Moose, Chris, Wayne,
I tried the No.1 Pentax extension tube tonight, indoors. As I mentioned
earlier, the basic lens alone will focus from approximately 4.5 feet to
infinity.
With the No.1 tube added, using very crude measurements, the focusing range
is 3 feet to 9 feet. This is not bad for insect work, and doesn't change
the exposure much, so I may give it a try. The length of this tube is
9.6mm. By Chris' formula, this gives about 7% magnification. The main
improvement is it reduces the lens to subject distance by 33%.
Jim Nichols
Tullahoma, TN USA
----- Original Message -----
From: "Chris Trask" <christrask@xxxxxxxxxxxxx>
To: "Olympus Camera Discussion" <olympus@xxxxxxxxxxxxxxxxx>
Sent: Saturday, September 03, 2011 8:22 PM
Subject: Re: [OM] IMG: More Insects, New Approach
> >
>> I have a set of 3 Pentax M42 tubes, which match this lens. Might give it
> a
>> try with the shortest one.
>>
>
> You probably already know this, but in case you don't the magnification
> will be roughly:
>
> MAG = (FL + EXT)/FL
>
> where FL is the focal length of the lens and EXT is the length of the
> extension tube. The drawback to using extesion tubes is that the amount
> of
> light falling on the film/sensor is the square of the reciprocal of MAG.
> This becomes a bit of a problem in low-light situations such as a flower
> in
> dense shade, especially if you need the short DOF advantage of a large
> aperture to defocus background clutter.
>
> Chris
>
> --
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