Inaccuracy in aperture?

[wiltw]

But does the aperture go to the correct position - or does it depend on the OM camera body lever being in the correct place in order to have the physical aperture match what is set on the ring? Could the problem originate with the adapter mount?
FWIW, when I had a Canon DSLR and used it with an adapter and my OM lenses, I didn't encounter any unexpected exposure inconsistencies.
The same applies to my wife's M43 Olympus OM-D DSLR.
I didn't use the OM lenses often with the cropped sensor Canon DSLRs I had and used for a bit. And I don't often use the OM lenses with the OM-D body now.

The aperture ring selects the aperture, the auto diaphram lever simply tells it to close to the preselected value. If the meter sees -2EV quantity of light comong thru the aperture, it SHOULD ask for +2EV in the shutter setting, but it is simply not acting linearly, it has nothing to do with the aperture closing to something other than the preselected aperture. If, for example, I had asked for -2EV but somehow the aperture only closed to -1.5EV, the meter should have detected -1.5EV instead and given a +1.5EV adjustment to the shutter speed to be centered.
I played with a lens (no adapter) and the aperture-actuated lever simply causes the lens to close to the preselected value and the adapter keeps the aperture-actuated lever at its extreme limit of its travel.

 

[MattKing]

The aperture ring selects the aperture, the auto diaphram lever simply tells it to close to the preselected value. If the meter sees -2EV quantity of light comong thru the aperture, it SHOULD ask for +2EV in the shutter setting, but it is simply not acting linearly, it has nothing to do with the aperture closing to something other than the preselected aperture. I played with a lens (no adapter) and the aperture-closed lever simply causes the lens to close to the preselected value and the adapter keeps the aperture-closed lever at its extreme limit of its travel.

I'm not convinced that my OM lenses stop down the same way when they are on the camera or a correctly functioning adapter as they do when I just manually turn the aperture ring on an unmounted ring - I think I see a difference in how the aperture blades position themselves.

 

[wiltw]

I'm not convinced that my OM lenses stop down the same way when they are on the camera or a correctly functioning adapter as they do when I just manually turn the aperture ring on an unmounted ring - I think I see a difference in how the aperture blades position themselves.

My lens apertures close to the preselected position before the auto-diaphram lever has moved to the limits of its travel, regardless of what the preselected aperture value has been set to. In posting this, I ahad moved the auto-diaphram lever manually multiple times just now, and I can see consistent closing is achieved before the limit of travel has been reached...if it were 1EV off in the size, it is noticeably different but I do not see a visible difference from actuation to actuation..

 

[MattKing]

But do they go to exactly the same position when you:
1) activate them manually when off a correctly functioning OM mount; and
2) activate them manually when on a correctly functioning OM mount?
In other words, does the adapter you are using affect the behavior - possibly incorrectly?

 

[wiltw]

But do they go to exactly the same position when you:
1) activate them manually when off a correctly functioning OM mount; and
2) activate them manually when on a correctly functioning OM mount?
In other words, does the adapter you are using affect the behavior - possibly incorrectly?

The adapter simply has a fixed protrusion which forces the aperture-stopdown lever of the lens and stay there when you mount the adapter. The aperture control ring then functions as if you have an OM mount manual aperture lens. There is no 'affected incorrect behavior'.
So the only 'variable' is the meter response to the light striking the sensor...a non-linear reaction to linear increase/decrease in the light admitted thru the aperture.

 

[Chan Tran]

The adapter simply has a fixed protrusion which forces the aperture-stopdown lever of the lens and stay there when you mount the adapter. The aperture control ring then functions as if you have an OM mount manual aperture lens. There is no 'affected incorrect behavior'.
So the only 'variable' is the meter response to the light striking the sensor...a non-linear reaction to linear increase/decrease in the light admitted thru the aperture.

That's right because on these old system the camera doesn't control the aperture so when it stops down it moves the stop down lever all the way and what f/stop the lens stops down to depend on the setting of the aperture ring. On new camera the camera would move the aperture stop down partly to stop the lens down to where it wants.

 

[koraks]

Guys. OP is using an EF-mount Samyang lens. There's no mechanical aperture linkage between camera and lens. There's not supposed to be one. It's an irrelevant tangent.

 

[wiltw]

Demonstrating a test shot today using a manual-aperture lens in Av automation mode, and the metered exposure result...not 'the same' exposure regardless of aperture, usually resulting a uniform midtone density in all shots...

This shot with Canon 7DII in Av, using a 300mm f/5.5 preset aperture lens (no linkage to any type of body in original mount), this series f/5.5 thru f/32
The first frame (f/5.5) result was closest to the midtone backgrund that Lightroom presents images against, f/32 was close but it bit more dense than midtone background.

 

[reddesert]

ASSUMING that the adapter pushes the lever enough to stop the lens down to whatever the aperture ring is set at, Wilt, what this says is either that your camera doesn't know how to do stop down metering (metering at taking aperture), or that there is a setting for it that needs to be activated.

This also might be related to the OP's problem. Or not. One needs to go into the camera manual to understand how these cameras that expect electronic communication of the aperture behave when a manual-aperture lens is mounted.

 

[wiltw]

ASSUMING that the adapter pushes the lever enough to stop the lens down to whatever the aperture ring is set at, Wilt, what this says is either that your camera doesn't know how to do stop down metering (metering at taking aperture), or that there is a setting for it that needs to be activated.

This also might be related to the OP's problem. Or not. One needs to go into the camera manual to understand how these cameras that expect electronic communication of the aperture behave when a manual-aperture lens is mounted.

You came to the same conclusion that I did!

• There are no instructions by the manufacturer (Canon) about how to do stopped-down metering...the electronically coupled lenses give you a wide-open reading assuming the body-selected aperture, and if you press the DOF Preview button the meter reading does not change. And just maybe Canon knows that manual aperture stopped-down reading does not work properly, which accounts for lack of instruction or comment (to 'save face')
• And using a preset aperture lens (no coupling to body) metering wide open at f/5.5 (with f/11 on the aperture control ring) gives (for example) 1/160 and then using the preset control to close the aperture -2EV results in 1/20 being displayed...a difference of +3EV rather than +2EV in shutter speed!

I have always used a handheld incident meter (rather than the in-camera TTL meter) to determine expsures when I am shooting architectural shots with my Canon and a mounted Olympus 24mm Perspective Control preset aperture lens, because I had never been able to get good exposure readings unless I metered with f/4 on the lens and manually determined shutter speed at the actual shooting aperture.

To the OP I propose that non-linear response of the metering system is what is resulting in bad exposure values with a manual diaphram lens, unless there is a documented method for stopped-down exposure measurement hat had not been written.