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08-14-2010, 08:21 AM   #1
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Lense Testing: Aperture reliability/accuracy

I propose the thought that while many people test the optical performance of a lense, including sharpness and out of focus areas, few seem to speak of aperture characteristics.

I wonder if there are any links or articles on the condition of a lenses aperture and testing for this. I feel that the following will influence the final photograph. Most of us know to watch out for misaligned blades and oil however I have come across two similar problems that I would like advice on as I have not been able to find any material on the matter.

The first is that I have found that when Lense A is on Camera A, that if you are in manual mode, you select the widest aperture and then use depth of field preview to stop the lense down, that sometimes while you are looking back through the lense, you can see the blades stopping down slightly, which in my opinion, should not happen. Thus the aperture mechanism may be slightly misalighned in some way?

The other instance is when you make a run of aperture shots of the same continuously lit scene and progress through the aperture range, where either due to the shutter or the aperture reliability, you may recieve different exposures, when in fact you should be taking the same exposure every time? Ie, 1/2 stop increments/decrements via shutter and aperture.

If anyone would like to share their thoughts on this, I would be grateful. Regards

08-14-2010, 09:11 AM   #2
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The appearance of the bokeh is always mentioned in lens reviews, but it's not easy to qualify explicitly. I guess that the most determining factor, apart from lens sharpness, is the number of aperture blades. If you combine a good number of blades with a long focal length (or shoter lens close to minimum focusing distance) and a large aperture, chances are that the bokeh will look very nice.

If you set the aperture using the aperture ring, then the exposure should never vary, unless, of course, your light source isn't constant (in which case a longer shutter speed would result in a brighter image).

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08-14-2010, 09:13 AM   #3
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QuoteOriginally posted by cps_goodbuy Quote
I propose the thought that while many people test the optical performance of a lense, including sharpness and out of focus areas, few seem to speak of aperture characteristics.

I wonder if there are any links or articles on the condition of a lenses aperture and testing for this. I feel that the following will influence the final photograph. Most of us know to watch out for misaligned blades and oil however I have come across two similar problems that I would like advice on as I have not been able to find any material on the matter.

The first is that I have found that when Lense A is on Camera A, that if you are in manual mode, you select the widest aperture and then use depth of field preview to stop the lense down, that sometimes while you are looking back through the lense, you can see the blades stopping down slightly, which in my opinion, should not happen. Thus the aperture mechanism may be slightly misalighned in some way?
I'm more inclined to believe that the aperture blades open all the way to a fixed point. When you press the DOF preview or make the exposure, the aperture is simply closing down to the set aperture. That is, an f2 lens, at rest (not stopped down at all) is open just a little bit wider than f2 by design.

QuoteQuote:
The other instance is when you make a run of aperture shots of the same continuously lit scene and progress through the aperture range, where either due to the shutter or the aperture reliability, you may recieve different exposures, when in fact you should be taking the same exposure every time? Ie, 1/2 stop increments/decrements via shutter and aperture.

If anyone would like to share their thoughts on this, I would be grateful. Regards
If you're suggesting making sure the test scene is metered exactly the same every time, I agree to some extent. Closing down the aperture and adjusting the shutter speed accordingly (slowing it down) may introduce camera shake so a tripod and remote (or 2 second Mirror Lockup) should be used.

08-14-2010, 09:20 AM   #4
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Thanks for the replies.

I am not speaking of bokeh or other lense characteristics, but rather the mechanics of the aperture mechanism and how this relates to the age or possible misalighnment of the aperture assembly.

I have found that due to the fault of the camera or lense that equal exposure settings, using incremental aperture shots do not produce the same exposure, which in theory, they should do. I have also found out that sometimes when the camera stops down the lense at full aperture (i.e the aperture the lense tells the camera) the aperture blades are hit a little, closing the aperture beyond what is considered a wide open shot (blades close down for the shot) which in a real shooting scenario, whould produce non circular highlighted bokeh.

What do you think of this?

08-14-2010, 09:32 AM   #5
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QuoteOriginally posted by JeffJS Quote
I'm more inclined to believe that the aperture blades open all the way to a fixed point. When you press the DOF preview or make the exposure, the aperture is simply closing down to the set aperture. That is, an f2 lens, at rest (not stopped down at all) is open just a little bit wider than f2 by design.
I see what you mean, but if the blades stop down a little bit wide open, then a perfectly round highligh would not be possible. Also, judging by older lense designs that used preset, and from testing a new lense, the blades do not move (stop down) when the camera takes the exposure at wide open aperture.


Change of thought regarding wideopen aperture:

When the camera is instructed to take a photo, I persume that the camera knows the chipped lenses wide open aperture, thus the camera would not stop the lense down at all. But when the camera is instructed to make a depth of field preview, the camera stops it down to the wideopen aperture physically via firmware commands, thus bumping lever and thus the aperture blades.

I will take test photos observing the difference in aperture behaviour between taking normal photos and against preview, as well as changing the custom mode for using the aperture ring as this may change the way in which the camera meters in review mode.

This is all a little hard for me to explain, appologies if it is not clear.

I have found that the (all things being equal) regarding taking the same exposure using different shutter speeds and apertures sizes, that the exposures produced are not equal, thus the following things may be happening:

The aperture assembly is worn, misaligned.
The camera aperture lever is inaccurate.
The cameras shutter curtain is inaccurate.
Or the camera may post process a little bit in camera?!

Meh, will go out and take some real photos


I have also observed that when taking a photo wide open the photo is taken but before the mirror returns for the next shot, the aperture is stopped right down to the smallest aperture. Does anyone know why this is? It seems really unnecessary.

Last edited by cps_goodbuy; 08-14-2010 at 09:41 AM.
08-14-2010, 09:48 AM   #6
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QuoteOriginally posted by cps_goodbuy Quote
I see what you mean, but if the blades stop down a little bit wide open, then a perfectly round highligh would not be possible. Also, judging by older lense designs that used preset, and from testing a new lense, the blades do not move (stop down) when the camera takes the exposure at wide open aperture.
Any Preset lens I've ever owned did not have a camera actuated system to stop down the aperture blades. They have 2 rings. One that acts as the preset Stop and another that actually moves the blades. You meter with the blades in the closed position and you can see your DOF right there. You can then open the blades to make your composition (or whatever reason you would have for opening them). The camera does Nothing to control this action.

I'm not suggesting that ALL presets work that way, only the ones that I have used.

08-14-2010, 09:54 AM   #7
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Sry, I meant lenses that have the auto/manual switch and the aperture pin.

QuoteOriginally posted by JeffJS Quote
Any Preset lens I've ever owned did not have a camera actuated system to stop down the aperture blades. They have 2 rings. One that acts as the preset Stop and another that actually moves the blades. You meter with the blades in the closed position and you can see your DOF right there. You can then open the blades to make your composition (or whatever reason you would have for opening them). The camera does Nothing to control this action.

I'm not suggesting that ALL presets work that way, only the ones that I have used.

08-14-2010, 10:11 AM   #8
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You mean the M42 screw mount type lenses (Takumars, etc).. Even then, they only work on the M setting do they not? That is, there is no mechanism to stop the lens down on a K mount camera.



08-14-2010, 10:15 AM   #9
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QuoteOriginally posted by JeffJS Quote
You mean the M42 screw mount type lenses (Takumars, etc).. Even then, they only work on the M setting do they not? That is, there is no mechanism to stop the lens down on a K mount camera.

Jeff, I think you are missing the point, I am taking about aperture assembly related problems. Doesn't really matter the camera that is stopping it down.
08-14-2010, 11:37 AM   #10
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QuoteOriginally posted by cps_goodbuy Quote
Jeff, I think you are missing the point, I am taking about aperture assembly related problems. Doesn't really matter the camera that is stopping it down.
No, I get your point, I was just clarifying (for myself) what type of lenses you were referring to when you said Preset. That's all.

08-14-2010, 01:16 PM   #11
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QuoteOriginally posted by cps_goodbuy Quote
Thanks for the replies.

I am not speaking of bokeh or other lense characteristics, but rather the mechanics of the aperture mechanism and how this relates to the age or possible misalighnment of the aperture assembly.

I have found that due to the fault of the camera or lense that equal exposure settings, using incremental aperture shots do not produce the same exposure, which in theory, they should do. I have also found out that sometimes when the camera stops down the lense at full aperture (i.e the aperture the lense tells the camera) the aperture blades are hit a little, closing the aperture beyond what is considered a wide open shot (blades close down for the shot) which in a real shooting scenario, whould produce non circular highlighted bokeh.

What do you think of this?
It is entirely possible.

I test each lens using a uniformly lit block wall, on each body, and then check exposure through the aperture range.

I find that some lenses drift upward in exposure as you stop down.

Manual lenses are much worse, because of errors in metering, which really cause problems, but it is still normal in my experience to see some exposure error as a function of F stop. I don't know if this is camera or lens induced since the camera accuracy may also be at issue.

as an example, my sigma 70-200F2.8 using aperture priority exposure has a +/- 10 greyscale drift arounf 110 greyscale, until F32 where all of a sudden it is +30 greyscale. this is about 0.7ev error at smallest aperture, and better than =/-0.35 ev across the rest of the aperture range. My tamron 28-75 F2.8 has a gradual drift in greyscale starting at 110 wide open and drifting up about 40-40 greyscale, i.e. 0.7 to 1.0 ev as I stop down.

every lens will be a little different, but it is normal for even the newest lenses to have some error, that's why I test.
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