Is the 50 1.4 supposed to be this bad?

Started Mar 11, 2013 | Discussions thread
tim73
Regular MemberPosts: 428
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Beware DoF expectations
In reply to The A-Team, Mar 11, 2013

Beware that the acceptable depth-of-focus depends very much on how you are viewing your images.

Most DoF calculators assume an image size of 10x8 inches and a viewing distance of 2ft.  And if I view your "bad" images at this size and distance they all look OK.  Put another way, the DoF calculators usually allow a blur diameter of 0.03mm at the sensor.  On your 6D this corresponds to roughly 5 pixels, which looks close to what you have even in the worst case.

At 100% magnification on a typical 92dpi screen your pictures are 5ft across -- or would be if they could fit on the screen at such a magnification.  The "acceptable" DoF at this print size is just 2 inches!  To focus that accurately you'll need to be extremely careful.  A tripod will be essential, and the focus-and-recompose technique will be noticeably inaccurate.

Finally, a word about why focus-and-recompose can be inaccurate, even on a tripod:

Most lenses are designed to have a flat plane of focus.  This means that if you photograph a flat subject (such as a brick-wall) face-on, it will all be in-focus even though the edges are further from the lens than the centre.  When using focus-and-recompose you would normally put the subject in the centre of the frame, lock focus, and then adjust the camera angle: the subject is now off-centre, but still at the same distance from the lens.  But we know from the brick-wall example that the lens focuses further away on off-centre subjects -- and the subject is now too close to the camera to be in perfect focus.

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