Schneider 240/420mm lens focal length

[mark_baylin]

Hi all!

 

Just picked up a rather nice Schneider 240/420 conv. lens and have a question or two...

I've looked elsewhere on photo.net and learned a great deal about this lens. I now realise

that I must remove the front element to be able to use the 420mm focal length and the

alternate F number scale on the barrel of the lens, as well as checking for focus shift.

However, I have tried the lens with only the front element and it does give an image

seemingly somewhere in between the 240mm and 420mm focal lengths.

 

Does anyone know approx what focal length this lens gives when you remove the rear

group AND how would one use the F scale when using the lens in this manner?

 

Thanks very much for any help!

 

Mark Baylin

[bob_gentile]

Pretty sure the answer is "undefined." The lens was designed to be used with either both elements or only the rear element. It was never intended to be used with only the front element.

 

That's <em>not</em> to say you can't do it... but if you do, you're on your own.

[michael_briggs2]

Since the front element by itself forms an image, it has a focal length -- the focal length isn't undefined, it's just that the value hasn't been published by Schneider.

 

Some older lenses were triple convertibles (including an earlier version of the Symmar, mentioned at http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=00E8CR), with three aperture scales, one scale for use with both cells, another for the front cell, and another for the rear cell. Typically the two cells had different powers so this gave three focal lengths.

With the trend to enlarging, the trend in lenses is away from convertibles and to always using the full lens to have the best corrections. The circa 1950s/1960s Symmar only has two of the scales. If you want to use it as a triple convertible, you will either have to guess exposure or create a third scale yourself. F-number is defined as focal length divided by aperture diameter. The aperture diameter is the easiest -- hold a mm ruler over the front of the lens and sight from a distance to measure the diameter of the aperture. In this case the front cell is in place so you are viewing the aperture through the cells to see the entrance pupil. (The entrance pupil might have a different diameter than the aperture because of magnification by the cell.)

 

Then you need to know the focal length. I am not sure of the most practical way to accurately measure the focal length. You can't just focus on infinity and measure the distance to the ground glass because you need to know where to measure to on the other side, i.e., where the principal plane is located. One method would be to focus on infinity, noting the position of the lens, then to refocus so that the image is life size, and measure how far you moved the lens. This would be one focal length.

[mark_baylin]

Thanks for your responses so far!

 

I just took the lens out for a test drive... some interesting things happened...

 

I took the front cell off and mounted the lens on my zone vi 4x5 and extended the bellows

as far as it could go. I then pointed the camera far out over the water at the hoizon. No

matter what i did, I could not get the distant horizon in focus... even with the camera's

bellows racked out to over 19 inches! Correct me if I'm wrong, but shouldn't a 420mm

lens focus at infinity somewhere in the 16-17 inch range?

 

The next thing I did is mount my 210mm lens and focused it at the same distance.

measuring the front and rear standard, it read a little bit past 8 inches... right on. I then

re-mounted the 240/420 lens with both cells attached and focused it at the same

subject... the bellows measured 240mm's... right on the money...

 

I then wated to make a reference of a building on my ground glass with my 210 lens... I

placed and memorized the top and bottom of the building against lined etched on my

ground glass. I then mounted the 240/420 lens WITHOUT the rear cell, and voila, the

building was easily twice as big on the ground glass as it was for the 210 lens...

 

It looks as though I cannot use this lens on my camera with the rear cell only, even at

almost 20 inches of extension. But, I can get the 420mm focal length with the front cell

only, even though the camera is only extended about 12 inches... go figure!

 

I guess the last thing to figure out is whether the 420mm F# scale on the lens will work if I

use the lens with the front element only...

 

Thanks all!

 

Mark

[pico_digoliardi]

<i>Does anyone know approx what focal length this lens gives when you remove the rear group</i><p>

 

Did you try measuring the bellows draw?

[gary_meader]

Hi Mark- I've got this lens and it's one of my favorites. I use it primarily for 5x7. The one

thing you didn't mention, so I don't know if you're aware of it is that when shooting B&W in

the 420 length, it's very important to use a yellow filter over the lens. I use a #15 deep

yellow to correct for softness and low contrast. It helps a bunch.

Otherwise, I don't think the two elements are symmetrical, so the front only configuration

shouldn't be the same as the back only. I don't shoot it with front only, so I don't have any

numbers. Mine tho, is on an 8x10 with a mile of bellows, so I can focus it no problem.

Maybe I'll work out the length and all that . Cheers

[michael_briggs2]

Schneider's intent for converting this lens was to remove the front cells and use the rear cell. So if you instead use the front cell only, the focal length won't be exactly 420 mm, and the aperture scale for the conversion mode won't be accurate for this setup -- the two cells don't have equal powers.

 

Theoretically using the rear cell results in better corrections, but it requires much more extension. As you have found, the advantage of using the front cell is the reduced bellows extension. This is exlained at http://www.photo.net/bboard/q-and-a-fetch-msg?msg_id=009cCz

 

Pico: the problem is where should he measure to? Where is the principal point in the single-cell configuration of this lens, which is highly asymmetrical?

[Troll]

Well, my Technika IV won't focus the rear element alone to infinity either. Best I can tell, using the front element alone gives about the same size image as the rear alone, with a lot less bellows draw. But the green aperture scale no longer applies. (I use a Horseman in-camera meter so it doesn't matter.) The image is quite satisfactory.

[gary_meader]

Mark- I just configured my copy of this lens with the front only and it measures 292mm give

or take. Without having made a neg, I can't attest to sharpness, but it looks good on the GG

at f16 at infinity. But I'd be aware of the corners-as has been mentioned, the function of the

rear cell is to correct one last time before the image hits the film. Hope this means

something to you. Cheers

[ole_tjugen]

The principal point of half a Symmar is a little bit outside the lens - away from the aperture. So to use the 420mm rear half you need enough extension to get the entire lens more than 42cm away from the film plane!

 

The front cell of the 240 Symmar has a focal length of about 500mm - I don't know the exact value. Maximum aperture is about f:14.