Aperture indexing on Zuiko lenses

I was wondering about one interesting question: How exact is the

indexing on Zuiko lenses?

 

It is obvious that a lever on the back of the lens tells the body "how

much will the lens stop down". So wide open all lenses should have

this lever in the same distance from the locking "pin". I compared all

my lenses and there was deviation at most 0.5mm (which could easily be

by measuring error).

 

But then there is something very strange:

comparing 50/1.8 with 135/3.5 - the movement of the indexing lever

between 1.8-2.8 and 3.5-5.6 is supposed to be the same (1.5 stops) but

in case of 50/1.8 the indexing lever moves much less than on 135/3.5.

I also compared the rest of my lenses - they were OK - 28/3.5 was the

same as 135/3.5, and the rest were also OK (35/2, 85/2, 200/4). This

is perhaps the problem of only 50/1.8 (mine is a black nosed MC

variant without "Made in Japan")

 

This is my hypothetical case: I want to shoot 50 @ 2.8 in manual mode,

the lens tells the body that it will stop down by 1.5 stops and the

body meters shutter speed. But what if the indexing lever on 50 @ 2.8

tells the body it will stop down by 1 stop? the body recommended

shutter speed will underexpose my shot if the lens in fact stops down

by 1.5 ! (lever says 1 stop, in fact the lens stops down by 1.5)

 

Or maybe the indexing lever really gives the body correct information,

but then the 2.8 designation on 50/1.8 is incorrect and the real

aperture at that setting is 2.5 or something like that...

 

So I really wonder: are all those Zuiko lenses really precise in

metering? Do they all give exactly the same reading of the same

subject at the same aperture? I am asking especially about the 50/1.8.

The lens is a mechanical device, the camera or the brain does the metering separately. How they are connected is not important only to say that it must be consistent.

It is not a linear scale, you will probably see that the 50/1.8 moves the same amount when

you go from 3.5 to 5.6! Does it actually give bad exposure?

 

I also think 3.5 - 5.6 is actually more like 1 2/3 stop and 1.8 to 2.8 is 1 1/3, but not entirely

sure.

 

I have always found them to be very precise. And this coupling is by far the most clever and

nicest to use of all manual systems.

This is like asking why (today) one 1976 Chevrolet has a top speed of 110 MPH, and another one, same year and model, same engine displacement does 115 MPH. You're testing something mechanical that was made 30 years ago. You could probably test 10 50mm/1.8 lenses of that era and get 5 different readings.

Actually, not quite, George: this was done by design, as can be seen by a look at the aperture rings of the lenses. The 1.8 index is visibly offset compared to the other positions on the 50mm lens.

 

Ondrej, I can't figure out the reason for this any more than you can. But I've been using OM1's since 1978 and have never found a metering accuracy problem in practice, so I think you'll be okay. Have you encountered any problems in use, or are you just taking measurements?

 

rick :)=

my worries were entirely hypothetical :) I was just wondering why the lever moves so litte... but recently I found that the 50/1.8 is actually 50/1.9 according to measurements in an old PopPhoto test of the lens.

 

Fortunately I haven't encountered any pictures ruined because of this - more ruined by the photographer behind the camera :)

even at f/1.9, or even f/2, the short movement seems odd. There is a similar quirk in Nikon's AI system, in which the position of the index tab is slightly different for certain high speed 50mm lenses (can't remember which ones offhand). It may be that something about the light distribution from these particular lenses affects the meter cells differently than others....

 

I suppose we just have to trust that the guys at Nikon and Olympus knew what they were doing......

 

:)=

Just a thought - is the relationship between the iris actuating lever and the iris opening cam linear between 1.8 and 2.0 as compared to 2.0 onwards? I have noticed that the actual iris opening of some of my 50mm f1.8 Zuiko's are not all the same (looking into the lens and comparing by eye.) I have seven, and if I set them all to f16 and move the lever by hand, the actual diameter of the iris opening varies from lens-to-lens. (Hope I'm making sense) The variation is most severe at the smallest openings.