I zoom and the viewfinder goes darker

[IlfordHPS]

I have a A700 and something as happened a few times in the past fortnight.
When I zoom the lens, as I zoom it activates the lens apertures, so as I zoom the viewfinder gets darker as if I have pressed the depth of field button, anyone else had this.

Thanks.

[Dr. Harout]

I presume that you are probably using a lens having focus lock/DOF knob on it, and quite probable that the adjustment of that knob is on DOF. While rotating the zoom ring, accidentally you might have pushed that knob which activated the diaphragm and hence the darkening of the viewfinder.
Anyway, I really hope that is your situation.

[IlfordHPS]

No the lens I had on at the time was a Sigma 10-20mm, no DOF button on it.
I also made sure I was not pressing the DOF button on the camera body, I was however on manual focus.

Thanks.

[Dr. Harout]

I would love to blame it on sigma , but if it doesn't repeat than better forget it.
Is it possible that the lens was not perfectly mounted (that is quite common with sigmas)?

[David Kilpatrick]

If your lens is accidentally not mounted fully, turning it in the mount can produce this effect. Dr Harout is right to point it out.

David