What does the AF/M body switch REALLY do?

Chris DiPetta said:

OK, I've looked all over, and can't seem to find a clear answer to this heavily technical question.

When using a screw drive lens, like an AF-D series, you are supposed to move both the selector on the lens to manual, and the AF/M switch on the body to M, or, as the manual states, you MAY damage the camera.

However, when you move the selector ring on the lens to manual, the manual focus clutch engages and as far as I can tell you are not turning the screw on the lens as you focus, and therefore not backdriving the motor in the camera.

I don't know if the screw in the lens locks up when the selector ring is in manual on some lenses, but I suppose if it does, then perhaps the body AF/M switch is intended to fully disable the motor to prevent it from trying to turn a locked up screw.

I've used remote control software, when the body switch was in AF, and an AF-D lens in M, and the only thing that seems to happen is that the focus motor spins and but the lens doesn't focus. It's not hitting resistance, because you can hear that when you get to a the maximum and minimum focus stops and try to focus past them(little clicking noises).

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Chris DiPetta said:

OK, I've looked all over, and can't seem to find a clear answer to this heavily technical question.

When using a screw drive lens, like an AF-D series, you are supposed to move both the selector on the lens to manual, and the AF/M switch on the body to M, or, as the manual states, you MAY damage the camera.

Click to expand...

Member said:

However, when you move the selector ring on the lens to manual, the manual focus clutch engages and as far as I can tell you are not turning the screw on the lens as you focus, and therefore not backdriving the motor in the camera.

Click to expand...

Chris DiPetta said:

So what does this switch actually do? Is it a mechanical disconnect? Electronic disabling of AF?

Click to expand...

afoton said:

Chris DiPetta said:

OK, I've looked all over, and can't seem to find a clear answer to this heavily technical question.

When using a screw drive lens, like an AF-D series, you are supposed to move both the selector on the lens to manual, and the AF/M switch on the body to M, or, as the manual states, you MAY damage the camera.

Click to expand...

Many screw drive lenses do not have a selector on the lens. Then it is only the selector on the camera.

Click to expand...

Chris DiPetta said:

So what does this switch actually do? Is it a mechanical disconnect? Electronic disabling of AF?

Click to expand...

Leonard Migliore said:

Chris DiPetta said:

So what does this switch actually do? Is it a mechanical disconnect? Electronic disabling of AF?

Click to expand...

Well, if you have a Nikon with a focus motor, you can see what it does if you remove the lens. When you turn the switch to M, the drive pin retracts back into the camera. So it is, in fact, a mechanical disconnect (that's presumably why the control is in its strange location). I assume that it also tells the camera about this so it doesn't try to run the focus motor in vain.

Click to expand...

Chris DiPetta said:

Leonard Migliore said:

Chris DiPetta said:

So what does this switch actually do? Is it a mechanical disconnect? Electronic disabling of AF?

Click to expand...

Well, if you have a Nikon with a focus motor, you can see what it does if you remove the lens. When you turn the switch to M, the drive pin retracts back into the camera. So it is, in fact, a mechanical disconnect (that's presumably why the control is in its strange location). I assume that it also tells the camera about this so it doesn't try to run the focus motor in vain.

Click to expand...

Ah ha, so it is a mechanical disconnect (in addition to an electrical disabling). I assume it does that because some lenses will drive the screw when in manual focusing mode, and therefore backdrive the motor. Thanks for that!

I noticed that on my AF-D lenses, the screw must be disconnected when put into M mode because it just spins freely, so there's no chance of damage by backdriving the motor even if the body switch is left in autofocus. I wonder if that's only an issue with very old autofocus lenses that don't disconnect the screw when in manual.

Click to expand...

mais51 said:

Chris DiPetta said:

Leonard Migliore said:

Chris DiPetta said:

So what does this switch actually do? Is it a mechanical disconnect? Electronic disabling of AF?

Click to expand...

Well, if you have a Nikon with a focus motor, you can see what it does if you remove the lens. When you turn the switch to M, the drive pin retracts back into the camera. So it is, in fact, a mechanical disconnect (that's presumably why the control is in its strange location). I assume that it also tells the camera about this so it doesn't try to run the focus motor in vain.

Click to expand...

Ah ha, so it is a mechanical disconnect (in addition to an electrical disabling). I assume it does that because some lenses will drive the screw when in manual focusing mode, and therefore backdrive the motor. Thanks for that!

I noticed that on my AF-D lenses, the screw must be disconnected when put into M mode because it just spins freely, so there's no chance of damage by backdriving the motor even if the body switch is left in autofocus. I wonder if that's only an issue with very old autofocus lenses that don't disconnect the screw when in manual.

Click to expand...

On my AF-D lens I have to put the CAMERA body switch to M to retract the drive shaft that drive the focus mechanism in the lens - I also must switch the A/M switch on the lens barrel to M for manual focus, leaving it on A will not allow me to manual focus at all - the focus ring will turn but focus is not engaged. Nikon lens that has built-in motor are the S and P lens, in which case you can leave the switch on M/A rather than just M.

Click to expand...

Chris DiPetta said:

mais51 said:

Chris DiPetta said:

Leonard Migliore said:

Chris DiPetta said:

So what does this switch actually do? Is it a mechanical disconnect? Electronic disabling of AF?

Click to expand...

Well, if you have a Nikon with a focus motor, you can see what it does if you remove the lens. When you turn the switch to M, the drive pin retracts back into the camera. So it is, in fact, a mechanical disconnect (that's presumably why the control is in its strange location). I assume that it also tells the camera about this so it doesn't try to run the focus motor in vain.

Click to expand...

Ah ha, so it is a mechanical disconnect (in addition to an electrical disabling). I assume it does that because some lenses will drive the screw when in manual focusing mode, and therefore backdrive the motor. Thanks for that!

I noticed that on my AF-D lenses, the screw must be disconnected when put into M mode because it just spins freely, so there's no chance of damage by backdriving the motor even if the body switch is left in autofocus. I wonder if that's only an issue with very old autofocus lenses that don't disconnect the screw when in manual.

Click to expand...

On my AF-D lens I have to put the CAMERA body switch to M to retract the drive shaft that drive the focus mechanism in the lens - I also must switch the A/M switch on the lens barrel to M for manual focus, leaving it on A will not allow me to manual focus at all - the focus ring will turn but focus is not engaged. Nikon lens that has built-in motor are the S and P lens, in which case you can leave the switch on M/A rather than just M.

Click to expand...

Out of curiosity, which lens does this?

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