>Vicente wrote:
>>
>> With a 20mm lens at 5m you get a magnification of M=0.0040, at f/8 you get
>> DOF from 1.4m to infinity.
>> With a 200mm lens at 50m you get at magnification of M=0.0040 too, af f/8
>> you get DOF from 40m to 66m (you don't reach infinity).
>Yes, but a 20mm lens at 1.4m has the same subject magnification as a
>200mm lens does at 40m. Check it out!
I checked it and it's not true.
How do you know that?
If you read my post:
>>With a 20mm lens at 5m you get a magnification of M=0.0040, at f/8
>>you get DOF from 1.4m to infinity.
>>With a 200mm lens at 50m you get at magnification of M=0.0040 too,
>>af f/8 you get DOF from 40m to 66m (you don't reach infinity).
>>In order to make a fair comparison, as the focusing distance is different,
>>just compare the range of magnification which will get DOF, using the 20mm
>>lens you get DOF from M=0.014 to M=0, using the 200mm lens you get DOF
>>from M=0.005 to M=0.003. Remember, both are focused at M=0.0040.
The magnification of a 20mm lens at 1.4m is 0.014, and the magnification of
a 200mm lens at 40m is 0.005. Quite different!
The crucial point is:
The magnification depends on focal length and subject distance. You can get
the same magnification with two different focal lengths. For instance, with
a 20mm lens at 5m and with a 200mm lens at 50mm you get in both cases a
magnification of 0.004. But the field of view of both lenses are quite
different. You will see the same in bot cases in the plane of focus, but
out of this plane, the image is different. Just test it.
If the field of view of the not focused regions is different, why has the
DOF to be the same?
Moreover, the physical aperture of a 20mm lens at f/8 is 2.5mm, the
physical aperture of a 200mm lens at f/8 is 25mm. Remember that f/8 means
equal exposure, not equal physical aperture. As DOF depends on the physical
aperture, why has the DOF to be the same?
Only in macro regions the differences compensate and DOF depends only on
magnification and f/ number.
>Mr. Bill
I think it's becoming quite off topic.
Best regards
Vicente
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