How do I set the f stop when I mount my 650-1300 telephoto lens via t-mount?

[Joelcase]

Camera says no lens attached?

[Samsen · Accepted solution]

Camera says no lens attached?

Make sure you are not on P or A or TV modes.
Only AV or M (Manual) are to be selected for when your lens is a manual one.

[Samsen]

You do it over the lens as there is no electronic communication between camera body and lens.

Then for metering you need to use either AV, so that camera will chose a correct speed for the given F stop, you set on the lens. To make more easy for camera, keep your ISO also on AUTO.

Or you may use Manual "M" for full manual setting of all parameters including Shutter speed, ISO and of course aperture as you have done before.

Pay attention to your focusing. You can get assistant via LiveView if your camera body has it or you can use Magnified view during focusing and in my personal experience, 10x does a great job for perfect focusing.

Post more with any problems on the way as you can run into something new when camera not fully on auto.

[Samsen · Accepted solution]

Camera says no lens attached?

Make sure you are not on P or A or TV modes.
Only AV or M (Manual) are to be selected for when your lens is a manual one.

[Scatterbrained · Accepted solution]

I'm assuming your adapter doesn't have a chip. If it did you should get 00 in the aperture display. You can purchase a dandelion chip for the adapter, this will allow the camera to "see" the lens while also telling it that there is a manual lens mounted. You'll have to use "stop down" metering, i.e. setting the aperture first and then metering the scene.