[DaveHolmes]

I had the DA 35mm f2.4 on my radar, but I think the 50mm f1.8 now looks more interesting, at least as far as autofocus lenses go.

If it's possible I'd go for both... 35mm is a really useful focal length and the f1.8 aperture of the 50mm will give you good lowlight ability as well as subject isolation (ability to use a thinner DoF)... Could be a good '2-prime' set-up...

[epqwerty]

I agree with daveholmes id go for both. I currently have the 35 but i will surely get the 50 when it comes out it might retire my m50 1.4 just because of the af and ae benefits with the da

[lytrytyr]

how on earth can stopping down make it harder to get things in focus?

Sharp focus at f/1.4 is a compromise, involving uncorrected aberrations.
Stopping down slightly, not enough to give good depth of field,
cuts out some of the aberrations, so the compromise "sharply focused" plane (or curved surface) moves.

Do you mean that "focus shift" is something inherent to some lenses, so that you'd have to stop down with DOF preview before focusing?

Yes, it's an issue with critical focusing of many fast lenses.

I'm not trying to convince you to get an EE-s screen, just trying to share my experience with OP.

I understand that, but your experiences are valuable for the forum as a whole.

[stevebrot]

Stopping down means an increased DOF.

I am not sure what lytrytr meant, but I do know that DOF is not the same as being in focus. You can miss focus just as easily at f/16 as at f/1.4. It all depends on viewing distance and magnification for the final image. DOF just means acceptable focus at an accepted magnification and viewing distance. A good example would be my avatar image. Looks sharp, eh? Here is the original at 800 pixels wide:



Now, I nailed the focus at about f/4 with an 85mm lens, but as you can see, the limited DOF of the original is only appreciated when it is viewed big. The same shot at f/16, but with missed focus viewed at the same 800 pixel width would be acceptably sharp all round (similar to my avatar), but if viewed VERY BIG would show the missed focus quite nicely.

Now, there is the matter of missed focus. In general, a wider aperture lens should allow you to get more precise focus, but often that target is illusive due to field curvature and such. There is also the matter of the stock focus screens in Pentax dSLR cameras. I don't pretend to know the full explanation, but in real terms the affective DOF is about the same as viewing the same subject at f/4. This is something that can be shown be simple examination with a manual aperture lens. Overall field brightness and DOF does not change from about f/4 to f/1.4. As a result, there is a huge loss of precision at wider apertures. That is why so many of us that use faster manual focus lenses have opted for after-market screens. (In all fairness, this is a problem on non-Pentax cameras too.) The suggested EE-s screen is a good one, though I prefer a screen with microprism or split-image focus aid.


Steve

[RioRico]

I have not found slower lenses (of any quality) to be harder or easier to focus than faster lenses (of any quality) when used at similar focus ranges. Other factors may interfere -- my Macro-Tak 50/4 (1x) is harder to focus a 3m than is my K50/1.2, but that's because the macro lens has a VERY short focus throw from 1m to infinity.

My delaminating eyeballs need some focusing tricks:

1) A katzeye-clone focusing screen is useful for well-lit contrasty subjects, to let me know I'm near
2) Focus Confirmation tells me I'm really close; Catch-In-Focus aka Trap-Focus nails the subject
3) With zone-focus or hyperfocus, I set the aperture and prefocus for my desired DOF range
4) Last resort: I measure or pace-off or guesstimate the distance, and set the lens accordingly

IMHO major differences between f/1.4 and f/1.7 glass aren't action-stopping ability or sharpness, but brighter VF images and more DOF control with the faster lenses. Using a high-ISO camera only affects the action-stopping and is irrelevant to the other factors. Even Kr users love the K50/1.2!

[lytrytyr]

I am not sure what lytrytr meant, but I do know that DOF is not the same as being in focus.

We were discussing the phenomenon of focus shift at wide apertures with certain fast lenses.
Apertures wide enough to give very little depth of field.
A focus setting on the lens barrel that gives sharp focus at full aperture
might no longer give sharp focus when the lens is stopped down one stop.
It's a lens aberration that people accept as a side effect of superior rendering qualities.