Sensor size & focal length question

Ysarex

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The focal length of the lens is the distance from the lens nodal point to the film/sensor when the lens is focused at infinity. The lens focal length does not have a fixed relationship with the lens's projected image circle. Two lenses of the same focal length can project different image circles. However on the same camera they will have the same angle of view.
Since we can "free lens" on a camera, the flange length is not fixed to one number by the lens (if it were, "freelensing" would be impossible because of the new flange length).

So since the length (50mm) is the distance from the nodal point (that is fixed?) to the sensor (which is not fixed), we cannot say that a lens is 50mm? We can only say that a lens is 50mm at a given distance from the sensor or film?

When you focus a lens on subjects progressively closer to the camera the lens moves away from the sensor whether you're freelensing or not. Focal length as inscribed on photographic lenses is the distance from the lens nodal point to the film/sensor when the lens is focused at infinity. That's true if the lens is physically mounted to the camera or not. It's true if you hold the lens and point it out the window and focus on a piece of paper in your other hand.

Joe
 

JerryLove

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JerryLove said:
I don't see how that is possible (unless you are cropping, which is not what I'm discussing).

If I put an infinite piece of paper behind the lens, how many arc-seconds of the terrain in front of me would be visible on that paper?

You used the term AoV (angle of view). I'm telling you how that term is commonly understood, defined and used by photographers. If you're discussing something else, use the right terminology for whatever that something else is.

Joe
What is the term that describes the number of arc-seconds (or similar measurements) of image from the front of the lens that is projected out the back of the same lens?

Does this value have a fixed relationship with focal length?
 

Ysarex

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If I put an infinite piece of paper behind the lens, how many arc-seconds of the terrain in front of me would be visible on that paper?

That will depend on the design of the lens and not just it's focal length. Two lenses of the exact same focal length can yield two very different results in this case.

Joe
 

Braineack

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JerryLove said:
I don't see how that is possible (unless you are cropping, which is not what I'm discussing).

If I put an infinite piece of paper behind the lens, how many arc-seconds of the terrain in front of me would be visible on that paper?

You used the term AoV (angle of view). I'm telling you how that term is commonly understood, defined and used by photographers. If you're discussing something else, use the right terminology for whatever that something else is.

Joe
What is the term that describes the number of arc-seconds (or similar measurements) of image from the front of the lens that is projected out the back of the same lens?

Does this value have a fixed relationship with focal length?

:popcorn:
 

Ysarex

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JerryLove said:
I don't see how that is possible (unless you are cropping, which is not what I'm discussing).

If I put an infinite piece of paper behind the lens, how many arc-seconds of the terrain in front of me would be visible on that paper?

You used the term AoV (angle of view). I'm telling you how that term is commonly understood, defined and used by photographers. If you're discussing something else, use the right terminology for whatever that something else is.

Joe
What is the term that describes the number of arc-seconds (or similar measurements) of image from the front of the lens that is projected out the back of the same lens?

Does this value have a fixed relationship with focal length?

It does not have a fixed relationship with focal length.

Joe

edit: You're talking about the projected image circle.
 

480sparky

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So since the length (50mm) is the distance from the nodal point (that is fixed?) to the sensor (which is not fixed), we cannot say that a lens is 50mm? We can only say that a lens is 50mm at a given distance from the sensor or film?


That is not the way a lens is measured for focal length.

The focal length of a lens is the measurement between the lens to a point where the parallel light rays will converge. As is happens, this just happens to be the same distance between the lens and sensor/film when the lens is focused at infinity. When the lens is focused closer, it is moved further away from the sensor/film, but that does not change the focal length.
 

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What is the term that describes the number of arc-seconds (or similar measurements) of image from the front of the lens that is projected out the back of the same lens?.....

Field of view.

.....Does this value have a fixed relationship with focal length?

It does have a relationship with focal length, but only a part of a formula.... the third variable being the size of the sensor. You can't just say "A 50mm lens has a xx° field of view" without knowing the size of sensor or film behind it.
 

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What is the term that describes the number of arc-seconds (or similar measurements) of image from the front of the lens that is projected out the back of the same lens?

Does this value have a fixed relationship with focal length?

You can call it the angle of coverage (angle of view being taken to mean the result of the combination of lens focal length and the film or sensor size), and you can also specify coverage as the diameter of the image circle. For example, you can say of a lens: "It has a 75 degree angle of coverage at f22. This results in an image circle of 189mm at f22"

Typically the angle of coverage is the same for a particular lens type - the above is for the Apo-Symmar-L series of large format lenses from Schneider. They all have a 75 degree angle of coverage, but they are available in different focal lengths. Each focal length has the same angle of coverage (more-or-less) but different image circle diameters - the example above is for a 120 mm lens. The coverage is usually quoted for focus at infinity, and it may be quoted for different f-numbers. Macro lenses may have their coverage quoted for closer than infinity focus.
 

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It does have a relationship with focal length, but only a part of a formula.... the third variable being the size of the sensor. You can't just say "A 50mm lens has a xx° field of view" without knowing the size of sensor or film behind it.
This is logically impossible. The number of arc-seconds of view out the back of a lens is independent of observer (sensor size). The sensor size might tell you how much of that circle you can actually capture, but it is not part of the lens measurement.

But I'm obviously lacking the language to describe what's inside my head (and, conversely, I've seen three different answers to one question).

mm does not tell you what would be circumscribed by a lens (both circles in the picture are properly 50mm even though they have different percentages of the outside visible)... which I suppose is where I was going to begin with.
 

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This is logically impossible. The number of arc-seconds of view out the back of a lens is independent of observer (sensor size). The sensor size might tell you how much of that circle you can actually capture, but it is not part of the lens measurement.

If it's 'logically impossible', explain why a given lens on an FX camera has a different FOV than the same lens on a DX body.

I'm sure the Big Manufacturers would be interested to know this.
 

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Come on boys and girls, keep it going, only 2 more pages to go to reach the magic number 5. :lmao:
 

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I'll bump. What am I bumping?
 

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