Swapping 150 and 135 Elements - will scales match up close enough?

**Frank Petronio** · 9-Nov-2010, 00:50

If you're buying a used lenses from me don't pay any attention to this post ;-)

If I screw in the front and rear elements of a 150mm/5.6 Symmar-APO into a late model Compur 0 shutter that originally held a 135mm Symmar-S.... are the scales going to be so darn close it won't matter if I change them or not?

I'm suspecting they will be off by about a 1/3 of a stop -- do you know in which direction?

**patrickjames** · 9-Nov-2010, 02:21

Why not just measure it?

**Steve Hamley** · 9-Nov-2010, 04:23

Frank,

Shorter lenses have smaller iris openings for a given f stop. This is easy to see just looking at two different focal length lenses (assuming someone else hasn't done the same thing you did ;^D).

So for a 150 with a 135 scale, open up 1/3 stop and see how that works. In practical matters, you'll be calibrating the shutter too, which could easily be off 1/3 stop depending on what speed you choose and how well the shutter actually works.

Cheers, Steve

**eddie** · 9-Nov-2010, 05:09

for you frank, it will not matter.

you are off a stop or so already!

**IanG** · 9-Nov-2010, 06:27

At full aperture they will be the same. At f45 (135mm scale) the true aperture is f50 and as you open up the difference is much smaller.

In practice I swapped a 135mm pre-WWII Tessar for more modern coated 1950's 150mm Tessar cells in a Compur shutter and there was no noticeable difference with B&W.

Ian

**Frank Petronio** · 9-Nov-2010, 07:50

Hmm Yeah I probably could get away with it for my lens, I wouldn't sell it without mentioning it.

Yeah heck since I scan I shoot at a higher ISO and my negs are thinner.

**Ole Tjugen** · 9-Nov-2010, 08:23

With the setting at f/16, you will be at f/14 or just about that. Real aperture will be a little larger than the setting. Not a big problem.

If it's crucial, just shoot your ISO 100 film at ISO 120.

**IanG** · 9-Nov-2010, 08:29

Wrong way Ole, the longer the FL the smaller the effective aperture because of the Inverse square law.

Ian

**Bob Salomon** · 9-Nov-2010, 08:49

Frank,

Just checked the scales on our 135mm Apo Sironar S and the 150mm Apo Sironar S and the 150mm Apo Sironar N.

As expected, the scales on the two 150mm lenses were the same and the scale for the 135mm S was different. So the short answer to your question is: No. They Won't be accurate.