Olympus 75mm f1.8 at f2.0 aperture is not round?

[axisig]

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Hello! On this photo you can see how looks aperture F2.0 and bokeh.
Аperture does not have a perfect circle F2,8 circle. Do you have the same?

 

[alex g]

Mine isn't perfectly round either.

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As you stop down, it develops a corner!

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If your lens is under warranty, I would send it to Olympus to repair.

 

[Mack]

My 75mm f/1.8 shows maybe a couple of blades influencing on the right side in the photo below. Left one is at f/2, and the other is at f/22 of a shot of the end of an LED at 8 feet, and thrown OoF as far as I could get it.

The weird part is for the non-roundness of the bokeh at f/2 on the right side of my photo (Maybe due to a couple of blades?), it seems the diaphragm's star spike on the right in the f/22 shot does not extend out as far as the other eight spikes do so some blade/s on the right is influencing the star spikes too. I rarely stop down that far to grab star spikes though so it's not an issue for me. Fwiw, the optimum roundness of mine - other than wide open - is about f/6.3.

I won't lose sleep over it though as the lens is pretty old as designs go. I do wish Olympus would make a Model 2 of this lens so it is weatherproof as well as using the AF/Manual clutch ring as on their other Pro lenses. Maybe add an ILS too.

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[cnyap]

I don’t see it would be possible to be perfectly round at every sized circle/aperture...the blades can’t change shape...would need the “exposed” part of the blade to move and be a different diameter. Do other lenses stay perfectly round? Never thought about this before,...

 

[archaeopteryx]

Do other lenses stay perfectly round?

Maintaining roundness is easy with any lens accepting Waterhouse stops, but that wasn't really what you were asking. Considering the 75 f/1.8 uses nine curved blades it could be better behaved than it is but, within my experience, the first half stop or so closing up from a fairly wide aperture is usually the worst for funky shaped bokeh balls.

I'm aware of lenses which use from 13 up to 30 aperture blades to keep the aperture close to circular, though how many blades it takes to be "perfect" is something you'll have to decide for yourself. Most of them aren't ILC lenses and, offhand, I can't think of any with autofocus. Among current lenses it's my impression nine rounded blades is something of a de facto standard.

 

[alex g]

I think there are two seperate issues here, (1) how closely can a regular polygon be made to approximate a perfect circle and (2) how precisely does the iris mechanism of a specific lens create that polygon.

The first point is what I believe @archaeopteryx is refering to — the more blades, the smoother the circle. Many designs also feature curved blades to soften the polygon.

The second point is what I think the OP was concerned about — the fact that on their lens (and mine for that matter) the precision with which the blades are positioned could be better. My copy appears to have one 'lazy' blade that lags behind the others somewhat. I don't know what causes it to be lazy, but I do know that it's the only one of my lenses that exhibits that fault.

 

[axisig]

Thanks for the feedback!

@alex-g, yes! On my Olympus 25/1,8, 45/1,8 and Nikkor 50/1.4(manual), the aperture is round at all values. That's why I was surprised

 

[frankmulder]

For what it's worth: this is how the one I got today looks at f/2.0:

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Not perfectly round either (but seems a bit better than what the OP has). Is this as good as it gets?

 

[Harvey Melvin Richards]

I purchased my 75 used from a site member several years ago. My blades aren't as far off as the OP's, but I also have 2 blades the show this behavior. One is at 12:00 and the other at 5:00. I'm not going to loose any sleep over it though.