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A galaxy that lenses another celestial body should as it moves away from Earth alter the sharpness of what it lenses by gravity. Could this be used to determine how quickly space is expanding?

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    $\begingroup$ Gravitational lenses don't bend light like glass lenses. They don't focus light to a point, and so they don't form a real image in the way that a glass lens does. So there is no "sharpness", and only virtual and distorted images of the lensed object. $\endgroup$
    – James K
    Jan 1 at 18:53

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Gravitational lenses don't form images that may be sharp or not.

In the diagram, the light from the point of the arrow is brought together at a point, forming an image. Notice that the light that passes through the middle of the lens is bend least. The "sharpness" results from whether you place your film (or CCD etc) exactly at the point that the image forms (in which case you'll capture a sharp picture) or not quite at that position (in which case the image will be less sharp.

But in a gravitational lens there is no image formed. The light that passes close to the lens is bent most. This is more complicated when the lens is not a single point, but the effect is that there is no image, so there is nothing that can be "sharp". What we see are multiple virtual images. For example we might see light that has come along paths "b" and "c", so we would see the object in multiple locations. The actual effect is to produce multiple, distorted virtual images, all of which are "sharp".

enter image description here

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    $\begingroup$ I didn't realise how far removed gravitational lenses are from glass lensing! $\endgroup$
    – user54398
    Jan 1 at 20:08

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