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08-14-2016, 08:17 PM   #1
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Quick question on AF adjustment

I'm trying to relate in my head the correlation of the autofocus adjustment number and the effect/impact:

Let's say I have a lens which is tending to front focus (which if I'm using the term right it focuses in front of the intended point - want to focus on eyes but tip of nose is in focus and the ears are blurred ...)

And let's say my AF fine adjustment for this lens is +5.

Would I want to move the number down to +4 or up to +6 to adjust it?

Does a positive adjustment number draw the focal point to you or push it away from you?

...

And while you are at it: does a West wind blow to the west or from the west?

08-14-2016, 08:34 PM   #2
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That's a good question. I always was under the impression that AF fine adjust wasn't the same as fixing front or back focus. So, your answers you get, will help me understand this too. Thanks for posting your question!
08-14-2016, 08:35 PM   #3
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With Pentax, you would enter -5 to bring it to zero. Envision it as moving the focus point even though this is not the case.
08-14-2016, 08:39 PM - 1 Like   #4
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I found this article here on PF.

Fixing Front and Back Focus - The Remedy - In-Depth Articles

08-14-2016, 10:36 PM   #5
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West wind blows from the west.
08-15-2016, 03:39 AM   #6
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QuoteOriginally posted by jbinpg Quote
With Pentax, you would enter -5 to bring it to zero. Envision it as moving the focus point even though this is not the case.
To be clearer in the case I cited I have already entered +5 for the particular lens so I can't enter -5 to make it 0. What I'm looking for is how should I change the entered number? Should I make it larger or smaller? And interpreting what you said I should make it smaller (change it from a +5 to a +4 for a small adjustment, 0 for a medium one and -5 for a large one for example) to move the sharp focus from tip of nose to eyes, making nose and ears equally out of focus...

---------- Post added 08-15-16 at 03:52 AM ----------

QuoteOriginally posted by photolady95 Quote
Thanks -- I remember reading that a while ago but going back I see this line on the page you cited:

If things in front of your subject are sharper than the subject itself, apply a -1 focus correction...
If things behind your subject are sharper than the subject itself, apply a +1 focus correction...


which tells me I should reduce my setting (maybe move it from +5 to +4) to move the sharpest focus from the tip of the nose to the eyes.

So if a lens is "front focusing" the sharpest focus is in front of where it is intended and the focus correction number needs to be reduced. So a positive correction number "pulls" the sharpest focus point towards the camera. Correct?
08-15-2016, 04:24 AM - 1 Like   #7
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QuoteOriginally posted by DavidDwight Quote
..

And while you are at it: does a West wind blow to the west or from the west?
I regularly had to teach this to students taking ecology. You face into the wind - the direction you are facing gives the name to the wind. Easiest way to remember (in this hemisphere): a north wind is cold.
BTW: For the non-science majors taking geology, I regularly had to teach them which sides of a map were N-E-S-W.
08-15-2016, 05:30 AM   #8
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Now that's just depressing.

QuoteOriginally posted by WPRESTO Quote
BTW: For the non-science majors taking geology, I regularly had to teach them which sides of a map were N-E-S-W.


08-15-2016, 06:17 AM   #9
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QuoteOriginally posted by WPRESTO Quote
I regularly had to teach them which sides of a map were N-E-S-W.
How old were they? 2? Geez, that doesn't say much for America's intelligence, eh?
QuoteOriginally posted by DavidDwight Quote
Thanks -- I remember reading that a while ago but going back I see this line on the page you cited:
QuoteQuote:
If things in front of your subject are sharper than the subject itself, apply a -1 focus correction... If things behind your subject are sharper than the subject itself, apply a +1 focus correction
... which tells me I should reduce my setting (maybe move it from +5 to +4) to move the sharpest focus from the tip of the nose to the eyes. So if a lens is "front focusing" the sharpest focus is in front of where it is intended and the focus correction number needs to be reduced. So a positive correction number "pulls" the sharpest focus point towards the camera. Correct?
I can't answer that.

How do you know which increase or reduction you should use? This was my questions about this all along.
08-15-2016, 08:05 AM   #10
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QuoteOriginally posted by DavidDwight Quote
To be clearer in the case I cited I have already entered +5 for the particular lens so I can't enter -5 to make it 0. What I'm looking for is how should I change the entered number? Should I make it larger or smaller? And interpreting what you said I should make it smaller (change it from a +5 to a +4 for a small adjustment, 0 for a medium one and -5 for a large one for example) to move the sharp focus from tip of nose to eyes, making nose and ears equally out of focus...

---------- Post added 08-15-16 at 03:52 AM ----------


Thanks -- I remember reading that a while ago but going back I see this line on the page you cited:

If things in front of your subject are sharper than the subject itself, apply a -1 focus correction...
If things behind your subject are sharper than the subject itself, apply a +1 focus correction..
which tells me I should reduce my setting (maybe move it from +5 to +4) to move the sharpest focus from the tip of the nose to the eyes.

So if a lens is "front focusing" the sharpest focus is in front of where it is intended and the focus correction number needs to be reduced. So a positive correction number "pulls" the sharpest focus point towards the camera. Correct?
Correct.
08-15-2016, 08:06 AM   #11
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QuoteOriginally posted by jbinpg Quote
With Pentax, you would enter -5 to bring it to zero. Envision it as moving the focus point even though this is not the case
Um, no. to bring it to zero you enter zero.
08-15-2016, 08:20 AM   #12
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QuoteOriginally posted by DavidDwight Quote
I'm trying to relate in my head the correlation of the autofocus adjustment number and the effect/impact:

Does a positive adjustment number draw the focal point to you or push it away from you?
Maybe a mental image will help.

An image is projected onto the sensor as a three dimensional cone, with the sensor as an intersecting flat plane "slicing" the cone to capture an image. Wherever the sensor intersects the 3d projection, that portion of the image will be in focus.

If the sensor is intersecting the image too "early" (front focus), you need to move the sensor BACK (negative numbers) in the projected image. If the Sensor is intersecting the image too "late" (back focus), you need to move it FORWARD (positive numbers).

Each adjustment moves the sensor within the image cone a small increment. The numbers are not relative, but absolute. You may move the sensor 10 "steps" forward or backward within the projected cone. Zero will always be the middle value and position of the adjustment range.
08-15-2016, 09:20 AM   #13
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I meant to bring it *toward* zero... one of the weaknesses of the English language.

---------- Post added 08-15-16 at 09:24 AM ----------

BTW, the direction of flowing fluids depends on your frame of reference. If you are planted with your feet on the ground and the wind is blowing over you then the wind direction is that from whence it blows. If you are riding with the fluid such as a raft on the ocean, then the direction of the current is the direction to where it is headed, not where it came from. One of the big differences between meteorology and oceanography.
08-15-2016, 10:36 AM - 1 Like   #14
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QuoteOriginally posted by photolady95 Quote
How old were they? 2? Geez, that doesn't say much for America's intelligence, eh?
College students, at least 18 years old. Two anecdotes that shocked me even more:
1) Many students confuse: [their, there, they're] and [through, threw] and [to, too, two]. When any of these appeared on a on a paper or exam, I'd cross out the mistake, write the correct word, and write the two or three words at the top of the page with the admonition "learn these." A student wrote "to" instead of "two," I inserted the correction and wrote the admonition, the student brought me the paper after and pointed to the "two" and said: "Are you telling me this is an English word?"
2) An early lecture in a non-majors course in historical geology is titled; "The Origin of the Universe and other Good Things." One connecting thread is the observation that when matter gathers because of gravity, it tends to spin and form a disk with most of the matter at the center. At very large scales, this creates a spinning galaxy with billions of stars. At a smaller scale it creates a star such as the sun with planets circling around it. At still smaller scales it creates a planet with moons circling around, or sometimes rings of debris are retained such as the rings of Saturn. A student came up to me after and said: "If I understand correctly, you're saying that the Earth moves around the Sun and the Moon moves around the Earth." A college student and those ideas were a revelation to him.
08-15-2016, 11:02 AM   #15
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QuoteOriginally posted by WPRESTO Quote
Many students confuse: [their, there, they're] and [through, threw] and [to, too, two]
I had a journalism college professor who stated at the beginning of the semester, if you mix up the words, it's and its, your A paper will become a C paper. Those that didn't get it, learned fast enough.

I was never one of those students because I knew the difference and their meanings.
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