[swanlefitte]

Wondering about infinity and registration distance. Is there a mathematical function of this? I assume that the 44.6 + xmm will determine max distance. I assume this is a function of registration distance + difference. It would be cool to see a graph. You could see what lenses for different systems would do mounted on different systems.

[GUB]

Wondering about infinity and registration distance. Is there a mathematical function of this? I assume that the 44.6 + xmm will determine max distance. I assume this is a function of registration distance + difference. It would be cool to see a graph. You could see what lenses for different systems would do mounted on different systems.

Not sure what you mean here but a 50mm lens has a 50mm distance from the theoretical centre of the lens to the sensor plane at infinity. The old school days optical formula diagram does a great job of visualising and calculating the extra distance needed for focusing at different distances. Just have to remember all the elements in a compound lens sits inside the plane that represents the optical centre of the lens. (the vertical oval here). The focal points described here represent the focal ength of the lens.

[swanlefitte]

So as you add tubes I think the elements don't matter. At least for magnification but infinity is different.

[GUB]

So as you add tubes I think the elements don't matter. At least for magnification but infinity is different.

Again not sure what you mean but it may boil down to the fact that the optical centre of a compound lens is a theoretical plane somewhere within the lens and probably just behind the front elements. Probably the other way round to what you are thinking here but the best way to find it is to set your lens to focus at infinity and that theoretical optical centre of the lens will be the focal length distance away from the sensor plane. So focal length minus registration distance - that far up from the base of the lens.

[stevebrot]

Wondering about infinity and registration distance. Is there a mathematical function of this? I assume that the 44.6 + xmm will determine max distance. I assume this is a function of registration distance + difference. It would be cool to see a graph. You could see what lenses for different systems would do mounted on different systems.

They are not related except that the rear elements must provide sufficient refraction to focus on at the focal plane without interfering with the mirror or other camera structures. I have several lenses where the rear element is located nowhere near the registration distance when focused to infinity. Such is usually the case with third-party lenses where identical optics were often used with many different mounts.

As for infinity. The calibration point for infinity focus is often arbitrary and for wide-angles not be particularly far away. I have several 28mm wide-angles originally intended for 35mm film where infinity focus is as close as 60 meters.



Steve

[swanlefitte]

Thanks, I guess this is why I haven't seen a graph.

[stevebrot]

Again not sure what you mean but it may boil down to the fact that the optical centre of a compound lens is a theoretical plane somewhere within the lens and probably just behind the front elements.

For a simple lens, the focal length is the distance from the lens primary plane (lens center) to the focus plane with plane of focus at infinity.

Edit: More correctly, the distance at which parallel rays (originating from a source an infinite distance away) intersect to form the focus point.

Below is a link to a series of articles on a site for coin macrophotography that make for a decent course on basic lens theory and design.

Optics Articles


Steve

[swanlefitte]

Ok got my reading for tomorrow. Looks like I need full concentration.

[.a.t.]

. . .
Below is a link to a series of articles on a site for coin macrophotography that make for a decent course on basic lens theory and design.

Optics Articles


Steve

An interesting link! Thank you, Steve.

[swanlefitte]

I read the first 11 articles and need to take it in. The use of f was confusing because I read fstop focal length and feet.
So why at f5 whoops 5x focal.
Why 2feet? Whoops 2x focal.
Now that I got that we move to fstop I decided to full stop for now.

[photoptimist]

Wondering about infinity and registration distance. Is there a mathematical function of this? I assume that the 44.6 + xmm will determine max distance. I assume this is a function of registration distance + difference. It would be cool to see a graph. You could see what lenses for different systems would do mounted on different systems.

If you have a lens of focal length, f mm, that is correctly focused at infinity for it's normal registration distance and you then shift the lens by x mm away from the sensor, the max distance will be:

MaxD = f*(2+ x/f + f/x)

That can be rewritten as:

MaxD = f^2/x + 2*f + x

A 28 mm lens shifted from its normal registration distance by 1 mm will have a new maximum will be 841 mm (about 2.8 ft)

A 300 mm lens shifted from its normal registration distance by 1 mm, will have a new maximum will be 90,601 mm (about 297 ft)

As you can see, the relationship is very sensitive to focal length and the graph is a parabola.

[swanlefitte]

Thanks for that. Is that just a simple lens or does that hold for multiple lens in multiple groups too?

[photoptimist]

Thanks for that. Is that just a simple lens or does that hold for multiple lens in multiple groups too?

It is almost that simple with the exception that more complex lenses might have a slight offset in MaxD equal to the distance between the nodal points inside the lens. But the shape of the curve will be the same.

[swanlefitte]

Cool to hear.