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09-10-2010, 09:42 AM   #16
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QuoteOriginally posted by Marc Sabatella Quote
Reducing aperture only increases DOF if everything else stays the same. Similarly, increasing focal length decreases DOF, if everything else stays the same. If you both reduce aperture *and* increase focal length, then these factors may cancel each other out.
Thanks Marc!

I think it is a bit more clear now.
Since both parameters are changing, and both pull DOF in opposite directions, the effect is not as dramatic as changing just 1 parameter by itself.

I see it like this:


Correct?

09-11-2010, 07:34 AM   #17
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QuoteOriginally posted by ismaelg Quote
Correct?
Excellent!

Except... DECREASE APERTURE is ambiguous. Because aperture numbering is inverted, DECREASE is unclear: a smaller fraction, or a smaller numerator? That's why we usually say STOP DOWN or OPEN UP when we talk about aperture adjustments. And the counterbalancing forces work like this: Increasing the FL or widening the aperture will thin-out the DOF; decreasing the FL or narrowing the aperture will thicken the DOF.

INCREASE FOCAL LENGTH <==< DOF >==> NARROW APERTURE
and
DECREASE FOCAL LENGTH >==> DOF <==< WIDEN APERTURE

It's easier to see than to explain. Ay yi yi.
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