[Erasmo]

As most of you already know, most lenses are not sharpest wide-open. Stopped down they are sharper, but how much?
For conveniences sake i've made a little list that you can print out and put into your camerabag if you always tend to forget them like me
The information that i've used is from Photozone.de:

Pentax SMC-DA 14mm f/2.8 ED [IF] - f/8
Pentax SMC-FA 20mm f/2.8 - f/5.6
Pentax SMC-DA 21mm f/3.2 AL Limited - f/5.6
Pentax SMC-FA* 24mm f/2 AL [IF] - f/5.6 center f/8 borders
Pentax SMC-FA 31mm f/1.8 AL Limited - f/4 center f/5.6 borders
Pentax SMC-FA 35mm f/2 AL - f/4
Pentax SMC-DA 40mm f/2.8 Limited - f/5.6
Pentax SMC-FA 43mm f/1.9 Limited - f/4
Pentax SMC-FA 50mm f/1.4 -f/4 center f/5.6 borders
Pentax SMC-DA 70mm f/2.4 Limited - f/5.6
Pentax SMC-FA 77mm f/1.8 Limited - f/5.6
Pentax SMC-FA 100mm f/2.8 macro - f/8
Pentax SMC-D FA 100mm f/2.8 macro - f/5.6
Pentax SMC-K 135mm f/2.5 - f/5.6
Pentax SMC DA* 200mm f/2.8 ED [IF] - f/5.6
Pentax SMC-FA* 300mm f/4.5 ED [IF] - f/5.6

Pentax SMC DA 10-17mm f/3.5-4.5 ED - f/8
Pentax DA 12-24mm f/4 AL ED [IF] - f/4 center f/8 borders
Pentax SMC-DA 16-45mm f/4 ED AL - f/5.6
Pentax SMC-DA 18-55mm f/3.5-5.6 - f/8
Pentax SMC-DA 18-250mm f/3.5-6.3 ED- f/11
Pentax SMC-FA 20-35mm f/4 AL - f/8
Pentax SMC-DA* 50-135mm f/2.8 ED - f/5.6
Pentax SMC-DA 50-200mm f/4-5.6 ED - f/11
Pentax SMC-FA* 80-200mm f/2.8 [IF] - f/5.6

Some notes:
- Were there was rather large difference i have both center and border sharpness (mostly WA)
- Zooms are the average best settings over the whole range, if you are looking for a specific part of the range please look up the the test @ Photozone.
- Yes, there are some lenses missing from the list

[rparmar]

Not to disparage this collation of facts, but I don't think it's that important.

Better is to use the aperture that gives the depth of field that makes the image work. Sacrificing a little sharpness is well worth it for a better composition.

If ultimate sharpness is what you want, just use the FA 50mm f/2.8 macro for every shot.

[thePiRaTE!!]

Only recently, I encountered a lens that really challenged me to learn that such information as above can be very useful, but there is still more too it than this.

What I learned about about the Zeiss T*25/2.8 that changed the way I use all my lenses:

- It is critically sharp from wide open on details up to and including near infinity - that is to say - objects close to me that are shot at infinity focus setting. As objects or structures fall away to medium and distant ranges (all still infinity) there is a fairly dramatic fall-off of sharpness until the lens is stopped down. One could characterize the lens as being somewhat near-sighted at wide apertures.

- Progressively, each stop increases sharpness on distant infinity subjects, like landscapes, until f11 where its peak seems to be reached in this regard. Edge clarity at smaller apertures is improved for near infinity subjects and within, but center sharpness is simply held at the high level set out from f2.8.

Basically, in 'discovering' this lens, I realized that it can be used for both near and far work at high levels of performance, but the best f stop to use is different for each application even from only a max sharpness perspective. For example, the total MTS for f11 is lower than that of f8 or even f5.6, but in practice, the lens is clearly sharper at f11 when shooting landscapes.

I hypothesize that it is perhaps the case that if the lenses K housing is designed even nanometers too long for Pentax registration distances (the Distagon is a convert to several manufacturers distances afterall) that only by opening the depth of field can it truely include ideal focus upon distant infinity. But this is simply a hypothesis, and the fact remains that other lenses I've scrutinized based on this model have also exhibited variable handling nuances that leave optimal aperture a thing of context.

The photozone charts are measured at a specific distance not factoring in what I've mentioned above, something to consider before simply accepting them at face value.

[k100d]

thanks for the big list
on the K10D and K20D, the sharpest points of the aperture should be coded into the lens information and usable in the MTF mode for FA and later lenses.

[beaumont]

Erasmo, thx for taking the time to do this. I will indeed test these recommendations.
PiRaTE...thx for adding another dimension to the train of thought. (The learning never ends...)