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01-27-2015, 05:06 PM   #1
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Aperture indicated and actual aperture settings

Well I have held off on this question long enough. I need to understand why on a lens the aperture indication differs from the actual lens aperture settings. For example I have been looking at a SMC Pentax-FA 28~80mm F3.5~4.7. However, on the aperture itself I can see the 3.5, but not the 4.7. So why would the numbers on the lens differ from the settings on the aperture? Inquiring minds want to know.

Thanks,

Tonytee

01-27-2015, 05:11 PM   #2
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The 3.5 setting becomes 4.7 when you zoom in to 80mm. In other words, the numbers on the lens no longer correspond to the actual f stop beyond 28mm. When you're zoomed in all the way, you have to factor in a loss of just over 1 stop.

Don't worry, though, because your DSLR is well aware of the change in aperture as you zoom, and will still meter correctly as long as the aperture ring is set to "A".

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01-27-2015, 05:58 PM   #3
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Thank you for your informative reply.

Regards,

tt
01-27-2015, 06:14 PM   #4
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Typically you take the focal length of the lens and divide it by the aperture rating and it will give you the size of the opening that lets the light through, so for a 50mm 1:1.4 you might expect to see the rear element ~35-36mm opening. Because the element is clearly a fixed size you would expect for the aperture to change as you zoom, as it is letting less light in given the extended focal length.

This is not always the case because it is not always possible to have openings large enough to accomplish this with standard design/mechanical processes so the elements will magnify for extra light. The more expensive constant aperture lenses use intuitive design and expensive elements to accomplish the task of letting more light in with the longer focal lengths.

01-28-2015, 02:04 PM   #5
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QuoteOriginally posted by Adam Quote
The 3.5 setting becomes 4.7 when you zoom in to 80mm. In other words, the numbers on the lens no longer correspond to the actual f stop beyond 28mm. When you're zoomed in all the way, you have to factor in a loss of just over 1 stop.

Don't worry, though, because your DSLR is well aware of the change in aperture as you zoom, and will still meter correctly as long as the aperture ring is set to "A".
In fact, most Pentax DSLRs will display the aperture in the viewfinder. So, you might set your lens at 28mm, use the camera to select f/3.5 in aperture priority mode, then change only the zoom ring on the lens to 80mm. The viewfinder should show f/4.7 as the aperture.
01-28-2015, 04:46 PM   #6
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QuoteOriginally posted by lsimpkins Quote
In fact, most Pentax DSLRs will display the aperture in the viewfinder. So, you might set your lens at 28mm, use the camera to select f/3.5 in aperture priority mode, then change only the zoom ring on the lens to 80mm. The viewfinder should show f/4.7 as the aperture.
Thank you for your comprehensive reply. I suppose I should have mentioned that I am still living in the Stone Age. By that I mean I am still hooked on film photography. Although it shouldn't matter as long as I follow the rules explained on this post. Again, many thanks.

tt
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