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Camera or lens calibration?

 
 
Bhogi
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      05-22-2007
My new Sigma 50-150 f2.8 has about 4cm of front focus. I purchased it
nevertheless because the lens can be calibrated.
What seems strange to me is when I focus it manualy using central
focus point on my Canon 20d, the focus is also confirmed about 4cm in
front. It's fair to say I don't know what I'm talking about, but I'd
realy expect when focusing manualy, there should be no lens focusing
logic involved for focus confirmation, only camera sensors.

In the viewfinder the confirmed point of focus is visualy less sharp
than 4cm in front at the distance of around 1m.

My only other fast lens is 50 1.8 which focuses sometimes in front
sometimes behind, which is to be expected from a cheap lens, but I
never noticed any focus problems at all.
I also tested the camera with my 500mm f5.6 mirror lens, which has
zero DOF at short distances, thinking if I could focus it acurately
using the viefinder, there's nothing wrong with my camera. The focus
was dead on.

Does this sound like the camera also needs calibration, or the lens
only? Sigmas are known for their front focus problem.
Otherwise the lens is great when focused in the distance but I'll use
it primarily for portraits and shallow DOF, for that it's useless the
way it is now.

Thanks in advance.

 
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Joseph Meehan
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      05-24-2007
Bhogi wrote:
> My new Sigma 50-150 f2.8 has about 4cm of front focus. I purchased it
> nevertheless because the lens can be calibrated.
> What seems strange to me is when I focus it manualy using central
> focus point on my Canon 20d, the focus is also confirmed about 4cm in
> front. It's fair to say I don't know what I'm talking about, but I'd
> realy expect when focusing manualy, there should be no lens focusing
> logic involved for focus confirmation, only camera sensors.
>
> In the viewfinder the confirmed point of focus is visualy less sharp
> than 4cm in front at the distance of around 1m.
>
> My only other fast lens is 50 1.8 which focuses sometimes in front
> sometimes behind, which is to be expected from a cheap lens, but I
> never noticed any focus problems at all.
> I also tested the camera with my 500mm f5.6 mirror lens, which has
> zero DOF at short distances, thinking if I could focus it acurately
> using the viefinder, there's nothing wrong with my camera. The focus
> was dead on.
>
> Does this sound like the camera also needs calibration, or the lens
> only? Sigmas are known for their front focus problem.
> Otherwise the lens is great when focused in the distance but I'll use
> it primarily for portraits and shallow DOF, for that it's useless the
> way it is now.
>
> Thanks in advance.


Well I guess I am not sure what exactly you are talking about either.


I will suggest one thing however. Don't worry about specifications,
rather worry about results.

Take a series of photos at different apertures and distances using both
manual and auto focus with any lens in question. Try to eliminate variables
like vibration (use a solid tripod) and then view the resulting photos.

Don't stop at one photo with auto focus or manual focus, take one shoot,
then refocus and take another. Make sure you true some different subjects
and lighting conditions as well.

Carefully view all the results and then if needed adjust to your
equipment or in some cases adjust the equipment. Don't worry about meeting
any standards other than the results you personally get.

Good Luck

--
Joseph Meehan

Dia 's Muire duit



 
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Bhogi
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      05-25-2007
Joseph Meehan wrote:
> Bhogi wrote:
> Well I guess I am not sure what exactly you are talking about either.
>


Hehe, I was just testing all the "big glass" lenses I have to figure
out if the camera also needs to be calibrated or not. I tested 4
different sigmas in the store and all had front focus. Too much of it
to be my error in testing.


> I will suggest one thing however. Don't worry about specifications,
> rather worry about results.


The results are very soft. Thanks to HSM I can correct the focus
manualy, then the results are good, but that's not a long term
solution.


> Take a series of photos at different apertures and distances using both
> manual and auto focus with any lens in question. Try to eliminate variables
> like vibration (use a solid tripod) and then view the resulting photos.
>
> Don't stop at one photo with auto focus or manual focus, take one shoot,
> then refocus and take another. Make sure you true some different subjects
> and lighting conditions as well.
>
> Carefully view all the results and then if needed adjust to your
> equipment or in some cases adjust the equipment. Don't worry about meeting
> any standards other than the results you personally get.
>
> Good Luck


Thanks!

 
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