Camera AF Microadjustment Maths

KIPAX

KIPAX

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KIPAX Lancashire UK
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Simple question is how much difference does -/+ 1 make?

Example
FF camera + 400mm lens at f2.8
If I take a pic at 50 feet and 500 feet

I am not convinced -/+ 1 makes much difference at 50 feet (I know it must but not noticable)... what sort of forward and backward difference does it make at longer distance..

I can test myself but its hard to see results.. I wondered if there is a math to this.. am guessing there is ? so subject 500 feet away eg -1 does it bring focus a foot forward.. an inch forward?
 
not sure of the exact calculation would be interested too cant see it makes a huge difference with those focal lengths tho from massive distances
 
It'll depend on your camera. On my Sony A77 +1 of focus adjustment at 50 feet on a 500mm lens seems to move focus roughly 1mm. On your camera just move it from max -ve to max +ve, measure the difference, and divide by the number of focus adjust units. It may well be that the inherent inconsistency of your AF is more than 1 unit of adjustment.
 
gramps said:
No idea on the maths but one of my lenses needed an adjustment of +4 and it's made a massive difference in near and far focus :)
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I was guessing and just checked.. football pitch can be up to 130 yards long so probably 400 feet (not staright line end to end)
 
chris malcolm said:
It'll depend on your camera. On my Sony A77 +1 of focus adjustment at 50 feet on a 500mm lens seems to move focus roughly 1mm. On your camera just move it from max -ve to max +ve, measure the difference, and divide by the number of focus adjust units. It may well be that the inherent inconsistency of your AF is more than 1 unit of adjustment.
Click to expand...

In all honesty... didnt follow that at all.. sorry :)
 
It will depend on the lens and aperture used at a certain distance, there is no one answer.

A 1DX with a 400mm set at f/2.8 focusing on a subject at 400ft away gives a focal depth of 51ft, at 300ft it drops to 29ft and at 200ft it's 12ft. Get a brightly coloured tennis ball and put it on the pitch 100ft away, this will give you just over 3ft depth of field, 1.5ft in front of the ball and 1.5ft behind.

I know this is not answering your question but it's the method I use.
 
I'm sure I read somewhere that on a Canon that one increment moves the point of focus 1/8 of the forward or backward DOF.
 
Gaz J said:
I'm sure I read somewhere that on a Canon that one increment moves the point of focus 1/8 of the forward or backward DOF.
Click to expand...

Agree and I was about to post the same comment. Can't remember where I saw it, or indeed whether it is true!
 
KIPAX said:
so subject 500 feet away eg -1 does it bring focus a foot forward.. an inch forward?
Click to expand...

I've read this before but could never get it to work, in theory at 500ft 1 click would shift the focal plane just over 10ft
 
This will work out your DoF

http://www.dofmaster.com/dofjs.html

You can get various Apps for your phone if you are out and about................look for depth of field calculator

EDIT: I see your link above is a DoF calc...........I didn't click on it

You'll just need to do the sums in your head...........or a fag packet :)
 
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If I shoot a football player 400ft away using a 1dx and a 400mm lens at 2.8... everything 30ft in front and 30ft behind isnt in focus...
 
I think your right I think it is 1/8 I'm sure I came across that somewhere also
 
KIPAX said:
If I shoot a football player 400ft away using a 1dx and a 400mm lens at 2.8... everything 30ft in front and 30ft behind isnt in focus...
Click to expand...

Sounds serious, the grass should tell you where the focus point is?

You got an image?
 
admirable said:
Sounds serious, the grass should tell you where the focus point is?
?
Click to expand...

haha.. I meant not that much is in focus....if i have a player stood 400ft away and another player stood 1 foot behinf him.. the player behind him isnt in focus.. let alone 30ft behind him :)
 
KIPAX said:
haha.. I meant not that much is in focus....if i have a player stood 400ft away and another player stood 1 foot behinf him.. the player behind him isnt in focus.. let alone 30ft behind him :)
Click to expand...

So it must be front focusing?
 
admirable said:
So it must be front focusing?
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I am guessing its me..usualy is...

.if i have a player stood 400ft away and another player stood 1 foot behind him.. the player behind him isnt in focus.. let alone 30ft behind him

.if i have a player stood 400ft away and another player stood 1 foot infront him.. the player infront him isnt in focus.. let alone 30ft infront him

So just the player is in focus as I would expect.. If i change to say f5.6 then players in front and behind start to come into focus.. the more f stop the more anything in front or behind comes into focus..

To suggest at f2.8 that everyhting around 30ft in front or behind would be in focus is...well ?
 
So what i am still looking for is how much the adjustment moves.. I am saying I would be suprised if its 1/8th of 30-40 feet as thats too much for micro surely ? woudlnt that be like 5 feet per notch ?
 
Gaz J said:
I'm sure I read somewhere that on a Canon that one increment moves the point of focus 1/8 of the forward or backward DOF.
Click to expand...

The 1/8th is correct but it's 1/8th of the single sided depth of focus (not depth of field).

Bob
 
Canon Bob said:
The 1/8th is correct but it's 1/8th of the single sided depth of focus (not depth of field).

Bob
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I see that the 2 articles I linked to refer to it in both ways. depth of focus makes a bit more sense though.
 
Canon Bob said:
The 1/8th is correct but it's 1/8th of the single sided depth of focus (not depth of field).

Bob
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Correct... Depth of Focus as relates to AF error is the depth of the acceptably sharp image at the sensor plane. It's a measurement of how far in-front of, and behind the sensor the image can be focused and still be acceptably sharp. This measurement is typically less than .1mm total. It's not the single sided measurement (i.e .05mm... it would be ±.05mm). This depth of focus doesn't really vary with distance.

How the AF shift of the projected image w/in the Depth of Focus affects the Depth of (focus) Field does vary with distance....because the DO(f)F varies with distance, but it's nothing like 1/8th.
 
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KIPAX said:
I am guessing its me..usualy is...

.if i have a player stood 400ft away and another player stood 1 foot behind him.. the player behind him isnt in focus.. let alone 30ft behind him

.if i have a player stood 400ft away and another player stood 1 foot infront him.. the player infront him isnt in focus.. let alone 30ft infront him

So just the player is in focus as I would expect.. If i change to say f5.6 then players in front and behind start to come into focus.. the more f stop the more anything in front or behind comes into focus..

To suggest at f2.8 that everyhting around 30ft in front or behind would be in focus is...well ?
Click to expand...
Only one point is ever "in focus."

DOF is a "generic" standard for "acceptably sharp" based upon a Circle Of Confusion (COC) limit. This COC limit is based upon certain assumptions, and those assumptions do not always hold true... The COC display "standard" only requires an ~2MP FF image for it to be "acceptably sharp" at *any size.*

DOF (as we are used to it) is not a fixed property of an image... it varies based upon the ratio of image size : viewing distance. It is technically wrong, and misinformed to critique an image and say something line "the image doesn't have enough DOF." What you're actually saying is "the image doesn't have enough DOF as I'm viewing it on my computer."
 
Gaz J said:
I see that the 2 articles I linked to refer to it in both ways. depth of focus makes a bit more sense though.
Click to expand...
The concept of "depth of field" repositioning (strictly the plane of sharp focus) is actually more tangible for someone who doesn't understand the true application because they can work under a false premise but still achieve the desired result.

Bob
 
Canon Bob said:
The concept of "depth of field" repositioning (strictly the plane of sharp focus) is actually more tangible for someone who doesn't understand the true application because they can work under a false premise but still achieve the desired result.

Bob
Click to expand...

That'll be me then Bob. I do it with a lens cal and some tins then check it at various distances up to the very max I'm likely to photograph at. Quick to do and best of all I don't need to understand the science. :)
 
KIPAX said:
I am not convinced -/+ 1 makes much difference at 50 feet
Click to expand...

You and me both. People seem to forget what the M stands for. When you're doing MICROadjustments small increments will make virtually no difference...

AFMA.jpg
 
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