Confused about F-number

Started Jul 10, 2002 | Discussions thread
Mike S
Regular MemberPosts: 410
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Did You Forget.....?
In reply to Netside, Jul 12, 2002

That the maximum aperture changes as the lens is zoomed It goes from f/2.0 at wide angle to f2.4 at telephoto. That is because the aperture does NOT open as you zoom

Netside wrote:
A.D., maybe I am crazy, but no one has been able to give me a
straight answer, in spite of all the camera lessons.

A.D wrote:

How can you find out the actual aperture diameter by looking
through the lens? It shows the shape of aperture opening by looking
through the lens but the different magnification factor doesn't
make it possible to measure the opening.

I never claimed I measured it. I did, however, calculate it, but it
doesn't agree with what I'm seeing. The aperture doesn't move when
zooming. If the calculations are right, the aperture should open
up. It doesn't.

When you set the aperture to f8, the hole looks like a hexagon.
It's quite distinctive. If you open up the aperture, it looses its
hexagon shape to a more circular shape. Now, I don't have the
foggiest idea how to actually measure the aperture with all the
lens surrounding it, but I CAN tell whether or not it opens.
What I see is that the aperture does not open when zooming, or when
the shutter closes.

Just zooming the lens doesn't change the aperture opening.The
aperture opening changes only when you half press the shutter. The
lens fully opens up to do the focussing and metering and then the
aperture adjusts to the selected value before closing again to
allow interline transfer mechanism. Do your same experiment again.
Select an aperture, select different zoom setting and half press
the shutter and see whether the aperture opening is changing or not.
On a side note changing the aperture ring doesn't show the depth of
field preview. The depth of filed preview is shown only after the
shutter is half-pressed and focus is confirmed by the camera.

I did what you said. It does open when half-pressing the shutter,
as you said, but it returns to its smallest opening when the
shutter closes. This is so easy to see and confirm, I can't believe
others don't see this. Maybe I AM crazy. (g)

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Michael Stone
http://www.pbase.com/mbstone
C2100
There are no strangers. Only friends we haven't met.

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