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Author Topic: Lens under exposing.  (Read 5368 times)
Justinr
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« on: June 01, 2014, 11:38:15 AM »
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I have a Nikkor 24-85 lens that I bought privately six months ago and it now appears to be under exposing by at least a stop on a regular basis. The only way I can be sure of getting a decent picture is to expose manually while the other two lenses I have are fine on the D3 body. Would there be anything simple that could be causing this before I send it away for repair?
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Slobodan Blagojevic
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« Reply #1 on: June 01, 2014, 02:50:37 PM »
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Justin, my immediate reaction was that lenses can not underexpose, cameras do. Then I thought what if the lens is not closing the aperture to what the camera tells it to, but closes it one stop less (i.e., the camera says f/5.6 and the lens stops to f/8)? Then again, that would happen if you set your exposure manually as well, unless by manually you mean stopping the lens down manually, not manual exposure mode on the camera. I am not too familiar with Nikon, but I know there are several generation of its lenses, some of which do not even have aperture ring on them (but rely on the camera to set it), so a bit more detail about what lens you have and the technique you use might be helpful.
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Justinr
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« Reply #2 on: June 01, 2014, 05:38:47 PM »
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That was pretty much my thinking as well to begin with, it must be the camera, but other lens work fine so I narrowed it down to a possible fault in the lens itself or a poor contact on the mount.

Since I first posted I've had another look at the camera and there was some slight movement of the lens in the mount which remounting the lens with a firmer twist eliminated, so I am now wondering if it's the lens not locking home properly rather than a fault within it. It's bedtime over here so I'll give it a little test in the morning, hopefully I'll have some good news.
« Last Edit: June 01, 2014, 05:55:21 PM by Justinr » Logged

Justinr
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« Reply #3 on: June 02, 2014, 05:30:32 AM »
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Justin, my immediate reaction was that lenses can not underexpose, cameras do. Then I thought what if the lens is not closing the aperture to what the camera tells it to, but closes it one stop less (i.e., the camera says f/5.6 and the lens stops to f/8)? Then again, that would happen if you set your exposure manually as well, unless by manually you mean stopping the lens down manually, not manual exposure mode on the camera. I am not too familiar with Nikon, but I know there are several generation of its lenses, some of which do not even have aperture ring on them (but rely on the camera to set it), so a bit more detail about what lens you have and the technique you use might be helpful.

A few casual shots this morning would suggest that all is well again now that I've made sure the lens is fully locked in.
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Ajoy Roy
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« Reply #4 on: June 15, 2014, 10:09:22 AM »
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The aperture is set by the small lever in the mount. When the lense is unmounted the aperture would be closed to the minimum value. Mounted the camera opens the aperture to its full vale. Moving the lever when the lense is unmounted, will open the aperture. If the lense mount is loose, then when the body moves the lever it may move it more or less depending on the lense/body design. In your case as you say, if the mount is loose then it is moving more, hence under exposing. To check if that is so, note how much the aperture opens up, when lense is unmounted and the lever is moved to extreme. Now mount the lense and see if the aperture is fully open. If the mount is the problem, the aperture will not be fully open. Twist the lense to seat it properly and see if the aperture opens up. If that was the problem then you have solved it.

In case the aperture does not open even then, then the lever may be slightly bent. Get it checked and adjusted.
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Justinr
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« Reply #5 on: July 04, 2014, 03:20:02 PM »
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The aperture is set by the small lever in the mount. When the lense is unmounted the aperture would be closed to the minimum value. Mounted the camera opens the aperture to its full vale. Moving the lever when the lense is unmounted, will open the aperture. If the lense mount is loose, then when the body moves the lever it may move it more or less depending on the lense/body design. In your case as you say, if the mount is loose then it is moving more, hence under exposing. To check if that is so, note how much the aperture opens up, when lense is unmounted and the lever is moved to extreme. Now mount the lense and see if the aperture is fully open. If the mount is the problem, the aperture will not be fully open. Twist the lense to seat it properly and see if the aperture opens up. If that was the problem then you have solved it.

In case the aperture does not open even then, then the lever may be slightly bent. Get it checked and adjusted.

Many thanks Ajoy and my sincere apologies for the delay in replying, I've only just noticed that you had.
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geezerhood
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« Reply #6 on: August 04, 2014, 12:34:12 PM »
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I discovered a similar issue with several of my lenses not too long ago but for me they were randomly over exposing.  

The mounts themselves were not loose, it was not caused by bent actuating levers, nor did the movement go away when the lens was more firmly locked in place. The problem for my lenses was the size of the opening where the lock pin enters the indexing slot in the lens mount.  One Nikon brand and two Tamrons. They had overly large indexing slots that allowed rotational movement that opened up the aperture blades significantly when the lens was rotated to the farthest stop point.  One lens had almost a full stop of movement.

I came up with a quick and simple fix that might be too Wile E. Coyote / Kitchen Table Tech for some of you but it worked for me and the lenses that had the fix applied are remaining tight.  Obviously don't do this to a lens that is still under warranty or on your $7000 used value 400mm f 2.8. Send those back to Nikon. The lenses I did this to showed no signs of wear in the indexing slot. No wear signs on the indexing pin on the body either. The body and one of the problem lenses were both almost new in fact.  The lenses fixed were all worth under a grand each so I was comfortable with the risk and it solved the problem quickly and permanently.   Ideally you would want to get a new mount with a properly milled index slot to replace the one that was too loose, but my guess is that even the new one might have the same problem, especially with non Nikon lenses.

Short version - Gradually and carefully peen the edge of the indexing slot exactly where the indexing pin is located. The side that needs the fix is on the counterclockwise side of the slot when viewed from the rear of the lens.  Increase the peened edge position slowly, testing after each strike until you reach the point where the movement is gone but the indexing pin engages easily and fully without having to apply a lot of torque to the lens to get it to lock.

If anyone would like me to post the procedure step by step in more detail with some photos either in this thread or in a new one, let me know. But, I am guessing this is not a method most would be comfortable with so I kept it brief.
« Last Edit: August 04, 2014, 12:37:42 PM by geezerhood » Logged
Vladimirovich
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« Reply #7 on: August 04, 2014, 12:46:00 PM »
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Justin, my immediate reaction was that lenses can not underexpose, cameras do.
technically they can - for example because the system meters wide open the faulty lens can in fact stop down more than expected
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Slobodan Blagojevic
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« Reply #8 on: August 04, 2014, 01:24:03 PM »
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technically they can - for example because the system meters wide open the faulty lens can in fact stop down more than expected

Isn't it what I said in my next sentence?

Quote
...what if the lens is not closing the aperture to what the camera tells it to, but closes it one stop less (i.e., the camera says f/5.6 and the lens stops to f/8)?
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Slobodan

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Vladimirovich
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« Reply #9 on: August 04, 2014, 04:26:56 PM »
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Isn't it what I said in my next sentence?

you absolutely did
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