Lens Focal Length Angle
Why measure Focal Length?
Unless you're testing and calibrating lenses, I'm not sure why you'd want to know the exact focal length of a lens other than simply to satisfy your curiosity. It's just something that some people want to know. If you pay for a 400mm lens, it's nice to know you have a 400mm lens, not a 370mm lens I guess. In fact most lens makers will tell you that the focal length marked on a lens is +/- 5%. That means your 400mm could be as short as 380mm and still be "within spec". Normally telephoto lenses err on the short side. It's MUCH more likely that a lens will be shorter than marked than longer.
Definition of "Focal Length"
Camera lenses are complex critters. In the case of a single element equi-convex thin lens, it's easy to measure focal length. You focus a point at infinity, and the distance from the center of the lens to the focal point is the focal length.
However life is not so simple with a camera lens. You can still focus an object at infinity OK, but what distance do you measure? From the focus to the back of the lens, from the focus to the front of the lens or from the focus to the middle of the lens? The answer is no to all three questions. You actually measure the distance from the focus to something called the rear (or secondary) nodal point of the lens. The strict definition is:
Where is the "rear nodal point"? Well it could be anywhere. It could be somewhere inside the lens, it could be out in front of the first element of the lens (for telephoto lenses) or it could be somewhere between the last element of the lens and the focus (for wideangle retrofocus lenses). This makes life complex.
If a lens actually had the focal length that was marked on it, the rear nodal point would be one focal length in front of the film (or sensor) plane when the lens was focused at infinity. If course if the lens had the focal length marked on it, you wouldn't need to measure it! the point of doing the measurement is to see what the true focal length is!
So to measure the focal length you either have to determine where the rear nodal point is, or you have to use a method of measurement which doesn't require you to know where it is.
There are a number of methods of finding the nodal points of a lens, but none are simple. I won't discuss them here. Instead I'll describe a couple of methods of measuring focal length.
The first method I'll call "the hard way" since it means setting up a small optical bench and making a number of linear measurements. It's the method I'd use to see what the true focal length of a close focusing zoom is. Close focusing telephoto zooms with internal focus often get that close focus by reducing the focal length. So when you have your 300mm zoom focused down to 12", it's probably only really acting as a 100mm lens. Does it matter? Well, if it does to you, this is how to measure it.
The second method I'll call "the easy way". It involves taking one photograph, followed by some fairly simple image measurements and calculations. It's the method I'd use to measure the focal length of telephoto lenses focused at infinity.
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Unless you're testing and calibrating lenses, I'm not sure why you'd want to know the exact focal length of a lens other than simply to satisfy your curiosity. It's just something that some people want to know. If you pay for a 400mm lens, it's nice to know you have a 400mm lens, not a 370mm lens I guess. In fact most lens makers will tell you that the focal length marked on a lens is +/- 5%. That means your 400mm could be as short as 380mm and still be "within spec". Normally telephoto lenses err on the short side. It's MUCH more likely that a lens will be shorter than marked than longer.
Definition of "Focal Length"
Camera lenses are complex critters. In the case of a single element equi-convex thin lens, it's easy to measure focal length. You focus a point at infinity, and the distance from the center of the lens to the focal point is the focal length.
However life is not so simple with a camera lens. You can still focus an object at infinity OK, but what distance do you measure? From the focus to the back of the lens, from the focus to the front of the lens or from the focus to the middle of the lens? The answer is no to all three questions. You actually measure the distance from the focus to something called the rear (or secondary) nodal point of the lens. The strict definition is:
Where is the "rear nodal point"? Well it could be anywhere. It could be somewhere inside the lens, it could be out in front of the first element of the lens (for telephoto lenses) or it could be somewhere between the last element of the lens and the focus (for wideangle retrofocus lenses). This makes life complex.
If a lens actually had the focal length that was marked on it, the rear nodal point would be one focal length in front of the film (or sensor) plane when the lens was focused at infinity. If course if the lens had the focal length marked on it, you wouldn't need to measure it! the point of doing the measurement is to see what the true focal length is!
So to measure the focal length you either have to determine where the rear nodal point is, or you have to use a method of measurement which doesn't require you to know where it is.
There are a number of methods of finding the nodal points of a lens, but none are simple. I won't discuss them here. Instead I'll describe a couple of methods of measuring focal length.
The first method I'll call "the hard way" since it means setting up a small optical bench and making a number of linear measurements. It's the method I'd use to see what the true focal length of a close focusing zoom is. Close focusing telephoto zooms with internal focus often get that close focus by reducing the focal length. So when you have your 300mm zoom focused down to 12", it's probably only really acting as a 100mm lens. Does it matter? Well, if it does to you, this is how to measure it.
The second method I'll call "the easy way". It involves taking one photograph, followed by some fairly simple image measurements and calculations. It's the method I'd use to measure the focal length of telephoto lenses focused at infinity.
Having problem with cbse sample papers class 12 physics keep reading my upcoming posts, i will try to help you.