This is where Ian Burley, DPNow's editor and founder, shares his unique thoughts and impressions on subjects that he hopes will be of interest to others on the subjects of digital photography and other related or loosely related topics! You can follow DPNow Editor's blog on Facebook and Twitter, too.
Diffraction in a small sensor compact system camera - the Pentax Q
Posted 20-03-12 at 03:02 PM by Ian
After yesterday's blog post focusing on sensor anti-aliasing and diffraction limiting, I thought a look at diffraction with a small sensor compact system camera might be interesting.
I have had a Pentax Q on loan (thanks to Jessops) for a while and it's going back today, so before I packed it away I have produced another set of crop images to explore the diffraction characteristics of this tiny camera. The Pentax Q Standard Lens is an 8.5mm focal length (5.6x crop factor making the lens equivalent to a 48mm full frame lens). Compared to yesterday's tests using Olympus DSLRs and a 50mm lens (100mm equivalent in full frame terms) I positioned the camera closer to the test subject so the crops are about the same in 1:1 terms but do bear in mind the much closer camera position.
Apologies for the variable density in these samples but the clouds were messing around with the sun at the time!
In theory, the really tiny 1/2.3 inch sensor (1/8th the area of a Four Thirds/Micro Four Thirds sensor and 1/30th the area of a full frame sensor) used by Pentax for the Q is diffraction limited from any aperture smaller than f/2.8. In the results above it's difficult to see much difference between f/2.8 and f/4, although the f/4 shot is more contrasty. But diffraction softening is clearly visible at f/5.6 and is detectably worse at f/8 - the smallest aperture you can select. Incidentally, at f/1.9 and f/2.0 I had to resort to the camera's built in Neutral Density filter to avoid over exposure even though the ISO setting was at its lowest - 125.
One problem is that even at optimum apertures the images are rather soft. I don't think the lens can be blamed as it's not a zoom. Although the test images were produced from Pentax Q DNG RAW files using Lightroom 3.7 (I really must get around to upgrading to 4.0!) it seems like Pentax must be using a fairly strong anti-aliasing filter.
So my advice to Pentax Q owners - if you want to avoid diffraction completely, stick to f/2.8 or brighter, but f/4 can be used with confidence the rest of the time.
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