A 24-50 aperture not entirely open from 24-35?

gdneil · 11-14-2018, 05:13 PM

Got an A24-50 recently and noticed that from 24-35 or 40mm the aperture is partially closed - when set at f4. At 50mm it opens completely. At 24 it would be closed to approximately f5? Moves along in between.

Anyone else have any issues like this?

Otherwise seems good, other than that I seem to be less adept than I had hoped at manual focus at wide angles.

-n

Adam · 11-14-2018, 05:21 PM

I think this is normal. Some lenses do this to help ensure the maximum aperture remains constant as you zoom.

Not a Number · 11-14-2018, 05:22 PM

It's probably because it is a constant or fix aperture zoom vs a variable aperture zoom. A variable aperture zoom would have different widest f-stops at either end of the zoom range.

**thix2112** · 11-14-2018, 06:33 PM

Now I’m curious. I checked my A24-50 f4 and yes, the aperture opens when zoomed to 50mm. By the way, it’s a great lens. Sharp and my standard go-to lens my K1ii
Correction - it’s an F lens with auto focus.
Tom
Yes my A manual focus lens does the same.

Last edited by thix2112; 11-14-2018 at 06:39 PM.

**volley** · 11-15-2018, 04:50 AM

Well, this is how it must be, no surprise.
constant f=4 means that the ratio between focal length and diameter of the aperture is constant and equal to 4. When the FL changes in a zoom the diameter has to chance as well. Otherwise it would not be a "constant aperture zoom"

D1N0 · 11-15-2018, 04:52 AM

I have the same with my 28-105 f4-5.6 pz.

gdneil · 11-15-2018, 10:19 AM

Ok, thanks everyone - good to know! I had seen this ever so slightly in with the A35-105 and others, but it was nowhere near as obvious... Of course I kind of wonder what the aperture would be at 24mm if that didn't happen? Hmm....

11-14-2018, 05:13 PM	#1
gdneil Veteran Member Join Date: Mar 2013 Location: Yukon Posts: 516	A 24-50 aperture not entirely open from 24-35? Got an A24-50 recently and noticed that from 24-35 or 40mm the aperture is partially closed - when set at f4. At 50mm it opens completely. At 24 it would be closed to approximately f5? Moves along in between. Anyone else have any issues like this? Otherwise seems good, other than that I seem to be less adept than I had hoped at manual focus at wide angles. -n

11-14-2018, 05:21 PM	#2
Adam Administrator Site Webmaster Join Date: Sep 2006 Location: Arizona Photos: Gallery \| Albums Posts: 51,595	I think this is normal. Some lenses do this to help ensure the maximum aperture remains constant as you zoom. Adam PentaxForums.com Webmaster (Site Usage Guide \| Site Help \| My Photography) PentaxForums.com server and development costs are user-supported. You can help cover these costs by donating or purchasing one of our Pentax eBooks. Or, buy your photo gear from our affiliates, Adorama, B&H Photo, KEH, or Topaz Labs, and get FREE Marketplace access - click here to see how! Trusted Pentax retailers:

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11-14-2018, 05:22 PM	#3
Not a Number Moderator Join Date: Mar 2012 Location: Venice, CA Posts: 10,526	It's probably because it is a constant or fix aperture zoom vs a variable aperture zoom. A variable aperture zoom would have different widest f-stops at either end of the zoom range.

11-14-2018, 06:33 PM	#4
thix2112 Site Supporter Join Date: Jan 2013 Location: North Alabama Posts: 237	Now I’m curious. I checked my A24-50 f4 and yes, the aperture opens when zoomed to 50mm. By the way, it’s a great lens. Sharp and my standard go-to lens my K1ii Correction - it’s an F lens with auto focus. Tom Yes my A manual focus lens does the same. Last edited by thix2112; 11-14-2018 at 06:39 PM.

11-15-2018, 04:50 AM	#5
volley Site Supporter Join Date: Jan 2009 Location: Springe Photos: Gallery Posts: 3,693	Well, this is how it must be, no surprise. constant f=4 means that the ratio between focal length and diameter of the aperture is constant and equal to 4. When the FL changes in a zoom the diameter has to chance as well. Otherwise it would not be a "constant aperture zoom"

11-15-2018, 04:52 AM	#6
D1N0 Pentaxian Join Date: May 2012 Location: --- Photos: Gallery Posts: 6,802	I have the same with my 28-105 f4-5.6 pz.

11-15-2018, 10:19 AM	#7
gdneil Veteran Member Join Date: Mar 2013 Location: Yukon Posts: 516 Original Poster	Ok, thanks everyone - good to know! I had seen this ever so slightly in with the A35-105 and others, but it was nowhere near as obvious... Of course I kind of wonder what the aperture would be at 24mm if that didn't happen? Hmm....