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11-25-2012, 05:51 AM   #1
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Constant Aperture Lens

Hi,

If you don't ask, you don't learn. OK, my DA 16-45 F4 lens is a constant aperture lens meaning that its maximum is f4. Under aperture priority then, I can set aperture from f4 to say f22, f32------- right? My F zoom 35-70 f3.5 -4.5 will change as I turn the zoom with max of f3.5 to f4.5 as you turn the zoom. So under aperture priority for this lens, say I have turned zoom to 50,then I can only set aperture from whatever the lens has set the aperture at 50, say f3.9 , from f3.9 to f22 etc. ??????????

Just need some verification of this.


Thanks

11-25-2012, 06:41 AM   #2
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QuoteOriginally posted by kiteman Quote
Hi,

If you don't ask, you don't learn. OK, my DA 16-45 F4 lens is a constant aperture lens meaning that its maximum is f4. Under aperture priority then, I can set aperture from f4 to say f22, f32------- right? My F zoom 35-70 f3.5 -4.5 will change as I turn the zoom with max of f3.5 to f4.5 as you turn the zoom. So under aperture priority for this lens, say I have turned zoom to 50,then I can only set aperture from whatever the lens has set the aperture at 50, say f3.9 , from f3.9 to f22 etc. ??????????

Just need some verification of this.


Thanks
Correct
As you have already observed with a variable aperture zoom, the f-stop may vary as you zoom in if the value you set is not available at the focal length you zoom too. If you select f4 at 16mm on the 16-45 then zoom to 45, you will still have f4. If you select f3.5 at 35 on your 35-70, then as you zoom the f stop will remain at the widest possible until it reaches f4.5 sometime before 70mm.

(NB If you Google f-stop you can find a number of sites that will give you the actual definition, and demonstrate that a constanf f stop in a zoom means that the actual diameter of the aperture varies, whislt a variable f-stop means the diameter of the aperture actually varies!)
11-25-2012, 07:06 AM   #3
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f stop is an equation involving the diameter of the opening in the iris and the focal length of the lens, so the same diameter opening will be one f at say 50mm and a different one at 70mm. Constant aperture lenses allow you more control and predictability in light management but are more costly (and complex) to produce.
11-25-2012, 07:23 AM   #4
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QuoteOriginally posted by Docrwm Quote
f stop is an equation involving the diameter of the opening in the iris and the focal length of the lens, so the same diameter opening will be one f at say 50mm and a different one at 70mm. Constant aperture lenses allow you more control and predictability in light management but are more costly (and complex) to produce.
Well said and I will also add that since f stop is a ratio of length and diameter of the aperture opening, constant aperture zooms will also be larger and heavier.

11-25-2012, 07:37 AM   #5
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QuoteOriginally posted by Docrwm Quote
f stop is an equation involving the diameter of the opening in the iris . . .
Technically, I believe, the diameter of the "entrance pupil,"
the (possibly magnified) image of the iris opening
that you see looking through the lens from the front.

Entrance pupil - Wikipedia, the free encyclopedia

So the entrance pupil diameter of a zoom lens
could at least theoretically change (depending on the design of the zoom),
even without a change of the physical iris opening diameter!
This would cause variation in both numerator and denominator
of the f/E_N equation giving the f stop.
11-25-2012, 09:15 AM   #6
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QuoteOriginally posted by lytrytyr Quote
Technically, I believe, the diameter of the "entrance pupil,"
the (possibly magnified) image of the iris opening
that you see looking through the lens from the front.

Entrance pupil - Wikipedia, the free encyclopedia

So the entrance pupil diameter of a zoom lens
could at least theoretically change (depending on the design of the zoom),
even without a change of the physical iris opening diameter!
This would cause variation in both numerator and denominator
of the f/E_N equation giving the f stop.
This gets more complicated by the minute!
Not to mention T stops may vary as the various elements move relative to each other during zooming! I hope the OP doesn't get too confused. In simple terms, the F stop you dial in Av mode stays the same on zooming, and all f stops are available at all focal lengths
11-25-2012, 01:47 PM   #7
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Not sure how well the rule of thumb goes, but I have an old 75-260mm f/4.5 manual constant aperture lens, and while I *can* shoot it at 260mm at f/4.5 doesn't mean I *should* shoot it at 260mm @ f/4.5.

Image quality tends to tank badly when I do that.
11-25-2012, 03:54 PM   #8
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To answer the original question: yes.

AF variable-aperture lenses report the changing max aperture to the camera. So as you zoom in aperture priority, the camera will limit the aperture range to the correct max aperture.

I'm not sure whether KA-mount manual focus lenses also do this...

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