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01-18-2011, 09:35 PM   #1
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Changing Focusing Screen -- Newbie

Hello All... Here is another Focusing screen question here.

A quick little back ground... I have a Pentax K-x. I am using a Tamron 75 -300mm 4 -5.6 lens with a 2x teleconverter. With out the teleconverter the auto focus works fine, but when I add the telconverter and have lens all the extended auto focus will not work. Even opening up the lens, changing the iso, and with plenty of sun light.

Which leaves me manual focus, which I don't mind. I am planning to get the KPS eye pc to help. But also changing the screen from katzeye or with the split screens.

I wanted to know would those screens does it effect the auto focusing if I do decide to which back to auto mode. And I read on some post, the screens effects the metering.

Does anyones know how does it effect the metering? Can the pictures be corrected in software like Light Room? or Can I change the setting on the camera to correct the offset metering of the screen. How is the metering effected? does it effect the white balance? color? I am no sure.

If any one have any insight please let me know...

Much Thanks!

01-18-2011, 10:02 PM   #2
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It won't affect white balance or auto-focus, but metering is typically affected. The light meter sits in the pentamirror/pentaprism assembly above the focusing screen.

Unfortunately, I bet your split screen will be black at f8 (f4 * 2x teleconverter), so I doubt it'd be much help to you. Split screens/microprisms work much better on faster lenses ~f2.8. I recommend you try out the KPS for now before buying a focusing screen that will likely be unusable. If that doesn't work, look for a ground glass focusing screen with no other focusing aids (like the Canon Ec-S).

To answer your question on metering, the narrower the max aperture of the lens (f8 in your case with the converter), the darker the center circle, and thus the more over-exposed the capture will be (moreso on spot meter than evaluative).

Last edited by jeffshaddix; 01-18-2011 at 10:09 PM.
01-18-2011, 10:05 PM   #3
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sterretje's Avatar

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In my experience they don't influence AF.

You can adjust exposure before taking the photo (use eV compensation); best to determine with test shots how much the screen affects the metering. And you can correct in PP.

Note that you might need to adjust the split prism screen with shims. I had to do mine on the K100D as well as on the K10D.
01-18-2011, 10:28 PM   #4
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That 2x converter costs you two stops of light. So all the way out that f5.6 becomes f11, which is beyond the autofocus system's designed capabilities.

Remember that aperture is a ratio of focal length divided by aperture diameter. So if you have a 100mm lens with an aperture of 25mm, for example, that is f4. Put on a 2x adapter, the 25mm aperture remains the same and then 200mm divided by 25mm is f8.

A 1.4x adapter costs you one stop. A 2x costs you two stops.

Last edited by Mike Cash; 01-19-2011 at 06:07 AM.
01-19-2011, 02:09 PM   #5
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Thanks for all the input!

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