Different aperture holes in similar lenses... Why?

Cabessius · 05-19-2020, 03:31 PM

I have two CZJ Pancolar 50mm f/1.8 of different generations.
Just noticed that when I set them at the same aperture, the holes left for light gathering are quite different, being bigger at the newer one.

Two pictures, with both set at f/22 (1st pic) and f/8 (2nd pic).

Is there anything wrong or is this normal?

Attached Images

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photoptimist · 05-19-2020, 03:43 PM - **1 Like**

That does not look right.

Two lenses of the same focal length and the same aperture should have the same visual size of aperture when compared side-by-side at a distance. Even if they are different brands or use different optical formulas, the numerical aperture defines the diameter of the light collected by the lens so it should be the same across all lenses of a particular focal length & aperture combination.

You can tell which lens is "right" by laying a ruler across the front of the lens, holding the lens at some distance (perhaps place it on the floor and stand over it), and eyeballing the visual diameter of the aperture. At f/8 with a 50mm lens, the visual aperture should be 50/8 = 6.25mm. At f/22, it should be 50/22 = 2.3 mm.

**BigMackCam** · 05-19-2020, 03:51 PM - **1 Like**

It's possible that one or both of the lenses have been disassembled and incorrectly re-assembled, or that one of them has an oily / sticky mechanism or diaphragm blades that are preventing it from closing down fully.

The upper (older) lens in both photos looks to have a more accurate aperture size for the apertures quoted...

Cabessius · 05-19-2020, 03:55 PM

Originally posted by photoptimist

That does not look right.

Two lenses of the same focal length and the same aperture should have the same visual size of aperture when compared side-by-side at a distance. Even if they are different brands or use different optical formulas, the numerical aperture defines the diameter of the light collected by the lens so it should be the same across all lenses of a particular focal length & aperture combination.

You can tell which lens is "right" by laying a ruler across the front of the lens, holding the lens at some distance (perhaps place it on the floor and stand over it), and eyeballing the visual diameter of the aperture. At f/8 with a 50mm lens, the visual aperture should be 50/8 = 6.25mm. At f/22, it should be 50/22 = 2.3 mm.

Thanks for the great answer.

Just measured it, in the older one the aperture is around 6mm (which is right).

In the newer one (the electric MC) it's almost 10mm.
If I set it to f/16 then it's identical to the other at f/8.

Cabessius · 05-19-2020, 03:59 PM

Originally posted by BigMackCam

It's possible that one or both of the lenses have been disassembled and incorrectly re-assembled, or that one of them has an oily / sticky mechanism or diaphragm blades that are preventing it from closing down fully.

The upper (older) lens in both photos looks to have a more accurate aperture size for the apertures quoted...

I have a few lenses with lazy/sticky blades and the behaviour is different. In this one, the blades open and close really fast.

Not a Number · 05-19-2020, 04:02 PM - **1 Like**

No it doesn't look right.

What kind of meter readings do you get? Easier than fussing around with a ruler

StarTroop · 05-19-2020, 04:09 PM - **1 Like**

Is it possible that the front elements have a magnifying effect on the aperture blades when viewed from the front? I'm not well versed enough to understand if that's really possible, but personally I wouldn't trust the apparent size of something just by eyeballing through curved glass.

Also, the advertised focal lengths and f/stop is rarely ever 100% accurate. Either of the lenses could be on opposite ends of the spectrum of "acceptably close to 50mm", like maybe one is really 48mm and the other is 52mm. Combined with a reasonable margin of error during production and some tolerance of inexact aperture measurements between the two designs, that could account for the apparent difference.
I think the best way to see if either one has an issue is to put them to the test. Take identical pictures with both lenses at each aperture and consistent ISO/SS then directly compare the level of exposure (and depth of field, if that's important to you.) Even though t/stops are the better measurement for exposure, both lenses should still be perceptibly identical at the same aperture.

Also, make sure that both apertures remain fully open when actuated at f/1.8. If one closes down slightly, then that would mean it was probably damaged or improperly assembled.

Cabessius · 05-19-2020, 04:19 PM

Thanks for all the answers, tomorrow I will shoot some photos with them and post them here.

Not a Number · 05-19-2020, 08:37 PM

Originally posted by Cabessius

I have a few lenses with lazy/sticky blades and the behaviour is different. In this one, the blades open and close really fast.

I've got a Takumar lens that behaves the same way - it won't close down all the way but the blades move quickly. I can see it has oil on the blades. The lack of a lens spanner and it's a 50mm f/1.8 lens keep me from taking it apart and cleaning it. Not enough motivation to buy a good spanner just for this lens.

**UncleVanya** · 05-19-2020, 10:13 PM

Originally posted by StarTroop

Is it possible that the front elements have a magnifying effect on the aperture blades when viewed from the front?

Even if that's the case, in photography aperture is defined using the entrance pupil size not the actual opening's physical size. This means how wide it appears from the front is what is used to calculate f values.

Your other point about actual focal lengths and inexact aperture could be a factor but if have to do too much math to determine if it could account for the amount of difference shown here.

Entrance pupil - Wikipedia

Paul the Sunman · 05-20-2020, 12:27 AM

Do they look the same wide open?

Bertrand3000 · 05-20-2020, 02:40 AM

Maybe the lens has been disassembled and now the aperture ring is off. Have you tried to set the aperture past the smallest (22) in the newer lens?

**Alex645** · 05-20-2020, 03:08 AM

Originally posted by Cabessius

I have two CZJ Pancolar 50mm f/1.8 of different generations.
Just noticed that when I set them at the same aperture, the holes left for light gathering are quite different, being bigger at the newer one.
Is there anything wrong or is this normal?

Originally posted by photoptimist

That does not look right.
At f/22, it should be 50/22 = 2.3 mm.

Originally posted by StarTroop

Is it possible that the front elements have a magnifying effect on the aperture blades when viewed from the front?

The aperture blades on the newer 'electric' lens on the left appear incorrect. The older version on the right looks correct around 2.3mm.

The newer lens had 6 elements in 5 groups while the older lens had 6 elements in 4 groups. Yes, the magnifying effect will distort the actual aperture size, but not to this degree because of 5 groups vs. 4 groups.

Pretty sure your tests will reveal about a +2 EV over exposure with the lens on the left with shots in the f/4-f/22 range.

**Just1MoreDave** · 05-20-2020, 03:59 PM - **1 Like**

It's a great reason to have several copies of the same lens.

zapp · 05-21-2020, 05:28 AM

Aperture size and reading do not agree for at least one lens. In this case one lens appears to be off. No two lenses have exactly the same opening that’s why ttl metering and electronic diaphragm control are so cool. Use stop down metering for exposure. For depth of field calculation ... you should find how far off the lenses are either by measuring the physical opening or by comparing or by comparing exposure values to a lens you trust. F-stop is focal length divided by aperture diameter.
Mechanical re-setting of the aperture ring will probably solve the problem.

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05-19-2020, 03:43 PM - 1 Like	#2
photoptimist Pentaxian Join Date: Jul 2016 Photos: Albums Posts: 5,125	That does not look right. Two lenses of the same focal length and the same aperture should have the same visual size of aperture when compared side-by-side at a distance. Even if they are different brands or use different optical formulas, the numerical aperture defines the diameter of the light collected by the lens so it should be the same across all lenses of a particular focal length & aperture combination. You can tell which lens is "right" by laying a ruler across the front of the lens, holding the lens at some distance (perhaps place it on the floor and stand over it), and eyeballing the visual diameter of the aperture. At f/8 with a 50mm lens, the visual aperture should be 50/8 = 6.25mm. At f/22, it should be 50/22 = 2.3 mm.

05-19-2020, 03:51 PM - 1 Like	#3
BigMackCam Digitiser of Film Join Date: Mar 2010 Location: North East of England Posts: 20,674	It's possible that one or both of the lenses have been disassembled and incorrectly re-assembled, or that one of them has an oily / sticky mechanism or diaphragm blades that are preventing it from closing down fully. The upper (older) lens in both photos looks to have a more accurate aperture size for the apertures quoted...

05-19-2020, 03:55 PM	#4
Cabessius Veteran Member Join Date: Jun 2013 Location: Ibiza, Spain Posts: 471 Original Poster	Originally posted by photoptimist That does not look right. Two lenses of the same focal length and the same aperture should have the same visual size of aperture when compared side-by-side at a distance. Even if they are different brands or use different optical formulas, the numerical aperture defines the diameter of the light collected by the lens so it should be the same across all lenses of a particular focal length & aperture combination. You can tell which lens is "right" by laying a ruler across the front of the lens, holding the lens at some distance (perhaps place it on the floor and stand over it), and eyeballing the visual diameter of the aperture. At f/8 with a 50mm lens, the visual aperture should be 50/8 = 6.25mm. At f/22, it should be 50/22 = 2.3 mm. Thanks for the great answer. Just measured it, in the older one the aperture is around 6mm (which is right). In the newer one (the electric MC) it's almost 10mm. If I set it to f/16 then it's identical to the other at f/8.

05-19-2020, 03:59 PM	#5
Cabessius Veteran Member Join Date: Jun 2013 Location: Ibiza, Spain Posts: 471 Original Poster	Originally posted by BigMackCam It's possible that one or both of the lenses have been disassembled and incorrectly re-assembled, or that one of them has an oily / sticky mechanism or diaphragm blades that are preventing it from closing down fully. The upper (older) lens in both photos looks to have a more accurate aperture size for the apertures quoted... I have a few lenses with lazy/sticky blades and the behaviour is different. In this one, the blades open and close really fast.

05-19-2020, 04:02 PM - 1 Like	#6
Not a Number Moderator Join Date: Mar 2012 Location: Venice, CA Posts: 10,526	No it doesn't look right. What kind of meter readings do you get? Easier than fussing around with a ruler

05-19-2020, 04:09 PM - 1 Like	#7
StarTroop Senior Member Join Date: Feb 2018 Posts: 175	Is it possible that the front elements have a magnifying effect on the aperture blades when viewed from the front? I'm not well versed enough to understand if that's really possible, but personally I wouldn't trust the apparent size of something just by eyeballing through curved glass. Also, the advertised focal lengths and f/stop is rarely ever 100% accurate. Either of the lenses could be on opposite ends of the spectrum of "acceptably close to 50mm", like maybe one is really 48mm and the other is 52mm. Combined with a reasonable margin of error during production and some tolerance of inexact aperture measurements between the two designs, that could account for the apparent difference. I think the best way to see if either one has an issue is to put them to the test. Take identical pictures with both lenses at each aperture and consistent ISO/SS then directly compare the level of exposure (and depth of field, if that's important to you.) Even though t/stops are the better measurement for exposure, both lenses should still be perceptibly identical at the same aperture. Also, make sure that both apertures remain fully open when actuated at f/1.8. If one closes down slightly, then that would mean it was probably damaged or improperly assembled.

05-19-2020, 04:19 PM	#8
Cabessius Veteran Member Join Date: Jun 2013 Location: Ibiza, Spain Posts: 471 Original Poster	Thanks for all the answers, tomorrow I will shoot some photos with them and post them here.

05-19-2020, 08:37 PM	#9
Not a Number Moderator Join Date: Mar 2012 Location: Venice, CA Posts: 10,526	Originally posted by Cabessius I have a few lenses with lazy/sticky blades and the behaviour is different. In this one, the blades open and close really fast. I've got a Takumar lens that behaves the same way - it won't close down all the way but the blades move quickly. I can see it has oil on the blades. The lack of a lens spanner and it's a 50mm f/1.8 lens keep me from taking it apart and cleaning it. Not enough motivation to buy a good spanner just for this lens.

05-19-2020, 10:13 PM	#10
UncleVanya Loyal Site Supporter Join Date: Jul 2014 Photos: Gallery \| Albums Posts: 28,404	Originally posted by StarTroop Is it possible that the front elements have a magnifying effect on the aperture blades when viewed from the front? Even if that's the case, in photography aperture is defined using the entrance pupil size not the actual opening's physical size. This means how wide it appears from the front is what is used to calculate f values. Your other point about actual focal lengths and inexact aperture could be a factor but if have to do too much math to determine if it could account for the amount of difference shown here. Entrance pupil - Wikipedia

05-20-2020, 12:27 AM	#11
Paul the Sunman Pentaxian Join Date: Aug 2011 Location: Melbourne Photos: Gallery Posts: 4,843	Do they look the same wide open?

05-20-2020, 02:40 AM	#12
Bertrand3000 Veteran Member Join Date: Feb 2019 Posts: 2,009	Maybe the lens has been disassembled and now the aperture ring is off. Have you tried to set the aperture past the smallest (22) in the newer lens?

05-20-2020, 03:08 AM	#13
Alex645 Site Supporter Join Date: May 2015 Location: Kaneohe, HI Photos: Albums Posts: 3,526	Originally posted by Cabessius I have two CZJ Pancolar 50mm f/1.8 of different generations. Just noticed that when I set them at the same aperture, the holes left for light gathering are quite different, being bigger at the newer one. Is there anything wrong or is this normal? Originally posted by photoptimist That does not look right. At f/22, it should be 50/22 = 2.3 mm. Originally posted by StarTroop Is it possible that the front elements have a magnifying effect on the aperture blades when viewed from the front? The aperture blades on the newer 'electric' lens on the left appear incorrect. The older version on the right looks correct around 2.3mm. The newer lens had 6 elements in 5 groups while the older lens had 6 elements in 4 groups. Yes, the magnifying effect will distort the actual aperture size, but not to this degree because of 5 groups vs. 4 groups. Pretty sure your tests will reveal about a +2 EV over exposure with the lens on the left with shots in the f/4-f/22 range.

05-20-2020, 03:59 PM - 1 Like	#14
Just1MoreDave Site Supporter Join Date: Feb 2007 Location: Aurora, CO Photos: Gallery \| Albums Posts: 6,340	It's a great reason to have several copies of the same lens.

05-21-2020, 05:28 AM	#15
zapp Pentaxian Join Date: Jul 2011 Posts: 2,385	Aperture size and reading do not agree for at least one lens. In this case one lens appears to be off. No two lenses have exactly the same opening that’s why ttl metering and electronic diaphragm control are so cool. Use stop down metering for exposure. For depth of field calculation ... you should find how far off the lenses are either by measuring the physical opening or by comparing or by comparing exposure values to a lens you trust. F-stop is focal length divided by aperture diameter. Mechanical re-setting of the aperture ring will probably solve the problem.