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[OM] Light loss in teleconverter (was:A-200 teleconverter)

Subject: [OM] Light loss in teleconverter (was:A-200 teleconverter)
From: Frank van Lindert <lindertv@xxxxxxx>
Date: Wed, 20 Sep 2000 11:44:02 +0200
Cc: per.nordenberg@xxxxxxxxxx
The maximum aperture of a lens or matched lens combination is
determined by the quotient of focal length and diameter of the front
element. This is only a rough rule of thumb: there may be other
limitations inside the lens!
 
The reason why well designed front-mounted teleconverters do not
significantly change the lens aperture is, that the diameter of the
front element of the converter can be made much larger than that of
the lens it is attached to - proportionally to the change in focal
length.
To keep the amount of light reaching the film equal, a 1.7x converter
will need a frontal element with the diameter being 1.7x as large as
of the lens it is mounted on. The cone shaped converters for the
Olympus IS-series do match this rule.

For rear mounted TC's the same rule applies: if the focal length is
doubled, the aperture is halved - since the diameter of the frontal
lens is not changed.
If a front mounted converter would be a cilinder (with equal diameters
of the front and rear glass) rather than a cone the result would be a
light loss equal to that in the rear mounted converter.

Frank van Lindert
Utrecht NL.


On Wed, 20 Sep 2000 00:00:27 +0200, "Per Nordenberg"
<per.nordenberg@xxxxxxxxxx> wrote:

>> I think it has to do with whether your teleconverter goes in front of the
>> lens or behind it.  ;-)
>>
>> Giles
>
>Correct Giles. This is the info I have:
>
>A conventional behind-the-lens converter altering the focal length of the
>prime lens affects the light *after* the limitation of the lens aperture.
>With front-mounted converters, the focal length of the prime lens is also
>altered by the afocal supplementary, but this is all done *before* the lens
>aperture, so does not affect the amount of light reaching the film. The same
>should be true of focal auxiliaries such as close-up lenses (but what about
>the close focus being affected then?)
>
>A front mounted afocal telephoto can also be referred to as a Galilean
>telescope (positive lens near the subject, negative near the lens); the wide
>attachment is a reverse Galilean telescope (negative front, positive rear).
>
>Dearly hoping this was of some help   ;-)
>
>Per Nordenberg


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