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06-28-2016, 08:21 AM   #31
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QuoteOriginally posted by UserAccessDenied Quote
Question:
Perhaps I'm overthinking this? I have only had one cup of coffee so far so I apologize if this question is "dumb"...

How do you fix a subject at infinity range?
I understand hyperfocal distance, but is this where I need to setup my subject?

I calculated for my DA*300 on a K-3ii that HFD is 3691.9 ft. There's no way I'll setup a subject at that distance!

To even reach the far limit of acceptable sharpness at infinity from 300mm my subject has to be a minimum of 2696.62 ft with a near limit of 1558.4 ft.

Am I overthinking this?!

I was planning on setting up my camera + lens on a tripod at one end of a football field and place a magazine on the fieldgoal post of the opposite endzone. Isn't this far enough for infinity focus, or not for 300mm?

Infinity focus on your lens is basically a stop point. Usually around 50 meters. Anything further than that, the lens just says, "I'm done." It's not the same as hyperfocal distance.

With that said, you definitely DON'T want to use hyperfocal distance when doing AF adjustments. You want your DoF to be razor thin so that you can see within the image itself the beginning and end of your DoF. That's usually about 3-4 INCHES at most for the tick marks on a basic ruler to be readable given the FoV of a 300mm+ lens.

Example: at 100 feet, the DoF for a 300mm lens at f4 on a k3 is 5 feet. (DoF at Infinity limit (150 feet) is ~12 feet). If your are using a yardstick (as an example) the entire yardstick would be IN FOCUS, giving you no inclination whether you are front or back focused. So using infinity for a lens calibration is cumbersome at best. Most people can't set up a 20 foot target at 150 feet of distance to calibrate a lens. I'm not even sure at 150 feet the numbers on the target would be readable anyway?

However, at 20 feet, your DoF is about 4 inches, meaning if you were focused on the 6 inch mark, you could very easily tell your lens is back focusing if the 9 or 10 inch mark was in focus and the 5 in mark was not, or vice versa.

That's why there's all the smirking about "I calibrate at infinity," comments. Maybe they actually do, but for most people calibrating a long lens at infinity requires space we don't have and pixel peeping resolution our cameras can't produce.


Last edited by nomadkng; 06-28-2016 at 08:38 AM.
06-28-2016, 09:02 AM   #32
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QuoteOriginally posted by nomadkng Quote
Infinity focus on your lens is basically a stop point. Usually around 50 meters. Anything further than that, the lens just says, "I'm done." It's not the same as hyperfocal distance.

With that said, you definitely DON'T want to use hyperfocal distance when doing AF adjustments. You want your DoF to be razor thin so that you can see within the image itself the beginning and end of your DoF. That's usually about 3-4 INCHES at most for the tick marks on a basic ruler to be readable given the FoV of a 300mm+ lens.

Example: at 100 feet, the DoF for a 300mm lens at f4 on a k3 is 5 feet. (DoF at Infinity limit (150 feet) is ~12 feet). If your are using a yardstick (as an example) the entire yardstick would be IN FOCUS, giving you no inclination whether you are front or back focused. So using infinity for a lens calibration is cumbersome at best. Most people can't set up a 20 foot target at 150 feet of distance to calibrate a lens. I'm not even sure at 150 feet the numbers on the target would be readable anyway?

However, at 20 feet, your DoF is about 4 inches, meaning if you were focused on the 6 inch mark, you could very easily tell your lens is back focusing if the 9 or 10 inch mark was in focus and the 5 in mark was not, or vice versa.

That's why there's all the smirking about "I calibrate at infinity," comments. Maybe they actually do, but for most people calibrating a long lens at infinity requires space we don't have and pixel peeping resolution our cameras can't produce.

Interesting!!

So the ruler should be laying on the ground, extending away from the lens?
Not sticking vertically out of the ground... This makes sense ^

Does the slope percent of my lens matter in relation to the subject?

Shooting from eye-level (~5.5ft) at a subject 20ft away = 27.5% slope

I imagine I should calibrate the lens at the slope percentage I typically shoot at. ~ between 5% and 27.5%
Or does that not effect the calibration?
06-28-2016, 09:14 AM   #33
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QuoteOriginally posted by UserAccessDenied Quote
Interesting!!

So the ruler should be laying on the ground, extending away from the lens?
Not sticking vertically out of the ground... This makes sense ^

Does the slope percent of my lens matter in relation to the subject?

Shooting from eye-level (~5.5ft) at a subject 20ft away = 27.5% slope

I imagine I should calibrate the lens at the slope percentage I typically shoot at. ~ between 5% and 27.5%
Or does that not effect the calibration?
Ideally, you want the lens completely horizontal, because tilting the lens down will actually increase to DoF. The ruler should be at a 45 degree angle to horizontal, with your target point (axis) even with the axis of the lens. Where the horizontal comes in, is often people will use a horizontal (flat plane) as an adjacent target for the center point AF to lock onto. The ruler or measuring device with be along side AF target. Just google "Len Align" for a visual. I use a printed and mounted black and white ruler which is good enough for the PDAF to lock onto without a target component.

But you don't have to get THAT OCD such that you run a string from you lens to the target, but close enough by eye is best. A lot of people use two tripods, if they have them available.

I typically just set up my ruler and lens next to each other, lock the tripod down and walk back 20 feet or so. It's close enough.

Last edited by nomadkng; 06-28-2016 at 09:21 AM.
06-28-2016, 10:08 AM   #34
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QuoteOriginally posted by nomadkng Quote
Example: at 100 feet, the DoF for a 300mm lens at f4 on a k3 is 5 feet. (DoF at Infinity limit (150 feet) is ~12 feet). If your are using a yardstick (as an example) the entire yardstick would be IN FOCUS, giving you no inclination whether you are front or back focused. So using infinity for a lens calibration is cumbersome at best. Most people can't set up a 20 foot target at 150 feet of distance to calibrate a lens. I'm not even sure at 150 feet the numbers on the target would be readable anyway?
You don't need to know whether you are back or front focusing - how do you translate the adjustment steps to focus distance error anyhow - all you need is to determine which AF adjust setting results in the most reliably sharpest image. It is easy with an 'infinity target' *. Try it.

* obviously not a useless chequer board / sloping ruler gizmo at 50 meters, but a distant building / tree / whatever.

06-30-2016, 09:27 AM   #35
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QuoteOriginally posted by kh1234567890 Quote
* obviously not a useless chequer board / sloping ruler gizmo at 50 meters, but a distant building / tree / whatever.
Yes...and 50 meters is hardly infinity*. As noted in comments above, evaluating infinity focus is extremely cumbersome, both due to DOF and the lack of mechanical precision** at that range for focus.


Steve

* Much depends on lens magnification. 50 meters is inadequate for evaluating infinity focus with a 200mm lens on APS-C, but works fine at 18mm.

** Both focus "lash" and the fact that AF systems use stepping motors with precision limits being directly related to the minimum step size.
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