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04-23-2022, 09:41 AM   #1
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Samyang 135mm aperture change problem in live view, manual mode

Recently I purchased a new Samyang 135mm F2.0 lens for my K-1 camera. I like the lens quite a lot. But there seems to be a problem when using it in Live View, in Manual Mode, which is the way I use lenses when mounted on a tripod (and sometimes even when not mounted on a tripod to allow shooting at waist-level).

Here's what's happening:

Step 1. Start with the lens wide open at F2.0. Set the ISO to whatever is appropriate (the problem is independent of the ISO). Set the exposure time (shutter speed) as appropriate. For example, just now I set the aperture to F2.0, the ISO to 800, and the exposure time to 1/30 sec. Note the brightness of the live view image.

Step 2a. Stop down to F2.0. Now the image is 1/2 stop darker.
Step 2b. Increase the exposure time to 1/15 sec to restore the original live view image brightness.

Step 3a. Stop down to F3.5. Now the image is again 1/2 stop darker, just like it looked in Step 2a.
Step 3b. Change the exposure time to 1/8 second to restore the original live view image brightness.

Step 4a. Stop down to F4.0. Now the image looks to be perhaps 2-3 stops darker than it should.
Step 5a. Don't change the exposure time. Instead stop down to F5.6. Now the image is actually brighter than it was in step 4a.

Sometimes I can stop down to F5.6 before the live view goes "too dark". Sometimes it takes going down to F8. Once it starts happening, it seems to happen about every increase of the F-stop number.

Taking a photograph does produce the right exposure for the "aperture/exposure time/ISO" settings.

Pulling the lever to stop down the image immediately restores the proper live view image brightness, and it stays restored when the lever is released.

So far this "too dark live view image brightness" doesn't seem to happen when making the aperture wider (eg going from F11 to F9.5 and so on). It doesn't happen in aperture priority mode.

Being on a tripod, or handheld and supporting the lens itself, either way the same problem happens.

Anyone else have or heard of this issue? Any idea what's causing it? It seems to be getting worse and I think probably I should return the lens while I still can.

04-23-2022, 09:59 AM   #2
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With my Samyang 135mm I made similar strange observations K-1 and Samyang/Rokinon 135mm f2 - PentaxForums.com . I returned it at the time and received another copy, the problems were gone then.
04-23-2022, 10:51 AM   #3
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Original Poster
QuoteOriginally posted by TSC992 Quote
With my Samyang 135mm I made similar strange observations K-1 and Samyang/Rokinon 135mm f2 - PentaxForums.com . I returned it at the time and received another copy, the problems were gone then.
Yes, I think returning for an exchange is the best course. Mine isn't exhibiting the exact same problems as yours was as it seems only to happen after stopping down a bit, and the resulting images are actually fine. But the behavior totally disrupts trying to shoot from a tripod. Sometimes it goes away, then comes back again. And just now pulling the stop-down lever actually made the image darker instead of restoring the proper brightness, though the resulting image was properly exposed.

I put the only other "A" lens that I have available at the moment - an SMC 24-50mm F4.0 zoom - on the K-1 and tried to elicit a similar problem, and it showed no problems at all.

On a possibly irrelevant note, the clicks and clacks made by the Samyang when changing the aperture in A setting (using the camera dial rather than the lens aperture ring), are considerably louder than the discrete clicks and clacks when changing the aperture on the 24-50mm "A" zoom lens. Maybe that's normal given the respective lens sizes, the old zoom being considerably smaller?

Maybe this might be relevant: My Samyang 135mm lens seems to work just fine when changing the aperture using the lens aperture ring (not in the "A" position). But that makes it an "M" lens, which doesn't allow a proper preview of the image when in live view manual mode.
04-24-2022, 05:44 PM   #4
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Hi eles,

I tested my copy of the Samyang 135mm in the same manner you described with a K1ii. ie. Camera in manual, lens in auto-aperture, the cycled up and down through the aperture range and made equivalent shutter speed steps to equalize the exposure.

The histogram always stayed in the correct spot and I didn't notice any significant changes to the exposure simulation brightness levels being displayed in live view. I could not reproduce the effect you are seeing unfortunately.

For what its worth, the aperture mechanism is quite noisy on my copy too, and I have noticed issues when mounting my lens on other older Pentax bodies with the auto aperture coupling arm. Basically I could not select the lens auto-aperture position on some bodies unless I had that position set when I mounted the lens. The auto-aperture mechanism has always worked correctly though once selected.

My take away is that generally, the aperture coupling arm on the Samyang may not be the most highly toleranced component.

Thats where I would be looking on your copy of the lens, as my guess is the aperture is not retracting fully open again in live view from certain selected apertures. Actuate it with your finger and see if it feels different opening from f5 vs opening from f10 (or whatever your problem range is).

For my usage, I don't really use the exposure preview functionality in live view, so if you have a good optical copy of the lens with good centering/collimation (ie you have already won the Samyang QA lottery), I'd personally think twice about moving to a second copy hahaha. Its in the top 3 sharpest lens I've used in K-mount, and thats what I'd try and focus on.


Last edited by Eye.n.Eye; 04-24-2022 at 05:52 PM. Reason: typos
04-26-2022, 10:40 AM   #5
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Original Poster
Hi Eye.n.Eye and thank you much! for checking your lens.

QuoteOriginally posted by Eye.n.Eye Quote
I tested my copy of the Samyang 135mm in the same manner you described with a K1ii. ie. Camera in manual, lens in auto-aperture, the cycled up and down through the aperture range and made equivalent shutter speed steps to equalize the exposure.The histogram always stayed in the correct spot and I didn't notice any significant changes to the exposure simulation brightness levels being displayed in live view. I could not reproduce the effect you are seeing unfortunately.
If I understand you correctly, using aperture priority in live view mode, my copy also doesn't have issues with suddenly getting darker - everything looks exactly as it should. But in manual mode, unpredictably live view will show 2-3 stops too dark (visually and also on the displayed exposure meter), when actually I only stopped down the aperture by 1/2 stop.

QuoteOriginally posted by Eye.n.Eye Quote
if you have a good optical copy of the lens with good centering/collimation (ie you have already won the Samyang QA lottery), I'd personally think twice about moving to a second copy hahaha. I
Exactly! an optically good copy is not easy to let go of! I've thought about this a lot, as my testing (test chart shots and also random subject shots) indicates that optically it really is a good copy. I was all set to just keep the lens, forget about manual mode, and start using aperture priority (or TAV, which seems very useful). However . . .

QuoteOriginally posted by Eye.n.Eye Quote
my guess is the aperture is not retracting fully open again in live view from certain selected apertures. Actuate it with your finger and see if it feels different opening from f5 vs opening from f10 (or whatever your problem range is).
Thanks! for suggesting this - it took a bit before I figured out that you mean using the aperture lever on the back on the lens mount, yes?

Until you posted I hadn't actually paid attention to that lever, didn't know it could be moved or that it had anything to do with the aperture. At first I thought you meant using the aperture ring on the lens, which reminded me that I had noticed that the blades looked increasingly "not symmetric" when stopped down - one segment being noticeably smaller than the other segments. But checking my other lenses, it seems at least two of them have this same issue, with my Pentax 77mm Ltd being the worst - a bit of searching the Pentax Forums revealed that a lot of Pentax lenses including Limited lenses do occasionally? often? have this issue.

But then I finally saw the aperture lever, realized what it is, and tried closing down using that for my various (6 total at this point) Pentax lenses - and all of them except the Samyang closed down smoothly, and made the same sort of very quiet (held up to one ear) sound when closing the blades.

Moving the Samyang aperture lever requires more force and the sound is louder - expected perhaps - it's a bigger lens. But it also is more uneven in the sound and the amount of pressure required to close the aperture, feels like the blades are somehow dragging unevenly. That's worrisome, so I've decided to return/exchange it and hope that the next copy is optically as good as this copy. Can I win the Samyang QA lottery twice? Here's hoping yes!
01-14-2023, 12:43 PM   #6
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Join Date: Jan 2021
Posts: 37
Original Poster
QuoteOriginally posted by eles Quote
Hi Eye.n.Eye and thank you much! for checking your lens.



If I understand you correctly, using aperture priority in live view mode, my copy also doesn't have issues with suddenly getting darker - everything looks exactly as it should. But in manual mode, unpredictably live view will show 2-3 stops too dark (visually and also on the displayed exposure meter), when actually I only stopped down the aperture by 1/2 stop.



Exactly! an optically good copy is not easy to let go of! I've thought about this a lot, as my testing (test chart shots and also random subject shots) indicates that optically it really is a good copy. I was all set to just keep the lens, forget about manual mode, and start using aperture priority (or TAV, which seems very useful). However . . .


Thanks! for suggesting this - it took a bit before I figured out that you mean using the aperture lever on the back on the lens mount, yes?

Until you posted I hadn't actually paid attention to that lever, didn't know it could be moved or that it had anything to do with the aperture. At first I thought you meant using the aperture ring on the lens, which reminded me that I had noticed that the blades looked increasingly "not symmetric" when stopped down - one segment being noticeably smaller than the other segments. But checking my other lenses, it seems at least two of them have this same issue, with my Pentax 77mm Ltd being the worst - a bit of searching the Pentax Forums revealed that a lot of Pentax lenses including Limited lenses do occasionally? often? have this issue.

But then I finally saw the aperture lever, realized what it is, and tried closing down using that for my various (6 total at this point) Pentax lenses - and all of them except the Samyang closed down smoothly, and made the same sort of very quiet (held up to one ear) sound when closing the blades.

Moving the Samyang aperture lever requires more force and the sound is louder - expected perhaps - it's a bigger lens. But it also is more uneven in the sound and the amount of pressure required to close the aperture, feels like the blades are somehow dragging unevenly. That's worrisome, so I've decided to return/exchange it and hope that the next copy is optically as good as this copy. Can I win the Samyang QA lottery twice? Here's hoping yes!
QuoteOriginally posted by eles Quote
Moving the Samyang aperture lever requires more force and the sound is louder - expected perhaps - it's a bigger lens. But it also is more uneven in the sound and the amount of pressure required to close the aperture, feels like the blades are somehow dragging unevenly. That's worrisome, so I've decided to return/exchange it and hope that the next copy is optically as good as this copy. Can I win the Samyang QA lottery twice? Here's hoping yes!
A very late update to this thread: I did exchange the Samyang 135mm for a second copy. Unfortunately the second copy was considerably worse than the first in terms of having a sticky/blades-dragging aperture. So reluctantly I decided to give this lens a pass and return it for a refund rather than trying again with a third copy of the lens. I didn't test the second copy optically as there wasn't any point.


As a point of reference, I checked several other lenses in the house for "sticky apertures", and none of them have this problem.
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