Camera Labs

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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Canon EOS 550D (with Magic Lantern)
18mm - 135mm Kit Lens, 50mm f1.8, Sigma 30mm f/1.4 EX DC HSM
430EX II Flash with Phottix Strato Wireless Tigger Set
Manfrotto 190CXPRO3 with 496 Ball Head. Gorilla Pod SLR
Canon Powershot A720IS with CHDK and intervalometer script. Pentax Optio W90

My Blog - http://www.ianganderton.wordpress.com

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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Canon EOS 500D + Canon EOS 5D Mark III
Canon EF 24-105mm 4L IS USM + EF 100-400mm 4.5-5.6L IS USM + EF 100mm 2.8L Macro IS USM + Sigma 12-24mm 4.5-5.6 EX DG HSM + Canon Speedlite 580 EX II + Nissin Speedlite Di 466

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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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.