Keplerian viewfinder

[john_bear]

<p>Can anyone explain what a Keplerian viewfinder is?<br>

I was looking at a Kowa SW on camera-wiki, and it's got this strange little viewfinder that is described as a Keplerian.<br>

<a href="http://camera-wiki.org/wiki/Kowa_SW">http://camera-wiki.org/wiki/Kowa_SW/</a></p>

[SCL]

<p>Google is your friend. Kepler was a scientist in the 17th century...his optics works included refracting telescopic designs - in this case I believe the reference is to clarify that the viewfinder is not of the single reflex design...probably a miniature reverse telescope as the lens is a wide angle lens.</p>

[john_bear]

<p>Ah ... if Google was my friend then this forum would not exist, because I (and everybody else) would be able to find the answer to every question. I've found plenty of references to Kepler, and Keplerian but I don't understand their wisdom in the context of looking through a little window on a camera. My brain can reason that it's a little reverse telescope without any help, but I want to understand it a bit more than that.</p>

[SCL]

<p>My physics is a little rusty after 50 years, but my recollection is that a Keplerian telescope design uses a small double convex lens ocular (eyepiece) and a larger double convex objective. When you reverse it and look thru the larger objective lens, you get a wide angle view, but because the smaller ocular lens is now the lens gathering light, the apparent brightness of objects in the field is reduced because less light from each specific object now reaches your optic nerve. This is often mitigated by the field being much larger, so our eyes are often fooled by the wider pattern (if one has a reasonably bright field) into perceiving overall increase in brightness. If you have a pair pf binoculars, you can simulate the effect (even though the design may not be Keplerian) by looking thru them backwards. One of the other drawbacks of the Keplerian design is that there is relatively little correction of chromatic distortion. This design, whether used for normal, telescopic, or wide angle (reversed viewing) is inexpensive to produce. Modern wide angle viewfinders have a larger objective lens, smaller ocular (like a typical telescope), and can be quite bright, depending on the design used, the relative sizes of the lenses and the corrections. A good example is the current Voigtlander or Leica wide angle viewfinders for rangefinder bodies....big, clear, bright images. I hope this helps.</p>

[donald_miller5]

Careful, the last time someone said that Google is your friend there was a big bru ha ha and it was the last straw for JDM.

[cees_jan_de_hoog]

Another camera with similar Keplerian viewfinder was the

Canon Demi. It used prisms to get a normal image, as a

Kepler telescope inverts the image. The benefit of this

setup was that the viewfinder could be made very small (as

on the Demi and Kowa) yet the image would be bright.

[Robin Smith]

<p>Yes, following on from Stephen, the other salient point is that a Keplerian viewfinder does not produce an upside-down image (unlike a Galilean telescope).</p>

<p>There was, indeed, a mighty hissy fit last time the friendliness of Google was asserted.</p>

[john_bear]

<p>Thanks for the explanations.</p>

<p>I've Googled other images of this camera, and having now seen its back-side, it's clear that the viewfinder eyepiece is offset ... not in line with... the viewfinder objective lens, indicating that mirrors or prisms are changing the path of the light.</p>

<p>I don't really care what a Keplerian finder is, who invented it, and what century they did it in: I simply wanted to know why it was different from any other viewfinder, hoped someone would know, and be willing and able to explain it.</p>

<p>JDM has my deepest sympathy.</p>

[James G. Dainis]

I believe it was JDM who instigated the "Google is your friend" brouhaha although he was trying to help.

 

http://www.photo.net/black-and-white-photo-printing-finishing-forum/00dGGx?start=0

[winfried_buechsenschuetz1]

<p>"Yes, following on from Stephen, the other salient point is that a Keplerian viewfinder does not produce an upside-down image (unlike a Galilean telescope)."<br>

It's just the other way round. Galilean telescopes produce an upright image, Keplerian telescopes will produce an upside-down image (which does not matter when these telescopes are used in astronomy). A Keplerian telescope always needs a device for reversing the image. This can be done by a set of reflective prisms (as in binoculars), or by using additional lens elements (which will increase the length of the telescope). <br>

The advantage of a Keplerian viewfinder is that you can easily add a frame limiting the field of view and the edges of this frame will appear as sharp as the image itself. <br>

More details are discussed here:<br>

http://www.openphotographyforums.com/forums/showthread.php?t=17828</p>

[chuck_foreman1]

While on the subject; a good source for describing viewfinders technology is Rick Oleson's great essay

 

http://rick_oleson.tripod.com/looking_forward.htm

[winfried_buechsenschuetz1]

<p>I think we did not answer the question yet WHY some camera manufacturers use Keplerian viewfinders. One point is that the length of a Keplerian telescope/viewfinder can be extended using relay lens elements (the "image erecting" lens system mentioned by me earlier doubles as such a device). So the path of light of the viewfinder can be bent by mirrors and the entry window can be placed virtually anywhere on the camera body, sometimes very close to the taking lens and thus avoiding parallax errors. All other viewfinder systems have a certain (limited) tube length with a given magnification factor. </p>