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peteronlantz



Joined: 31 Mar 2016
Posts: 3
Location: NJ

PostPosted: Mon Apr 11, 2016 5:24 pm    Post subject: Jump in ISO Reply with quote

Zeiss 24-85 lens. As I increase from 24 - 50 ISO increases slightly (to about 22). When I reach 70 it jumps to 3200 and over-exposes. Any ideas?
mounted on Canon 5Ds.
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conurus
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Joined: 15 Aug 2006
Posts: 795
Location: Vancouver, Canada

PostPosted: Fri Apr 15, 2016 7:40 am    Post subject: Reply with quote

Please test the DoF preview function in either A or M exposure mode on the mode dial on top of the camera.

Try various aperture settings and watch the aperture blades behind the glass elements as you hold down the DoF preview button. Does the aperture blade correspond to the setting on the camera?

One of the weak points of the 24-85 which is prone to failure is a cable running to the electromagnetic aperture diaphragm. If that weakens and cracks start developing, aperture operation will become erratic at certain focal length. It may gradually worsen over time until it completely breaks, at which point the aperture diaphragm will become inoperative.

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