# AFMA Query



## TeT (Dec 23, 2014)

I have a choice; focus is dead on between +1 & +2, which way should I go... Dont confuse me with back and front etc... you know what +1 is & what +2 is, so keep it simple for me...

Not a lot of sports or wildlife, mostly pics of ppl or architecture or flowers or my cat.

Long end of 70 300 L

+1 or +2

John


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## neuroanatomist (Dec 23, 2014)

Did you test at only one distance? If so, what was that distance and how does it compare to the distance(s) at which your subjects are usually positioned with that lens?


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## takesome1 (Dec 23, 2014)

Adding one thing to what Neuro said about distance.

Add in lighting, good lighting will change your test.

Did you test twice and had a +1 and +2?
Did you test 100 times with different light at different distances and have a +1 and +2?
It is an easier decision when you have additional results.

I would probably go for the +2 but I doubt you notice much of a change from one to the other if you are getting both results.


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## TeT (Dec 23, 2014)

neuroanatomist said:


> Did you test at only one distance? If so, what was that distance and how does it compare to the distance(s) at which your subjects are usually positioned with that lens?



I was unable to test the distance that I usually shoot with that lens from. I did testing at 25 x focal length. Moved in closer to confirm what I thought I was seeing from 25x. Used remote viewing to computer which allows me to zoom in hard.

At the distances and F stop I am shooting +or- .5 wont signify... 

reset focus both long and short of target as I know lens focus shows different results depending upon if they were focusing in or out to target.

The wide end (70mm) is solid at +1.


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## TeT (Dec 23, 2014)

takesome1 said:


> Adding one thing to what Neuro said about distance.
> 
> Add in lighting, good lighting will change your test.
> 
> ...



Tested multiple times with good light from different distances.

I know for a fact that the .5 will make no difference whatsoever now that I think about it.

so it turns into a philosophical question. Philosophical is probably not the right word but you know what I mean...

BTW: I left it at +2


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## takesome1 (Dec 23, 2014)

TeT said:


> neuroanatomist said:
> 
> 
> > Did you test at only one distance? If so, what was that distance and how does it compare to the distance(s) at which your subjects are usually positioned with that lens?
> ...



Maybe you should describe your technique or method you are using to test. example; focal, lens Align a box of post toasties and a ruler.


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## TeT (Dec 23, 2014)

takesome1 said:


> Maybe you should describe your technique or method you are using to test. example; focal, lens Align a box of post toasties and a ruler.



Post toasties look like it has nice crisp text on it.

Used a black and white ruler with a target to side aligned at an angle that would allow camera to hold focus for a 1/8" front and 1/8" back of point at f4 which is my widest available. Level Tripod, targets square to camera...

3 lights 250 f4 800ISO


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## takesome1 (Dec 23, 2014)

Give you an idea using Reiken FoCal.

This weekend I tested a 500mm on my 1D IV.
Using 2 Halogen, 2 x 250 watt with the Halogens 8' away I would get a -1 to a -2. 
Move the lights 5' away and add more 250 watt and I consistently had a 0 to 1.
I checked several distances but my house isn't big enough.
0 ended up being the number. 

+1 or +2 either will probably work.

My suggestion, take your camera out for a day and shoot a few portraits. If your constantly hitting the eyelash and getting good detail you found the right setting.


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## TeT (Dec 23, 2014)

takesome1 said:


> Give you an idea using Reiken FoCal.
> 
> This weekend I tested a 500mm on my 1D IV.
> Using 2 Halogen, 2 x 250 watt with the Halogens 8' away I would get a -1 to a -2.
> ...



Will try that, was at zero, wasn't getting the crisp eyeballs which is why I undertook the operation. I knew none of my lenses were off by more than 1 or 2 to begin with but wanted better.


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## Mt Spokane Photography (Dec 24, 2014)

Unless you have super vision and a 4K monitor, you won't be able to tell the difference in a image that is +1 or +2. That number will change at different focus points in any event. So many things can affect AFMA that its just a imprecise estimate at best. Light levels, color, detail in the image, all these can change the perceived afma value.

Canon generally consider a +/- 5 to be in tolerance.


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## Marsu42 (Dec 24, 2014)

Mt Spokane Photography said:


> Unless you have super vision and a 4K monitor, you won't be able to tell the difference in a image that is +1 or +2. That number will change at different focus points in any event. So many things can affect AFMA that its just a imprecise estimate at best. Light levels, color, detail in the image, all these can change the perceived afma value. Canon generally consider a +/- 5 to be in tolerance.



+1000, this information should go as sticky to the top of the list. Even if FoCal would loose some sales, it would stop people wondering about "My pictures look blurry! I have to adjust afma!". In hindsight, I probably saved myself some headaches as my first camera (60d) didn't have afma at all and I was mostly shooting af f5.6+

If you focus & recompose (like you have to with mediocre af systems like 5d2,6d,...) there's an additional source of error so a spot-on afma is "nice to have" if you happen to shoot at the same distance as the afma adjustment was, but nothing to write home about.


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## rpt (Dec 24, 2014)

Is 0 worse or +3? I guess that should solve it for you. If you have FoCal you can see the curve and it becomes amply clear.


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## TeT (Dec 24, 2014)

Marsu42 said:


> Mt Spokane Photography said:
> 
> 
> > Unless you have super vision and a 4K monitor, you won't be able to tell the difference in a image that is +1 or +2. That number will change at different focus points in any event. So many things can affect AFMA that its just a imprecise estimate at best. Light levels, color, detail in the image, all these can change the perceived afma value. Canon generally consider a +/- 5 to be in tolerance.
> ...



makes sense, would not have been able to determine the difference without the monitor. Mainly it came up and since I was doing it, wanted to finish it.

Thanks for the info...


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## Mt Spokane Photography (Dec 24, 2014)

TeT said:


> Marsu42 said:
> 
> 
> > Mt Spokane Photography said:
> ...


 
I recall reading that 1 point on AFMA = 1/8 DOF, since AF accuracy tries to be within 1/3 DOF, that comes to about +/- 2.5 points or 5 total to move it about 1/3 DOF.


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## neuroanatomist (Dec 24, 2014)

Mt Spokane Photography said:


> I recall reading that 1 point on AFMA = 1/8 DOF, since AF accuracy tries to be within 1/3 DOF, that comes to about +/- 2.5 points or 5 total to move it about 1/3 DOF.



With the caveat that the 'F' stands for focus – one unit of AFMA is 1/8 the depth of focus at max aperture. Depth of focus is the sensor-side counterpart to depth if field, but the former is basically insensitive to subject distance.


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