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12-31-2009, 08:44 AM   #1
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Vivitar 24-48mm Series 1 repair question

I recently won a Vivitar 24-48mm Series 1 lens off fleabay, unfortunately it arrived with some major issues.

When you were near the 24mm end of the zoom range the rear lens group put pressure on the aperture adjustments and linkages and would prevent the lens from stopping down. As soon as you zoomed in the pressure would be released and everything would work properly. (At least I thought so at the time)

I decided to take the lens apart and see what I could make out. The screws came out quite a bit easier than I would have thought for a lens this old? Once I got pretty deep into the lens I saw some scratch marks used to help in re-assembly. Great someone had already had this lens apart, and by the looks of it completely apart.

Threw trial and error I was able to determine that they had put the lens back to together incorrectly. The lens group that was causing the problem had been threaded at the wrong start point into the helicoil. (that might be the wrong term?) Therefore my lens was actually more like a 21-45mm or worse when I got it.

Success... well unfortunately that wasn't the end of it. Looks like the reason they went this deep into the lens was because they wanted to clean the oil off the blades. Perhaps like myself they didn't realize that you could simply unscrew the lens into 2 pieces and have direct access to the diaphragm in 5 seconds flat. (live and learn)

No more oil on the blades and they are no longer stuck... and now I'm the one stuck. During their fantastic lens repair they played with the diaphragm adjustment screws and now I have no idea where it should be set. They put some scratch marks as a guide but the adjustments are so small that it doesn't help much.

So here's my question after my long winded story. Does anyone know what size the aperture opening should be at each f-stop? Is it the same for all lenses or does it vary? The other thing is noticed was that if the aperture was at say f8 when I was at the 24mm end, when I zoom in the aperture looks like it opens up a bit. Is that how they make it a constant aperture?

I assume it's not a big deal on a dslr because the camera stops down the lens to get a reading and has no idea what the f-stop should be anyway. But I was hoping to use this lens on my new super program when it arrives. I assume I'll end up with consistent over or under exposure on a film camera?

Sorry that was a lot of reading to get to the point. Any help is appreciated.

Thanks -Dave-

12-31-2009, 04:31 PM   #2
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QuoteOriginally posted by dmoon911 Quote
...So here's my question after my long winded story. Does anyone know what size the aperture opening should be at each f-stop? Is it the same for all lenses or does it vary? The other thing is noticed was that if the aperture was at say f8 when I was at the 24mm end, when I zoom in the aperture looks like it opens up a bit. Is that how they make it a constant aperture?
A particular f-stop should be a ratio between focal length and the diameter of the aperture opening. A 50mm lens set at f4 should have an aperture diameter of 12.5mm. A 25mm lens at f4 should have an aperture diameter of 6.25mm. So it looks like your lens is supposed to be that way.

[Then again, I only have one constant-aperture zoom I have where I can see the aperture while zooming - in other words, only the front elements zoom. I don't see any aperture size changes from 70 to 200mm. Who knows if this is right or not.]

I would consider discarding the theory and measurements for trial and error. Set the aperture so it appears to have a full range of motion. Wide open is pretty easy to hit. Then see if exposures with the Vivitar match another lens at identical focal lengths. Ideally your comparison lens would be very similar, such as having the same number of aperture blades. If you can hit on exposures that are close between lenses, that should be good enough.
12-31-2009, 05:41 PM   #3
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Would the size of the Filter ring make a difference? This one is a 77mm would the aperture opening on it be the same as a 49mm filter ring lens? It looks like the formula is focal length/f-stop, if that's true it will help a lot.
01-01-2010, 10:24 AM   #4
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QuoteOriginally posted by Just1MoreDave Quote
A particular f-stop should be a ratio between focal length and the diameter of the aperture opening. A 50mm lens set at f4 should have an aperture diameter of 12.5mm. A 25mm lens at f4 should have an aperture diameter of 6.25mm.
QuoteOriginally posted by dmoon911 Quote
It looks like the formula is focal length/f-stop, if that's true it will help a lot.
The formula for the diameter of the stop is focal length / f-stop as Just1MoreDave said.
However the diameter you have to measure is NOT the diameter of the diaphragm as seen with the lens dismantled. The diameter you need to be concerned about is the entrance pupil diameter. The entrance pupil is the image of the aperture stop as seen from the front of the lens. Due to the magnifying effect of the front lens group(s) the aperture need not be the same size as the entrance pupil.

QuoteOriginally posted by Just1MoreDave Quote
Then again, I only have one constant-aperture zoom I have where I can see the aperture while zooming - in other words, only the front elements zoom. I don't see any aperture size changes from 70 to 200mm. Who knows if this is right or not.
Do you mean that the size of the aperture diaphragm doesn't change as you zoom (do the blades move as you zoom) or the entrance pupil diameter doesn't change? The stop diameter doesn't have to change (the blades don't have to move) for the aperture to be constant as long as the optical arrangement causes the entrance pupil size to change as the lens is zoomed.

QuoteOriginally posted by dmoon911 Quote
So here's my question after my long winded story. Does anyone know what size the aperture opening should be at each f-stop? Is it the same for all lenses or does it vary? The other thing is noticed was that if the aperture was at say f8 when I was at the 24mm end, when I zoom in the aperture looks like it opens up a bit. Is that how they make it a constant aperture?

I assume it's not a big deal on a dslr because the camera stops down the lens to get a reading and has no idea what the f-stop should be anyway. But I was hoping to use this lens on my new super program when it arrives. I assume I'll end up with consistent over or under exposure on a film camera?

Sorry that was a lot of reading to get to the point. Any help is appreciated.

Thanks -Dave-
If its like zoom lenses that I have dismantled previously then the aperture diaphragm will be widest open when the lens is zoomed to its longest focal length. If you zoom the lens to 48mm and set the aperture to it widest setting then you can adjust the diaphragm screws untill the aperture is just fully open and then lock everything up tight. This should be close to the right settings (although check that everything is at least close to the scribed marks to make sure it is not all way off).

Although with a DSLR in manual mode the exposure shouldn't be a problem as it stops down to meter, but if the aperture settings are too far open then you may get a lot of extra aberations etc ruining the images.

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