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18-55mm stock lens with Canon 550d/T2i issue


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#1 Chris Lambert

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Posted 22 February 2012 - 11:39 AM

Hello

Firstly I don't shoot too often with this lense generally I use Nikon manuals but haven't coughed up for the anything wider than 24mm at present. I was using the 18-55 stock lens today and was wondering if anyone else has seen or experience this when zooming to the long end of the lens I am experiencing a weird iris issue even on apertures slower that the long end of the lens 5.6 it seems to ramp up and down throughout the range with a noticeable shift in light. Is this normal iso and shutter were not set too auto or anything like that. tried both the switches on the lens IS and Auto focus but I am still getting the issue as well as hearing the opening and closing of the iris.

Is this normal?
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#2 Phil Rhodes

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Posted 22 February 2012 - 12:00 PM

If you look at the front of the lens, if it's the one I'm thinking of, it says "F=3.5-5.6", so at the wide end, it has a maximum aperture of 3.5; at the long end, 5.6. So yes, it's normal.

An F stop is nothing more than the ratio between the diameter of the aperture and the focal length, so if the aperture stays the same diameter and you zoom in, the image will get darker due to the increased effective stop. This is why stop numbers get bigger as the image gets darker, which is counterintuitive.

Many lenses also insist that you stop down at the long end of the zoom to maintain optical quality, which may actually be more of an issue in that sort of lens.

P
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#3 Chris Lambert

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Posted 22 February 2012 - 06:09 PM

Sorry Phil maybe I wasn't clear I already stopped the lens down beyond 5.6 and zoomed throughout the range and the issue is still there
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