How does a lens work to form an image on the camera's sensor...

Started Mar 7, 2014 | Discussions thread
57even Forum Pro • Posts: 10,842
Re: How does a lens work to form an image on the camera's sensor...

You need a good primer on basic optics. The diagrams in these examples are next to useless.

The best explanation is to imagine how a lens would focus a single point, then two points, then all the points that make up a specific object.

Cameras capture light reflected off an object which radiates in all directions, including down your lens. If you imagine light streaming in straight lines from a given point, they will diverge. What a lens does is to refocus divergent rays to a single point....

In the diagrams above are three point objects. For simplicity only four of the billions of light rays from each object are shown.

In A the object is focused on the sensor. The curvature of the lens has the effect of bending light towards the centre of the lens (angle of refraction). However the angle through which it bends depends on the glass type (refractive index) and the curvature of the lens. If the distance from lens to sensor changes, the image will blur because the rays of light are not fully converged at the right point. (In this case I moved the sensor, in a camera you move the lens).

In B the closer object has rays which hit the lens at a steeper angle so they will not converge as quickly and will focus behind the sensor - the image will therefore be a blur covering all the points where light rays from that object hit. The further object will focus in front of the sensor, but the effect will be the same.

In C when you close down the aperture, you cut off the rays from the edges, so the blur radius also gets smaller and the objects appear sharper. Stop down more and they look sharper still (until diffraction starts to impact the overall sharpness). Effectively you are just using the centre part of the lens. Using a smaller lens would have the same effect. Of course, you also reduce the amount of light, so you need a longer exposure or higher ISO.

Please note most cameras have multiple elements in the lens, some which move and which don't. This so called compound lens corrects for the fact that (1) light of different wavelengths does not refract the same amount (chromatic aberration) and (2) that ground lenses do not focus exactly in the same point across the surface (spherical aberration). However a well corrected compound lens will combine multiple elements to get very close to a perfect lens. It also explains why aspherical elements are used in some complex lenses.

Hope that helps.

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