[pathdoc]

Yep. A lot of the mirrorless cameras are now creeping up into the small SLR size bracket - or at least it is difficult to distinguish some of them from the smaller film SLRs of old - they even have pentaprism-like structures for their flashes and viewfinders!

There are two things which IMO are undeniable when it comes to this issue.

1)

This thread wouldn't exist if the AF wasn't an issue at least for some users.

I did not want to be one of those users.

2)

Any buyer of the Sigma 18-35 will have to make their own assessment of risk as people do when purchasing the SDM DA* lenses.

It's not just the risk but the ease (or otherwise) of having the problem resolved one way or another (swap versus refund) if you're one of the unlucky ones. Strike one, I would have been willing to return it for another copy; strike two, I would have had to conclude this wasn't the lens for me and ask for my money back - but BOTH strikes would have involved repacking it for transport and hoping Canada Post were careful enough both ways. Those living in big cities with bricks-and-mortar camera stores close by are at a major advantage here; some others of us, not so much.

[MadMathMind]

I suppose I should clarify where I see issues with my various lenses. I push them to their limits in my cosplay sessions. (It's not as hard as I push the cosplayers sometimes in trying difficult to hold poses, but they always love the results!) Here's where I have problems:

FA31 - about 7' away or so I have trouble getting the face in 100% focus at f/2.2. Improvement at f/2.5 because DoF. I often get the costume perfect and very slight blur on the face, even though I aim the focal point there. For close-ups, it's spot on. If I back up a bit, no issues whatsoever.

FA77 - from long distances where the person is small. If I'm doing a pure portraiture where her upper torso is 80% of the photo, no problem. In fact, this probably the most pinpoint lens in the bag.

Sigma - Close distances at low f stops. No problems at f/4 or higher ever or with f/2 from 6'+ away. It seems to do better than the FA31 with distance shots at low apertures.

I had an incident taking a close shot with the Sigma just recently. I took several a half dozen of them and they looked on site so I started to leave. I decided to review and realized that the DoF and focus point was not where I wanted. I went back and did it with the FA31. Shots were mostly identical. Result? Same focus points, same lack of DoF, except I bracketed it more the second time. But at f/2.2, I saw the same wrong point out of both lenses.

So yah, I see all sorts of issues and know how to work within them. I take tons and tons of duplicate shots with small apertures just to make sure the focus is nailed; it's AF button, shutter, AF button, shutter, AF button, shutter. Almost all the time, I get one of them good, if not all of them.


Where I have the most problem with getting focus on faces is with (East) Asians. One would think that a Japanese camera would be better at this--in fact, be *best* at it, but it's not. I guess it's the softer facial features and silkier skin that makes it hard on the camera? No clue. A lot of my cosplayers are Asian as is my wife so I see this often, but man...the camera just fights with the focus on them. And the color. I usually have to hit the LR orange saturation down to make them come out right.

[rawr]

I bracketed it more the second time

I keep on wishing that one day we may see focus bracketing as a feature with Pentax viewfinder AF, just like exposure bracketing ... Maybe even with built-in stacking.

A quick automated pass, recording an image at various points across the AF adjust range (+10, +5, 0 -5, -10), should be easily doable via the camera firmware, I would have thought.

Could often be useful.

[drypenn]

I keep on wishing that one day we may see focus bracketing as a feature with Pentax viewfinder AF, just like exposure bracketing ... Maybe even with built-in stacking.

A quick automated pass, recording an image at various points across the AF adjust range (+10, +5, 0 -5, -10), should be easily doable via the camera firmware, I would have thought.

Could often be useful.

+1, but does that mean we have to lose the mirror to get that feature? Or can that be implemented similar to Sony's translucent mirror concept?

[rawr]

Or can that be implemented similar to Sony's translucent mirror concept?

A little bit different.

The K-3/K-5 etc have an AF adjust feature built into the camera software already that lets you tune the AF across a + / - range, to adjust for front or back focus. A 'Focus stacking' feature would just mean using the existing AF adjustment capability to move the focus point back and forward a bit for a bracketed set of shots.

For instance, press shutter button, AF gets a lock, camera fires a burst of 3 shots, the first with the AF zeroed (or at the currently recorded AF adjustment point for a lens), the 2nd at +5 from shot 1, the 3rd at -5 from shot 1.

Then either save the images individually, or stack them all together.

[drypenn]

A little bit different.

The K-3/K-5 etc have an AF adjust feature built into the camera software already that lets you tune the AF across a + / - range, to adjust for front or back focus. A 'Focus stacking' feature would just mean using the existing AF adjustment capability to move the focus point back and forward a bit for a bracketed set of shots.

For instance, press shutter button, AF gets a lock, camera fires a burst of 3 shots, the first with the AF zeroed (or at the currently recorded AF adjustment point for a lens), the 2nd at +5 from shot 1, the 3rd at -5 from shot 1.

Then either save the images individually, or stack them all together.

Got it! Nice!

To continue with the OT.

Will that require a (little) longer than correct shutter speed, such that it will necessitate a tripod, or can a faster processor do the trick? [Thanks! I'm learning and is tickled by the ingenious idea! ]

[rawr]

Will that require a (little) longer than correct shutter speed, such that it will necessitate a tripod, or can a faster processor do the trick?

When I make AF adjustments to a lens, the changes get implemented as quickly as I can get out of the camera menu. So I think focus bracketing this way should be able to work quite quickly, using all existing shooting settings (shutter speed, aperture etc). Maybe even do it at 8fps, like the K-3 is capable of.

As long as nothing else changes very dramatically in the scene (eg the subject moves a lot, like an insect in flight), or you don't set it to do a 20 shot bracket (which even at 8fps would mean 2 or 3 seconds elapse between shot 1 and shot 20, maybe longer to factor in card-write delays). But a 3-5 shot focus bracket should be able to happen in less than a second, I reckon. ...

No doubt a tripod would help, just like it does for other bracketed shooting (eg HDR), but depending on subject motion a tripod may not always be necessary.

[drypenn]

When I make AF adjustments to a lens, the changes get implemented as quickly as I can get out of the camera menu. So I think focus bracketing this way should be able to work quite quickly, using all existing shooting settings (shutter speed, aperture etc). Maybe even do it at 8fps, like the K-3 is capable of.

As long as nothing else changes very dramatically in the scene (eg the subject moves a lot, like an insect in flight), or you don't set it to do a 20 shot bracket (which even at 8fps would mean 2 or 3 seconds elapse between shot 1 and shot 20, maybe longer to factor in card-write delays). But a 3-5 shot focus bracket should be able to happen in less than a second, I reckon. ...

No doubt a tripod would help, just like it does for other bracketed shooting (eg HDR), but depending on subject motion a tripod may not always be necessary.

I hope that this comes into a fruition soon!

[UncleVanya]

I hope that this comes into a fruition soon!

(RE: AF bracketing)

In macro work - particularly handheld you can simulate this. Set the camera to manual focus and focus on the subject then pull back slightly. Have the camera in full speed burst mode and press the trigger and slowly move the camera towards the subject.

[Zafar Iqbal]

After a whole bunch of weddings over the past few months, my conclusion is, that my K-3 II's do not focus properly. It's not just with the 18-35, but other lenses as well, mostly noticeable at wide-angle and when attempting to focus at distant subjects.

It could very well be that the Sigma 18-35 has an issue too, but I can't diagnose it properly without functioning cameras :/

[botzi]

Hello,
I'm a Pentax user, have a K3, and K3 II (demo) for test, and i consider buying a Sigma 18-35 f1.8 for weddings photography. But now I'm reading for so many problem with PDAF focusing with this lens. If there any solution, i'm looking for help?

[Digitalis]

But now I'm reading for so many problem with PDAF focusing with this lens. If there any solution, i'm looking for help?

Buying a good copy is the ticket - but the chances of that are certainly against you. Testing the lens is rather difficult, there are still optical centering issues present with the ART series lenses despite sigma saying otherwise. The lens dock is the closest thing to a solution as you will ever get, and even then it is a band aid at best.



The lens dock will set you back about $100, a high quality polariser will also cost an extra $200.A proper lens calibration target will set you back about $80

If you do choose to get it, spend some time getting to know its quirks, and the situations where it is likely to be less than accurate. Don't waste your time with in-camera AF corrections, as it is only really designed with primes in mind.

Get the Sigma lens dock, calibrate your lens properly with an accurate target*, and be warned that focusing on anything at a distance between 2m and infinity has at least a 90% of being out of focus at f/1.8 - and the bokeh from this lens is rubbish at that distance, it only delivers smooth results with acceptable consistency at focus distance of 0.5m or shorter @ f/1.8. Remember to stop the lens down, I know you probably want to use this lens at f/1.8 all the time, but the reality is at the f/4 ~ f/11 aperture range you will have a higher keeper rate.



At least the calibration software is free. It can be downloaded from the sigmaphoto website.

Even if you do all of this perfectly, there is still a high chance your lens will never be absolutely reliable or accurate at apertures wider than f/4

To be honest: if you are buying this lens hoping it will perform perfectly out-of-the-box to document a once-off** event like a wedding this, this particular lens isn't worth the trouble, It is simply too unreliable.


*Brick walls don't cut it. You need a high contrast, target for accurate repeatable calibration at f/1.8.
**hopefully, with divorce rates being what they are..and all that Ashley Madison controversy going on.

[botzi]

Buying a good copy is the ticket - but the chances of that are certainly against you. Testing the lens is rather difficult, there are still optical centering issues present with the ART series lenses despite sigma saying otherwise. The lens dock is the closest thing to a solution as you will ever get, and even then it is a band aid at best.



The lens dock will set you back about $100, a high quality polariser will also cost an extra $200.A proper lens calibration target will set you back about $80

If you do choose to get it, spend some time getting to know its quirks, and the situations where it is likely to be less than accurate. Don't waste your time with in-camera AF corrections, as it is only really designed with primes in mind.

Get the Sigma lens dock, calibrate your lens properly with an accurate target*, and be warned that focusing on anything at a distance between 2m and infinity has at least a 90% of being out of focus at f/1.8 - and the bokeh from this lens is rubbish at that distance, it only delivers smooth results with acceptable consistency at focus distance of 0.5m or shorter @ f/1.8. Remember to stop the lens down, I know you probably want to use this lens at f/1.8 all the time, but the reality is at the f/4 ~ f/11 aperture range you will have a higher keeper rate.



At least the calibration software is free. It can be downloaded from the sigmaphoto website.

Even if you do all of this perfectly, there is still a high chance your lens will never be absolutely reliable or accurate at apertures wider than f/4

To be honest: if you are buying this lens hoping it will perform perfectly out-of-the-box to document a once-off** event like a wedding this, this particular lens isn't worth the trouble, It is simply too unreliable.


*Brick walls don't cut it. You need a high contrast, target for accurate repeatable calibration at f/1.8.
**hopefully, with divorce rates being what they are..and all that Ashley Madison controversy going on.

Thanks for the advice. And what zoom do you recommend me to buy for wedding photography with K3/k3 II and the coming new FF body?

[Digitalis]

And what zoom do you recommend me to buy for wedding photography with K3/k3 II and the coming new FF body?

the nearest alternatives for the Sigma 18-35mm ART Pentax K mount aren't full frame capable, among third party makers full format lenses for K mount have been dwindling over the past few years.The 18-35mm f/1.8 isn't able to cover full frame until 32mm. I'd suggest that you go through the lens reviews on this site. But If I had to make a recommendation for an alternative to the sigma 18-35mm f/1.8 ART, the sigma 17-70 contemporary lens would be a good choice.

[Class A]

Hello,
I'm a Pentax user, have a K3, and K3 II (demo) for test, and i consider buying a Sigma 18-35 f1.8 for weddings photography. But now I'm reading for so many problem with PDAF focusing with this lens. If there any solution, i'm looking for help?

If you look around a bit, you'll find many users who do not complain one bit about the lens.

Despite making his claims with great conviction, Digitalis has no actual way of knowing how hard or easy it is to get a good copy of the lens. Note that he kept the lens despite having all the money in the world to go for the very best equipment only. Go figure.

Why don't you get a copy and see for yourself?