Symmar 135/235 Convertible

[mikxer]

Hey Everyone,

Newbie question here...

I recently purchased a Tachihara and the Symmar 135/235 Convertible lens. The question I have is when I convert the lens to the 235 (unscrew front element), I don't seem to have enough bellows extension. Am I doing something wrong?

Thanks.

[Mark Woods]

No. The focal length is the distance from the center of the lens design (more or less) to the focal plane when the lens is focused at infinity. If you want to focus closer, the bellows will need to extend until the subject is in focus.

MW

[Ole Tjugen]

The focal length is the distance from the rear optical node to the focal plane. with converted lenses the rear optical node is somewhere in free air behind the whole lens thingy, necessitating a longer bellows draw than expected. Look at http://www.schneiderkreuznach.com/ar...f/sr_5.6_a.pdf - your 235mm half of the 135mm Symmar needs 285mm flange focal distance at infinity.

The really nice thing with the 135mm Symmar is that it's in a #0 shutter, and it's said to be optimised for 1:3. So if you nees an extreme macro shot at 3x enlargement or thereabouts, you can simply swap the lens cells and have a lens optimised for 3:1!

[Bjorn Nilsson]

To clarify what Mark said. You have to extend the bellows draw to a minimum of 235mm. Now, if I remember correctly, when using the Symmar back cell, you have to extend a little further to get infinity in focus, so start by extending the camera to 250mm or so. I havn't used a Tachi, but it should extend to 300mm or so.
But even though the lens is called "convertible", there are not many who uses the lens in the converted state. The quality isn't much to write home about. Contrast is down the drains, but a contrast filter, e.g. a yellow filter or stronger will help. (Please note that the lens usually is excellent when using both cells.)

//Björn

[Andrew]

Hi'ya...
I've got a Tach and I don't have your lens but I do have a 150/265 and I've got a 210mm
the 210 focuses fine but the convertible at 265 won't quite focus at infinity
I'd guess your lens at 235 would just focus infinity but you probably won't have extension to bring it down to a closer working distance

how close are you trying to focus?

on reflection:
you may be able to get that lens to work at 235 with a "top hat" lens board to get you a couple of extra cm extension
but my understanding of the convertibles is that they aren't very satisfactory at the longer focal length
perhaps just bite the bullet and get a 210mm which should work fine... ?

[mikxer]

Thanks everyone for the replies.

Andrew, to your question, I've only tried to convert the lens and focus at infinity. Which I can barely do, thus suggesting to me that I won't be able to focus at anything much closer than infinity. I think Ole's point about infinity focusing at 285mm is what I am experiencing; and the Tachi only draws out to 300mm.

I'm getting the sense that using this lens at 235mm on my Tachi is not going to work for most practical uses.

[Bill_1856]

Try taking off the REAR element. It won't require as much bellows extension, but it also won't match the aperture (in green) on the shutter.
Also, you'll need to focus carefully AFTER the lens is stopped down for the shot.
It sounds complicated, but it's really a good way to work.
(Excellent lens, by the way.)

[mikxer]

Bill, when I first got the lens (couple weeks ago) I thought the right way to convert was to take the rear lens off. I did this and did get a nice shot. I wasn't sure if this was technically correct or not, but it did work with reasonable contrast but pretty soft.

Thanks everyone for the replies.

[Ole Tjugen]

Converted Symmars are not "low contrast", but they show quite a bit of chromatic aberration. That's the reason for the recommendation to use a yellow filter.

Here is an example, shot with a converted 150/265mm Symmar.

[JW Dewdney]

I wouldn't be surprised if you get something like twice the 'sharpness' if you use a fairly steep (meaning steep cutoff slope in the absorption spectrum) filter... like a red or dark green or something...