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View Full Version : Magic Lantern lens-Working out the F stop/Focal lengh



sapata
25-May-2010, 16:40
Hi,

I just got hold of a Magic Lantern lens for a very good price and in great condition, the lens has no makers so the chances that it's a Petzval is probably zero.

I'm planning to use on my 4x5 camera. I did some research in order to find the aperture and focal lengh and before I start shooting I just want to make sure that what I did is correct:

I placed the lens near a white wall and let the image focus on it. The distance from the wall to the rear of the lens is 195mm, I then divided this number for 40mm which is the front lens' diameter, the result is F4.8... Am I correct or missing something ?

Is this lens too big for a 4x5 negative ? I'd love to get the full effect on the film sheet...

Thanks all !

goamules
25-May-2010, 18:59
Correct on the approximate aperture. It should cover 4x5 fine. If you mean you want to see the edge aberrations on 4x5, the only way to know is to put it on the camera. Some have more falloff than others. The faster the lens, the less coverage and more chance of swirl, in my experience.

Pete Watkins
25-May-2010, 22:52
Mauricio,
The chances are that it IS a Petzval lens. There are plenty of them around in England if you know where to look. If you're looking for "that Petzval effect" some are better than others and if the price is right (10 - 30) I would always take the chance. Look out for balsam break down on the front pair though.
Best wishes,
Pete.

Steven Tribe
26-May-2010, 02:49
As Pete says the chances of it not being a Petzval are very small. And you seem to have got one with both rear lenses intact ( bigger worry). Triplets were mostly introduced after the magic lantern era was over. You have got the commonest size - where lens opening is 40mm and the brass barrel is probably around 2". Often dimensions are almost standardised so a magic lantern petzval missing its mount fits on another 2" mount. Some have rack and pinion - others have pressure mounts.

Jim Galli
26-May-2010, 08:41
A picture might help to identify. If it's brass it's probably a petzval. Triplets are from the same era that Model T's turned black, and any of them are fairly easy to identify.

Here's how I measure. No black magic at all. Focus on something far enough away to be at infinity focus. At the midpoint between the front group and rear group measure to the ground glass. That's the focal length. Measure the width of the aperture. 40mm is probably a good number. Divide that width into the focal length and you're there. For studio focus just do the same thing over. Focus on an apple. Measure from GG to mid lens. Lets say for illustration you're now at 640mm focus. 40 into 640 is 16, you're at f16. No bellows factors needed.

sapata
26-May-2010, 10:12
Thanks guys... !

goamules... I made a lens panel with some thick cardborad to test before and I could see on the glass of my camera the image distorted on the edges, more or less like an "Holga" camera effect, hopefully will turn out nice !

Pete , this is good news to me since I have been trying to get a Petzval for ages but on ebay the prices are up to 500.00 !!! I luckily bought two very cheap, the other one is slightly bigger.

Steven, I've opened the lens to clean and there were so many inside that I can't even remember !:D One of them has got the rack and pinion the other hasn't.

Jim, here are the pictures...

I'm going to load some 4x5 tonight and get the camera ready for the weekend, in the mean time I might put the roll film back on the camera and try out some 120 film to get used to the exposure times, which is also going to be a bit tricky...

Many thanks for all of your help !

Asher Kelman
30-Jun-2011, 09:48
A picture might help to identify. If it's brass it's probably a petzval. Triplets are from the same era that Model T's turned black, and any of them are fairly easy to identify.

Here's how I measure. No black magic at all. Focus on something far enough away to be at infinity focus. At the midpoint between the front group and rear group measure to the ground glass. That's the focal length. Measure the width of the aperture. 40mm is probably a good number. Divide that width into the focal length and you're there. For studio focus just do the same thing over. Focus on an apple. Measure from GG to mid lens. Lets say for illustration you're now at 640mm focus. 40 into 640 is 16, you're at f16. No bellows factors needed.

Jim,

That's so practical and elegant!

Asher