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[OM] Re: OM Teleconverter 1.4xA?

Subject: [OM] Re: OM Teleconverter 1.4xA?
From: "Darin" <d.rhein@xxxxxxxxxxx>
Date: Sat, 17 Feb 2007 08:32:58 -0800
Thank you for the explanation Chuck. I was surprised to see that big a 
difference between 500mm and 600mm, I was only  expecting a gain or 5 or 6 
meters.

Thanks again,
Darin

Chuck wrote:
> 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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