Question Must Be Answered ONCE AND FOR ALL: Step down-rings

Started Dec 11, 2009 | Discussions thread
Joseph S Wisniewski Forum Pro • Posts: 34,312
Telecentric lenses, determining parameters...

LensEnvy wrote:

Joseph S Wisniewski wrote:

That's the whole point of the telecentric stop. It moves the entrance pupil out to infinity: the perspective is orthographic, size does not change with distance.

You can move the camera, and focus will shift, but perspective will not shift.
Have a look at the two diagrams on this page.

OK... the concept is finally sinking in. The URL visuals and explanation helped a lot. This "flat view" is very different from how we're used to seeing the world, from the perspective of a single focal point.


So, does the focal plane become flat as well, or do the lens characteristics make it stay parabolic?

It's as flat as the lens's focal plane normally is, the telecentric stop doesn't really affect curvature of field. Most lenses are pretty good, the focal plane isn't particularly "parabolic".

And it seems a side-effect is that the lens has effectively a zero-degree "field of view" that's the diameter of the front element?


I can totally see the benefit in machine vision for things like looking down a hole without it being in the center of the lens. Very cool. Now it makes sense how you can slide the whole assembly on the rail to adjust focus, but the perspective doesn't change.

Now to the practical application...

How do you determine how large to make the "stop", where to position it between the lenses (where the "principal rear focus point" is), and how far apart to gap the lenses when they're reversed?

Where is easy. You can find the front principle plane (where you put the stop on the reversed lens) by simply taking the lens, setting the aperture wide open, pointing the rear element at something bright and far away, and holding something transluscent (an old focusing screen, a bit of home made ground glass, a piece of waxed paper) in front of the front element and moving it back and forth until you get an in-focus image. The more precise you want the result to be, the more stable you have to make your measuring rig. Mounting the lens to a Cokin P-flange and using a solid, flat projection screen like a piece of ground glass will give you the most accurate reading.

How big is a bit more difficult. You figure the stop size based on diffraction and effective aperture. And there's other math to throw in for a parameter of the lens called the "papillary magnification factor". For a 50mm f1.4, you can ignore the papillary mag factor part, and you can probably also ignore it for a 35mm f1.4 or 2.0, but you have to consider it for a shorter lens.

Basically, then, you're considering the light from an in focus macro subject to be totally parallel when leaving the lens, so you use the diameter of that parallel column to determine aperture. The 50mm in front of the 200mm has a "bellows factor" of 5. That's the amount the effective aperture increases. You figure it as

BF = rear fl/front fl + 1 = 200mm/50mm + 1 = 5

So, if you decide f11 is a good effective aperture for preserving detail without too much diffraction, and it will work with your lighting...

diameter = front fl/f stop * BF = 50/16 * 5 = 15.6mm.

Aside from that, you want to get the lenses as close together as possible.

I'm sure there's loads of math behind it, but I imagine it can be distilled to a process, no?


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Rahon Klavanian 1912-2008.

Armenian genocide survivor, amazing cook, scrabble master, and loving grandmother. You will be missed.

Ciao! Joseph

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