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06-08-2011, 02:13 AM   #1
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DA 35 f1.4 & Sweespot question

I would have thought sweetspot is the position in which my lens will produce the sharpest as my understanding goes.

Does this mean, we may not be given a chance for landscape shots in this spot? Why I ask this?

I hear the f stops always being mentioned for sweetspots are neither wide open nor closed. I see the numbers f8 f16 being mentioned, which I feel is okay for f16 but when it comes to f8 I asking to myself given the DOF in the f stop how much will be sharp and presumably I guess the spot that is in focus to get the sharpest.

It even confuses me more when somebody mentioned in another thread that the sweetspot for DA 35 F2.4 is at F4!! Which means the lens will give the best sharpness here and given the DOF I once again presume the spot in focus to be getting this advantage and so comes my new question - will I not be able to do a sharp image for landscape compared to the sharpness that it produces at F4?

Also, please confirm - to find the Sweetspot I will have to click at each f stop the same scene to identify. Is this the only way or is there some online tool or been recorded somewhere online the sweetspot for each lens.

EDIT: Blue,thanks for spotting the wrong fstop entered by me, now stands corrected


Last edited by sany; 06-08-2011 at 11:22 AM.
06-08-2011, 02:21 AM   #2
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You have sharpness and sharpness. It might very well be that the 35mm f/2.4 is at its sharpest at f/4, but wide-open, it's still a heck of a lot sharper than i.e. the kit lens, esp. in the corners of the frame. The kit is renowned for being sharp at f/8-f/11 at 35mm, but I would expect the plastic wonder to at least match the kit in terms of sharpness at the same f-stop.

In short, it's all relative. Both have sweet spots, but primes will nearly always be sharper than zooms. And the 35mm f/2.4 is one sharp lens, that's for sure.

For landscapes at 35mm, I think you probably want to go to at least f/8. Wider angles are more common in landscape photography, in which case e.g. f/5.6 can also be used without a problem.
06-08-2011, 03:11 AM   #3
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There are two separate issues. Depth of field and sharpness. You choose your aperture based on the depth of field you want and your light level. There are on line calculators that will tell you what your depth of field will be for a given focal length, aperture and distance to subject.

As far as sharpness goes, web sites like photozone test lenses and find these things out (Pentax SMC DA 35mm f/2.4 AL - Review / Lens Test - Analysis). If you go to the site, you find that the DA 35 f2.4 really had maximal sharpness at f4 and continued to maintain sharpness till f11. This is pretty typical performance for a good prime. Stop down a stop to a stop and a half to get maximal sharpness (although it looks like the DA 35 is pretty sharp at f2.4).
06-08-2011, 06:04 AM   #4
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QuoteOriginally posted by sany Quote
. . .

It even confuses me more when somebody mentioned in another thread that the sweetspot for DA 35 F1.4 is at F4!! Which means the lens will give the best sharpness here and given the DOF I once again presume the spot in focus to be getting this advantage and so comes my new question - will I not be able to do a sharp image for landscape compared to the sharpness that it produces at F4?

Also, please confirm - to find the Sweetspot I will have to click at each f stop the same scene to identify. Is this the only way or is there some online tool or been recorded somewhere online the sweetspot for each lens.
Wow, Pentax finally released a new fast glass.

06-08-2011, 08:56 AM   #5
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Easiest way to learn the characteristics of a lens is to look at the MTF (sharpness) numbers in the review on photozone.de. You'll be able to see for yourself what aperture is sharpest for the center and for the corners (often, these are two different apertures).

But it's a mistake to think about this to much. DOF and shutter speed are far more important considerations in choosing an aperture than tiny differences in sharpness. If you need f/16 to get the DOF you want, then you shoot f/16, even if that makes the center slightly less sharp than it might have been at f/4. And if you need f/2.4 to get a fast enough shutter speed to combat blur, then you shoot f/2.4.
06-08-2011, 09:11 AM   #6
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QuoteOriginally posted by Blue Quote
Wow, Pentax finally released a new fast glass.
my thoughts to at the moment of clicking.

Well there almost no lenses that have their prime resolution/sharpnes at wide-open aperture. There is always some gain in image quality when stopping down.

When you set your camera to MTF setting then you will see what aperture is preferred with this lens.
06-10-2011, 10:10 PM   #7
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thanks all!
06-13-2011, 11:36 PM   #8
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QuoteOriginally posted by RonHendriks1966 Quote
When you set your camera to MTF setting then you will see what aperture is preferred with this lens.
As its what I would have suggested, i will elaborate a little on this concept. In the menu setting, you have the ability to set the program line. The Program line is the bias that the onboard computer will use while calculating the aperture and shutter speed for the metered light. For instance you can favour high shutter speed, or large DOF. If you set Program line to MTF, with Pentax lenses DA, DFA, FA, FAJ, the lens communicates its MTF chart to the camera body, enabling the computer in the camera body to select the optimum aperture for the attached lens for the shooting conditions. In effect, the camera will select the sweetspot of the lens for you automatically. Note that this only works with the mentioned lenses, not with lenses from third parties. I think it also works with F*300/4.5 which is probably the first lens it was implemented on.

06-14-2011, 02:30 AM   #9
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selar, thanks for getting your thoughts in. I will need to play with the MTF setting to understand this. I kept reading 'program line' on the manual but never understood what it meant, now it starts vaguely clear the clouds out...
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