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D40 Body - small button

Glyn Sumner , Jul 30, 2009; 06:37 a.m.

This maybe a silly question but I have limited experience with DSLRs. On the Nikon D40 body there is a small button beside the lens mount (photo attached)

Could anyone please tell me what the purpose of this small button is?


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Matt Laur , Jul 30, 2009; 06:42 a.m.

That's the release button that unlocks the lens from the mount so that you can remove it.

Glyn Sumner , Jul 30, 2009; 06:55 a.m.

No not that button its on the opposite side, lower left as you look at the lens mount

D40 lens mount

Sjoerd Leeuwenberg , Jul 30, 2009; 07:03 a.m.

I think that is the button that lets the camera know that the lens is at it's narrowest aperture. Only for non-G (no aperture ring) lenses. This is needed so the camera can have the full range of apertures available.

Shun Cheung , Jul 30, 2009; 07:03 a.m.

That little switch detector is on all AF Nikon SLRs that have no aperture follower tab; i.e. the one that cannot meter with lenses that have no built-in CPU. On Nikon lenses that have an aperture ring, there is a little post with the fancy name "EE Survo Coupling Post." When the lens is set to its minimum aperture, that post will press on this little switch to inform the camera. Otherwise, the camera would display the Fee error. The switch is useless with G lenses since they have no aperture ring and is automatically set to the minimum aperture.

On Nikon bodies that have the aperture follower tab so that they can meter with no-CPU lenses, there is no such switch, e.g. the D200, D300, D700, etc.

Sjoerd Leeuwenberg , Jul 30, 2009; 07:06 a.m.

Cannot believe I beat shun with an answer, but only by a few seconds, haha.
And your answers is also a lot more elaborate.

Shun Cheung , Jul 30, 2009; 07:08 a.m.

Sjoerd, it is 4am here in California. Gimmy a break. :-)

Glyn Sumner , Jul 30, 2009; 07:22 a.m.

Thanks very much guys for clarifying that, my curiosity is satisfied.

Raden Munim , Jul 30, 2009; 07:59 a.m.

Good question, and good answer, Shun. I never thought this small tab is all it takes for the lens to tell the SLR that it's at the smallest aperture. I thought it involved a complex protocol using the 7 data contacts.

Shun Cheung , Jul 30, 2009; 08:18 a.m.

Here is an image comparing the D100 vs. D200 mounts. You can see the aperture follower tab in the 1 o'clock position on the D200, while the D100 has the little switch in discussion here in the 7, 8 o'clock poition.

You may also notice the different mirror size. The D100 was an early DSLR, converted from the Nikon F80/N80 film body so that it retains the mirror for a film/FX SLR. The D200 was designed from scratch as a DX DSLR so that it only needs a small mirror.

Nikon D100 vs. D200 Mount Area

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