Forgot Password
Pentax Camera Forums Home

Show Printable Version Search this Thread
04-07-2013, 03:59 AM   #46
Junior Member

Join Date: Apr 2013
Posts: 36
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
Na Horuk's Avatar

Join Date: Feb 2012
Location: Slovenia, probably
Photos: Gallery | Albums
Posts: 10,867
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.
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.

  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
40mm xs, experiment, f2.8, f3.5, guys, k-mount, pentax lens, picture, shutter, slr lens, xs
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Similar Threads
Thread Thread Starter Forum Replies Last Post
Cap holder for Pentax-DA 40mm F2.8 XS lens mastermisho Pentax K-01 5 07-22-2013 10:23 AM
For Sale - Sold: SMC Pentax-DA 40mm F2.8 XS Guakala Sold Items 6 03-23-2013 11:52 AM
Pentax Normal Lens Shootout- 40mm ltd, 40mm xs, 35mm f2.4 Adam Pentax SLR Lens Discussion 6 07-11-2012 08:09 AM
For Sale - Sold: DA 40mm F2.8 XS relyt Sold Items 7 06-06-2012 05:24 PM

All times are GMT -7. The time now is 12:39 PM. | See also:, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]