Why is f/2 dimmer than f/2.8 on a Zuiko lens?

[sreed2006]

I have been running exposure and development tests on Efke 50 b&w film, using D-76 1+1 developer.

There were three development times, 4:30 (blue), 9:00 (red), and 18:00 (yellow). Measurements of negative density were taken for each frame, and plotted as Density versus f/stop.

This chart shows an Olympus 28mm F2 lens. When I change the f/stop from f/2.8 to f/2 on the lens, the aperture obviously opens up, so the aperture is working properly. Yet, in the plots, f/2 clearly lets in about the same amount of light, or slightly less light, than f/2.8. That is shown at the right hand side of each line, where it levels out or drops. I expected it to go up.

What causes f/2 to let in less light than f/2.8, even if the aperture really is wider open?


Zuiko 28mm F2 graph by sreed2006, on Flickr

 

[rscheffler]

Were the measurements from the center of the frame? What immediately comes to mind is vignetting, which with a wide angle lens set wide open, can influence areas of the frame quite far from the edge.

 

[sreed2006]

The spots measured were near the center of the negatives. Visually comparing the negatives, by holding them up to the light, I see very little difference between the f/2 and f/2.8, over the entire frame.

The lens was focused at a distance of about 15 inches from the test card.

 

[aizan]

bizarre...

another thing that's odd is the uptick from f4 to f2.8.

 

[sreed2006]

So, maybe f/2.8 is too wide open? I hadn't noticed that before, but if the line should be straight, then f/2 is just a little bit below the line, and f/2.8 is way above the line.

Thanks for noticing that.

 

[dberger]

It doesn't look so much like f2 is dimmer, rather f2.8 seems too bright. If you remove the f2.8 data, you get what looks like a fairly linear response. Also, is the camera auto stop-down actuator working correctly at all stops?

 

[jesse1dog]

Agree with'aizan' - exclude the 2.8 readings and all three graphs look as they should to me. Is the issue at 2.8 rather than 2.0?

 

[sreed2006]

Still, the aperture does get wider open from f/2.8 to f/2. It leaves me baffled why less light gets through the lens at f/2.

 

[MRohlfing]

What does the lightmeter in the camera show when you open up from 2.8 to 2?

I guess the camera is an OM with focal plane shutter set to manual?

 

[sreed2006]

What does the lightmeter in the camera show when you open up from 2.8 to 2?

I guess the camera is an OM with focal plane shutter set to manual?

Just ran these tests:
On an Olympus OM-2n in Manual mode, with the lens at f/2 (wide open) I set the shutter speed to center the needle. Pressing the DOF button on the lens does not move the needle. That is what was expected.

I set the lens for f/2.8, centered the needle by adjusting the shutter speed, pressed the DOF button to stop down the aperture, and the needle does not move but just a tiny fraction in the downward direction. Maybe 1/10 of a stop.

Then, set the lens to f/4, centered the meter needle, pressed the DOF button on the lens, and the needle moves down 2 stops (to the base of the triangle on the negative side of meter display). That is expected.

F/5.6 also causes the needle to dip.

So, it appears the aperture is not closing enough for f/2.8, since f/2.8 doesn't change the meter needle position.

 

[Merlijn53]

What I remember from testreports in the time this lens came out, is that 2.0 is not really 2.0, more like 2.3. But that still does not really explain your graph. Did you use auto exposure?
Frank

 

[sreed2006]

It doesn't look so much like f2 is dimmer, rather f2.8 seems too bright. If you remove the f2.8 data, you get what looks like a fairly linear response. Also, is the camera auto stop-down actuator working correctly at all stops?

The actuator moves way too fast for me to see it complete its travel, but the mirror goes all the way up, and the camera sounds just like it should. I compared the camera I used for the tests above with another OM, and the actuator looks like it is in the same position on both bodies (with the lenses off).

 

[sreed2006]

What I remember from testreports in the time this lens came out, is that 2.0 is not really 2.0, more like 2.3. But that still does not really explain your graph. Did you use auto exposure?
Frank

I used an OM-1n, which does not have automatic exposure. I didn't use the meter on the camera to meter the card, that was done with a spot meter, and checked frequently.

At first I started to do the exposure tests in front of a large north facing window. Then a bunch of clouds came in and made the light change so much and so often that I abandoned that approach.

In a darker room, with the blinds closed, I set the OM-1n on a tripod, the test target on another tripod, and set up a strobe (flash) that runs on A/C power. The camera was set to 1/30 second shutter speed, which didn't change. For each line, then, the only change made was just in the f/stop selected.

 

[sevo]

At what exposure time? It might be a matter of aperture (actuator) bounce...

 

[sreed2006]

At what exposure time? It might be a matter of aperture (actuator) bounce...

All exposures were done at 1/30 second, with light from a strobe.

I do not follow what you mean by actuator bounce. What do I look for?

 

[sevo]

All exposures were done at 1/30 second, with light from a strobe.

I do not follow what you mean by actuator bounce. What do I look for?

Try again with long times and no flash. Apertures and aperture actuators may bounce just like shutters - that is, they hit the stop and bounce back, and a short exposure time or the flash would freeze them in that position.

written on the road

 

[sreed2006]

Try again with long times and no flash. Apertures and aperture actuators may bounce just like shutters - that is, they hit the stop and bounce back, and a short exposure time or the flash would freeze them in that position.

written on the road

Okay, I can do that test. Thanks for the information.

 

[jcb4718]

As you say, it looks like something is amiss at f2.8. Have you got a hand held lightmeter with a flash measuring capability (I have a Gossen Variosix F which measures flash levels)? You could place it at the film plane with the back door open. That way, you are measuring light levels independent of anything except how the camera stops down when you press the shutter. If f2.8 is the highest reading, there's definitely something wrong with how it stops down at f2.8. It's plausible that if the aperture blades were sticking a bit it would be most likely to affect f2.8. Open is open of course -no aperture blade movement. Once the blades start to move they might be OK -so f4 and smaller is OK. That leaves f2.8 where the blades are starting to unstick...

 

[sreed2006]

There are two ways to resolve the issue with this lens.

A. Have it repaired, so that the click stop for f/2.8 sets it at true f/2.8.

B. Leave the lens like it is. To set true f/2.8, set the aperture ring so that the indicator points at the "2" in "2.8" instead of at the ".". The click stop at f/2.8 is incorrect and will give over-exposure 1 stop in automatic mode on an OM-2 or OM-4.

I choose Plan B.

Thank y'all for your assistance and insights.