[digitaldaydreams]

Hi
MY QUESTION: What's the sharpest 20mm FF lens (third party lens ok) option for k1 that has filter threads (no funky hood thread adapters) and little or no vignette issues when using the screw on filter GND.

Here's what I'm doing with it.
Now that I have a Pentax K1, I would like to produce an image using the whole sensor (not crop mode) to produce the largest sharpest print I've ever made. This is to be a landscape shot with the subject matter in each corner that needs to be sharp in the extreme corners. This will be a pixel-shift shot with nothing moving in the scene. This will have nothing closer than 20ft in the foreground and all in focus out to the clouds, so the f stop may be about 11 to help with corner sharpness and DOF.

Thanks for any help.

[pepperberry farm]

the only FF 20mm lenses that come to mind are the Pentax A/FA 20/2.8 and the Sigma 20/1.8 EX DG....

I have the Sigma and it is very good, but I've never used it on FF (APS-C only here)...

[biz-engineer]

Sharpness of all lenses fall off towards the edges and especially in the corners. The wider the lenses, the more pronounced is sharpness fall off towards the edges. MTF charts show between 20% up to 50% drop of resolution for any lens, usually around 20% to 30% for wide angle lenses. Prime lenses tend to be better, but still not perfect. If I wanted a wide angle image with sharp corners, I wouldn't use a 20mm wide angle lens, I'd use a 30mm lens, shoot 3 vertical images and stitch them (eventually crop to 4:3 ratio to remove corner and improve stitching alignment accuracy). That said, for the K1, there is a Samyang 20mm f1.8, should be quite good stopped down. I used the D-FA15-30 , it has a fairly even sharpness across the frame at 20mm, but it is rather big and not a cheap lens.

[JensE]

The 20mm f/1.8 Samyang is likely your best bet for a new lens matching your specs, which natively supports a K-Mount - if you manage to get a good one. There's the rare 21mm Zeiss ZK (or even the 18mm), which may be an excellent choice stopped down, good luck finding one though.


BUT, if you scene doesn't move and doesn't have very close objects, have you considered stitching shots with longer focal length, which you may have already? A 20mm lens has about 62° vertical field of view, similar to e.g. the 60° "horizontal" of the FA 31mm LTD, so a single-row panorama would cover basically the same field-of-view, giving you great sharpness and even higher resolution if mounted and stitched carefully. This will likely outperform any 20mm lens, if only for the reason of much less geometric stretching of the scene on the sensor, i.e. much higher resolution captured to start with at the edges.

[bdery]

I think the way you're looking at this might be restricting your options.

You have looked at primes, and considered mostly older lenses. These, in general, offer sharpness that is less uniform across the frame. Center might be superb, while corners might be soft. Newer lenses are designed (again, in general), with sharpness across the frame as more important. In particular, field curvature is taken into account better than in "the old days".

Also, you're looking at primes, while there are many zooms that could cover your needs.

In addition to the lenses already mentioned, I'd recommend taking a look at the FA 20-35mm, which you can find used. Stopped down its corner sharpness is quite good. Also, and perhaps more importantly, look at the DFA 15-30mm, which is a stronger performer. Sadly it lacks a filter thread, but you could easily use a Cokin system on it. Regarding sharpness it's your best bet.

[Sandy Hancock]

I reckon my SMC Pentax-FA 1:2.8 20mm fits your requirements.
Next question?

Personally, I'd prefer to use my DFA15-30 with the Lee 150 filter holder, but you've ruled that out.

[Roadboat24]

I use a Rokinon 20mm 1.8 (Samyang) and the Cokin "P" holder on my K1. When you get to 20mm and wider on full frame your options are limited.

---

Last edited by Roadboat24; 04-29-2019 at 05:51 AM.

[Papa_Joe]

You have looked at primes, and considered mostly older lenses. These, in general, offer sharpness that is less uniform across the frame. Center might be superb, while corners might be soft. Newer lenses are designed (again, in general), with sharpness across the frame as more important. In particular, field curvature is taken into account better than in "the old days".

Sorry I have to contradict to this statement. I think there is no such general rule to the uniformity of the sharpness about the whole frame. For every new lens designed the various characteristics of a lens are set by its developer newly. And if a developer pronounces center sharpness in a new lens you will not have uniform sharpness.
This is absolutely true for my DA 18-135 which has absolute no sharpness at the borders at 135 mm. The priorities where somewhere else obviously.

One may argue that pimes are better in this way, until having a look at the Sigma AF 30 mm / 1.4 which behaves like the worst portrait lens ever in this respect. And that is a full frame lens, not one designed for APC as a standard lens with portrait in mind. This lens is clearly beaten in uniformity across the frame by the much older Tokina RMC 17 / 3.5 which is on par with the SMC DA 15 / 4.0 on the borders and corners at 5.6 but falls behind the DA 15 in the center of the image where this outperforms the Tokina and it's own borders clearly. No uniformity again. But of course better performance over the older lens in all.
All tests cited can be found here: Pentax K Lens Tests

This aside, what I am really interested in is, will the stiching solution work? Has anyone done it already? Don't you have problems with the sharpness at the seams?
Using a 50 mm macro would be the obvious solution then, wouldn't it?

[JensE]

This aside, what I am really interested in is, will the stiching solution work? Has anyone done it already? Don't you have problems with the sharpness at the seams?

I wouldn't have recommended it if I didn't. My wife e.g. has a 160cm wide glossy print in her office, created using a K-5 with the 18-135mm at ~35mm out of maybe 7 vertical shots in a single row. It has basically undetectable seams, showing a Scottish landscape scene, even when looking very closely. Depending on the subject, it can work very well, but with subtle complex textures or any kind of movement (e.g. leaves in the wind) it can be tricky. I did spend a few hours on post-processing, e.g. manually setting control points, to compensate for less than ideal exposures. Precise capture in the first place helps to reduce the effort. Using a small number of frames makes this easier and there is definitely less room for mistakes in a single row shot.

[Papa_Joe]

Thank you Jens!

[SteveinSLC]

If you can live with 24 instead of 20, the Samyang/Rokinon 24 is very sharp if you get a good copy.

[Docrwm]

If you want older lenses (MF) the Vivitar 19/3.8 is rather nice.

[UncleVanya]

Stitching is higher resolution and offers more flexibility. It is slow and takes practice.

[geedee]

Don't forget the Voigtländer 20mm f3.5. You should stop it down though to achieve this freaking sharpness you want.

[Kerrowdown]

It may be of interest to you to have a look at this thread, it may help you with your choices.

I think nearly all of mine, certainly the later examples are with the "Good Lady”(Pentax-A 20mm F2.8) and a K1s, many others were were also shot on FF.

20 mm Lens Club. - PentaxForums.com