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04-22-2016, 11:43 AM   #16
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QuoteOriginally posted by jatrax Quote
I've read 40 to 50 times the focal length. So for a 50mm lens you should be about 2.5 meters. Personally I use much less distance than that more like 10 to 20 times focal length but the official recommendation from Canon and the lens align makers is about 50x.
So, if I want to test my FA* 400mm f5.6, it would have to be 20 metres (62.6 feet) from the calibration chart?

04-22-2016, 11:58 AM   #17
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QuoteOriginally posted by pete-tarmigan Quote
So, if I want to test my FA* 400mm f5.6, it would have to be 20 metres (62.6 feet) from the calibration chart?
According to Syder, maker of the LensCal you should be 30 to 50 times focal length. So for the 400: 12 to 20 meters

According to LensAlign you should be 25 to 50 times focal length

According to Canon you should be no less than 50x the focal length

I seem to use closer to the 20x value because at longer ranges I cannot see enough difference to make a determination. Also my studio is not large enough to do 50x on anything larger than 50mm
04-22-2016, 12:26 PM   #18
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Of course one also has to allow for the temperature of the lens/camera. And for barometric pressure, RH and air temperature as these also affect the air refractive index.
04-22-2016, 01:59 PM   #19
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QuoteOriginally posted by kh1234567890 Quote
And for barometric pressure, RH and air temperature as these also affect the air refractive index.
Don't forget small-scale temperature and water vapor gradients when testing those long lenses. A certain member of these forums (to remain unnamed) tests lenses (even wide angles) at 100 yards across a sun-drenched cultivated field 1/4 mile from the shores of the North Pacific...mirages in the making...fog too...


04-28-2016, 06:31 PM   #20
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Should I use 50x crop sensor equivalent focal length or 50x FF focal length for a K3?
04-28-2016, 07:45 PM   #21
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QuoteOriginally posted by DavidDwight Quote
Should I use 50x crop sensor equivalent focal length or 50x FF focal length for a K3?
Forget 'equivalent', David.

Just 50x what it says on the lens. ☺

04-28-2016, 09:34 PM   #22
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I'll concur with others here, that after experimenting over a period of time with my K-5, I found the following to be crucial:

- Camera locked on center point focusing.
- Tripod mounted.
- Daylight color temperature (seemed to be crucial back in the K-5 days, unless my most crucial shooting was to be in other lighting, e.g. incandescent).
- Lens to target distance approaching 20 to 50x focal length (or alternately most important subject distance, e.g. if a portrait lens).
- An AF target that provides only a single plane perpendicular to the camera axis and a single central flat target design (e.g. thick horizontal bar) of high contrast on which to lock focus (having a zone of no-contrast surrounding the AF center target).
- Ideally a symmetrical high contrast pattern nearer the perimeters or four corners of the target image frame that can help account for errors in alignment and provide a pattern (e.g. test images or text fonts) that facilitate the ability to distinguish focus resolution.
- Defocusing between images.
- Refocusing accomplished with a single lock on target (without hunting or step-wise adjustment).
- Image comparisons (e.g. pixel peeping) should be made considering errors in alignment (e.g. comparing all four corners of target).
- Zoom lens tested at minium, median, and maximum focal length (then judging which to give priority to based on best compromise or most important use of the lens).

For defocusing, a couple of options:
- Pan to the side then auto focus on a consistent spot either closer or farther distance than the AF target, then pan to recenter with the AF target and auto focus for the test image.
- Without panning by switching to manual focus and adjust the lens focus point by hand to something consistently closer or farther than the AF target, then switch back to auto focus for the test image.

Things that didn't work well:
- Mixed lighting (for a camera such as the K-5 that was AF sensitive to lighting temperature).
- A very close focus scale (e.g. 45 degree camera on tripod to floor).
- Attempts to use a sophisticated 45 degree target at longer distances (approaching 20x).
02-22-2019, 06:52 PM   #23
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About the internal adjustment

I have a K3ii and it came with 18-135 DC WR lens. I had a K-X before for almost 10 year and I was using with 16-50 DA* . So in my case the DA* lens was good and on focus also I have Sigma 30 mm art and I never had a problem but once I used the new Pentax lens I had a big issue. First the lens has no scale to show you where is the focus also and different focal length the focus is also changing. I tried using it for astro tracing but every time it was out of focus. I tried focusing on a illuminated object to infinity then locking the focus but nothing ever worked. Today I discover this internal adjustment for the auto focus and now finally after I put the lens to -10 it became sharp. Obviously the lens gets really soft once you start looking in to the corners but this is to be expected for a zoom range the good thing is that it is sharp in the centre . I almost sold the lens brand new for a 100 $ but now I discover that the lens is not that bad.
I hope this helps.


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