In article , ari@shore.net says...
>In article <5gva0o$sfl@ns2.image.dk>, dkisspro@ibmmail.com wrote:
>>Mark Walberg wrote:

>>> As I understand Nikon AI metering, the coupling between the lens
>>>mount ridge and the tab on the body only tells the body the relative
>>>difference between the selected aperture and the lens max aperture, so
>>>that the camera knows how many stops to stop the lens down for
>>>exposure. (So, the camera does not know the actual max aperture for
>>>those lenses with a CPU, such as AF lenses.)

>>Yep - that's how it works. The camera knows how many stops you have
>>stopped the lens down.

>Here's a related question. I have a 3.5" 1000mm f11 Meade telescope for
>which I got a T-mount adapter. How do I communicate the fact that the lens
>is f11 to my Nikon (an n90s, if that matters) so that I can use TTL
>metering?

No need to - the AI camera defaults to "stop-down" metering mode when
the lens attached does not engage the body AI tab. This would normally
require you to press the DOF preview button while metering, but since
the mirror has no diaphragm, it is "automatically" set at the taking
aperture, so nothing need be done to meter with the telescope.
Hope This Helps