Aperture question
 

Are lens apertures reliable from lens to lens ? In other words, if I were to take a good interchangable lens camera (video or still) and keep the Iso (or Gain) and the shutter speed constant, would the the exposure be the same for any compatable lens set at an aperture of f 4 ? Does this hold for any family of cameras using the same sensor ?

Re: Aperture question
 

You'd need to factor in any transmission losses, this is what T (Transmission) stops do. With modern multi coated lenses you can lose about a 1/3 stop, in the past on some zoom lenses it could've been around a stop. Cine lenses usually have T stops, rather than f stops.

Any DOF calculations use the f stop

Re: Aperture question
 

T-stops are more reliable than F-stops, but F-stops are physically the same from lens to lens. T-stops take into consideration the light loss of the lens itself, called transmittance. F-stops are purely based on the size of the aperture.

Re: Aperture question
 

With still camera lenses, the aperture isn't even consistent from shot to shot. This can cause flicker in timelapses. The reason is that the lens opens up wide for focusing and then closes down for the photo. This is no big deal for big apertures, but if you're at f/16 or f/22 (as one often is for timelapses), a micro-variation for each blade can cause large changes in the opening as well as its shape.

For Canon lenses, I set the aperture, hold the DOF preview button, and untwist the lens just to the point at which the camera turns off. This locks the aperture into a single position. Now I can shoot a timelapse with no change in the exposure or bokeh. :)

So yeah, if aperture varies from shot to shot with a single lens, it definitely varies somewhat from lens to lens. And the bokeh will almost certainly change as different lenses have different numbers and shapes of blades.