teleconvertor with fisheye?

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default - 22 Jul 2006 09:10 GMT

Just an observation I decided to share.

I just tried putting my Peleng 8mm f/3.5 180 degree circular fisheye onto a
2x teleconvertor.

Wow, suddenly it is a 16mm f/7 full-frame fisheye. The diagonal is was
close to 180 degrees FOV. Seems obvious, except that the Peleng only has a
25mm image circle yet the image filled the full 36x24mm frame when mounted
on the teleconvertor.

Apparently the teleconvertor works by doubling the projected image diameter
to give the appearance of doubling the focal length and explains the two
stop light loss. Is this correct?

I had often read not to use a teleconvertor with wide-angles, and it is
pointless of course since you are better off just using a longer lens, but I
haven't seen problems mounting wide lenses on the teleconverter other than
the general stupidity of it.

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Prometheus - 22 Jul 2006 09:28 GMT

>Just an observation I decided to share.
>
>I just tried putting my Peleng 8mm f/3.5 180 degree circular fisheye onto a
>2x teleconvertor.

Experimentation is good.

>Wow, suddenly it is a 16mm f/7 full-frame fisheye. The diagonal is was
>close to 180 degrees FOV. Seems obvious, except that the Peleng only has a
>25mm image circle yet the image filled the full 36x24mm frame when mounted
>on the teleconvertor.

The 2x TC doubles the exit angle.

>Apparently the teleconvertor works by doubling the projected image diameter
>to give the appearance of doubling the focal length and explains the two
>stop light loss. Is this correct?

How else would a 2x TC double the effective length of a lens than by
doubling the exit angle, of course it might cut off too much if you try
to use a 35mm TC on MF.

>I had often read not to use a teleconvertor with wide-angles, and it is
>pointless of course since you are better off just using a longer lens, but I
>haven't seen problems mounting wide lenses on the teleconverter other than
>the general stupidity of it.

You probably loss some light and introduce distortion by doing it.

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Ian G8ILZ

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Toby - 04 Aug 2006 18:09 GMT

>>Just an observation I decided to share.
>>
[quoted text clipped - 28 lines]
>
> You probably loss some light and introduce distortion by doing it.

With any lens you basically enlarge all the problems (lack of resolution,
aberrations) 2x with a 2x teleconverter, not to mention those added by the
teleconverter itself and the loss of two stops. Take an expensive 14mm f2.8
lens, add a converter and you end up with a very expensive, very poor 28mm
f5.6.

Toby

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default - 05 Aug 2006 03:03 GMT

> With any lens you basically enlarge all the problems (lack of resolution,
> aberrations) 2x with a 2x teleconverter, not to mention those added by the
> teleconverter itself and the loss of two stops. Take an expensive 14mm
> f2.8 lens, add a converter and you end up with a very expensive, very poor
> 28mm f5.6.

Yup, it certainly has an image quality cost. The only practical use of a TC
where it creates something you don't already have and the results turn out
better than simply cropping half the picture out and resizing. This
generally has to be determined by experiment. Certainly you never get
results as good as simply using a lens of double the focal length.

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