I have a general question regarding how stop-down metering works. My camera is the K-5 IIs, if that matters at all.
My understanding is that when you use a lens newer than the A series, it works like this: you dial in the aperture electronically, but the lens is not physically moved to that aperture until you actually press the shutter. If you look at the front of the lens, you see no differences when you dial up/down the aperture. It's always wide open until the very moment you shoot. That means all metering is done with the lens wide open, but because the camera knows the aperture you will use, it computes how much less light (e.g., on a 1.4 lens, if you set it for 2.8, then incoming light will be half) and adjusts the shutter speed accordingly. In short, the aperture doesn't actually figure into the
metering but only the shutter speed, which is a number the camera calculates after taking the metered value (taken wide open) and plugging it into some formula that includes the aperture.
Let me know if that's not correct.
What I don't understand is why Av setting with K/M lenses will result in incorrect metering values unless the lens is wide open. When I dial in the aperture on my K lens, I can see the blades physically move. Here it is wide open:
Attachment 199961
Here it is at f/2:
Attachment 199962
and here it is at f/8:
Attachment 199963
If you change the aperture on a modern lens and look right into the lens, you see no change whatsoever. It will always look like the first image.
What I don't understand is the necessity for stop-down metering with this sort of lens. My understanding of it is that the camera pretends to take a photo by having the diaphragm set the aperture as it would on a modern lens and then taking a measurement of the actual amount of light that comes in. But isn't that what I am doing when I use the aperture ring? When I dial in a new aperture, shouldn't the amount of light coming into the camera change immediately--after all, the hole got a lot smaller--and thus change the metering appropriately?
That's what I don't get. I thought I was setting the diaphragm when I use the aperture ring. Is that not what is happening? What exactly am I seeing happen in the above pictures?
Last edited by MadMathMind; 10-09-2014 at 09:44 PM.