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Recomposing?
In all manuals and books there is a technique for setting the sharpness using central AF point when the object is not in the center of planned photo. It is suggested to set the focus on the object using one of the AF points and then recompose the image using focus hold (half pressed shutter button, sometimes other buttons as well). This brought me to a question - if the focus is set on 2 meters, does it mean that all objects that are 2 meters from me will be sharp?I red somewhere on the internet that it is not true. If the focus distance is set to 2 meters it mean that all objects that are crossing the plane perpendicular to lens optical axis and distand 2 meters from the sensor/film (measured along the optical axis) will be sharp. This is quite obvious. If we consider shooting a wall that is 1 meter from us (therefore we set focus to 1 meter) and the above statement is untrue only the center part of the photo would be sharp. I didn't notice that.
The test
I must confess - I was trying really hard to show "refocus blur" and I do not believe that it is a problem in real life photography. I took two shots, one using recomposin (I used central AF point to set the focus, recompose the image and took the shot), and second one using a non-central AF point setting AF on exactly the same area. The image was not recomposed between those two shots, camera was fixed on a tripod. Image on the left shows one of the shots, and area where the focus was set (the crop shows this area in 100% zoom).
It turned out that my 24mm Sigma gives the same sharpness on both shots, the same with 50mm Minolta (at f/1.8 and f/1.4 respectively), the effect was visible only on my 105mm Sigma (a macro lens) set on f/2.8 at (nearly) minimal focusing distance. There was virtually no depth of field, therefore the change in sharpness between the shots is clear, still this effect is hardly visible in other than macro types of photography.
The image below is 100% crop from the shot take using recompose technique (notice the colorfull dots on the crop are drops of ink, not visible by naked eye). Hover the mouse over it to see the same crop from a shot when focus was set using off-center AF point.
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