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01-28-2009, 03:22 PM   #1
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Sigma 18-125 DC Question

I just recieved my Sigma 18-125 lens from KEH (thanks!) today, and I've got a question as to the behaviour of the lens when attached to a K10D. Basically, when I set an F-stop of 8 on the camera (using AV mode), then turn the camera around, and set the lens to 18 mm, and then hit the DOF switch, I get the aperature blades closing down properly to F8. However, when I extend the lens to 125 mm and set the aperature to F8 and observe the blades, they don't seem to close, yet when the lens is extended to 125 mm, the maximum aperature becomes 5.6 (versus 3.5 at 18 mm).

My main question is: Is there something wrong with the lens or is this perfectly normal behaviour?

01-28-2009, 03:34 PM   #2
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Have you tried actually taking pictures with the lens at these settings and checking the exposure? The difference from 3.5 - 8 is a lot more than the difference from 5.6 to 8. Don't forget that the physical size of F8 at 18mm becomes F13 (or so) at 125mm. (F13 is a guess, 1.5 stops)
01-28-2009, 03:52 PM   #3
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Yes I have tried taking pictures (flash only sofar, as the weather and the light out is crap). So far, it appears that the lens is working properly. It's just perplexing as to wether or not the lens is communicating properly with the camera or not. Both the contacts on the camera and the lens are spotless, and the aperature lever on the lens is able to move freely.
01-28-2009, 04:00 PM   #4
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I don't have this lens, but can hazard a guess. That lens is a variable aperture zoom, as you probably know, with a min of 3.5 and max of 5.6. As you zoom the lens, the aperture remains wide open, the blades don't move, but the effective aperture changes from 3.8-5.6. What I suspect you're seeing is that at 18mm, the blades need to move and stop down a substantial amount to close the aperture from f/3.5 to f/8 (2 1/3 stops). When you extend the zoom, the aperture only needs to close a small amount (1 stop) to reach f/8. So it probably moves, just not all that much.

In the days of yore, with mechanical aperture rings, when you set f/8 on a lens with those specs you would actually be setting 'stop down 2 1/3 stops'. The effective aperture at 18mm would be f/8, but the effective aperture at 125mm would actually be f/13 (f/5.6 - 2 1/3 stops) even though the ring says f/8!

Its also possible that the difference may be exaggerated by optics depending on the lens--sometimes at one side of the zoom the blades are very visible and at the other they appear tiny.

So the answer to your original question: this is probably normal. Perhaps you have another variable aperture zoom on which you can verify similar behavior. If the aperture blades move freely and snappily by moving the lever on the back of the lens with your finger, and appear to open and close fully, you're probalby fine.

01-28-2009, 04:03 PM   #5
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I have had the older version this lens for sometime. I believe the F3.5 is only available at 18mm and it reverts f4.5 after that....not sure where the cut off point is though.
01-28-2009, 05:52 PM   #6
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Thanks for the replys all. I'm gonna play around with the lens over the next few days to see if it's a-ok.
01-28-2009, 08:52 PM   #7
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I like the theory that the blades *are* closing, but not as far to get from f/5.6 to f/8 t 125mm as they had to in order to get form f/3.5 to f/8 at 18mm. Try f/16 and you'll prbably have no trouble seeing the blades close at any focal length.
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