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12-03-2013, 05:35 PM   #1
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Focus calibration - who to believe?! (hex or focus screen)

Hi everyone,

I have a K5 with a handful of old primes (K 50 1.4, K 135 2.5 and K 135 1.2), as well as a few modern lenses (17-70 and 12-24).
To help with manual focus I recently installed a a split-screen focussing screen with micro-prism.
Calibrating with a Lenscal, I found I needed to install 2 small shims (it was back-focussing), and for my manual lenses, a setting of around -4 seems to work nicely.

Now, that's great, but my main concern is that the focussing screen is just ever so slightly out of whack with the hexagon focus indicator.
It's slight, but it's enough to be mildly irritating. I'm not sure whether to use the focussing screen to get 'rough' focus right, then rely on hex if I'm using a really wide aperture and need to fine tune.
Kinda feels like that defeats the purpose of having a focussing screen in the first place though.

How would I go about getting the focussing screen to match the hexagon indicator? Is that to do with the shims?

Any advice greatly appreciated!

12-03-2013, 05:57 PM   #2
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The hex is the boss. The red screen indicator is just a guideline to show which focus point is being used.
As far as the focus screen? Sorry, no ideas from me. :-(
12-03-2013, 05:58 PM   #3
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You should not have to do a focus adjustment for the lens. Work with the shims and just get the focusing screen adjusted correctly.. Then just depend on it instead of the hexagon.
12-03-2013, 06:20 PM   #4
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QuoteOriginally posted by heartattackandvine Quote
Hi everyone,

I have a K5 with a handful of old primes (K 50 1.4, K 135 2.5 and K 135 1.2), as well as a few modern lenses (17-70 and 12-24).
To help with manual focus I recently installed a a split-screen focussing screen with micro-prism.
Calibrating with a Lenscal, I found I needed to install 2 small shims (it was back-focussing), and for my manual lenses, a setting of around -4 seems to work nicely.

Now, that's great, but my main concern is that the focussing screen is just ever so slightly out of whack with the hexagon focus indicator.
It's slight, but it's enough to be mildly irritating. I'm not sure whether to use the focussing screen to get 'rough' focus right, then rely on hex if I'm using a really wide aperture and need to fine tune.
Kinda feels like that defeats the purpose of having a focussing screen in the first place though.

How would I go about getting the focussing screen to match the hexagon indicator? Is that to do with the shims?

Any advice greatly appreciated!
Unless the camera is in control of the lens's AF motor, the hex af confirmation is only going to be approximate. It will say the image is in focus with a small about of tolerance.


Adam
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12-03-2013, 06:22 PM   #5
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QuoteOriginally posted by amoringello Quote
The hex is the boss.
Except that the AF is not particularly precise (or accurate for that matter).

As mentioned above, most screens will not require changes to the shims if the factory screen was properly collimated. Test the screen collimation against a moire' target or using liveview under magnification. In both cases the target should be flat and parallel with the sensor. When properly collimated, a screen with split image will provide superior accuracy and also be much more precise that the AF system. Once the split image focus is confirmed, calibrate your various AF lenses against the it.


Steve

FWIW...manual focus does not suffer from "back/front focus" issues per se. Those are AF-related terms that reflect the mechanical/optical characteristics of the various lenses. That is why all AF adjustments are on a lens-by-lens basis.

Last edited by stevebrot; 12-03-2013 at 06:34 PM.
12-03-2013, 06:40 PM   #6
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Right, sounds like I'm going to have to recalibrate everything, starting with the shims.
I'd relied on the hex for calibration adjustments.

I think I'm a little bit confused still.
The focus fine-adjustment (-10 to +10) applies to all lens, right?
And shims affect focussing (thicker shim produces more front focus), right?

From my measurements so far (factory), I have back focus on manual lenses.

So, do I just choose a focus fine-adjustment value (say -5), then adjust the shim thickness until I get a properly in-focus image when the split focussing screen lines up?

Then, what does that mean for my AF lenses?
I suppose I change the fine-adjust in camera and 'apply to one' for each of those lenses, then I can either use AF and ignore the focus screen (if it doesn't align) or use manual focus and treat the same as manual lenses?

Rather confusing...
12-03-2013, 07:12 PM   #7
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Using any lens--in manual focus (best is likely a fast macro) shim the screen till the focus is correct.

Then the adjustments (+/- not shimming) are for AF--nothing to do w/ manual focus. (As I understand it the focus confirmation suffers same uncertainty/error as the AF.)
12-03-2013, 07:34 PM   #8
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For the Auto Focus fine adjustment you can choose between applying the adjustment value to all lenses (apply all) or just one (Apply one) and you can do up to 20 different lenses (page 126 of K5 manual). But the apply one option will one be display if the camera can identify the lens mounted on it. By that I assume it won't work for old lenses and I am not sure if will work for the K series.

12-03-2013, 08:30 PM   #9
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Oh I get it now...
AF adjustments dont affect front/back focus at all when you're using the focussing screen to get focus....
I only thought that was happening, because initially I was focussing using the hexagon, which is affected by AF fine adjustments.

No wonder I was getting such odd results.

Once I get the focus screen sorted (using shims), then I can fiddle around with AF fine adjustments to get my AF lens' to match the focus screen.

Hopefully I'll have this done shortly - and I hope it's worth it! - I've been fiddling for too many hours.
12-03-2013, 09:14 PM   #10
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QuoteOriginally posted by heartattackandvine Quote
I've been fiddling for too many hours.
Part of the learning process
12-03-2013, 09:38 PM   #11
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Done and sorted!
Thanks for the advice everybody.

For anyone else who might read this later...

I was using Nikon model 'K5' focussing screen. The correct setup for me was to replace the steel factory installed shim, and replace that with 2 thin shims that came in the package.
There were three plastic shims supplied (2 thin, 1 thick), and the above was the correct installation for accurate manual focus.

AF lenses and AF fine adjustment didn't need any tweaking in the end. Very slight back focus on my 17-70. Honestly, probably not enough to bother adjusting for. And, the hexagon lights approximately matching the focussing screen. Of course, the AF lenses have a much more sensitive focus ring, so it's not really the feasible to do manual focus on those. It's the price you pay for convenience of AF, and I'm okay with that.

Thanks again!
12-03-2013, 10:37 PM   #12
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QuoteOriginally posted by heartattackandvine Quote
The correct setup for me was to replace the steel factory installed shim, and replace that with 2 thin shims that came in the package.
You probably would have been fine simply using the factory shim. The plane of focus should be the same for both the original and the K5 screen. But if you are good now, that is excellent. Have fun with your new screen. I will make a world of difference with your manual focus lenses.


Steve
12-03-2013, 11:52 PM   #13
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QuoteOriginally posted by stevebrot Quote
You probably would have been fine simply using the factory shim. The plane of focus should be the same for both the original and the K5 screen.
I can't speak specifically about the old Nikon screen, but many aftermarket screens are a different thickness than the factory screen. Different thickness screens need different shims. Also, I can't speak for others, but my old K10d had the wrong thickness shim from the factory.
12-04-2013, 11:29 AM   #14
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QuoteOriginally posted by Steve Beswick Quote
I can't speak specifically about the old Nikon screen, but many aftermarket screens are a different thickness than the factory screen. Different thickness screens need different shims. Also, I can't speak for others, but my old K10d had the wrong thickness shim from the factory.
I only have the stock screen from my K10D and the Katz Eye (essentially the same as the Nikon K5) that replaced it and did not need to shim. I would be very surprised if screen thickness were an issue. The underside (nearest the mirror and registered to the mirror box by a metal frame) is intended to be at the plane of focus. As a result thickness should not be part of the equation. Unfortunately, poor factory screen collimation seems to be a common problem. If the factory shims are wrong, an adjustment (add/remove shims) would be in order.


Steve
12-05-2013, 01:34 AM   #15
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they're two separate animals

if your focus screen is shimmed correctly - leave it alone

if your focus sensor is out - adjust it under the options


also, both of these will drift slightly depending on light temperature

given a fixed subject I still favor manual focus over auto - even on the Kr pentamirror

moving objects is a different story
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