Zoom lens with N-70

I just noticed something odd (perhaps it is normal but I never

noticed before. I use an N-70 and just purchased a Sigma 28-105 3.8-

4.6 to use with it and my FM. When moving focal length of the lens

when on the N-70 in manual mode (and all others for that mater) the

camera registers changing f-stops. at 45mm it may read 5.6 and when

moved to 105mm it may read f8 or f8, even if the apature ring on the

lens is set to 5.6.

I would expect to see this at mimimum apature, but not in the

middle. Any advice or explanations? Will my FM read this apature

propery when coupled to the meter?

Thank you

Ed

That's normal, and it does happen with my micro lenses as well, depending how close you focus. The camera always displays the effective aperture which isn't always the same as the aperture you set on the lens. I don't know however how the effective aperture is calculated.

cheers,

Ed,

Most of todays zoom lenses are variable aperture designs. The aperture of these lenses varies throughout the aperture range. As an example, my 70-300mm f/4~5.6 AF-D looses a full stop going from 70mm to 300. This is true at all apertures.

If you mount your lens on your FM and watch the exposure indicator as you zoom from one extreme to the other, you will see that the meter does react to the change. If you change the focal length after setting the exposure, you will want to check the exposure again.

As long as you frame your shot and then set the exposure, your FM should have no problem with the variable aperture.

I hope this helps.

Vernon

Thank you. Makes sense, and what if suspected, but I just thought I would confirm.

Ed