Focal length confusion

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GaryPolter

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I think something finally clicked in my brain. So as Helen said, the thin lens equation assumes a fixed focal length.

So let's say I tell my lens to have some fixed focal length of 50mm. This means for "infinitely" far objects I need to have the lens 50mm away from the image sensor. If an object is 1000mm away, then to focus it, the lens will move, but the focal length stays the same. Put another way, the focal ring will only move the lens closer/further from the image sensor. When I use the focal ring to move the lens 52.6mm away from the sensor then this image will be in focus.

To ask if other objects will be in focus is not a good question because for that I'll need to calculate the depth of field which requires other values such as aperture.
 

Helen B

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I think something finally clicked in my brain. So as Helen said, the thin lens equation assumes a fixed focal length.

So let's say I tell my lens to have some fixed focal length of 50mm. This means for "infinitely" far objects I need to have the lens 50mm away from the image sensor. If an object is 1000mm away, then to focus it, the lens will move, but the focal length stays the same. Put another way, the focal ring will only move the lens closer/further from the image sensor. When I use the focal ring to move the lens 52.6mm away from the sensor then this image will be in focus.

To ask if other objects will be in focus is not a good question because for that I'll need to calculate the depth of field which requires other values such as aperture.

Yes, you have got it. It is not just theoretical thin lenses that maintain their focal length. All lenses that focus as a unit (ie there is no change in the relative positions of the elements within the lens) maintain constant focal length. The formula 1/f = 1/v + 1/u applies to those lenses as well, if u and v are measured from the correct points. The object distance must be measured from the front (or first) nodal point and the image distance must be measured from the rear (or second) nodal point. In most cases you probably don't need to know the exact location of the first nodal point for your particular application.

As an aside, the focus markings on lenses are usually made for the distance from the image plane, call it D. If the distance between the nodal points is s, measured in the direction of light travel, then

D = u + v + s

Best,
Helen
 

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