da* 16-50 f2.8 artificially restricted to f2.8?

Let me answer some/all you quetsions

1 the stopper you see sets the lens to f2.8@50 a cam follower shuts the irsi down as you zoom to maintain constant aperture.

2 If the lens are individually calibrated (rather than mass knock out) then every lens will have a slightly different stopper position, This is caused by slight changes in linkages and/or blade position.

Though the difference in Brightness your seeing coudl be an improperly calibrated iris I suspect not.....see below.

3 A lens 'Brightness' is related to T factor not F stop.

F stop = pupil size

T factor = light transference

In photography we only talk F-stop but in reality the exposure is F-stop + light lost through elements.

This T rating can affect lens anything up to a stop, in you example it shows the 16-50 is a more complex beast than the 50-135 .

It one of the things that should make the da560 a clever design by Pentax (less elements = more light)

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