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08-23-2016, 03:37 AM   #1
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Is it normal behavior of aperture in Pentax DA*16-50 ??

Hello everybody. I have Pentax DA*16-50 mm lens for testing. I was wondered that at 16 mm the aperture is nor fully open (about f3,2-3,5). It opens to full extent (f2,8) at zooming to 50 mm.
My question is: Is it normal?
Thank you for answers.


Last edited by Medex; 03-11-2017 at 04:09 AM.
08-23-2016, 03:44 AM   #2
pjv
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Hi Medex. I just checked my copy and it is the same. Will be watching this thread with interest.
08-23-2016, 03:47 AM   #3
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QuoteOriginally posted by pjv Quote
Hi Medex. I just checked my copy and it is the same. Will be watching this thread with interest.
thank you. I noticed the same behavior with DA* 60-250 mm. At 60 mm aperture is not fully opened also.
08-23-2016, 03:55 AM   #4
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QuoteOriginally posted by Medex Quote
I noticed the same behavior with DA* 60-250 mm. At 60 mm aperture is not fully opened also.
My 60-250 also. As both our lenses are behaving the same way, they must be correct. ( I hope !!! )

08-23-2016, 05:02 AM - 1 Like   #5
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I think this is to keep constant f/stop throughout the zoom range. Remember the f/stop number describes the aperture diameter in terms of focal length divided by that number. The longer the focal length, the wider the aperture diameter needs to be for the same f/stop.
08-23-2016, 05:04 AM   #6
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Absolutely normal. That's the price you pay for having a constant aperture lens.
08-23-2016, 05:50 AM   #7
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Yes, some constant aperture zooms do this -- artificially close down the aperture at wide angle so they can be marketed as "constant aperture".

That lens could be 16-50 mm f2.4-2.8 but then people would complain.

It's the price you pay for marketing.
08-23-2016, 07:40 AM   #8
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But does this actually affect the image? You can take a photo "wide open" and then look at the bokeh highlights to see if it is round and so on. The little closing down of aperture at the wide end might not actually affect the image circle on apsc.

08-23-2016, 07:58 AM   #9
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QuoteOriginally posted by Na Horuk Quote
The little closing down of aperture at the wide end might not actually affect the image circle on apsc.
The stopping down with focal length change is intentional and effective. Similar mechanisms are used in all constant aperture zooms. Whether there is an affect on the appearance of OOF highlights is easy to test. To the best of my knowledge, nothing of this sort has been noted by users on this site.


Steve

Last edited by stevebrot; 10-05-2016 at 12:42 PM.
10-05-2016, 12:06 AM   #10
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Here's a good article about that, scroll to fixed aperture zoom.
Get The Most Out Of Variable Aperture Lenses - Outdoor Photographer
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