Pentax A-50mm / f1:1.4

I took a walk around Lerwick harbour in Shetland last week and gave the 50mm lens some exercise.

You can see from the pictures below, that the performance at f1.4 was very poor, but very good at f5.6. The conditions were probably darker than they appear in the images, ISO was 800 for both, 1/40 @ 5.6 and 1/320 @ 1.4.
Both handheld using SR.

Both pictures are 'as-shot' no post processing, just reduced in size to 800px wide.

Should I expect better quality at f1.4 or is this typical ?

If people are paying extra for the faster f1:1.4 and f1:1.2 lenses, should they not perform better at these wide apertures ?

f1.4




f5.6

Galoot,
Can't work out the f/1.4... but the DoF will be incredibly shallow. At 2m it is pretty unforgiving

Personally, I found I messed up the SMC-A a lot... turned out it was mostly because I was using the red dot as a focus aid. Doh! Should use the green octagon at the bottom of the viewfinder for focus confirmation.

When I got a portrait manually focussed with f/1.4 it was sharp, razor sharp, but with a DoF of a couple of mm (or so it seemed). No room for error!

I personally haven't tried taking landscape style shots at f/1.4.
It's a different beast, and takes a little getting used to, but the optics should be the same between the SMC-A and SMC-F/FA versions.

Hope that helps!
Matt

Looks like a focusing problem to me. The number on the side of the red boat is nearly in focus. You said it was quite dark and this may be the reason for the error. Now, if we had split screen focusing like in the good old days, this probably wouldn't happen.

This is a question I've been trying, unsuccessfully, to answer for myself as I weigh up whether to get a DA* 16-50, since unless it's good at F2.8, I don't want to pay £500 for it.

I had a quick look at the online DOF calculator which suggests that the DOF when focused, say at 50 feet, is such that it's acceptably sharp only from about 40 to about 60 feet at F1.4.

I can't really tell how far away your subjects are, but that's not much DOF at that distance. If it's focused at 30 feet, the DOF is only +/- 3 feet!

That said, your first picture doesn't look all that sharp anywhere, so I have no good answers really. At 1/320s, there shouldn't be any camera shake, but who knows. I assume you took several at that sort of exposure, and they were all much the same.

I have to say, I have nowhere near enough confidence in how to go about assessing these things, to go out and buy an expensive lens.

But if I was going to try and do tests like this, I'd be using properly repeatable conditions, which this is clearly not.

I took about 10 pictures at f1.4, and they all resemble the first picture above. Even with the shallow DOF, it should be sharp somewhere !
As soon as I stopped down to f2.8 and below, the quality improved greatly, even with the resulting slower shutter speed.

I'm stuck in the office in Edinburgh for the next few days, but I'm up in the Orkney Isles next week, so I'll do some experimentation and let you know the results.

I was sure you did.

I've been put right off getting the 16-50 by the reports elsewhere of the variability in its quality.

I can hear George tutting even as I type, but sadly it's not his £500 I'm thinking of spending...

This lens is well known for being soft at 1.4.

See http://tinyurl.com/3xmxao (photozone.de) (FA optically the same as A).

Photozone say the central sharpness is "very good" at F2, "excellent" at F2.8 and "stella" at F4.

F1.4 does at least provide more scope when the light is very low, plus it makes the viewfinder wonderfully bright. More importantly it's arguably a better lens in general than the 1.7 at medium apertures. The fact that it goes up to 1.4 is mostly academic.

Soft?

If Galoot's example is typical (bearing in mind that it's about a quarter of the size of the original), then it's not soft, it's c**p!!!! I'm still hopeful that it's technique at fault. No discredit, just that in general I'd rather it's my being rubbish than the kit's, if you know what I mean.

There's something funny going on, I'm sure. Haven't we seen examples that were much sharper than this at F1.4?

I may be making all this up, but I have a memory that it was Matt's FA 50 1.4 - isn't it supposed to be optically the same as the A-50?

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Taking any picture with depth to it at f1.4 will be disappointing, especially one like this example where you would have to really search for the sharp plane of focus.

Try a flat subject and focus very, very carefully....

PENTAX FA 50mm F1.4

*ist DS, RAW, Silkypix with default "Natural Sharp", default Tone, -.8 Exp Bias on garden shots.
All ISO 200, Av mode, uncropped apart from the 100% crops.
Except for last picture, same settings within each set apart from aperture and shutter speed.

F1.4 1/40s tripod



F2.0 1/25s tripod



F2.8 1/13s tripod



Focusing and spot metering on the bird, best of several attempts
F1.4 1/2000s



F2.0 1/1250s



F5.6 1/160s



100% crop: F1.4 1/2000s



100% crop: F2.0 1/1250s - this one maybe be misfocused



100% crop: F5.6 1/160s



Same as above with some extra Silkypixing

In my opinion this lens is indeed cr*p at 1.4. Doesn't bother me since it's so brilliant at smaller apertures - which are still larger than what my zooms can provide.

I can believe it's possible to get better results than myself and Galloot have managed at 1.4 - but I think you would have to go to a lot of trouble to get much better.

Any idea how it compares with the 1.7 at 1.7 and smaller? If there's not much in it then I'll definitely cross the manual focus 1.4 off my list of "maybe sometime".

Everyone can judge the sharpness of the images for themselves, but don't forget that one sample of a lens may or may not be indicative of the general performance. This is one of the problems with magazine tests - they only have one sample.

Having said that, all four 18-55mm lenses we have had have all performed exactly the same.