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06-30-2012, 03:27 PM   #1
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Sigma zoom on Kx

I need a little professional help. I recently bought a Sigma 75-300 mm f/4.5-5.6 and am checking it out on my Kx. It's an old lens, not APO, and was reviewed in the Pentax Forums. When I put it on my Kx it asks for the focal length. To get started, I inputted 200 mm, set the lens on A, set the camera to auto focus, and the dial to P. It seems to work well and the shots are sharp if I take the time to focus and hold the camera as steadily as possible. Even when I change the focal length on the lens, the pictures still seem sharp. Am I expected to input a new focal length on the camera with every change I make? The only way to do this is to switch off the camera so that it asks for the new focal length. This is a pain of course. When I upload my pics to my computer, it is clear that the lens contacts are compatible with the camera since the apertures, speeds, and ISOs are accurately specified. However, the focal length is 200 mm as originally inputted. I tried to use it on manual settings by selecting an aperture, setting the dial to M, and the camera to manual focus. I depressed the AF/AE-L button while exposing but I just got vast under-exposure. I hit the green button before exposing and got the same result. How do I work with this lens?

06-30-2012, 03:49 PM   #2
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QuoteOriginally posted by P. Soo Quote
Am I expected to input a new focal length on the camera with every change
The focal length is needed by the camera to calculate the shake reduction needed. So, yes you sorta need to adjust it when you change focal length. I think most folks either set it in the middle or at whatever focal length they expect to use the most. One other important use though is that the image EXIF will report the focal length as whatever you input.

QuoteOriginally posted by P. Soo Quote
switch off the camera so that it asks for the new focal length
This is not true, you can change it in the menu, I forget where on the k-x.

QuoteOriginally posted by P. Soo Quote
manual settings by selecting an aperture, setting the dial to M, and the camera to manual focus. I depressed the AF/AE-L button while exposing but I just got vast under-exposure. I hit the green button before exposing and got the same result.
Green button should work if you are in manual mode, unless you have it set to do something else. On the k-x, I think you can use the "+/- Av" button to set exposure not the AF/AE-L button. Also do you have "Use Aperture Ring" set to "yes" in the menu?
07-01-2012, 05:09 AM   #3
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Thanks Jatrax. Yes the aperture ring was set to "yes". I didn't realize that the focal length input was for shake reduction, and not for sharpness of the image. Your advice to set the focal length to some average value is good since I don't have to keep fiddling around and making adjustments before every shot. I'll stick to the auto exposure mode instead of manual.
07-01-2012, 08:20 PM - 1 Like   #4
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Well, shake reduction *does* affect sharpness. If you have it set to 200mm but actually shoot at 75, the mechanism will overcorrect by enough that your pictures may well be blurrier than if you used no SR. Better to understimate than to overestimate - although a little overestimation won't hurt much.

07-01-2012, 10:43 PM   #5
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QuoteOriginally posted by P. Soo Quote
Your advice to set the focal length to some average value is good since I don't have to keep fiddling around and making adjustments before every shot.
I'm just saying that is what many people do, not necessarily recommending it. If you want the SR to be the most helpful you need to tell it what focal length to use. And yes, I know that is fiddly. But like Marc says if you set it wrong it can make things worse not better. I have lots of manual primes and no manual zooms so it is easy for me, just set it when you change lenses and forget it. If I were using a manual zoom like you have I would be tempted to just turn SR off unless I really needed it. Remember when that lens was made "shake reduction" meant drinking less coffee before taking pictures.
07-02-2012, 04:25 AM   #6
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Thanks again, Marc and Jatrax. I'll try both the no-SR and the "fiddly' focal-length-change-at-every-shot techniques to find which one I can live with.
07-03-2012, 10:58 AM   #7
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Did someone say something about switching SR off? What would be the point of that? The idea is to choose a focal length that is either less than or equal to the correct focal length most of the time, and if it's too high, have it be only a little too high. For a 75-300, somewhere around 100 would probably work well enough.
07-03-2012, 11:37 AM   #8
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Roger!

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