Mamiya 43mm f/4.5 wide-open Aperture Design

[rwreich]

Hello All,

So, I was just looking over this lens and wondering if anyone else has noticed that wide-open at f/4.5 is not quite wide-open.

"wide-open" =

f/5.6 =

f/8 =

f/11 =

f/16 =

f/22 =

As you may be able to see, the apertures do get smaller with every click, but is f/4.5 supposed to be this way, or is my copy acting funny? FWIW, my 80mm f/4 does appear to open all the way.

 

[shutterfinger]

f=Fl/ep. f-f stop/aperture marking; Fl-lens focal length; ep-entrance pupil- the aperture opening diameter as viewed through the front lens cell.
I do not know how to accurately figure the entrance pupil on a non circular aperture. Measuring at the center of the lens will give a rough measurement that will calculate close to the actual f stop.

ep=Fl/f 43mm/4.5=9.5mm.

Be careful not to scratch the lens when measuring ep.

 

[rwreich]

I had not intended to measure the aperture, but that seems like it would be just as well to begin there.

In order to view the aperture, I simply held the shutter open on Bulb mode while adjusting the aperture ring. I was hoping that other owners of this lens would do the same and then offer a visual confirmation of normal/abnormal operation.

 

[EdSawyer]

I can check mine later. This sort of design is not that unusual. The image cone may not require a fully wide open stop in any case.

 

[bernard_L]

Looks like the aperture could be larger, when viewing on-axis (or close to-). But, would that larger aperture still be in effect off-axis; and that is quite a wide-anngle lens. Probably the lens would suffer severe vignetting (or, more severe than at f:4.5). es

 

[rwreich]

I can check mine later. This sort of design is not that unusual. The image cone may not require a fully wide open stop in any case.

I would really appreciate it if you could check, Ed. It'd make me feel a lot more confident about my own copy.

Looks like the aperture could be larger, when viewing on-axis (or close to-). But, would that larger aperture still be in effect off-axis; and that is quite a wide-anngle lens. Probably the lens would suffer severe vignetting (or, more severe than at f:4.5). es

Part of me is wondering if the designers realized that a more open aperture would result in a loss of quality and thus likited it to f/4.5 and not f/4.

 

[John Koehrer]

Don't mess with it yet. A lot of lenses have blades that come into the edges slightly. It's not a mistake, they were made that way.

 

[EdSawyer]

Haven't checked yet, but the size of the image cone in the center of a biogon type lens is tiny. Look at the cross section some time.

 

[itsdoable]

My 43mm lens looks identical to yours at f/4.5.

 

[summicron1]

don't worry about it. It is entirely possible that it was made that way on purpose -- the lens designers probably could have made the lens slightly wider, but kept it at 4.5 to maintain image quality.

I have a very old canon 25mm lens for my leica that looks, wide open, as if it could go an extra full stop, judging by the amount of blade still showing. However, wide open as it is now it still goes really interestingly soft -- you have to stop it down to 5.6 or so to get really great sharpness.

Modern lenses designed by computer, using modern glasses, have really good sharpness wide open. Older lenses did not.

 

[rwreich]

My 43mm lens looks identical to yours at f/4.5.

Perfect, that's just the confirmation I was looking for. Thanks!

 

[rwreich]

don't worry about it. It is entirely possible that it was made that way on purpose -- the lens designers probably could have made the lens slightly wider, but kept it at 4.5 to maintain image quality.

I have a very old canon 25mm lens for my leica that looks, wide open, as if it could go an extra full stop, judging by the amount of blade still showing. However, wide open as it is now it still goes really interestingly soft -- you have to stop it down to 5.6 or so to get really great sharpness.

Modern lenses designed by computer, using modern glasses, have really good sharpness wide open. Older lenses did not.

That's the line of reasoning that I suspected. I wasn't sure, though, and the lens is new to me.