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[OM] Re: [photos] OT (taken with digital camera)

Subject: [OM] Re: [photos] OT (taken with digital camera)
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Sat, 16 Jul 2005 08:33:52 -0400
Earl Dunbar wrote:

> And depth of field is an issue for me as a result of the sensor size.  
> Now lenses need to be ~ a stop faster in some situations to get shallow DOF.
----------------------------------------------------

Yes, DOF can be something of a problem at times.  Actually, the problem 
is a bit worse than you state since maintaining the same DOF requires 
that the equivalent aperture be computed by dividing by the infamous 
"focal length multiplier".

For example, using a 35mm lens of 80mm focal length with aperture set at 
f/5.6 and focused at 5 feet gives DOF ranging from 4.8 to 5.2 feet (to 
the nearest 1/10th foot).

If we're using a C*n*n with its 1.6 multiplier the equivalent focal 
length is 50mm.  Dividing f/5.6 by 1.6 yields a stop of f/3.5 and this 
will deliver the same DOF range of 4.8 to 5.2 feet.

The Oly is a bit worse in this respect since its multiplier is 2 and 
requires shooting at 40mm opened up two stops faster to 2.8 instead of 3.5.

On the other hand, the DOF "problem" is sometimes your best friend. 
Case in point is the wonderful macro shots shown here yesterday taken 
with Richard's wife's D-560 which has a multiplier of about 4.4.

ps:  Note when computing DOF for these digital thingies that it's 
necessary to apply the multiplier against the focal length of the lens 
and also the resolution.  The standard resolution assumption for 35mm 
film is that we need to resolve 30 lines/mm to make a decent 8x10. 
Since making an 8x10 from one of these digital sensors requires greater 
enlargment of the image the required resolution number needs to go up as 
well.  For the 1.6 multiplier we need to use a resolution number of 48 
lines/mm and for the Oly a value of 60 lines/mm.  The little D-560 has 
to resolve an amazing 130 lines/mm.

Chuck Norcutt


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