Fine focus adjusted but still back focuses!!!


richandfleur

Link Posted 06/02/2019 - 23:09
Focus adjustment can simply be very tricky with a zoom lens. Worst case scenario is things are out + one end of the zoom and out - at the other.
Personally I'd be aiming for the end of the zoom I'm most likely to use, and aiming to focus adjust there.

johnducguz

Link Posted 07/02/2019 - 17:44
I'd read about fine focus of a zoom being tricky which is why I went for the 70mm end as that's what I use the most. I'm going to wait for the weekend & try the zoom on my other body to see what happens then.

pgweber

Link Posted 07/02/2019 - 17:50
Isn't it better to optimise for the long end of a zoom rather than the short end?

i.e. wide angle > greater depth of field.
Peter

Pentax K5
Pentax DA 18-55 Mk1, 50-200 (Samsung), 16-45, , 55-300, 35 f/2.4
Pentax MZ6 + FA28-90, FA50 f/1.4, M 50 f/1.7
Tamron 80-210mm & 28mm
Last Edited by pgweber on 07/02/2019 - 17:51

redbusa99

Link Posted 07/02/2019 - 18:13
does the lens focus accurately in live view?
K3 II and the odd lens or 2

Flickr
ppg

johnducguz

Link Posted 07/02/2019 - 18:23
I did optimise the long end, it's a 24-70. Not tried live view, I've never used it. Never thought i'd need to if the body & lens could manage to work together. If I've got to start manual focusing I may as well get a 6x7.

JAK

Link Posted 07/02/2019 - 18:34
redbusa99 wrote:
does the lens focus accurately in live view?

johnducguz wrote:
I did optimise the long end, it's a 24-70. Not tried live view, I've never used it. Never thought i'd need to if the body & lens could manage to work together. If I've got to start manual focusing I may as well get a 6x7.

It doesn't involve manual focussing (unless you want to.)
You'd better have a read of this:
https://www.imaging-resource.com/PRODS/K5/K5LIVEVIEW.HTM

You appreciate there isn't a digital 6x7. 645D or Z perhaps and you'll be free of Sigma lenses.
John K
Last Edited by JAK on 07/02/2019 - 18:35

johnducguz

Link Posted 07/02/2019 - 19:05
Had a quick read, thanks a lot, but unless I'm missing something, which is possible 'cause it's the 1st time i've ever read about Live View, you wouldn't be able to use it when shooting a model for example who is changing their pose every second. For a still life I think I get it but is it a useful tool for a model shoot. Focus here needs to be quick & accurate, will 'live view' allow that? I had an RB back in the day, miss it terribly.

JAK

Link Posted 07/02/2019 - 19:12
johnducguz wrote:
you wouldn't be able to use it when shooting a model for example who is changing their pose every second.

Try it and see! Some prefer live view (normally mirrorless fans) and some don't. There's no reason for it not being able to do what you want if you can cope with the image on the back screen. The focussing method need be no different to your normal workflow.
John K

derek897

Link Posted 07/02/2019 - 19:56
You can use live view to check the accuracy of your af, and keep adjusting till both are the same.
Live view is not affected by front or back focus issues. Take a test shot using live view, this should be accurate. Now switch back to vf, and fine tune to match your test shot.
I know what i like, If not always why.

johnducguz

Link Posted 07/02/2019 - 20:15
Trouble is I think, I'm old school, I just can't see me shooting by using the rear screen or checking focus using live view. I'm still thinking that the compromises with a zoom may not be worth the faff, they had their issues in film cameras also. Would a prime give me these issues? Is it ok to accept focus issues when you pay out like photographers do? If a manufacturer made an affordable camera with manual, aperture & shutter priority without video (I wouldn't even know how to switch video recording on) & spent the savings on auto focus I'd buy it like a shot. A wide, mid & tele prime plus a set of legs may well be my answer. No one in the replies has dissed primes but problem solving with a zoom seems to crop up a lot. Bit of devils advocate playing re zooms going on here, I may well try the zoom on a k3ii yet.

JAK

Link Posted 07/02/2019 - 21:18
If that zoom lens is put of calibration, contact Sigma and ask if it is possible to get it recalibrated.
https://www.sigma-imaging-uk.com/repairs-support/
If its duff they may replace it for you or some other suggestion.
John K
Last Edited by JAK on 07/02/2019 - 21:19

derek897

Link Posted 07/02/2019 - 22:11
johnducguz wrote:
Trouble is I think, I'm old school, I just can't see me shooting by using the rear screen or checking focus using live view. I'm still thinking that the compromises with a zoom may not be worth the faff, they had their issues in film cameras also. Would a prime give me these issues? Is it ok to accept focus issues when you pay out like photographers do? If a manufacturer made an affordable camera with manual, aperture & shutter priority without video (I wouldn't even know how to switch video recording on) & spent the savings on auto focus I'd buy it like a shot. A wide, mid & tele prime plus a set of legs may well be my answer. No one in the replies has dissed primes but problem solving with a zoom seems to crop up a lot. Bit of devils advocate playing re zooms going on here, I may well try the zoom on a k3ii yet.

You wouldn't have to shoot using the rear screen / live view. It would only be for the purpose of fine tuning the af,
It would do 2 things for you.
1 give you an accurate target to compare your af against, so it can be fine tuned properly.
2 it will show up if the af cannot be fine tuned enough to correct for front or back focusing.
In which case you could try Jak' s suggestion.
I know what i like, If not always why.

richandfleur

Link Posted 07/02/2019 - 22:50
The issue is brought about by using autofocus and an optical viewfinder. You're seeing a view through the lens, and not what the sensor will see when the mirror is folded up and the shutter is opened. The autofocus process has to be adjusted to precisely match where you are pointing the camera, so that it all works when the image is taken.

Two things spring to mind here. One is are you pointing at something with sufficient contrast/edging to allow for the autofocus to be accurate? You mention as an example taking portraits, and I've often had issues targeting the eye and not the eyebrow with autofocus.

Second is an issue with the lens itself and/or the calibration. Again it takes a bit of time to correctly calibrate the lens adjustment, so don't rush this. I personally did this with a focus target I printed off, stuck to a box and then ran a silver metal ruler on an angle along the end of the box towards the camera. The target is very contrasty, so there can be no uncertainty for the camera around where the intended focus point was.

Live view does not require the autofocus module inside the camera, as the sensor is exposed to the image the whole time. You need to give it a go, but understand it has it's uses and it's weaknesses. Live view focusing on most Pentax's uses contrast detection only, so it will struggle if there's not a lot of variation around the point of focusing. Live view is excellent for manual focus work, where you can enable digital zoom and focus peaking to help precisely located the focus point. For static type shots it's excellent, but it can be a bit hard to work on moving subjects or in bright day light.

Whether using the optical viewfinder, or the rear screen in live view, it's still the same sensor so it doesn't impact on image quality. That's one reason why I personally wouldn't mind a mirrorless Pentax camera, especially for landscapes/anything static shot or tripod based, where I use Live view to focus precisely almost 100% of the time. Live view is another tool included on modern Pentax DSLRs so doesn't hurt to learn how it can help you. I use in tripod based portraits too, so I can target the eye I'm after, and I'd see it as more traditional getting involved in taking the shot than handing this component over to Auto Focus, which may or may not get it right. I'd love to see Pentax's get Eye Auto Focus capabilities, but again that's another story.

Re the zoom vs prime discussion, yes calibrating a prime is likely to be more effective, as it's characteristics won't change across the zoom range. That said, others have successfully calibrated zoom lenses, so it's not to say it can't be done.

Good luck!

JAK

Link Posted 07/02/2019 - 23:47
Quote:
Second is an issue with the lens itself and/or the calibration. Again it takes a bit of time to correctly calibrate the lens adjustment, so don't rush this.

Problem is the K-5 doesn't recognise the lens, other than its a Sigma. Sigma have used the same code number for a variety of different lenses including: Sigma 18-200mm F3.5-6.3 DC; Sigma DL-II 35-80mm F4-5.6; Sigma DF EX Aspherical 28-70mm F2.8; Sigma 70-300mm F4-5.6 Macro; Sigma 55-200mm F4-5.6 DC so to the camera it isn't a valid ID. The only solution (as I've already said several times) is to contact Sigma for advice as to how to get it corrected, maybe they can update the firmware and adjust it. It isn't Pentax's fault that Sigma haven't registered the lens with Pentax to obtain an official ID code for it, instead using a generic one. That's the downside of purchasing third party lenses from a manufacturer who back engineers the firmware for their lenses without working with the camera manufacturer. Issues come home to roost. In other brand cameras often Sigma lenses just cease to work any more unless Sigma have a firmware update available and the lens is updated.
But the OP needs to do another test to confirm the test he did remains valid in other conditions.
John K
Last Edited by JAK on 07/02/2019 - 23:57

richandfleur

Link Posted 08/02/2019 - 00:16
JAK wrote:


Problem is the K-5 doesn't recognise the lens, other than its a Sigma.

Thanks for this info.
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