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

Started Mar 7, 2014 | Discussions thread
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Re: How does a lens work to form an image on the camera's sensor...

GBJ wrote:

I hope the information I added to the diagram above is correct...if it is not, please correct any misperceptions. I believe the focus point representation is off.

I know the sensor never moves forward or back, and it is the lens elements and the Focal length distance from the sensor that moves, so in my diagram I do not intend to make it seem as if the sensor would move forward to the 16mm distance, rather in reality the lens elements move backwards towards the sensor to create the 16mm focal distance.

Great information you have posted here, thanks again 57 Even! This is very generous of you to take time to share this knowledge with others.
Thanks also to everyone else.

One of the images from a suggested website helped my perception of points of focus. I'll attach it.

The first diagram is a bit confusing because it is not showing light rays focusing on the sensor, it is merely showing view angle. The lines shown are only those passing through the lens' optical centre which are not refracted (as the lens is symmetrical). The second diagram shows more light rays passing through other parts of the lens onto the focal plane.

The focus point of a lens is where it focuses (ie the sensor, not the optical centre). The optical centre is the point in the lens which is exactly 1 focal length away from the focus point. Only in a symmetrical convex lens will this point be in the centre of a lens. The optical centre of a compound zoom lens (like in the second diagram) is quite hard to define just by looking at it. You have to know the relative focal lengths of each element and the distances between them at a given point. Because the focus group can move this sometimes affects the true focal length of a lens when focusing at less than infinity (typical example is Nikon 70-200). Focal length of a varifocal camera lens is ONLY defined at infinity for this reason.

You are correct that to get the equivalent view angle on APSC to match a 24mm lens on FF would require a 16mm lens. (24x0.67).

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