measuring image circle

[alex from holland]

is there a way to measure the image circle of a lens. I don't mean calculating it, but really measuring it
This without having a HUGE groundglass. So maybe by projecting on a wall or so?
I have been looking for pictures or a film about this but can't find anything.

Alex

[Leigh]

The image circle and the circle of illumination are not the same thing.

It's easy to measure the circle of illumination, given an imaging surface of sufficient size.

The image circle as defined by the manufacturer is usually defined as the area within which the image quality meets
the maker's specifications. That would be pretty difficult to measure, even if you knew the specs in the first place.

- Leigh

[Bob Salomon]

Turn out all the lights in the room. Close all drapes and blinds, point the lens out the window at an object at infinity. Get a very large piece of white, stiff cardboard. Place it behind the lens and move it towards and away from the lens till you see a sharp image. That is the coverage of your lens. Now just measure it.

The lens must not move, just the cardboard.
The image circle will grow larger as you stop the lens down.
You will start to enter diffraction at some point as the circle grows and that will limit the aperture range for maximum performance optically.

You will probably need to use all 4 of your hands or at least two people depending on how long the lens is and how large the circle is.
But as pointed out. Not all of this circle will really be useable due to optics, fall off, distortion, etc.

[alex from holland]

So it works the same way as m easuring the focallenght than?

[Leigh]

Yes. The light coming from the lens is a cone.
Its height is the focal length, and the diameter of its base is the circle of illumination.
As mentioned previously, the image circle (as defined by the manufacturer) will fall inside the circle of illumination.

- Leigh