Actually, 600mm works out better since a 7mm tube is a smaller
percentage of its focal length. To compute this stuff you need the
formula for *object distance* given *image distance*. Object distance
is the distance to the thing you're shooting as measured from the
optical center of the lens. Image distance is the distance from the
film plane to the optical center of the lens. The image distance for
furthest possible focus would have the lens focused as for infinity and
would normally be 600mm (the lens focal length). However, we've got to
add the 7mm from the extension tube for a total of 607mm.
The following page <http://scubageek.com/articles/field_of_focus.pdf>
discusses the calculation of depth of field (here called field of focus)
and in the process it gives us what's called the "thin lens formula" or
the "lens makers formula" which is 1/O + 1/I = 1/F (where O = object
distance, I = image distance and F = focal length). Rearranging terms
for the object distance formula we get I = OF/I-F. Solving for our
particular problem we get (607x600)/(607-600) or 364200/7 or some
humongous number in millimeters which, when converted to meters, gives
us the object distance of about 52 meters. A more precise answer for
the 500mm lens would have been 36.2 meters rather than 35 meters... but
we have to remember that these numbers are to the optical center of the
lens which, for these long teles is 0.5 to 0.6 meters in front of the
focal plane of the camera. But precision here is not called for unless
we're shooting macro where the distinctions about distance measured from
what really mean something.
Chuck Norcutt
Darin wrote:
> Not that it makes much difference, but I'm working with a 600mm lens rather
> than 500mm. Actually, 35 meters would probably work for me at least 80% of
> the time. I'm not really looking for a longer reach, just tighter framing
> for small wildlife and bird photography. If that makes any sense? BTW,
> what's the formula used to figure this kind of thing out? I'm sure I could
> find it in one of my photography books, but it's faster just to ask, if it
> isn't to difficult to explain.
>
> Darin
>
> Chuck Norcutt wrote:
>> Forgetting about the TC for a moment, a 500mm lens set at infinity but
>> on a 7mm extension tube should be focused at about 35 meters. The depth
>> of field at f/8 and 35 meters is less than +/- 1.5 meters on either
>> side. The TC starts its magnification task from this point. Sorry,
>> guys... time for plan B.
>>
>> Chuck Norcutt
>>
>> Darin wrote:
>>> Actually, I was wondering the very same thing.
>>>
>>> Darin
>>>
>>> ----- Original Message -----
>>> From: <usher99@xxxxxxx>
>>> To: <olympus@xxxxxxxxxx>
>>> Sent: Thursday, February 15, 2007 2:30 PM
>>> Subject: [OM] Re: OM Teleconverter 1.4xA?
>>>
>>>
>>>> Wonder how you would do with a 7mm ext tube between the lens and the
>>>> very
>>>> nice Oly 1.4 TC? Couldn't be too far from infin focus. I bet someone
>>>> on
>>>> the list could determine the dof at infinity .
>>>> Mike
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