A question about focal length

[RichardC27]

This is possibly a little bit dumb, but it's been bugging me on and off for a while now. Where do focal length numbers (e.g 300mm) come from? I recently sold my 75-300mm lens, and it was nowhere near a foot long when it was fully extended. So how does it end up being a 300mm lens?

 

[who?]

I think that the term 'Effective Focal length' should really be used. The combination of lenses in a camera lens has the same effective focal length as a single lens with a 300mm focal length.

If you look at the specs for any camera lens you see that there are a a veriety of individual elements, it is the total effect of these that makes the lens up.




ie its a fix.

 

[rpcrowe]

Normally, focal length was calculated from the optical center of the lens to the film plane (sensor these days) when the lens was focused on infinity. This made the lenses physically very long. I am going to try to attach a picture of a 1,200 mm Canon lens of this type.

Telephoto lenses of today optically compress the distance between the optical center of the lens and the film/sensor plane. Thus a 300mm telephoto lens is not 300mm between lens and film plane but, a shorter distance.

 

[Canon Bob]

The focal length is measured from the rear nodal point of the "lens group" (all the glass in the tube, in simple terms). For a simple bi-convex lens, the nodal point is the centre of the lens but designers can move this nodal point by using a mix of convex and concave profiles together with glass with different refrangability properties. To get a physically short "lens group" the nodal point is pushed out in front of the group.....the further it goes, the bigger the distortion correction required.

Bob

 

[mrgubby]

refrangability

Very OT but that is an Excellent word , will have to slip it into a conversation today

 

[Canon Bob]

Very OT but that is an Excellent word , will have to slip it into a conversation today

Report back if you manage it.....then we'll have identical post counts again.

Bob

 

[RichardC27]

Thanks gents, answers my question perfectly

 

[Flash In The Pan]

According to one sales assistant I overheard in a Glasgow branch of Jessops 300mm refers to the maximum distance an object can be from the lens to be in focus........ :shrug:

 

[Grockle]

Wonder if that's the same git that tried telling me that a 1.6x crop sensor meant that the size of the sensor was 1.6x BIGGER than a frame of 35mm film.