Hi all. I'm practising with my 50mm 1.8 and trying to nail focus consistently, when using f1.8. I know the lens may be sharper if I stop down a bit but i'm simply trying to improve my technique. I think the camera (Sony a580) is not back or front focussing, but with the small dof, I am finding it hard. Some shots are pin sharp, others focus just in front or just behind. Am trying not to move when taking shot. Any tips
F1.8 - hard to focus
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Nawty
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Going to AF-C rather than AF-S may make a difference - the camera might be able to predict the slight movements you can't help making and compensate for them. Other than that, it's either stop down for an inch or 2 of DoF rather than the few mm you get wide open at f/1.8 (F/1.4 has even less wide open). Or practice, practice, practice until your technique is better.
markmullen
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Also select a focus point manually, focus and recompose will give you hit and miss results.
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TBH, wide open portraits aren't that flattering anyway IMO - that shallow DoF throws too much out of focus for my tastes. If the eyes are spot on (as they usually should be) even the tips of the lashes are out of focus, and as for the nose and ears... IMO, f/5.6 gives far better results -at least most of the features are within the DoF zone. Most lenses are getting close to their sweet spot by then too - few are stellar wide open.
Goody13
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I'm curious as to why focus and recompose is "hit and miss". I always use back button focussing using the centre focus point. I was under the impression that would stop other parts of the shot being picked up by other focus points. 
I'm using centre point focus. When recomposing, the danger is that you move forward or back slightly. When dof is so shallow, that can throw out the focus. Agree that 1.8 is sometimes too wide. This all started with me trying to decide if the camera was back focussing. I decided it isn't, but my technique is the issue.