Close up lens, how do they work?

[erian]

I am trying to figure out how (in the sense of why) the close up lens work.

I have some theory but the available information mostly focuses only on the usage aspect.

I have gathered from the bits and pieces that close up lens may work by reducing focal length but I am not confident that this is the case. It does make sense for the limited lens extension cameras - smaller focal length lens can get closer using the same lens extension.

[reddesert]

The close up lens adds additional power to the primary lens. You can think of this as making the combined focal length shorter - shorter focal length lenses are more strongly curved and bend light more sharply. The effect is relatively similar to putting on reading glasses so your eyes can focus closer.

It's also possible to think of the close up lens as taking light diverging from the subject and bending it so it is less strongly diverging, ie it makes the primary lens able to focus on this less strongly diverging light (as if the subject were further away).

See for example the focal length combination formula and the diagram of a bent light ray at: https://physics.stackexchange.com/questions/247617/how-is-focal-length-defined-for-a-two-lens-system-separated-by-a-distance-d

[Emmanuel BIGLER]

Hi!

To understand how a close-up lens works, I prefer thinking in terms of images instead of thinking in terms of changed focal length or modified optical system.

A close-up lens is a single positive lens element with a given focal length "f", e.g., 1 meter, f = 1m.

Suffice to consider the object-image correspondence in a single lens element.
If the subject, e.g. a flower, a butterfly or a mushroom, is located at a distance of one meter (generally speaking = one focal length) in front of this close-up lens, by definition of the focal point, the image delivered by the close-up lens is located at infinity.
The camera's taking lens set-up for infinity, sees this image at infinity and eventually delivers a sharp image in its own focal point, since the new "object" (actually: the image delivered bu the close-up lens) is located at infinity.

A close up lens with f=1m brings the image of an object located at a distance of one meter to infinity.
A close up lens with f=0.5m brings the image of an object located at a distance of 0.5m = 50 cm to infinity.
A close up lens with f=0.33m = 1/3 m brings the image of an object located at a distance of 33 cm to infinity.

Hence, no need to compute the equivalent focal length of the combined optical system (close-up lens + camera lens).

Now a more subtle issue is: with a close-up lens, do I need an exposure correction?
The answer is simply: no, with a close-up lens added in front of the camera's taking lens, no exposure compensation or correction is needed.

(the reason is a bit difficult to explain, but will be explained here in detail, with pleasure, upon request ;) )

[Bob Salomon]

Hi!

To understand how a close-up lens works, I prefer thinking in terms of images instead of thinking in terms of changed focal length or modified optical system.

A close-up lens is a single positive lens element with a given focal length "f", e.g., 1 meter, f = 1m.

Suffice to consider the object-image correspondence in a single lens element.
If the subject, e.g. a flower, a butterfly or a mushroom, is located at a distance of one meter (generally speaking = one focal length) in front of this close-up lens, by definition of the focal point, the image delivered by the close-up lens is located at infinity.
The camera's taking lens set-up for infinity, sees this image at infinity and eventually delivers a sharp image in its own focal point, since the new "object" (actually: the image delivered bu the close-up lens) is located at infinity.

A close up lens with f=1m brings the image of an object located at a distance of one meter to infinity.
A close up lens with f=0.5m brings the image of an object located at a distance of 0.5m = 50 cm to infinity.
A close up lens with f=0.33m = 1/3 m brings the image of an object located at a distance of 33 cm to infinity.

Hence, no need to compute the equivalent focal length of the combined optical system (close-up lens + camera lens).

Now a more subtle issue is: with a close-up lens, do I need an exposure correction?
The answer is simply: no, with a close-up lens added in front of the camera's taking lens, no exposure compensation or correction is needed.

(the reason is a bit difficult to explain, but will be explained here in detail, with pleasure, upon request ;) )

Not all are single element, Minolta and Heliopan, among others, made achromat close up lenses.

[Emmanuel BIGLER]

Not all are single element, Minolta and Heliopan, among others, made achromat close up lenses.

Good point, Bob, this howerer does not change at all my previous analysys, a cemented doublet is of course much better than a single lens element, since red, green and blue foci of an achromat are very close to each other, unlike a poor single lens element ;)

[erian]

Hi!

To understand how a close-up lens works, I prefer thinking in terms of images instead of thinking in terms of changed focal length or modified optical system.

A close-up lens is a single positive lens element with a given focal length "f", e.g., 1 meter, f = 1m.

Suffice to consider the object-image correspondence in a single lens element.
If the subject, e.g. a flower, a butterfly or a mushroom, is located at a distance of one meter (generally speaking = one focal length) in front of this close-up lens, by definition of the focal point, the image delivered by the close-up lens is located at infinity.
The camera's taking lens set-up for infinity, sees this image at infinity and eventually delivers a sharp image in its own focal point, since the new "object" (actually: the image delivered bu the close-up lens) is located at infinity.

A close up lens with f=1m brings the image of an object located at a distance of one meter to infinity.
A close up lens with f=0.5m brings the image of an object located at a distance of 0.5m = 50 cm to infinity.
A close up lens with f=0.33m = 1/3 m brings the image of an object located at a distance of 33 cm to infinity.

Hence, no need to compute the equivalent focal length of the combined optical system (close-up lens + camera lens).

Now a more subtle issue is: with a close-up lens, do I need an exposure correction?
The answer is simply: no, with a close-up lens added in front of the camera's taking lens, no exposure compensation or correction is needed.

(the reason is a bit difficult to explain, but will be explained here in detail, with pleasure, upon request ;) )

Thank you for this explanation. Can you also explain how it would work as a combined system? What happens when I focus my lens closer than infinity?

I would like to understand if I could shorten the focal length with a close up lens.

Of course I want to know why exposure compensation is not necessary. :)

[Bob Salomon]

Thank you for this explanation. Can you also explain how it would work as a combined system? What happens when I focus my lens closer than infinity?

I would like to understand if I could shorten the focal length with a close up lens.

Of course I want to know why exposure compensation is not necessary. :)

When you focus closer then infinity then the subject can be a little closer to your camera with a CU lens. It will not let you general scenic photography. That would require a different focal length lens if you want to bring the forest closer or further away from your camera position.
No exposure compensation is needed as you have not increased the lens to image plane. If you extend a bellows or use extension tubes you would need to increase the exposure.

To reiterate. A CU lens does not change a lens into a shorter focal length wide angle lens.

[reddesert]

On a fixed mount camera (like a 35mm SLR type camera), if you put a close-up lens in front of a normal lens, you can focus close-up but not at infinity.

On a view camera, if you put a closeup lens in front of your lens, the combination does have a shorter focal length. Because it's got a focusing rail rather than a fixed mount, you can then try to bring the lens closer to the film plane, and you might be able to focus at infinity. But the image quality will not be good (even with a nice achromatic doublet like Bob mentions). You can expect spherical aberration and off-axis aberrations (curvature of field, astigmatism, etc). It might be capable of soft-focus special effects if that is your thing.