At 18:35 08.11.01, C.H.Ling wrote:
Check the following DOF table you will know it is true. Try to input some
figures e.g a 85mm lens for 35mm system and 150mm for 6x6 system. They
both have more or less has the same angle of coverage, so that at the same
magnification the object distance is the same. You can see the DOF in 6x6
system is smaller and most important the 150mm lens's out-focus effect has
much sharper change, the background will be very soft even they are not far
away from the object (focus point).
You should be warned that this DOF calculator uses inaccurate values for
the circle of confusion, so I'll try to show what's going on:
1) Using the same DOF calculator,
http://members.home.net/gillettm/DOF.html, enter realistic values for a
normal lens on 4x5 film: 150mm, f/6, and a distance of 2 meters/feet. Note
the DOF.
2) A normal lens for 35mm is 3 times shorter (50mm), so the circle of
confusion should also be 3 times smaller if the final print on the wall is
to be the same size (less magnification required). The numbers in this
program are a little off, so you'll have to enter 645 as the film format to
have the correct relative value for COC. Use 645 film, 50mm, and f/2 to
keep the absolute lens opening constant (150/6=50/2, very important!), and
finally keep distance constant at 2 meters/feet. Note DOF, it's practically
unchanged.
3) Now imagine the same situation for a digital camera with a normal lens
of 10mm (to keep the numbers easy). Everything would be easy to scale,
except the lens opening. It would require an f/0.4 to keep the lens opening
constant!
Obviously, untill somebody improves on the laws of physics this is not
going to happen. You're stuck with a 10mm f/2 normal lens (a 5mm opening),
roughly equivalent to a 50mm f/11 on 35mm with regards to DOF.
4) Note that this is an approximation only (but a good one for focus
distances longer than the focal lengths). The situation gets a little worse
for the digital camera as you focus closer, and that is of course a really
nice feature when you're really close, as in macrophotography. But it
really s*cks for portraits.
Regards,
Thomas Bryhn
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