Sigma 50 1.4 from Microglobe


lemmy

Link Posted 17/07/2011 - 12:15
I bought a Sigma 50mm f1.4 from Microglobe in Bloomsbury a couple of weeks ago, decent price at 315.

On getting it home I did what I normally do with new lenses and shot a sequence of pix of a neighbour's rooftop with my K5 and the new lens wide open and the focus adjustment from -10 to +10. The Sigma needed an adjustment of -7 for the sharpest image.

Over the next few times I used the lens, I had a high proportion of fuzzies so did some further tests. Basically, the lens focus was inconsistent. It front focused sometimes, rear focused at others. It's always difficult to get good focus with such a wide aperture lens but at a certain point you realise there's definitely something wrong.

Then I researched a bit on the web and it seemed there were problems with examples of this lens. I thought maybe I'd swap it for the Pentax equivalent (55mm, a lot more expensive and with less good performance according to the tests). Unfortunately, according to various testers there are focus problems with this lens too. Must be something to do with the wide aperture, I suppose.

Either way, I took it back to Microglobe, where they suggested I bring my camera in. They had four more examples in stock and I could try them to find one I was happy with. I duly did this, taking my iPad and camera adaptor with me to check the results.

The first lens I tried was perfect, focus smack on everywhere, wide open or not. Having used it for a couple of days now, I'm very happy with my choice. It is astonishingly sharp at f1.4 as the mag tests suggest and at f4 crisp right across the frame.

I thought that was a nice touch at Microglobe, too, to let me try different examples until I found a good one. The original is now on it's way back to Sigma for re-calibration.

I now have a couple of Sigma/ Pentax lenses, this one and the 105mm f2.8 macro which on side by side testing with the Pentax 100 I bought in preference.
lemmy
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Algernon

Link Posted 17/07/2011 - 13:01
I've noticed people with similar problems with the Sigma 30mm
although I've never seen anyone on here with a duff one.
Half Man... Half Pentax ... Half Cucumber

Pentax K-1 + K-5 and some other stuff

Algi

Mike-P

Link Posted 17/07/2011 - 14:20
Algernon wrote:
I've noticed people with similar problems with the Sigma 30mm
although I've never seen anyone on here with a duff one.

I have a duff one

I bought it off Flossie (on the strict understanding the lens was not focusing correctly) as I thought it was worth a punt to see if it was ok on my K-5 but it is exactly the same as Lemmys.

Got it spot on with an adjustment of 7 but on the next shot its miles off. Most strange.
No equipment list here but thanks for taking an interest. My Flickr

lemmy

Link Posted 17/07/2011 - 14:41
Mike-P wrote:
[quote:3496ace15f="Algernon"]
Got it spot on with an adjustment of 7 but on the next shot its miles off. Most strange.

Sigma can put it right I'd think. I know they can the 50mm. They should maybe sort out their QC, too.
lemmy
My Home Pages, Cartoons and Videos

Mike-P

Link Posted 17/07/2011 - 14:51
I bought the lens at a very decent price with the knowledge that should it be a problem on the K-5 then Sigma will fix it for an amount that when added to the original price I paid meant the total would be about right for a used 30mm f1.4. Plus I would have 6 months warranty.

Just a pity it isn't the bargain I hoped for.
No equipment list here but thanks for taking an interest. My Flickr

Algernon

Link Posted 17/07/2011 - 14:55
I assume you've tried tapping it with a hammer!
Half Man... Half Pentax ... Half Cucumber

Pentax K-1 + K-5 and some other stuff

Algi

lemmy

Link Posted 17/07/2011 - 15:27
Mike-P wrote:

Just a pity it isn't the bargain I hoped for.

Worth a punt, though. Either you get a bargain or you have paid around the normal price with a little inconvenience added.

I'd wondered whether it was a camera/ lens combination fault or just the lens. From your experience, it's just the lens.
lemmy
My Home Pages, Cartoons and Videos

Mike-P

Link Posted 17/07/2011 - 16:18
Just the lens, does the same thing on my K-7.
No equipment list here but thanks for taking an interest. My Flickr

Smeggypants

Link Posted 17/07/2011 - 17:11
How would the AF be calibrated on the lens itself?

surely the camera is the device deciding when focus is achieved and turns the barrel until that is satisfied?

Am I missing something?
[i]Bodies: 1x K-5IIs, 2x K-5, Sony TX-5, Nokia 808
Lenses: Pentax DA 10-17mm ED(IF) Fish Eye, Pentax DA 14mm f/2.8, Sigma 17-70mm f/2.8, Pentax-A 28mm f/2.8, Sigma 30mm F1.4 EX DC, Pentax-A 50mm f/1.2, Pentax-A 50mm f/1.4, Pentax-FA 50mm f/1.4, Pentax-A 50mm f/1.7, Pentax DA* 50-135mm f/2.8, Sigma 135-400mm APO DG, and more ..
Flash: AF-540FGZ, Vivitar 283

karma mechanic

Link Posted 17/07/2011 - 17:32
Yes.

It does that with contrast AF, as in Live View.

With phase AF it measures the difference between the out-of-focusness as seen by two sensors and calculates how much to tell the lens to move the focus. If the lens says it has got there it doesn't always mean it is perfectly focussed.
My own website is www.richardgaddphotography.com

Also on 500px

Anvh

Link Posted 17/07/2011 - 17:38
lemmy wrote:
Unfortunately, according to various testers there are focus problems with this lens too. Must be something to do with the wide aperture, I suppose.

here you go, read the first two parts.
http://toothwalker.org/optics/spherical.html
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
Last Edited by Anvh on 17/07/2011 - 17:39

Smeggypants

Link Posted 17/07/2011 - 17:54
karma mechanic wrote:
Yes.

It does that with contrast AF, as in Live View.

With phase AF it measures the difference between the out-of-focusness as seen by two sensors and calculates how much to tell the lens to move the focus. If the lens says it has got there it doesn't always mean it is perfectly focussed.

How does the lens say it has got there? Surely the camera decides when focus is achieved, and the lens is only responding to instructions from the camera via mechanical coupleing ( electrical with SDM )?

http://graphics.stanford.edu/courses/cs178/applets/autofocusPD.html

Sorry if I'm missing something here, but I always think of the lens as the passive device in this
[i]Bodies: 1x K-5IIs, 2x K-5, Sony TX-5, Nokia 808
Lenses: Pentax DA 10-17mm ED(IF) Fish Eye, Pentax DA 14mm f/2.8, Sigma 17-70mm f/2.8, Pentax-A 28mm f/2.8, Sigma 30mm F1.4 EX DC, Pentax-A 50mm f/1.2, Pentax-A 50mm f/1.4, Pentax-FA 50mm f/1.4, Pentax-A 50mm f/1.7, Pentax DA* 50-135mm f/2.8, Sigma 135-400mm APO DG, and more ..
Flash: AF-540FGZ, Vivitar 283

Anvh

Link Posted 17/07/2011 - 18:01
It got me too Smeggy but it's know that there is a focus tolerance in the lens as well, don't know precisely how it works though.
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ

johnriley

Link Posted 17/07/2011 - 18:08
It's an electronic instruction to a mechanical device. There must be feedback to say "I've arrived at the point of focus" but of course there will be mechanical tolerances.

Deceleration also is a factor - nothing stops instantaneously. So a lens with a long throw may reach higher speeds and overshoot more, just as a lens with short throw won't offer the potential for accuracy because of the shorter scale. Going from infinity to the point of focus may also give a different final point than going from closest focus to the point of focus.

Then we have different distances affecting accuracy, different zoom positions. Don't forget to zoom first and then focus with modern lenses - very few of them hold focus properly. They are often varifocal lenses, not parfocal. The old idea of focusing at the longest length and then zooming out will cause a focus shift, in fact an enormous one with some lenses.

Try it and see!
Best regards, John

Anvh

Link Posted 17/07/2011 - 18:26
johnriley wrote:
It's an electronic instruction to a mechanical device. There must be feedback to say "I've arrived at the point of focus" but of course there will be mechanical tolerances.

That's the point Smeggy is making.

Smeggypants wrote:
How does the lens say it has got there? Surely the camera decides when focus is achieved, and the lens is only responding to instructions from the camera via mechanical coupleing ( electrical with SDM )?

From what i read about the patents of the AF system it works a bit like this.

1. AF reads the focus error
2. estimate the amount the AF needs to move and estimate the number of turns of the AF motor
3. When motor has done his job, the AF module check again and either says it's on focus or try to estimate the number of turns it needs to get the focus.

So it goes on and on till the AF sensor say it's in focus.

Even if the lens or motor overshoot, the sensor will say it's out of focus and will try to get the focus right.

ps. Pentax does even do a double focus check by slightly moving the lens out of focus and back on focus again when after the first focus lock.
Stefan


K10D, K5
DA* 16-50, DA* 50-135, D-FA 100 Macro, DA 40 Ltd, DA 18-55
AF-540FGZ
Last Edited by Anvh on 17/07/2011 - 18:28
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