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Comparison: FA 31mm f1.8 Limited to DA 40mm f2.8 Limited

he question was brought up on Pentax Forums recently “Is the 31mm Limited really 2.8 times better than the 40mm Limited?” As you can guess, the debate has been raging on for many pages.

FA 31mm f1.8 Limited to DA 40mm f2.8 Limited together:


The 31mm and 30mm with a Canadian coin.


To start things off, that photo describes why I advocate buying both. While their usage can overlap, the sheer size difference puts them in different categories.

FA 31mm differences:
Almost the perfect general purpose lens with a very normal focal length in APS-C.
No quick-shift focus system is a pain sometimes.
Average sized and gives your left hand something to hold.
Wide aperture for background blur and low-light photography.
Big front element with the build in hood isn’t ideal for current Pentax DSLRs.
Has an aperture ring.
Front element and filter threads are a little different. Took me three tries to find a filter that I could safely use (Hoya HMC and S-HMC are good, I opted for a S-HMC with this lens). Many of the cheaper filters have the glass too close to the thread end, so the filter glass and the center of the front element on the 31mm would touch.
Less vignetting at f2.8 than the 40mm, even with a 9mm wider view.

DA 40mm differences:
  • 40mm on APS-C is a bit tight sometimes. My biggest issue.
  • Quick shift focus system is great.
  • Super small in size, which is usually good, but sometimes bad because there isn’t much for your left hand to hold.
  • Screw on lens hood is good and bad. I like how it protects the front element, but it takes a long time to screw on and off. Sometimes I use a plastic clip on lens cover from another lens.
  • No aperture ring.
  • Very low chromatic aberrations, less than the 31mm throughout the aperture range. 
How are they alike?
  • Produce (in the words of Steve Jobs) magical images, which are sharp and full of contrast.
  • Have solid metal construction.
  • Metal lens hoods and caps.
  • Have fast screw-drive auto-focus.
  • Should be able to cover full-frame cameras (DA 40mm referenced test on PF).
  • Have very minimal barrel distortion.
  • Don’t have weather resistance. The big image comparison:

FA 31mm f/5.6

FA 31mm f/1.8

FA 31mm f/5.6

FA 31mm f/2.8

FA 31mm f/16

FA 31mm f/1.8

FA 31mm f/2.8

FA 31mm f/16

DA 40mm f/2.8

DA 40mm f/5.6
DA 40mm f/16

DA 40mm f/16

Each lens was set to a range of aperture values, but the shutter speed and ISO were consistent between lenses/aperture values. All images were taken at ISO 400, so f16 required long shutter speeds. You can see all of that information in the EXIF of course.


In conclusion, when I want to one lens to do everything, I use the 31mm. When I want the smallest setup possible, I use the 40mm. I actually bought the 31mm after owning the 40mm for a few years. My justification on holding out was that that the 31mm is expensive, but I see now that it is worth the asking price. After owning both I have to say I never plan on selling either of them. They both have unique and useful qualities and are great photography tools.
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