[IMG]http://dpnow.com/imagelib/html/8101/product_views/pentax-q_04.jpg[/IMG]
After [URL="http://dpnow.com/forum2/blog.php?b=305"]yesterday's blog post focusing on sensor anti-aliasing and diffraction limiting[/URL], 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.
[LIST][*][URL]http://dpnow.com/files/forum/anti-aliasing_comparison/PentaxQ_IMGP0739_f_1.9.jpg[/URL][*][URL]http://dpnow.com/files/forum/anti-aliasing_comparison/PentaxQ_IMGP0751_f_2.0.jpg[/URL][*][URL]http://dpnow.com/files/forum/anti-aliasing_comparison/PentaxQ_IMGP0750_f_2.8.jpg[/URL][*][URL]http://dpnow.com/files/forum/anti-aliasing_comparison/PentaxQ_IMGP0749_f_4.0.jpg[/URL][*][URL]http://dpnow.com/files/forum/anti-aliasing_comparison/PentaxQ_IMGP0748_f_5.6.jpg[/URL][*][URL]http://dpnow.com/files/forum/anti-aliasing_comparison/PentaxQ_IMGP0746_f_8.0.jpg[/URL][/LIST]

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.