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PostPosted: Thu Feb 02, 2012 2:09 pm 
So as I've already posted I'm having a front focus problem with my 550D anf sigma 30mm f1.4 combination

Just been doing some tests and it seems that with live view it focuses acurately but using the centre focus point through the view finder is focusing short

The live view focus seems to take a lot longer, hunting more

I assume they use differnt methods, can anyone explain in simple terms what these are?

Any ideas on realistic ways to solve this problem?

Ian


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PostPosted: Thu Feb 02, 2012 2:24 pm 
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Location: Speyer (Germany)
I can't really explain the phase detection autofocus that's done in the "viewfinder mode" but in live view the camera is using the contrast based system. It's more accurate because it's using the sensor itself checking for the highest contrast (sharpness).
If you are using the viewfinder, the light is sent "somewhere else" so you can't be sure the distance is 100% the same as the sensor, that's why there can be some differences.

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PostPosted: Thu Feb 02, 2012 5:33 pm 
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Live view uses the main sensor image output. It looks at the contrast and tries to get it so whatever you're pointing at is sharp. With DSLRs though, this is a generally slow process as they were never originally designed to do this. Assuming you don't have a complex shape or pattern, then it will get it right more often than not, and there is not much room for error.

Phase AF uses a separate sensor located in the bottom of the mirror chamber. This gives the body more information about distance and amount of defocus to go on, but you're relying on a fair few assumptions here to give the same focus at that point as the main sensor.

On the front focus, from what I recall of previous threads the "error" wasn't that great anyway, with the actual focus lying on the edge of the depth of field of the desired focus. I don't know about other countries but Sigma UK will adjust a lens to a body if you supply them with both. The other option is to replace the body with one that supports micro-adjust, so you can set the offset yourself. Practically speaking that means either a 50D (not 60D) or 7D.

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