If shooting at f/22, is a fast 1.8 lens still considered fast?

Started Apr 17, 2011 | Discussions thread
noirdesir Forum Pro • Posts: 13,580
Re: its funny

Ronj2 wrote:

I think what edwardaneal is saying is that the AF sensor does not see the brightness effects of opening and closing the apeture of the lens only the DOF effect. The AF sensor is outside of the sensor plane and does not see the effects of a lens opeing up more than F5.6. The light the AF sensor sees does not get brighter, only darker with any given lens. I just have a hard time visulizing how it avoids the light effects of aperture changes larger than F5.6

Just expand on my flashlight thought experiment.Imagine you take a number of flashlights and place them along a line parallel to a glass plane (ie, all having the same distance to the glass pane). Now, point them all such that they hit the same point on the glass pane. Now, place a short distance behind the glass plane baffles (as indicated in green in my drawing). It should be obvious that the light from the flashlights representing the outer lens areas (ie, the f/2.8 area) clearly illuminate the sensor plane but not the AF sensors. The same is true for 'flashlights' representing the small apertures (f/8 and beyond), ie, the ones in the center.

Post (hide subjects) Posted by
(unknown member)
(unknown member)
Keyboard shortcuts:
FForum PPrevious NNext WNext unread UUpvote SSubscribe RReply QQuote BBookmark MMy threads
Color scheme? Blue / Yellow