posted and all...