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04-07-2013, 03:59 AM   #46
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My 40mm xs on a k-5 changes. if i keep exposure time constant and change aperture from 2.8 to 3.2, then to 3.5, the images get darker.

In addition, if i set aperture to 2.8 and turn the main switch to the right (optical preview), nothing happens as expected. if i set aperture to 3.5 and turn the switch, the finder gets darker and i can clearly hear the aperture change.

04-07-2013, 07:28 AM   #47
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A couple things to keep in mind. The 40mm XS has rounded aperture blades, so the bokeh won't become drastically different between 2.8 and 3.5. Some other lenses have a big change in bokeh, because they have few and straight blades, or a star shape aperture, but not the 40mm XS. Next, you might not see the aperture blades very clearly because they don't really do much at those apertures, they only touch the corners, so look at the sides of the diaphragm.
Here is another silly way to test it: Use the 2 second timer, but look into the lens. When you press the shutter, the camera will lift the mirror and set the aperture and wait for 2 seconds. In that time you can hear or see the aperture moving.

Though, a lot of older lenses with aperture rings don't have half stops between the "wide open" and the next one, maybe this is a remnant of that. But who knows. Taking photos of measuring tapes to figure out the bokeh and DoF probably won't work, because the differences are so small.

Edit: I've just checked with my 40mm XS and K-r. The change in photos is pretty much nonexistent, between f2.8 and f3.5. But using the 2 sec thingy, you can see a small difference.
40mm/2.8=14,3
40mm/3.2=12,5
40mm/3.5=11,4
So the difference in the lens between f2.8 and f3.2 will be 1.8mm, spread around the circle (because diameter is reduced on all sides). So 0.6mm on a side is all you can notice. Even at f3,5, the aperture blades will only stop down 3mm, which is 1,5mm on the side.

Last edited by Na Horuk; 04-07-2013 at 07:44 AM.
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