Sigma 35mm f/1.4 user feedback needed

Started Jun 10, 2013 | Discussions thread
Mais78 Contributing Member • Posts: 870
Focus and recompose vs using outer points

Focus and recompose is quicker and generally works very well with lenses at f2.8 or slower.

With a f1.4 lens things change. A 35mm lens has a c.54 degrees angle of view, assuming you focus on the subject and then recompose to have it on the rule of thirds you are rotating by about 9 degrees (54/3/2). The shift of the focus plane is the difference between the cosin and the distance from the subject given a certain degree. Assuming the subject is 1m away the shift of the focus plane for a 9 degree movement is 1.25cm (assuming you can rotate perfectly without moving the camera forward or backward). With a DOF calculator, if you assume circle of confusion at 0.006mm (c. pixel size of a 5DMkIII, to define DOF as sharpness at pixel level, 100% view), your DOF at 1m is 1.32cm at f1.4, so no margin for error. I would say at f1.4 to be safe you should not recompose if your subject is closer than 2-2.5m.

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