How to add aperture to a lantern lens?

[papageno]

Finally I got tired of that boring sharpness and bought a petzval magic lantern lens. It has a fixed aperture inside (almost wide open). In many situations however there is a need to stop it down further. Unscrewing front element and putting some sort of a washer inside (against existing wide aperture ring) does not seem quite practical, especially when shooting people. My friend who is a machinist can make a slot in the main barrel for waterhouse stops and machine a rectangular opening in the outer barrel. The only problem that I see here would be ruining a historical value of that lens (see that nice Darlot Paris sign that would have to be gone).
Of course the slot for waterhouse stops may be made just outside the outer barrel, without making any damage to it, but in such case focus will have to be adjusted by camera's bellows extension. And what seems to be really more important here: aperture will not be placed exactly in the light rays coincidence point. Looks like the slot for waterhouse slots will have to be made about 10mm from that point.
I have some technical education, but not exactly in optical design, so I can not predict the effect of placing the aperture outside of that coincidence point. I have a feeling that it may increase edge fall-off.
I would welcome any comments from folks with some optical design knowledge or experience with lenses modifications.

[CCHarrison]

fabricate some stops out of metal - if you can, or cardboard and find a machinist who will make a slice in the middle of the barrel. Insert stop, shoot.

where you exactly place the stop really wont matter - I doubt you could see the difference it will cause....and it most likely will only impact the distortion the lens will produce.... experiment !

[Ash]

Get yourself a 'wiggly worm' type clip. It's basically bendy metal with a clip either side.

Clip it to the front of the lens, make yourself square aperture stops out of anything you like, and clip them to the other end of the arm.. bend the stop over the front. This will let you use any stop you like, without damaging the lens.

Otherwise build or buy yourself a G-clamp with the same intent, attach the clamp to the barrel, and have an arm or bracket to hold the stops in front of the lens.

[Jim Galli]

roll the lens all the way forward in it's sleeve and make a slot just in front of the sleeve that is there on occasion when needed but disappears inside when the lens is rolled back. How close would that get you to where the factory baffle is?

[Mark Sawyer]

I'd hate to cut up a Darlot. Since Petzvals are now most often used wide open, on the occassions you do want to stop down, I'd just unscrew the front element and put the stop inside, then reassemble and expose.

BTW, are you sure it's a Petzval? Many magic lantern lenses were triplets, but I don't know about the Darlots. Does it have one or two elements up front?

[Joe Smigiel]

One of my favorite lenses is a Darlot Petzval that takes stops with small bent tabs which fit into a recess on the inner barrel. This allows the stops and inner tube to be racked beneath the outer tube.

http://my.net-link.net/BA/D5/jsmigiel/images/technical/collodion/Darlot_lens.jpg

http://my.net-link.net/BA/D5/jsmigiel/images/technical/collodion/Darlot_lens_02.jpg

This leaves the outer tube intact. You only need a small slit and the recess cut in the inner tube as long as the stops incorporate the bent tab.

Joe

[Ash]

I still think retrofitting a bracket on the front will be better than devaluing and possibly damaging the lens.

Think Cokin filter holder, but bigger.... then think square stops with centre-cut holes.


Your machinist friend could still make that for you.

[papageno]

Thanks for help, everybody. Addressing questions posted in replies:

- It looks like a Petzval design. Has two cemented elements in front and two elements in the rear separated with a ring. One is a thin positive lens and the other is thicker meniscus? element. Unfortunately, when cleaning the rear group of lenses I forgot the order in which they were mounted and now I am not sure if the assembly is correct. But I've seen some drawings on the web somewhere, so it shouldn't not be a problem to check it out.

- As far as I remember, the distance from the internal buffle (its purpose is probably to serve as an aperture to limit edge fall-off) to the place where the slot can be made (without damaging outer tube) would be 10mm or slightly more. Since the lens is pretty long (155mm), that should not matter, I hope.

- I know little of optical design, but still can't believe that aperture placed in front of the lens would result in something else than light fall-off (instead of increasing the range of sharpness).

And a general remark: I am under impression, that since switching to large format I have to deal more with technicalities, than with making photographs.

[Mark Sawyer]

And a general remark: I am under impression, that since switching to large format I have to deal more with technicalities, than with making photographs.

Not so much "have to" as "given the choice to". What other format would so happily allow you all the fun, challenges, experiences, and aesthetics of using a hundred-plus-year-old old magic lantern petzval?

[Ash]

Seconding Mark's points.

You're moving into a much older technology that is much less "automatic". You aren't faced with technicalities, you are experiencing choice and full control. Modern cameras and smaller formats do a lot more for you, and as a result you think less about the taking of the picture, more about taking the picture. That said, you have options in large format where you can up and shoot with the speed of a 35mm.

Having chosen a very old lens, you're allowing yourself to plunge into optics and all sorts.