Setting aperture manually for automatic speed calculation?

Your camera doesn't need to know the aperture of the lens to calculate the exposure automatically. That's just there for your display, and for you to control. The camera calculates the exposure value based on knowing how much light is hitting the sensor based on calibrated factory values. Modern cameras are constantly metered, so you never have to guess the correct exposure.

If you're in 'A' mode with a non-native lens, the camera adjusts the shutter speed automatically as you manually adjust the lens aperture in order to meet whatever EV you set. That's it. In 'M' mode, the EV adjusts as you adjust the individual settings to tell you how far off neutral exposure you are as you adjust those settings.

 

Some cameras do that. That's why the histogram is so useful to see while you're composing. It allows you to adjust the EV appropriately to match the lighting you want. I've heard people say to 'expose to the right' for some of these cameras, which would match your experience with underexposed photos. Try setting your EV to +2/3 or something and see if your photos comes out properly exposed to your liking. As an aside, I can't say I've experienced these issues on my GH1.

 

Expose to the right

A little off this thread's topic, but:

"Expose to the right" refers to a technique that maximizes the dynamic range of the camera's sensor when shooting RAW images (I think it is not as useful when shooting JPG). Check out this link.

(Might be more exact to say: its a way to maximize the signal-to-noise ratio in your final image by using the most of the camera sensor's dynamic range -- but this makes me sound like a nerd (which I am, :smile )

Brian