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I've had the Sigma 30mm f/1.4 a few weeks now and it performs quite well, except when using autofocus. When focusing on a distant subject the autofocus goes beyond infinity. Today I received my new Sigma 20mm f/1.8, and guess what? It does exactly the same thing, focuses well beyond the subject when the camera says it is in focus. This isn't noticeable at f/8 due to the large DOF, but wide open the results are truly abysmal.
Now I couldnt imagine both lenses would suffer the same problem, so maybe its the camera. This evening I did some tests with the kit lens at 20mm and 30mm (and discovered in the process that it doesnt record 20mm or 30mm, but jumps from 18 to 22 and 29 to 31 in the EXIF data)
Its much slower speed gave a greater DOF, but the problem is still there, so it is the D70 and not the Sigma lenses that has the problem. I also did the test with the 105mm, and it seems to be ok. So, the D70 has a serious autofocus problem with wide angle - normal lenses.

Has anyone else encountered this problem?

Here is a test pic with the 30mm @f/1.4 where the problem is most obvious due to the shallow DOF. Cropped from the original RAW image with no post processing.

The true focus is about twice as far from the camera as where the autofocus set it.
At infinity the photos are hopelessly out of focus with the fast lenses, but just slightly soft with the kit lens due to its great DOF.
The PVC pipe (a good contrasty focus target in low light) is a bit over 2m away, best focus almost 5m.



Gordon

a while ago I read somewhere that some d70 cameras had back focus issues. wonder if this is what you are experiencing??

maybe a member here has had the same thing happen to them??

Steve

yes I remembered that vaguely too... I'm hoping to find out what their resolution to the problem was.

Gordon

those poor souls in sydney had to endure mr maxwell

basically it was return for service, although I did see some guy publish an article on do it yourself back focus repairs, and I thought "big chance"

Steve

you may have to send for repairs and limit yourself to the observatories schmidt camera for a while
Steve

hehehe maybe I will
16Mpixels, but no focus other than temperature and flexure adjustment... and no colour!

Actually I would seriously look at doing the adustments myself as per
http://www.leongoodman.com/d70focus.html rather than not have my camera while Maxwells fiddle with it


Gordon

The problem with focussing issues is that when you send it in for focus calibration, you really need to send it in with the lens that is causing the problem - otherwise it can't be calibrated. And Nikon won't take in a Sigma lens to check it for you.

Hi Gordon,

I might be touching the obvious ?
Please tell us exactly what were your focus settings on the D70.
How did you focus etc ...
Was it single focus, what sensor did you select, did you focus on the front
object, hold focus then recomposition ?
Questions ...

HTH,
CD

Gordon, I'd DIY (If you've the guts). I did so for mine, and it's no scarier than waxing one's ears.

In the guide you linked to will show you precisely how. The rear hex screw is the one to fiddle with. Keep in mind VERY subtle changes will lead to big differences in the AF, hence possibly why the calibration was out in quite a number of the original D70 bodies.

Just make sure that after you're done trialing the focus adjustments with your Sigma lenses, that you haven't upset the focus positioning of other lenses in your posession (but as you've discovered, the DOF of the kit lens can masks focus errors quite well).

Each time I sent my D70 into Maxwells, they'd do the focus adjustments as part of their courtesy checks (along with sensor clean, etc), but each time I'd have to set things back myself, as they would alter it just enough to be detectibly 'off' with my set of lenses even though they consider it to be "calibrated".

christiand wrote:Hi Gordon,

I might be touching the obvious ?
Please tell us exactly what were your focus settings on the D70.
How did you focus etc ...
Was it single focus, what sensor did you select, did you focus on the front
object, hold focus then recomposition ?
Questions ...

HTH,
CD

from my menu:
AF-S
single area
AF assist on
AE/AF Lock on

focused in the centre, focus locked while I recomposed to include more background, looking down the track, to keep the best focus area near the middle of the frame.

The problem is very obvious when taking images at infinity, its so far out of focus I may as well just use a beer bottle bottom instead of a lens!

Strangely the autofocus seems to be OK with the 105mm, so I'm not sure what to make of that. If adjusting the screws for the 20,30 and 18-70 throws the 105 out, it wont be too much of a problem, as I generally manually focus for macro photogrpahy.

I'm going to have a play with the adjustments this morning before another trip to Tiger Snake country

Gordon

Hi Gordon,

I also have the 30mm F1.4. Researching this lens I found a lot of people were complaining about the infinity focus. Do a search on Fred Miranda for the Sigma 30mm, lots of great topics and responses.

I believe that people were sending the lens back to Sigma and having it adjusted. But also my impression was that it was an issue only with the 30mm, I haven’t read up on the 20mm so I couldn’t say for sure.

I have found my auto focus to be quite good, but I don’t think I have really tried a distance beyond infinity. Well maybe here on this thread http://www.dslrusers.net/viewtopic.php?t=13610&highlight=nbm, but the panorama’s weren’t really focusing on the distance.

Let us know how you get along.

Cheers

Wocka wrote:but I don’t think I have really tried a distance beyond infinity

That would be difficult Wocka

Personally, I can't see how a problem with autofocus could be lens specific. It's the camera that adjusts focus based on contrast, so even if a lens is badly calibrated with respect to its focussing ring, that shouldn't affect autofocus. The camera would make +ve / -ve adjustments until maximum contrast was achieved in the focussing area.
Looks like you have a strange phenomena there Gordon...

leek wrote:Personally, I can't see how a problem with autofocus could be lens specific. It's the camera that adjusts focus based on contrast, so even if a lens is badly calibrated with respect to its focussing ring, that shouldn't affect autofocus.

It's not unusual John, if you consider that these days the lens to body interface is via CPUs communicating with each other. The AF sensors may still rely purely on optics to determine point of maximum contrast, but the commands of adjustment 'steps' to bring that point backwards or forwards are relayed via the chips.

And not to pick on third party or Sigma lenses specifically, but they have been other issues reported between their lenses and Nikon bodies (no support for AF-On for the D200 for instance).

Onyx wrote:

leek wrote:Personally, I can't see how a problem with autofocus could be lens specific. It's the camera that adjusts focus based on contrast, so even if a lens is badly calibrated with respect to its focussing ring, that shouldn't affect autofocus.

It's not unusual John, if you consider that these days the lens to body interface is via CPUs communicating with each other. The AF sensors may still rely purely on optics to determine point of maximum contrast, but the commands of adjustment 'steps' to bring that point backwards or forwards are relayed via the chips.

And not to pick on third party or Sigma lenses specifically, but they have been other issues reported between their lenses and Nikon bodies (no support for AF-On for the D200 for instance).

True for AF-S lenses I suppose... I guess Gordon should try an old screw type focus lens to see if the problem occurs then...

I have emerged form the dark inner recesses of my camera... ok I was only just behind the mirror, but dark inner recesses sounds better
I can report that its an easy job and focus adjustment is no reason to let Mr Maxwell take your camera away from you for several weeks!

I have only done relatively near post adjustment focus tests with the 30mm, due to the difficulty of finding somewhere dark enough to use f/1.4 at infinity at this time of day... if only Nikon would let us use ISO50!
The focus point has changed slightly with the 105 MicroNikkor, but it still works well, these pics just taken using autofocus, all ~50% crops from the original, and downsized.

small Golden Orb Weaver


Grevillea


Crested Pigeon



Gordon

Gordon, impressive, I believe you are the first to follow in the relatively untrod footsteps of Chi

Its a much easier process than what we had to do to get our 16Mpixel CCD camera perpendicular to the optical axis in the Uppsala Schmidt telescope
That took quite a few nights of effort to get right, and we settled on "close enough" in the end.

I took a stack of photos in the bush over the past couple of days and the camera is focusing nicely now

Gordon