Originally posted by Cuthbert Why? It has already been said that with K and M lense the K-1 can't meter full open, I still have to understand if at least it would work with A lenses (no green buttom!).
That depends what you mean by 'meter full open'. I think there may be a couple of cross wires in this thread. Read this post a couple of times, and hopefully all will become clear!
In all cases (M/K/A/F/FA/DA/DFA) lenses will be wide open when you are metering.
With all lenses with the 'A' setting on the aperture ring (ie everything apart from 'M' and 'K'), you will need to ensure that the aperture ring is set to 'A'. To choose the aperture you will typically use the rear dial on the camera body itself (IMPORTANT: the camera knows the f-stop), unless you are using an AUTO mode of some sort (the camera will still know the f-stop!). This does not require a green button. It's easy, and a nice user experience. The only difference between 'A' lenses and all the later lenses, is the lack of autofocus.
With 'M' and 'K', things differ slightly. Metering still happens with the lens fully open, BUT! The camera has no way of telling what F-Stop you've got selected (the crippled K mount removed the mechanical linkage). Because it does not know the f-stop, it can't guess the correct shutter speed. So if you are in AV/AUTO/TV etc, the selected f-stop on the lens is ignored completely, and the shot will always be taken wide open, and the exposure will always be correct.
IF you want to select an f-stop, then you will need to use the 'M' mode on the camera and the green button. Choose the aperture you want on the lens, compose and focus your shot, hit the green button to meter (this is called 'stop down metering'), and then click the shutter to take the shot.
So with 'M' and 'K' glass, you can use the K-1 as a point and shoot by simply selecting say AV mode, and click away to your hearts content. Metering will be correct, but the lens will always use the largest aperture. If you want to use a different aperture, you'll need to use the green button and manual mode.
With 'A' glass, just set the camera to AV. The rear dial will now control aperture. Everything will meter correctly. If you have FA or DFA lenses, the behaviour is the same, except you also have autofocus available.
I hope that makes sense?