Originally posted by Jonathan Mac I think I'm missing something - f/4 at 40mm is always f/4 at 40mm (let's forget the extra 3mm of the Limited prime in this case). Given the same subject distance and sensor size, while the out-of-focus area may have different characteristics between lenses, the degree to which the background (or foreground) is out of focus at a given aperture will always be the same. f/4 is f/4 regardless of whether you need to stop down to get there or that's the maximum aperture of the lens in question.
What you're missing is the difference between a lens having more glass/elements than another, this can have quite a different impact on how the bokeh renders and appears (as well as I imagine other things such as grouping and placement of the elements). If you click on that other link I posted you can see a fair bit of difference between the FA31 and FA35 on how the bokeh resolves around the jug. (
Hd fa35/2 - Page 3 - PentaxForums.com)
FA35, 6 elements, 5 groups
FA31, 9 elements, 7 groups
I have come to test and see for myself that the FA ltds seem to do a really nice job with how it handles bokeh. Those extra elements seem to account for a bokeh that you would normally associate from shooting at wider apertures, yet it's still able to put out that kinda look when stopped down. This is why these lenses are such a joy to use, you can shoot them a little more stopped down than wide open, gain the advantage of that (increased sharpness, more of the subject in focus) yet the compromise from bokeh that you would normally have to put up (from stopping down) does not quite apply the same. Really nice lenses for subject isolation, hence their preference for portraiture.
Originally posted by nocturnal F4 is wide open on DA20-40 Ltd so the aperture is a perfect circle with no blades forming a pattern which introduces a different kind of diffraction you get when stopping down on the FA 43 f1.9...
So 20-40 @ f4 @ 40mm has a perfecr circular aperture
and the FA 43 @ f4 has an octogal 8 sided aperture
Correct, and that comes with pros and cons. On the one hand the 20-40 is giving nice circular bokeh at stopped down apertures, on the other hand the roundness of the blades leads to less than stella star bursts (if that's your thing). Having compared how a FA ltd renders star bursts I would say it quite substantially trumps the 20-40 in this regard.
It's not a competition tho, I mean I own both glass for a reason, the 20-40 is after all also WR, compact and silent AF, my biggest reasons for adding it to my collection. But hopefully the picture of the ducky above can help show people how different lenses do behave differently at the same apertures and approximate distance to subject (and the jug comparison shots are helpful also in the other thread).
One of my favourite shots that I think really shows the FA43 at its best is this shot I took here;
Obviously not comparing with a 20-40 here, how can it? This is a f1.9 shot and the 20-40 at 40mm would be f4. But what we're looking at here is
how the bokeh is rendering. If we look at the wine glasses bottom left behind the girl the bokeh is layered, textured and rich. Having owned a DA50/1.8, FA50.1.4, DA40/2.8, I can honestly say that whilst those are great lenses and priced attractively with fantastic value to be had, the bokek for me was not resolving the same way. It was flatter and a little more... 'boring'.
The interesting thing about this shot is that even greater difference would have been seen had I stopped down to say f2.8, and then compared that with a DA40/2.8 or something, I think you will see how much more the subject appears isolated and the bokeh still retaining a more dreamy fade compared to a DA40/2.8 that starts to show that 'stopped down' look (similar to the jug examples).
Hope this helps! Sorry all for the slight deviation from the wonderful 20-40! Let's get back on track