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MF Lense Usage (Especially M42)
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PostPosted: Thu Nov 12, 2009 8:23 am    Post subject: Reply with quote
Shrek wrote:

I think you're complicating it :) I do this:

1. Switch to AV mode but don't set aperture on the camera
2. Focus wide open (it's easier)
3. Stop the LENS down to f8 manually
4. Take the picture


This is indeed the correct way. (Although to clarify step 1; with an AF confirm adapter, make sure the largest aperture, i.e. smallest number, is set in camera.)

Shrek wrote:

Various DSLRs behave differently when taking this simple approach e.g. Sonys tend to under expose with each lens stop down. I use exposure compensation to sort this.


Depends on the Sony, the lens, and the metering mode. I find centre-weighted and spot metering are better than multi-segment, and the A900 more accurate than the A100. And for some reason some lenses get more accurate results than others.


PostPosted: Thu Nov 12, 2009 8:32 am    Post subject: Reply with quote
Tharos wrote:

Let's say you want to have F8 and arrange the lens manually accordingly.
In AV mode if you set F8, until you shoot the aperture is at lowest in AF lenses so the cam receives the light at maximum aperture (lets say F1.4) and than a CPU calculates the light for F8 which you desire and when you take the picture the aperture closes to F8 and there you go. Theres maths going on inside the cam.


The camera always uses open-aperture metering with AF lenses, meaning that it measures the amount of light with the lens wide open, and if the aperture is set to something other than wide open, it compensates for the stopping down, but only actually stops down when the picture is taken.

Tharos wrote:
So I am not so clear how the camera decided correctly on the shutter speed in AV mode, as the calculation will not be accurate (as the cam can not control the lense)...


Since the camera cannot control the lens, you must select the largest aperture (i.e. wide open, smallest available f-number) and only stop down the lens itself (manually). The camera will then be fooled into thinking that it's darker than it really is (because it “thinks” that it sees through the lens wide open, even though it's stopped down already). Because the widest aperture is selected, the calculation will (in theory) be accurate—the camera does not need to know the actual aperture, only the difference between the measuring position and the shooting aperture, which you are making zero by selecting the widest aperture.

Tharos wrote:
If you set the aperture on the AV mode as F1.4 than it is more correct but still I am not sure if it is perfect though.


The f/1.4 was merely an example; you need to select the widest aperture available with your AF confirm adapter. This depends on the programming of the adapter. In theory exposure will be “perfect” if you use the widest setting, but in practice different lenses may fool the camera. (In this case use exposure compensation, not a different aperture value.)
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