As I noted earlier, the focal length is the distance between the second principal plane and the focal plane when the lens is focusing a plane wave (like light coming from infinity). No nodes anywhere.

I figured I would try and get an understanding about how optics work but I can't quite wrap my head around focal length. So it is my understanding that focal length is the distance between the converging point of a lens and the sensor/film, and the flange distance is the distance between the lens mount and the sensor/film. Okay, great.

Telephotos are far easier to design than wide angle lenses and have a far simpler arrangement of elements and groups in the optical path. Wide angle zooms are even more complicated and harder to design.

I figured I would try and get an understanding about how optics work but I can't quite wrap my head around focal length. So it is my understanding that focal length is the distance between the converging point of a lens and the sensor/film, and the flange distance is the distance between the lens mount and the sensor/film. Okay, great.

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Not great. You have the flange distance right but not the focal length. What's the "converging point" of a lens? The focal length is defined as the distance between the second principal point of a lens and the place where parallel rays would focus. The second principal point is a somewhat abstruse mathematical concept. It's the place where, if you put a pinhole, it would generate an image the same size that the lens is making.

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

It is also difficult to design a very long focal length lens that is physically short. Telephoto lenses are shorter than their focal length, but not much less than half. A 400mm pancake lens is not practical.

Not great. You have the flange distance right but not the focal length. What's the "converging point" of a lens? The focal length is defined as the distance between the second principal point of a lens and the place where parallel rays would focus. The second principal point is a somewhat abstruse mathematical concept. It's the place where, if you put a pinhole, it would generate an image the same size that the lens is making.

The principle behind this optical engineering feat is well known. The artangement of lens elements and groups bend the light path so the nodal point of the lens is moved towards the front of the lens. This known as a "reverse telephoto" design and with some ultra-wide angle lenses the nodal pount can actually be in front of the first element.

As I noted earlier, the focal length is the distance between the second principal plane and the focal plane when the lens is focusing a plane wave (like light coming from infinity). No nodes anywhere.

Prime lenses 50mm and up to 100mm or more are usually simple lenses with most glass elements in the vicinity of the focal length.  Obviously this doesn't work with short focal length lenses.  Nikon's 10.5mm is going to be more complex, no way to get the glass elements that close to the sensor.

I seemed to be under the impression that for example a 135mm lens had to be 135mm (about 5.3 inches) from the center point of the lens to the sensor. That's what basic diagrams like below seemed to imply.

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The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

I figured I would try and get an understanding about how optics work but I can't quite wrap my head around focal length. So it is my understanding that focal length is the distance between the converging point of a lens and the sensor/film, and the flange distance is the distance between the lens mount and the sensor/film. Okay, great. But considering the large flange distance in SLRs, how can you design a lens to have a smaller focal length than the flange distance of the mount? For example the Canon EF mount is 44mm. How can you design a lens with a 24mm focal length for such a relatively large flange distance? Aren't you inherently limited by flange distance?

But considering the large flange distance in SLRs, how can you design a lens to have a smaller focal length than the flange distance of the mount? For example the Canon EF mount is 44mm. How can you design a lens with a 24mm focal length for such a relatively large flange distance? Aren't you inherently limited by flange distance?

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

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The 10.5mm is not overly long but some other lenses carry this to more lengths.  Take a look at the size of the new Sigma 18-35 ART.

I figured I would try and get an understanding about how optics work but I can't quite wrap my head around focal length. So it is my understanding that focal length is the distance between the converging point of a lens and the sensor/film, and the flange distance is the distance between the lens mount and the sensor/film. Okay, great. But considering the large flange distance in SLRs, how can you design a lens to have a smaller focal length than the flange distance of the mount? For example the Canon EF mount is 44mm. How can you design a lens with a 24mm focal length for such a relatively large flange distance? Aren't you inherently limited by flange distance?

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Well, there are simple lenses, like magnifying glasses or spectacle lenses. A simple lens focuses light from infinity just about one focal length from the lens. But if you put simple lenses together you can change all that because each lens in a train of lenses works with the image produced by the lens in front of it. So the focus can be anywhere or, for that matter, nowhere.

The principle behind this optical engineering feat is well known. The artangement of lens elements and groups bend the light path so the nodal point of the lens is moved towards the front of the lens. This known as a "reverse telephoto" design and with some ultra-wide angle lenses the nodal pount can actually be in front of the first element.

The article you linked seemed to say that it's about bending the light to give a desired field of view equal and it has nothing to do with how far the center of the lens is to the sensor.

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As noted in Roger's web page, a lens with a given focal length can be very near or very far from the sensor. Optics is rather complex and counterintuitive in many ways.

Telephotos are far easier to design than wide angle lenses and have a far simpler arrangement of elements and groups in the optical path. Wide angle zooms are even more complicated and harder to design.

Nevertheless: wide angle lenses on Single Lens Reflex cameras  (including DSLRS) are by necessity reverse telephoto designs. For cameras that do not  use a reflex viewing system  ( mirrorless digital, rangefinders and view cameras) they do not need to be.

Sorry, i did not have the rigjt language at hand. I was trying to express in insufficient language the same idea you are expressing, and you clearly have more experience in this area than I do.

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

The nodal point is not relevant to this discussion. You measure focal length from the second principal point (or plane). The second principal point in a retrofocus lens is typically behind the rear element.

But considering the large flange distance in SLRs, how can you design a lens to have a smaller focal length than the flange distance of the mount? For example the Canon EF mount is 44mm. How can you design a lens with a 24mm focal length for such a relatively large flange distance? Aren't you inherently limited by flange distance?

I seemed to be under the impression that for example a 135mm lens had to be 135mm (about 5.3 inches) from the center point of the lens to the sensor. That's what basic diagrams like below seemed to imply.

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High-efficiency Blacklight10W black light spotlight, with 24 units LEDs and 90 degree wide beam angle, high powerful efficiency, 270-degree up & down ...

Telephotos are far easier to design than wide angle lenses and have a far simpler arrangement of elements and groups in the optical path. Wide angle zooms are even more complicated and harder to design.

The working distance (WD) is defined as the free space between the sample (object plane) and the first surface of the first element in a complex lens (optical ...

I figured I would try and get an understanding about how optics work but I can't quite wrap my head around focal length. So it is my understanding that focal length is the distance between the converging point of a lens and the sensor/film, and the flange distance is the distance between the lens mount and the sensor/film. Okay, great. But considering the large flange distance in SLRs, how can you design a lens to have a smaller focal length than the flange distance of the mount? For example the Canon EF mount is 44mm. How can you design a lens with a 24mm focal length for such a relatively large flange distance? Aren't you inherently limited by flange distance?

The article you linked seemed to say that it's about bending the light to give a desired field of view equal and it has nothing to do with how far the center of the lens is to the sensor.

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

Not great. You have the flange distance right but not the focal length. What's the "converging point" of a lens? The focal length is defined as the distance between the second principal point of a lens and the place where parallel rays would focus. The second principal point is a somewhat abstruse mathematical concept. It's the place where, if you put a pinhole, it would generate an image the same size that the lens is making.

The article you linked seemed to say that it's about bending the light to give a desired field of view equal and it has nothing to do with how far the center of the lens is to the sensor.

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The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

Well, there are simple lenses, like magnifying glasses or spectacle lenses. A simple lens focuses light from infinity just about one focal length from the lens. But if you put simple lenses together you can change all that because each lens in a train of lenses works with the image produced by the lens in front of it. So the focus can be anywhere or, for that matter, nowhere.

I figured I would try and get an understanding about how optics work but I can't quite wrap my head around focal length. So it is my understanding that focal length is the distance between the converging point of a lens and the sensor/film, and the flange distance is the distance between the lens mount and the sensor/film. Okay, great.

The nodal point is not relevant to this discussion. You measure focal length from the second principal point (or plane). The second principal point in a retrofocus lens is typically behind the rear element.

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

In practice, however, there is a limit. If you design a very short focal length lens for an SLR with a large flange distance, the lens will be very complicated and expensive.

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

Not great. You have the flange distance right but not the focal length. What's the "converging point" of a lens? The focal length is defined as the distance between the second principal point of a lens and the place where parallel rays would focus. The second principal point is a somewhat abstruse mathematical concept. It's the place where, if you put a pinhole, it would generate an image the same size that the lens is making.

I seemed to be under the impression that for example a 135mm lens had to be 135mm (about 5.3 inches) from the center point of the lens to the sensor. That's what basic diagrams like below seemed to imply.

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

What’s the best camera for around $2000? This price point gives you access to some of the most all-round capable cameras available. Excellent image quality, powerful autofocus and great looking video are the least you can expect. We've picked the models that really stand out.

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The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

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A faster sensor, improved autofocus and video see Nikon's third-gen mid-range full-framer go toe-to-toe against Canon's EOS R6 II and the Sony a7 IV. We feel its all-round ability lifts it to the top of the pile.

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

The focal length of a lens determines two interrelated characteristics: magnification and angle of view.

I figured I would try and get an understanding about how optics work but I can't quite wrap my head around focal length. So it is my understanding that focal length is the distance between the converging point of a lens and the sensor/film, and the flange distance is the distance between the lens mount and the sensor/film. Okay, great.

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

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The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

I suggest that people who don't know about optics shouldn't talk about lens design. It's not all that important unless you're trying to design a lens. I design lenses so I have some background in the terms and concepts used in that discipline. But I still have to look them up.

But considering the large flange distance in SLRs, how can you design a lens to have a smaller focal length than the flange distance of the mount? For example the Canon EF mount is 44mm. How can you design a lens with a 24mm focal length for such a relatively large flange distance? Aren't you inherently limited by flange distance?

But considering the large flange distance in SLRs, how can you design a lens to have a smaller focal length than the flange distance of the mount? For example the Canon EF mount is 44mm. How can you design a lens with a 24mm focal length for such a relatively large flange distance? Aren't you inherently limited by flange distance?

The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

dbo:abstract. This article summarizes equations used in optics, including geometric optics, physical optics, radiometry, diffraction, and interferometry. (en).

The principle behind this optical engineering feat is well known. The artangement of lens elements and groups bend the light path so the nodal point of the lens is moved towards the  front of the lens. This known as a "reverse telephoto" design and with some ultra-wide angle lenses the nodal pount can actually be in front of the first element.

As noted in Roger's web page, a lens with a given focal length can be very near or very far from the sensor. Optics is rather complex and counterintuitive in many ways.

I know how simple optics work but I'll admit I don't know how the more complicated lenses work.  But I am interested to learn so I will read the responses here from more knowledgeable people and the links they have posted.

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The Classic Sequential Tail Light (CST) kits are direct retrofits and replacements for incandescent light bulbs in classic vehicles. These kits inc...

I figured I would try and get an understanding about how optics work but I can't quite wrap my head around focal length. So it is my understanding that focal length is the distance between the converging point of a lens and the sensor/film, and the flange distance is the distance between the lens mount and the sensor/film. Okay, great. But considering the large flange distance in SLRs, how can you design a lens to have a smaller focal length than the flange distance of the mount? For example the Canon EF mount is 44mm. How can you design a lens with a 24mm focal length for such a relatively large flange distance? Aren't you inherently limited by flange distance?

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I suggest that people who don't know about optics shouldn't talk about lens design. It's not all that important unless you're trying to design a lens. I design lenses so I have some background in the terms and concepts used in that discipline. But I still have to look them up.

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