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Author Topic: accuracy of "focus and recompose"?  (Read 9771 times)

Jim Pascoe

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accuracy of "focus and recompose"?
« Reply #20 on: May 21, 2009, 03:30:09 am »

Quote from: R_Medvid
Hi Jim,

Thanks for your posting. It's not often that you have a quick real-world example of the topic discussed here.

However, looking at the flower out of focus, I am thinking if it being out of the center of the frame (out of the sweet spot) and closer to the edge of the frame (where MTF characteristics worsen especially at the open aperture) could have contributed to the blurr-ness of the flowers.

Just a thought.

THNX

I agree that we are not looking at the best spot as far as the lens is concerned, particularly as it is at the maximum aperture of f2.  But the difference between the two pictures is the point.  The one where I focussed with the centre spot on the flower head, and then re-composed, is noticeably less sharp than the close up of the flower head where
I composed the picture, then used a focus point very near to the flower head.  Remember that the two flower heads are 100% crops from the main picture.  I did not bother to upload both of the full frame pictures because of course at that size the difference in focus would not be visible.

This test was done because the shooting range used is one I encounter frequently when photographing people.  Imagine that the flower bloom at the top is the eye, and the full frame picture is a half-length portrait.  In the past I would have used the centre spot to focus on the eye, and then re-composed.  Result, the eye would be soft.  At least, at wide aperture it would.  And that is how I usually shoot

I only used a flower and a tree because my wife did not want a 100% crop of her eye posted for the world to see!  
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pegelli

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accuracy of "focus and recompose"?
« Reply #21 on: May 25, 2009, 04:47:52 am »

Quote from: Wayne Fox
Of course, you are using another extreme ... a 1.4 lens at wide open so almost 0 depth of field,

Wayne, agree, something needs to be extreme for this effect to be noticable, either a large swing or a narrow dof (and practically both).

Just out of interest I did the math for the OP, with a 100 mm f 2.2 on a MFDB

For that lens at 5 meter distance and full open the dof is roughly from 4,80 to 5,22 meter

Using the same formula a "swing" of 1,50 meter would result in the focus point actually being right at the edge of the dof. So if you're shooting a standing person and focus on his face and then swing to his (her) feet will just render the face oof, indeed quite extreme.

As you say, something to be aware of, but not something you need to lose sleep over in practice when you use "smaller" swings and close your aperture one or two stops for better sharpness and dof.
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pieter, aka pegelli
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