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Should the aperture value change when in A or M mode when focussing a prime macro lens on a D70s or is it a fault?

For avoidance of doubt, I've tried this with both the Nikkor 65mm and Nikkor 105mm primes, so this definitely isn't anything to do with maximum aperture changing as a lens zooms.

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    Just to make sure: you literally mean "any lens"? Not just a zoom or two, right? – Wayne Jan 9 '15 at 19:07
  • please clarify if that only happens when you start out at minimum aperture. set it to eg. f/11 and try again. – ths Jan 9 '15 at 20:11
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    Also please clarify the following points: you do mean focusing, not zooming, do you? As James's answer will only apply if the latter. Also, does this not happen in other modes (e.g. aperture priority, shutter priority)? In order to rule out certain behaviours more information is required. – NickM Jan 9 '15 at 22:56
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    Thanks @Nick When I say 'any ' lens I mean of the few I have, and yes focussing not zooming.As the auto focus is moving the barrel, the aperture reading alters when in manual mode and in aperture priority mode. The lenses I have tried are primes nikor af 60mm and af 105mm .Following your comments I have tried different apertures and found the problem starts at f4.5 through to f2.8. When focussing manually the problem is the same.-Jan – Jan Davies Jan 10 '15 at 20:24
  • Well then I'd say this is far from 'normal' behaviour, possibly a software fault with the camera body. It's impossible to say how straightforward it would be to fix, but if your camera is new you should be able to get it replaced (if is found to be faulty) – NickM Jan 10 '15 at 20:32
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The Nikon 60mm and 105mm lenses are macro lenses. As you focus closer than a certain point, the aperture decreases. For normal distances, both lenses should remain at 2.8, but as you get close to macro distance, you'll lose "effective" aperture.

There's a mathematical equation to calculate the loss of light and some more information here:

http://en.m.wikipedia.org/wiki/F-number#Working_f-number

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    I can't upvote, so you'll have to take a virtual. If the displayed aperture doesn't change as you focus closer, you're either using a lens that doesn't communicate properly with the camera (that is, it fibs a bit about either the focus distance or the iris size, which is not unheard of in third-party lenses) or the focal length of the lens is being severely reduced as you focus closer (internal focus) while the iris effectively "floats" with the lens elements, remaining the same physical size. – user35658 Jan 11 '15 at 6:53
  • photo.stackexchange.com/questions/20300/… This answer appears to address the same issue – NickM Jan 11 '15 at 14:03
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When you adjust the focal length using the zoom ring, the aperture read out will change for most lenses, this is normal and expected behaviour. It happens as a result of the way the optics move mechanically inside the lens. You can tell which lenses will exhibit this behaviour as their aperture will be listed as a range of values such as the AF-S DX NIKKOR 10-24mm f/3.5-4.5G ED.

A prime lens like the AF Nikkor 14mm f/2.8D ED has one focal length and cannot zoom so will not exhibit this behaviour. Neither will a 'constant aperture' zoom like the AF-S DX Zoom-Nikkor 12-24mm f/4G IF-ED.

Your camera is working as it should, don't worry.

Update: Based on feedback in the comments this is not the problem in this instance as it happens with prime lenses but I'll leave it here as it's likely to be seen by others who are seeing this behaviour.

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