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 Post subject: DOF Question
PostPosted: Tue Oct 06, 2009 1:51 am 
OK, need some help. Might be getting the 50mm f1.4 in 2 weeks and there is something I'm not sure about with regards to DOF (read a lot about it, but it doesn't seem to clearly answer my question). And after reading my question, you might not be so sure yourself!

If I'm in a large room or hall, with a group of persons on a stage (lined up roughly in a straight line), that is about 10-15m from where I'm shooting from (at f1.4), will I get significant blurring horizontally across the frame on APS-C or will people be reasonable sharp, i.e. does DOF increase relative to distance?

If my DOF is 1ft @ f1.4 from about 5ft distance from my subject, will it increase to say, 10ft @ f1.4 if I'm 50ft from my subject (using the same focal length, with my subject occupying proportionally the same amount of the frame)?

I ask because, I know DOF is the amount of the frame in sharp focus and that areas before the point of focus and after are essentially "outside" the DOF, but if I'm shooting a large enough scene that occupies the same relative area in the frame, could I assume that the DOF will remain the same?

Confusing, but I don't know how else to ask this question.


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 Post subject:
PostPosted: Tue Oct 06, 2009 2:04 am 
The biggest issue you'll face with distant subjects at wide open f/1.4 will be border softness. f/1.4 should provide relatively good DOF that far away, assuming the angle of the sensor is pretty perpendicular. Still, I'd say f/2 would be a good safe bet, still damned fast with good border performance.


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 Post subject:
PostPosted: Tue Oct 06, 2009 6:30 am 
DOF increases with your perpendicular distance from the subject, not horizontal distance across the frame. If you get a DOF of 1m in the centre of the frame, you should get roughly 1m at the edges too but you will need to watch out for field curvature and edge softness.


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 Post subject:
PostPosted: Tue Oct 06, 2009 6:52 am 
Use http://www.dofmaster.com/dofjs.html for all your DoF calculations.


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 Post subject:
PostPosted: Tue Oct 06, 2009 8:53 am 
WOW thanks pgtips for the DOF calculator,


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 Post subject:
PostPosted: Tue Oct 06, 2009 7:56 pm 
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1. grahamnp is right: around the plane of sharpest focus you have equal dof at any point. That also means: point your camera as perpendicular to the line of persons on the stage as possible!
2. dof increases by the square of the increase in distance. So at 10m dof is 100 times the dof at 1m!

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 Post subject:
PostPosted: Wed Oct 07, 2009 12:09 am 
Wow, thanks for the great info guys!

I'm still a little foggy on it, mainly because from the lens I have (an f4 minolta), I do notice some blurring horizontally, but as grahamp said, its probably some really bad field curvature.

I guess my only "practical" option is to go out and try it! I won't have the lens until about 2 weeks, but as soon as I get it (and hopefully my spanking new A850!) I'll be out taking pictures to try and nail down DOF.

Not having any large aperture lenses (f4 is my largest), I think this is probably a gap in my photographic understanding, even if I understand it in "theory", its just not the same compared to getting it in practice.

Thanks again, you guys really do pull through for me time and again!


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