d3100 body, lens mount 'catch'

[paulph]

Hi folks,
Could someone tell me what that little catch on the D3100 body, by the lens mount does, please? I'd guess it's lens info that the camera needs to know, just not sure what!
Thanks -Paul

[Dave Humphries]

Not sure I know what you mean Paul.

Do you mean the button you press to de-mount the lens?



Marked with the minus magnifier above?

Or were you referring to something inside the lens throat?

Ah, now I found a better picture;



You mean that thing at 8 o'clock, don't you?
I have one on my D5000, but it doesn't seem to 'engage' with anything on any lens I own, nor do anything in camera if I depress it with a finger nail. (it is spring loaded)

No idea - can someone enlighten us please?

Credit: Pictures from DPReview site review of D3100, my "go to" place for detail like this.

Cheers,

[MattNQ]

Because no matter how good your Nikon is, there is ALWAYS a catch.....

Sorry, that was no help at all, but I couldn't resist

[AB26]

You mean that thing at 8 o'clock, don't you?

No idea - can someone enlighten us please?

Dave I am taking a wild guess here as I cannot find an answer on the net.

I would say that is a little switch to tell the camera when a non-CPU lens is attached to the body.

[Inkanyezi]

It's a feature of the AI (Auto-Indexing) mount, see image in fifth entry (Tommy Lee) in http://cameragx.com/2009/09/12/a-his...nets/dsc_2803/
See also http://cameragx.com/2009/09/12/a-his...nets/dsc_2803/

So with AI lenses, the "catch" makes sense. It's there for backward compatibility with older lenses.

[Dave Humphries]

Dave I am taking a wild guess here as I cannot find an answer on the net.

I would say that is a little switch to tell the camera when a non-CPU lens is attached to the body.

Hi Andre,

I would guess the same, although the only such device I was previously aware of was the 12 o'clock AI-S notch and 1 o'clock tab for passing the selected aperture to the meter in the camera body.

Having more time today, I looked on the web and found this (I haven't read it all yet) http://www.aiconversions.com/history.html

If you follow the N60 link in the fourth paragraph on that page, it takes you to this image showing a similarly located push device on the body - but I still couldn't determine what it is supposed to do (given it is likely to be specific to a small number of lens models produced decades ago).

Cheers,

[Dave Humphries]

It's a feature of the AI (Auto-Indexing) mount, see image in fifth entry (Tommy Lee) in http://cameragx.com/2009/09/12/a-his...nets/dsc_2803/
See also http://cameragx.com/2009/09/12/a-his...nets/dsc_2803/

So with AI lenses, the "catch" makes sense. It's there for backward compatibility with older lenses.

Hi Urban,

Yes, now I have also 'played', I can see that it should* get depressed when the aperture ring of an older lens is at minimum aperture, but that being the case, the only real purpose it serves is to fix a limit for the metering system.

Here it is with on an old Nikkor 105mm f/1.8 on my D5000 (at f/16), selecting a wider aperture moves the index tab on lens away from this switch (i.e. left).



* However, when I try to rotate the aperture ring to f/22, the tab hits the sloping edge but does not depress it, although it can be be encouraged down with a finger nail thus:



The switch is quite easy to depress, not stiff in any way and the tab hits it on the slope, but the slope angle looks to be too steep. Good job I don't need this 'feature' to work, eh?

Since this lens is a completely mechanical device and they won't meter on a D5000 anyway, it seems odd they put the switch on the camera body at all, perhaps there are some semi-electronic lenses that still need this feature.

Apologies for the shoddy handheld 'macro' photography and dust

Here is a pic of the "AI ears" (as I call them).



For later generations of camera that didn't detect aperture via a pin engaging between the ears, it is my current belief that the step change in level in the foreground (by f/11) is what was used to pass the information, since the tab seen in the first two images (top left corner here) doesn't engage with anything that rotates about the lens throat - but I may be wrong.

Cheers,

[GrahamS]

The purpose of the little tab is to inform the camera's metering system that a non-G type lens is mounted with the manual aperture ring set to the minimum aperture. This will allow the camera to operate in "P" or "S" mode, which require the camera to have full control of the entire aperture range of the lens.

[Dave Humphries]

The purpose of the little tab is to inform the camera's metering system that a non-G type lens is mounted with the manual aperture ring set to the minimum aperture. This will allow the camera to operate in "P" or "S" mode, which require the camera to have full control of the entire aperture range of the lens.

Thanks Graham,

So if the lens is stopped down, another mechanism i8n the camera allows it to control aperture?

I'm not aware of anything that could do that, but perhaps it is only fitted on the better models.

Cheers,

[Inkanyezi]

On Nikon, the angle of the actuating arm governs the F-stops, so there is a particular angular movement for one stop. It can be controlled mechanically.

[GrahamS]

On Nikon, the angle of the actuating arm governs the F-stops, so there is a particular angular movement for one stop. It can be controlled mechanically.

There's your answer, Dave.

[GrahamS]

On Nikon, the angle of the actuating arm governs the F-stops, so there is a particular angular movement for one stop. It can be controlled mechanically.

There's your answer, Dave.