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[OM] Shutter speed requirements [was: 3x21 tele converter]

Subject: [OM] Shutter speed requirements [was: 3x21 tele converter]
From: Dan Mitchell <danmitchell@xxxxxxxx>
Date: Tue, 27 Feb 2007 08:17:29 -0700
Moose wrote:
> [1/lens-length rule of thumb]
 >
> Third, the rule of thumb assumes an unmoving subject, which is not true 
> of most critters. And even if you shoot a wooden decoy at the top of a 
> tall tree, the tree is always moving, more or less, depending on weather 
> conditions.

  I'm not sure I follow this step. Sure, static subjects are easier to 
shoot then moving ones, but how does one factor lens length come into it?

  It seems like subject distance is as important as lens length in this 
case, and I could almost make the case that standard composition would 
imply that longer lenses are _better_. Say I'm taking a shot of a car, 
and I want the car to fill the frame -- if I'm using a 17mm lens at 
1/17th, then the car will have moved a lot during that time, but if I'm 
using a 500mm lens at 1/500th, there's that much less time for subject 
motion to happen.

  Now, if I'm using 1/500th for both shots, and say it's moved 1 inch 
during that time -- if the car is filling the same portion of the frame 
in both cases, then the amount of motion blur from 1" of motion will be 
the same in both cases, right?

  I suppose one thing is that the DOF at 17mm will be larger than the 
DOF at 500mm, so if the car's driving towards or away from me, then I've 
got a much better chance of keeping it in focus with a shorter lens, 
where with a longer lens I either need to be panning (I suppose with a 
fixed-focus lens, running backwards.. :) ), or it may just move out of 
the range of the world that's in focus.

  There's also different distortions (filling the frame with a car at 
17mm will look very different to filling it with the frame at 1/500th).

  -- dan

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