# AF Queries



## dlee13 (Sep 7, 2014)

What I am wondering is, can a lens that suffers from say -5 back focus still focus accurately at 0 on certain objects? 

I've recently noticed that my 35 seems to be much sharper at -5 when focusing on people. At 0, it is still tack sharp when focusing on objects such as text although the images are nowhere near as sharp when focusing on people when it is set to 0.

So it is possible for a lens to focus accurately at different AFMA values on certain objects such as text?


----------



## neuroanatomist (Sep 7, 2014)

Subject shouldn't matter. Distance to that subject definitely _does_ matter.


----------



## dlee13 (Sep 7, 2014)

neuroanatomist said:


> Subject shouldn't matter. Distance to that subject definitely _does_ matter.



So if I was shooting at say 5 meters compared to the MFD then it may not focus accurately at 0?


----------



## neuroanatomist (Sep 7, 2014)

Correct – AFMA varies with distance. It changes most at close distances; MFD to a few meters can make a big difference, a few meters to infinity not much if any.


----------



## dlee13 (Sep 7, 2014)

neuroanatomist said:


> Correct – AFMA varies with distance. It changes most at close distances; MFD to a few meters can make a big difference, a few meters to infinity not much if any.



Thanks for the clarification. Now that you mention it that makes a lot if sense since the Sigma dock can AFMA at different distances. 

I previously thought my lens had only recently started back focusing but now I think of it, I'm pretty sure it has always been at -5 and I just didn't realise or test it.


----------



## Mt Spokane Photography (Sep 7, 2014)

dlee13 said:


> neuroanatomist said:
> 
> 
> > Subject shouldn't matter. Distance to that subject definitely _does_ matter.
> ...



Most lenses tend to front focus near MFD. As soon as you get back a ways, things should be better. However, when Canon adjusts a lens AF, there are adjustments for various distances, so it it is a big issue, they can fix it. They consider =/- 5 to be a acceptable variation.


----------



## takesome1 (Sep 7, 2014)

At f2 with a 35mm at 15' you have a DOF over 7'. 
Accuracy doesn't mean much when you shoot at the broad side of a barn.

Personally I am concerned with the accuracy 4 or 5 foot.

Unless you are shooting pics of people reading news papers, why does text matter as long as you are getting good portraits.

I have always noticed wide lens are a bit harder to nail down, and for the 35mm I always have preferred that it back focus just a bit. It helps keep subjects in the narrow DOF at close range.


----------



## dlee13 (Sep 7, 2014)

takesome1 said:


> At f2 with a 35mm at 15' you have a DOF over 7'.
> Accuracy doesn't mean much when you shoot at the broad side of a barn.
> 
> Personally I am concerned with the accuracy 4 or 5 foot.
> ...



I mostly use my lens at close distances, like MFD to 2-3 meters and now that I have it set at -5 it's spot on aside from user error.

When I say shooting text, I mean shots like this:





Text doesn't matter for portraits but my main point was AF accuracy shooting different subjects. 

My 35mm f2 IS is tack sharp even wide open, if I can't see every tiny facial detail then I figure something is wrong!


----------



## takesome1 (Sep 7, 2014)

Shooting a vertical flat subject as your target with the inclined text object laying next to it would give you a better idea of where it is hitting. Also if you are using multiple points or with some bodies single points with assist this shot you posted may not be accurate. 

I don't think it matters because you know -5 works for you, but you might try the vertical object out of curiosity.


----------

