[StiffLegged]

I have an old FA 80-320mm which occasionally gives me nice results but is often just a bit too soft. It's also very sensitive to manual focus. Someone recently posted on a thread about checking focus calibration by the DotTune method and I thought I'd give it a go.

BAM! +8 correction on my old K-7 and Sunday afternoon's trial shots are decently sharp for a change! I wonder how many check this before posting less than complimentary reviews?

Just for completeness I had to test my trusty A 35-105 and the two Pentax-Ms. No adjustment necessary, I dunno...

Youtu.be

Youtu.be

[microlight]

I use the dot tune method too - works for me.

[Pentax Syntax]

I too like the 80-320 on the K-1. I found that if I am going to use 320mm focal length it is best to fine tune AF at that length. I use a variation of the technique described here as dot tune. Been doing it ever since live view became reliable and precise. I think many people have used this method without knowing that it had been "named" as a specific approach. Kudos to the poster for giving a detailed presentation of the approach though!

[Topsy]

Just for completeness I had to test my trusty A 35-105 and the two Pentax-Ms. No adjustment necessary, I dunno...

I'd be surprised if this method would change focus confirmation for manual lenses?

It's funny, I was playing around with it just yesterday, annoyed by having to look at the display all the time when shooting. (one thing that made me enjoy film more recently)
Messing around with it seemed to improve the confirmation somewhat but I put that down to placebo/a wide range of what the camera thinks is "in focus" and luck with getting the sharper end of it a few times in a row.
Nearly started a topic but thought better of it last night!
But I may not entirely understand how it works with MF lenses to be honest!

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Last edited by Topsy; 09-14-2020 at 06:54 PM.

[gerardbaron]

I use the dot tune method too - works for me.

I don’t know this dot calibration.
Can you please explain
Thank you

[Apet-Sure]

I don’t know this dot calibration.Can you please explain

The video posted above does a good job of explaining it. I find this method to be simple and effective.

[kiwi_jono]

It certainly does make a big difference if the AF is dialed in. Suddenly your keeper rate goes up and satisfaction with it.

Interesting method of tuning AF, have not used that but it makes sense and easier to do in the field (as long as you have a tripod) if you need to. I will give it a go and compare as it looks simpler than my current process with focus charts.
Have not viewed 100% of that video yet (maybe it does cover this) but the distance to the subject while tuning has bearing on the results too (not too close and not too far) - it depends on focal length (typically something like 25 x your focal length works pretty well).

Thanks for posting!

[gerardbaron]

The video posted above does a good job of explaining it. I find this method to be simple and effective.

Thank you

[rawr]

Dot-tune seems just an evolution of a simpler approach to AF adjust, with no LiveView required:
- use a sturdy tripod + flat contrasty target (eg newspaper page),
- shoot a shot of the target using centre-point AF for each AF adjustment stage (-10 to +10), ie 20 shots in sequence,
- pick which image (and the matching adjustment stage) achieved the sharpest result,
- lock that adjustment value in.

[Mooncatt]

I finally got around to calibrating my lenses, but now have a dilemma with the 18-135mm WR. I initially calibrated it at a "happy medium" because it's a zoom lens. When I tested at each end of the focal range, it looked bad. When I calibrated to one end of the range, the other end became worse.

Is this the nature of the beast and should I just use this lens in live view whenever possible? Or is there something I'm missing in dealing with calibrating a zoom lens? Stopping down isn't ideal due to being in low light a lot when I use that lens.

[Tesla]

I finally got around to calibrating my lenses, but now have a dilemma with the 18-135mm WR. I initially calibrated it at a "happy medium" because it's a zoom lens. When I tested at each end of the focal range, it looked bad. When I calibrated to one end of the range, the other end became worse.

Is this the nature of the beast and should I just use this lens in live view whenever possible? Or is there something I'm missing in dealing with calibrating a zoom lens? Stopping down isn't ideal due to being in low light a lot when I use that lens.

I haven't tried this yet, but on a zoom maybe calibrate at the half way mark, about 58mm. Is that what you did?

[Wheatfield]

I finally got around to calibrating my lenses, but now have a dilemma with the 18-135mm WR. I initially calibrated it at a "happy medium" because it's a zoom lens. When I tested at each end of the focal range, it looked bad. When I calibrated to one end of the range, the other end became worse.

Is this the nature of the beast and should I just use this lens in live view whenever possible? Or is there something I'm missing in dealing with calibrating a zoom lens? Stopping down isn't ideal due to being in low light a lot when I use that lens.

I think with zooms it's best to dial the AF in at the long end of the zoom range.

[Mooncatt]

I haven't tried this yet, but on a zoom maybe calibrate at the half way mark, about 58mm. Is that what you did?

Yes, more or less. I don't remember the exact focal length. That is what was recommended on the video, but the results were still quite noticeable at the extreme ends.

[Mooncatt]

I think with zooms it's best to dial the AF in at the long end of the zoom range.

I'll give this a try. I went to the short end of the zoom range (no real reason, just what I happened to try first) and then gave up when I saw it made the long end worse. I figured the same would happen vice versa.

[Yoda boy]

Since the Pentax branded lenses are not made "in house", I suspect that there could be a slight tolerance issue that comes up from time to time. Probably a "best practices" protocol when anyone gets a new lens is to do a check on the auto focus accuracy with the camera where this lens would be typically used.

---------- Post added 05-18-21 at 04:56 PM ----------

Dot-tune seems just an evolution of a simpler approach to AF adjust, with no LiveView required:
- use a sturdy tripod + flat contrasty target (eg newspaper page),
- shoot a shot of the target using centre-point AF for each AF adjustment stage (-10 to +10), ie 20 shots in sequence,
- pick which image (and the matching adjustment stage) achieved the sharpest result,
- lock that adjustment value in.

Excellent advice and not a bad idea to do anytime one gets a new lens or camera body.