Is this backfocussing?


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wong1979

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Aug 16, 2005
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#1
Lately, after changing the whole IS and optical system of my EF70-300 IS, i felt my shots look oof even though i'm quite confident that i hv it sharp in the viewfinder. So just now, i did a test.

Slap on the 500d CU filter,undo IS, mount on tripod with cable release, set to manual focussing and adjust to the minimum focussing distance to get the highest magnification. This is what I get at different F-stops, very strange...











No.1 to no.5 is F 5.6, F 8, F 11 F 16, F 22

And from shot 1 thru to shot 5, I've ensured thru the viewfinder that the original focussed line stayed in focus.

It seems that as my F-stop gets higher, the back gets sharper than the original focussed point. I'll be glad if u can tell me it's something wrong.

Thanks!
 

wong1979

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dude... you are not testing back/front focusing.. you are testing DOF and sharpness @ different aperture...
Let me put it this way, if i were to shoot at F5.6, the object to be in focus stays in focus.

But if i were to shoot the same object at F11, the back gets more focussed than the object.

If u r to examine my series carefully, don't u feel that the focus have shifted backwards as the F stops gets higher, especially from F5.6 and F11?
 

raptor84

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Dec 6, 2005
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#4
I dont think it is since the main target is still in focus. The DOF seems to be moving back cause you alredy mentioned you are at the MINIMUM focuing disance so anything nearere will confir be OOF right? Try testing again slightly farther away?

Perhaps when you did your shots you recomposed so they became OOF?
 

Paladin

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#5
Do a search on the internet...there is a method do check for back focusing...it was couple yrs back when I did it...cant remember the site now...
 

wong1979

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#6
I hv tried shooting on a planar text at around 45 degrees at F/11. Most of the time, it appears that the top line is sharper. Maybe the above test is flawed somewhere, but I was pretty sure that the selected text was in focus all the way. I'll try again.
 

lsisaxon

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Nov 29, 2004
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I hv tried shooting on a planar text at around 45 degrees at F/11. Most of the time, it appears that the top line is sharper. Maybe the above test is flawed somewhere, but I was pretty sure that the selected text was in focus all the way. I'll try again.
Do one more test. First perform an AF and once the focus lock, switch to MF. without affecting the camera position, perform the tests again at different aperture. This way you are certain the focus didn't change at all. AF has a certain tolerance which you cannot see from the viewfinder.
 

scenar

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Aug 23, 2005
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#8
if im not wrong, DOF extends more to the front of the focusing point than the back, so really, your results look perfectly fine to me.
 

lsisaxon

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Nov 29, 2004
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if im not wrong, DOF extends more to the front of the focusing point than the back, so really, your results look perfectly fine to me.
I think you meant extends more to the rear than to the front. ;) Still, for such a close distance, the difference should be quite small. In wong1979's case, it does seem that at some aperture the line further away was in focus instead.
 

wong1979

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#11
Maybe I'm just being paranoic just now, even if i was wrong, i think i owe everyone a retest, so here it is. AF locked and switched to manual.









 

Stoned

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May 7, 2004
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#13
You really don't need to stop down your lens to test for backfocusing. In fact, you should always keep it wide open to minimise DOF.

Conclusion, if your focus point was "in store", your lens is not backfocusing. If it was "sized", your lens is backfocusing.
 

wong1979

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Aug 16, 2005
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#14
You really don't need to stop down your lens to test for backfocusing. In fact, you should always keep it wide open to minimise DOF.

Conclusion, if your focus point was "in store", your lens is not backfocusing. If it was "sized", your lens is backfocusing.
Yup, i know. But the point I'm trying to illustrate here is it seems that my lens will backfocus at a higher F-stop, which is unheard of. Best to compare my shots at F5.6 and F16, don't look at others. See the lower half of the focussed line seems to fade away? Or maybe it's just my imagination :dunno:
 

creampuff

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Jul 11, 2006
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#15
Bro, please go and read up on what depth of field is all about because the examples you have posted merely illustrate the effect of stopping down on depth of field, it does not demonstrate back focusing at all. Back focusing relates to a problem whereby the camera's AF tends to focus just behind the intended subject. So in your case, how can there be back focusing if you had focused manually on the line "...be in store..."?

An actual case of back focusing would be if you intended to focus on the line "...be in store..." using the camera's AF to focus but the focus point was behind the intended target.
 

wong1979

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Aug 16, 2005
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#16
Bro, please go and read up on what depth of field is all about because the examples you have posted merely illustrate the effect of stopping down on depth of field, it does not demonstrate back focusing at all. Back focusing relates to a problem whereby the camera's AF tends to focus just behind the intended subject. So in your case, how can there be back focusing if you had focused manually on the line "...be in store..."?

An actual case of back focusing would be if you intended to focus on the line "...be in store..." using the camera's AF to focus but the focus point was behind the intended target.
Hi, of coz i know what dof is, but will u pls analyse carefully. Take for example, the set with the word "in store". No doubt I want to focus the word "in store" and it's already focussed correctly as depicted in Pic no.1.

As u can see, at F5.6, the upper and lower portion of the word "store" is a bit OOF. ie I've locked focus in the centre part of the word. And look carefully towards the end. If I hv increased DOF, shouldn't the word become clearer and clearer? Instead, it was biased towards the back of the word. Particularly at pic no.3 which is at F/11, the lower portion of the "store" becomes blurer than no.2

But you may not notice it at F22, as DOF is sufficiently large enuff to cover it assuming that my assumpition "on backfocussing on higher F-stop" is true.

I hope someone understands what I'm trying to put across.
 

creampuff

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Jul 11, 2006
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#18
To me you don't have a problem with focus. The slight variations you mention I would attribute to slight variances in the lens's optical sharpness at varying apertures.
 

V

vince123123

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#19
I was thinking the same thing - DOF extends further behind faster than in front, so it looks like your focus point shifted back (cos you were probably thinking it should be in the middle).

doesnt look like its back focussing
front 1/3 and back 2/3 is sharp
looks alright to me
 

Canew

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Jul 26, 2005
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#20
Most importantly, the word 'in store' is in focus throughout the f-stops. The DOF for zoom lens tend to shift more to the rear as the aperture gets smaller. All you need to take note is the the point of focus is sharp throughout the f-stops.
 

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