Glass and mounts for fs100

[Brian Bang Jensen]

Originally Posted by Alister Chapman

That's not quite correct. The focal length is the amount that the lens converges light or the optical power of the lens.

The distance between the rear of the lens and the sensor, known as the flange back will vary depending on the design of the camera. On a Canon EF-S camera it is 44mm on a Nikon it is 46.5 and E-Mount is 18mm. That's why you can put a Nikon lens on a Canon or Sony, but not the other way around. A 50mm lens will always be a 50mm lens no matter whether it has a 18mm flange back or 44mm flange back. If you try to use a lens at the wrong distance from the sensor you won't be able to focus it (or at least the focus scale will be off).

It's all very confusing, particularly because people often talk about applying a conversion factor to the lens when really the conversion factor should be though of as applying to the camera as it is the camera and the size of it's sensor that changes, not the lens.

Normally, I think you are very knowledgeable but at this point I partly disagree.
A lens focal length is equal to the distance from film plane to the theoretical focal point inside the lens.The focal point is the point where the light crosses inside lens.FB is the distance to be adjusted to achieve the right focus length for the lens, thus putting the lens in focus. But maybe we mean the same thing and it is language difficulty that separates us :-)

[Brian Drysdale]

Perhaps this covers the definition

Flange focal distance - Wikipedia, the free encyclopedia

Often called the flange distance.

[Brian Bang Jensen]

And this.

Focal length - Wikipedia, the free encyclopedia

I can see that we just use different expressions to describe the same topic.

[Alister Chapman]

Originally Posted by Brian Bang Jensen

The focal point is the point where the light crosses inside lens.

OK, optics are not my forte but according to my understanding the focal point is where collimated light that passes through a single lens is focussed and it will be some distance behind the lens. See Focal point - Wikipedia, the free encyclopedia

For a single convex element in air it is true that this distance, from the lens vertex (center) to focal point is the focal length. The focal plane would normally pass through the focal point and the sensor or film would be placed at the focal plane. But we are not talking about single element convex lenses, we are talking about multi-element camera lenses and this is probably where our confusion with each other comes in.

With camera lenses it becomes very complicated, especially with wide angle lenses.
In your earlier post Brian you said that "On a 50mm lens the distance to the focal plane is always 50mm". My interpretation of that is that you are saying that there would be 50mm from the center of the lens to the focal plane, i.e. where the film or sensor is placed. That is fine in theory, but it implies that 300mm lens has to be 300mm from the sensor and what about zoom lenses with a variable focal length? In practice it's obvious that this is not normally the case and this is confirmed by the table provided by Brian.

How for example do you get a 16mm lens to work on a camera with a lens to focal plane distance greater than 18mm (Sony NEX) or even 44mm (Canon DSLR)? How can you get the focal plane behind the focal point? Well it's done using multiple lens elements (wide retrofocus lens) and it's one of the reasons why good wide angle lenses are so expensive and demonstrates how the indicated focal length is not the actual focal length but a theoretical one know as the "effective focal length". Most stills and video lenses give their EFL (effective focal length) as opposed to the true focal length and it is calculated according to the magnifying power of the lens. See Camera lens - Wikipedia, the free encyclopedia

So IMHO the confusion stems from the fact that when a camera manufacturer gives the focal length of a lens it is a calculated equivalent distance based on the lenses magnification and not the actual focal length.

I think we all agree that a 50mm lens is a 50mm lens, no matter what size the sensor it is still a 50mm lens.
That you can have almost any flange back distance i.e. distance from lens rear to the sensor or film plane. This is determined by the camera manufacturer.
That the flange back distance does not change the focal length and you can have a 50mm lens that is only 18mm from the sensor, or 44mm or 46mm etc.
The focal length determines the magnification factor of the lens.
It is the size of the sensor that determines the FOV, not the flange back distance or any other factor.

[Brian Bang Jensen]

Alister I totally agree.
It is the theoretically focal length. I forgot the word “theoretically” in my post, my mistake.

Regarding wide lenses, before retrofocus where implemented in the design, the lenses actually protruded far into the camera and the mirror had to bee locked in the upper position, preventing it to hit the rear of the lens and a long lens were actually a drain pipe..