[rawr]

I would like to know what the sweet spot is for these 4 lens ie what fstop is best/sharpest.

You can study reviews of Pentax lenses on web sites like PhotoZone or Lenstip to get a feel for the aperture settings, focal length etc that work best for sharpness/resolution on lenses. Between them those sites have, I believe, tested most of the lenses you have (or very similar models).

[clickclick]

clickclick, I have the genuine pentax m42 adapter. The problem is I don't want to switch back and forth all the time. If/when I use them it will be for a month straight just to test/try them out. And I'm worried about dust getting into my sensor.

super tak 1:1.8/55
vivitar 1:2.8/35mm
revuenon 1:1.8/50
vivitar 1:3.5/200mm

paid less than $100 combined

I understand the concern, but I change lenses all the time. Otherwise, my LBA issues would be without merit . I just try to do it in as controlled an environment as possible, even if that means putting my back to what I perceive as the wind and holding the camera face down. I've got a copy of the 1.8/55 Takumar in my collection. Nice lens. I don't have any experience with the others, but have a go and see how you like them.

[normhead]

Go to any lens testing website and you'll see results like this
Pentax HD DA 16-85mm f/3.5-5.6 ED DC WR - Review / Test Report - Analysis

If you want the sharpest possible image you'll shoot ƒ5.6.
The FA 31 is sharpest at ƒ2.8, the DA 70 is sharpest at ƒ4, the DFA 50 macro is sharpest at ƒ4 as is the DA 35 2.4.

However that being said, because of Depth of Field more of the subject may be in acceptable focus at ƒ8 than ƒ5.6, which may mean more of the image is sharper, even if it's not as sharp. But then the differences are probably not really noticeable in all the lenses mentioned. You're talking about differences you won't really see.

The 55-300 probably goes the other way being sharpest at ƒ8.

For the lenses I own, I shoot almost everything at ƒ5.6 when DoF permits. All these lenses have started the downward march to diffraction fall off by ƒ11.. most have fallen from excellent to very good by ƒ11. No lens good or bad is at it's best at ƒ11. You shoot ƒ11 for wider depth of field, not sharpness, although the shamrpness may possibly meet your needs for a particular shot.

The DFA 50 macro, sharpest at ƒ4....out of the excellent zone by ƒ11, falling off the charts by ƒ22, fairly typical excellent lens behaviour.

[Wheatfield]

Hi I have a K50 body and I picked up some lens. I would like to know what the sweet spot is for these 4 lens ie what fstop is best/sharpest.

1: smc pentax fa 1:4-5.6, 35-80mm

2:: tamron af 18-250mm, 1:35-6.3 if macro

3: smc pentax dal 55-300mm 1:1:4-5.8 ed

4: smc pentax dal 18-55mm 1:3.5-4.6 al wr

should I go by the f7-11 rule or just keep them all at f9

thanks

Take each lens and shoot the same picture at each aperture, then look at the images.

[enyaw]

@ wheatfield, thanks but that would take quite awhile. and I thought each lens has a sweet spot so to say. I'd be happy with that instead of shooting 100 pics just to see where each lens is sharpest and I might even get that wrong. Normhead thanks for the link to that lens site, I'm going to bookmark it. I'm looking for center sharpness or the highest # on the scale that you pasted in your response. That is what I'm aiming for. rawr I'll look at those sites too. I think I'm well armed now

thanks,

ps, I thought those lens testing sights were to be taken with a grain of salt because their results/comparisons aren't consistent. ie diff bodies, diff..... etc

[normhead]

@ wheatfield, thanks but that would take quite awhile. and I thought each lens has a sweet spot so to say. I'd be happy with that instead of shooting 100 pics just to see where each lens is sharpest and I might even get that wrong. Normhead thanks for the link to that lens site, I'm going to bookmark it. I'm looking for center sharpness or the highest # on the scale that you pasted in your response. That is what I'm aiming for. rawr I'll look at those sites too. I think I'm well armed now

thanks,

ps, I thought those lens testing sights were to be taken with a grain of salt because their results/comparisons aren't consistent. ie diff bodies, diff..... etc

Ya, there are lots of reasons to second guess their results, the biggest being most of them don't test enough lenses to compensate for sample variation. But, at least they ran some constant tests on some lenses on the same bodies giving you some comparison points. To me, even with doubts about accuracy etc. I have fewer doubts than someone who has run less rigorous tests on a few lenses they own. The may be worth a grain of salt, but it's bigger grain of salt than a lot of what' is out there. Bottom line, you have to be happy with the end. What these tests measure doesn't always ad up to a commentary on how the lens renders, or how much you are going to like it's rendering.

As you can see in tests like this, people don't always prefer the rendering of the sharpest lens.Regardless of which lens is the sharpest, most people don't prefer it, because the majority of the voters didn't prefer any single lens.
35mm find that prime. - PentaxForums.com

One of my favourite lenses. the FA 35-80, I don't use much because the build quality is so bad, I'm afraid I'll break it. I use it 2 or three times a year.

[enyaw]

lol, thanks. I've lowered the fstops to 5.6-8. Should be good. I'll play in that ballpark

[enyaw]

norm, is that the same lens as my 35-80mm if so what do you shoot it at? and at that price use the hell out of it. How does the macro come into play. at what range on the mm scale? can't find much data on that lens, she's old.

[kh1234567890]

should I go by the f7-11 rule or just keep them all at f9

If you are using an automatic Pentax lens just stick your camera on the MTF program and shoot away ...

[BrianR]

ps, I thought those lens testing sights were to be taken with a grain of salt because their results/comparisons aren't consistent. ie diff bodies, diff..... etc

You have to be aware of their limitations, but for the simpler question of "which aperture is the sharpest for lens X", many of the problems are gone by default - photozone gives a chart for a given lens at multiple apertures tested with the same procedure by the same person on the same body (the do re-test some lenses on multiple bodies). You aren't comparing a number spat out by one tester with a number spat out by someone completely different.

The biggest question remaining is whether all copies of a given lens will have their 'sweet spot' at the same aperture. I don't know, but it likely doesn't matter as they don't often have huge drops a stop up or down from the sweet spot. You can always use the test charts to narrow down your own test apertures to investigate, i.e. if it shows f/5.6 is the 'best', then try f/4 to f/8 and see which you prefer or which you find acceptable. This doesn't require hundreds of shots and shouldn't take long at all to get an eyeballed result. You might also find that you don't have to be super picky about selecting the 'best' aperture.

[normhead]

You might also find that you don't have to be super picky about selecting the 'best' aperture.

There are two ways to select aperture. Select the widest aperture you can for the depth of field you need, or stopped down as far as possible for the "everything in focus" look, common in landscape. I even go to ƒ16 on 50mm at times for that, or as close to wide open as possible to blur your background. I generally shoot ƒ5.6 by default, but I don't hesitate for even an instant if anything less than ƒ22 will be more appropriate to the shot. You can know where the lens is sharpest, but you still don't want to try and make a living there.

It's not that uncommon for anything from ƒ1.4 to ƒ16 on APS-c, to give you the best image in a particular circumstance.

[CarlJF]

There are two ways to select aperture. Select the widest aperture you can for the depth of field you need, or stopped down as far as possible for the "everything in focus" look, common in landscape. I even go to ƒ16 on 50mm at times for that, or as close to wide open as possible to blur your background. I generally shoot ƒ5.6 by default, but I don't hesitate for even an instant if anything less than ƒ22 will be more appropriate to the shot. You can know where the lens is sharpest, but you still don't wan to try an make a living there.

It's not that uncommon for anything from ƒ1.4 to ƒ16 on APS-c, to give you the best image in a particular circumstance.

This is exactly it! The optimum sharpness for a lens, although good to know, should not dictate which aperture you use. You should decide the aperture based on the required DOF.

I would even say that what's need to be know isn't the point of optimum sharpness, but rather the other way. It's much more important to know the conditions under which the lens IQ is crap enough that you don't want to use it...

[normhead]

This is exactly it! The optimum sharpness for a lens, although good to know, should not dictate which aperture you use. You should decide the aperture based on the required DOF.

I would even say that what's need to be know isn't the point of optimum sharpness, but rather the other way. It's much more important to know the conditions under which the lens IQ is crap enough that you don't want to use it...

I use my 18-135, which has very soft borders from about 70mm on, only when a centre sharp image with a blurred background is needed. Then the softness helps smooth out the out of focus areas, and I still get excellent sharpness in the centre. If I want edge to edge at 135 for a landscape, I'll probably switch to the 60-250. You have to learn to shoot each lens to it's strength.

Most modern Pentax lenses are useful for something. The trick is to not buy one that's useful for one thing, and try and use it for something else. I love my 60-250 for landscape but because it's about the same as 135mm lens for subjects 10 feet away. For birding I ended up buying a couple of 2.8 lenses that give me much more magnification for small close subjects, the Pentaxx DA*200 and TRamorn SAF 300. The 60-250 is a great lens for what it does sharp everywhere, but if you buy it for birding you are going to be disappointed, even with the 1.4 on it. For larger wildlife that you are further away from, its awesome.

That's why my advice in these types of situations is "find someone who shoots the way you want to shoot and buy what they have." There are many characteristics besides sharpness that come into play. You won't find many people shooting birds with a DA*60-250. I should have paid more attention to that before i bought the lens. But, it's such a good lens I use it for other things and it's a great walk around lens on the K-1 now that I have one. Still I had that month of disappointment when I was thinking "I can't believe I paid $1400 for a lens that doesn't even do what I wanted it to do." Fortunately it does a lot of other things extremely well, and I've never for a second considered selling ti.

[BrianR]

There are two ways to select aperture. Select the widest aperture you can for the depth of field you need, or stopped down as far as possible for the "everything in focus" look, common in landscape.

Or shoot at the sharpest aperture as the OP is trying to do, or by the shape of the out of focus highlights your want (nice stars, clean round balls, etc) or any other number of things you can choose to prioritize.

My point was to investigate a stop in either direction of the 'best', where 'best' was defined by the OP's sharpness criteria (partly in the hopes that starting a simple test might lead to a more thorough investigation others had suggested). It's a compromise as you move away from the sharpest stop, how much of one to be determined by pretty easy testing. Once you know how much of a compromise it is, you can better weigh it against the gains you get in other areas, e.g. if f/5.6 is the sharpest, f/8 might still be "sharp enough" for your use, and the extra DoF may be worth it, same for f/11, or f/16, or f/22 or however far into diffraction land you want to go. Likewise the other direction for faster shutter and/or lower iso. There's rarely a one size fits all compromise, and the only way to be sure is to try it out yourself, either with some kinda dull tests, or just setting out to shoot your usual subjects at a few different settings and learn as you go.

[wildman]

I would like to know what the sweet spot is for these 4 lens ie what fstop is best/sharpest.

for most practical purposes "sharpness" is not a issue between f/4 to f/11 for modern.glass but DOF may be.