View Full Version : Removing rear of lens: f/stop unchanged ?

Ken Lee
2-Jan-2012, 08:15
I read that we can remove the rear portion of a (for example) plasmat lens, and get something akin to an old-fashioned portrait lens.

I've tried it with a Sironar, and something happens, that's for sure.

To determine the new focal length of this lens, we can focus at infinity and measure the bellows draw.

How do we determine the new f/stop ?

Example: let's say a 150mm f/5.6 lens becomes a 300mm lens. It is now 2x the length.

Does that mean that instead of being f/5.6, it is effectively f/8 (1 stop less) ?

2-Jan-2012, 08:24
yep. that should get you real close.

2-Jan-2012, 08:25
if the measureed focal length is correct then your method is sound if you have use the front cell mounted in the front of the shutter...if you put it in the back, then there may be a slight difference...but considering the method for measuring focal length, it will be close enough for government work

2-Jan-2012, 09:25
The calculation of "f-stop" is a ratio of focal length to aperture. If the the focal length changes, the "f-stop" value changes also.

2-Jan-2012, 09:30
As cdholden pointed out, that relation would be plain wrong. Twice the focal length at constant aperture diameter is exactly twice the numerical aperture. That is, when going from 150 to 300mm, the aperture would change from f/5.6 to f/11, and when increasing the focal length by sqrt(2), in our example from 150 to 212mm, the aperture would change by one stop.

But the assumption of a doubled focal length is wrong as well. IIRC classical Plasmat halves had 1.75 times the focal length of the whole lens, and the slightly asymmetric modern ones often have sth. like a 1.6/1.85 ratio for the halves.

And you'll have to reckon in that the aperture is now outside the primary plane of the halved lens, so that its projected and physical size aren't identical any more.

Going by my older Symmars where the spec sheets mention a aperture for "converted" use, I'd expect about 1 1/2 to 1 2/3 stops to fit the bill, but official figures for newer post-"convertible" Plasmat types don't seem to exist.

Ken Lee
2-Jan-2012, 10:06
Excellent - Thanks !!!!!

Dan Fromm
2-Jan-2012, 11:03
Ken, if your lens is convertible, check with the manufacturer's specs.

If it wasn't sold as convertible, here are some examples:

Boyer Zircon/Saphir BX (plasmat type), complete lens f/5.6, rear cell f/12; complete lens f/7.7, rear cell f/15.4; convertible Symmars' rear cells also have f/#s 2x the complete lens' f/#

Boyer Saphir (double Gauss type), complete lens f/2.8, rear cell f/5.6

Boyer Beryl (dagor type), complete lens f/6.8, rear cell f/13