Aperture control

[Mel Davies]

I suppose this applies to all vid cams as well as stills models.
In manual mode we are given a descrete range of "f" numbers to chose from. In auto mode does the cam have just the same range, or does it control the aperture in a infinately variable way i.e can set any lens opening within its range?
Also in the XM2 (GL2) why is f8 the smallest aperture that can be set - a big jump from a suitable setting to fully closed!

Mel.

[Chris Hurd]

Hi Mel,

If the aperture were infinitely variable, that would be analog, wouldn't it. In fact, I think infinitely variable *is* the definition of analog, right? In the XM2/GL2, the aperture like everything else is controlled digitally, in a series of discrete steps. According to page 76 of the NTSC GL2 owner's manual, under point #5, manual exposure control on the GL2 is offered in a series of 19 steps from f/1.6 to f/8. The same info is probably in the XM2 manual.

With f/8 being the smallest aperture value that can be set, an optical problem called diffraction (which softens the image at very small apertures) is avoided.

[David Ennis]

Mel, to avoid any confusion if you're experimenting, note also that as you zoom in, the largest aperture setting allowed by the cam reduces.

[Mel Davies]

Originally Posted by Chris Hurd

Hi Mel,

If the aperture were infinitely variable, that would be analog, wouldn't it. In fact, I think infinitely variable *is* the definition of analog, right? In the XM2/GL2, the aperture like everything else is controlled digitally, in a series of discrete steps. According to page 76 of the NTSC GL2 owner's manual, under point #5, manual exposure control on the GL2 is offered in a series of 19 steps from f/1.6 to f/8. The same info is probably in the XM2 manual.

With f/8 being the smallest aperture value that can be set, an optical problem called diffraction (which softens the image at very small apertures) is avoided.

Any system involving digital processing can be thought of as discrete steps, but if you make the "digital resolution" high then it tends to get near the realms of analog. Take auto focus - that does not appear as discrete steps, but is precise in the analog sense.
Was just wondering in the case of auto aperture if the steps were really aproaching the realms of analog - a variable range of f1.6 - f8 without the discrete steps of manual mode.
It is all for interest sake really. Will try to physically look at the iris in both modes and see if I can detect the stepiness or otherwise of the aperture, and will report back.

I wasn't aware that a small aperture can cause softening of the image. Thought that a lens was (so to speak) more perfect at the centre than at the edges. I'm no optical expert though!

Mel.

[Mel Davies]

Have consulted Canon about the range of control of aperture in auto mode.
Their reply is as follows:-

"The aperture control when in the automatic mode uses 1/8th steps, so is not infinitely variable but has much greater flexibility than when under manual control."

Mel