Why does it seem like focal ratio is rarely talked about?!

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about. A 50mm F/1.4 & 100mm F/1.4 for example; the 100mm can let in more let despite the same aperture. It seems to me like people think F/stop is the main deciding factor in light gathering ability. Myself being someone who enjoys astrophotography sometimes, it's a great thing I know about focal ratio, but it's also a reason why I tend to go with lens of a long focal length rather than a wider lens, even if their apertures are the same.

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See the discussions of throughput and Entendue elsewhere in PS&T.

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about.

Click to expand...

In photography, "aperture" usually means f-number (also called f-stop), defined as the focal length divided by the entrance pupil diameter.

I can't recall seeing the term "focal ratio". I have no idea what it means. How would you define it?

Tom Axford said:

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about.

Click to expand...

In photography, "aperture" usually means f-number (also called f-stop), defined as the focal length divided by the entrance pupil diameter.

I can't recall seeing the term "focal ratio". I have no idea what it means. How would you define it?

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I think the OP should explain what the numerator and denominator of the ratio he’s talking about are.

The aperture is actually the entrance pupil. I like to refer unambiguously to the aperture diameter, which could be f/4, for example. For example, if f = 100 mm, then the aperture f/4 is 25 mm. I guess the focal ratio is 1.4 in the OP's example. The f stop is f/1.4, which also happens to be the aperture. As far as I know, this is consistent with practice in other areas of optics. Note that in general, the aperture doesn't even have to be circular. It could be square or some other shape -- even for photography.

In the OP's example, the 100 f/1.4 mm lens has twice the aperture of the 50 mm f/1.4 lens (twice the aperture diameter to be perfectly clear), not the same aperture as the OP indicated. Common terminology is rather ambiguous in this respect.

Note also that although use of terminology has gotten rather loose and lazy, it should be expressed as f/1.4, not F/1.4. As I recall, the relative aperture is defined as entrance pupil diameter divided by f, i.e., 1.4/f. The terminology of photographic lenses is a little murky, in my opinion.

The OP's suggestion to use "focal ratio" is probably a good one in principle. Good luck trying to get anyone to adhere to more precise terms, although I don't think it matters much the way people use it most of the time.

Focal ratio is f-stop.

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about.

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These three are often interchangeable with f-number.

MacM545 said:

A 50mm F/1.4 & 100mm F/1.4 for example; the 100mm can let in more let despite the same aperture.

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F/1.4 is the same aperture setting, but 50mm /1.4 = 35 mm and 100mm /1.4 = 70 mm are different aperture diameters.

MacM545 said:

It seems to me like people think F/stop is the main deciding factor in light gathering ability.

Click to expand...

It is true when people talk about the light collected by the whole sensor. This is relevant when photographing a grey card filling the whole frame or a landscape.

However, It is the diameter of the entrance pupil that defines "light gathering ability from the subject, provided the subject is completely within the frame". This is relevant when the moon or a duck is taking a small portion of the frame. In this case we are cropping anyway, so the size of the sensor is not relevant.

MacM545 said:

Myself being someone who enjoys astrophotography sometimes, it's a great thing I know about focal ratio, but it's also a reason why I tend to go with lens of a long focal length rather than a wider lens, even if their apertures are the same.

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People use "aperture" for both "aperture diameter" and "aperture setting", hence the confusion. It is less important what you call it, if it is clear if you need units of length or not to measure it.

Dem Bell said:

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about.

Click to expand...

These three are often interchangeable with f-number.

Dem Bell said:

A 50mm F/1.4 & 100mm F/1.4 for example; the 100mm can let in more let despite the same aperture.

Click to expand...

F/1.4 is the same aperture setting, but 50mm /1.4 = 35 mm and 100mm /1.4 = 70 mm are different aperture diameters.

Dem Bell said:

It seems to me like people think F/stop is the main deciding factor in light gathering ability.

Click to expand...

It is true when people talk about the light collected by the whole sensor. This is relevant when photographing a grey card filling the whole frame or a landscape.

However, It is the diameter of the entrance pupil that defines "light gathering ability from the subject, provided the subject is completely within the frame". This is relevant when the moon or a duck is taking a small portion of the frame. In this case we are cropping anyway, so the size of the sensor is not relevant.

Dem Bell said:

Myself being someone who enjoys astrophotography sometimes, it's a great thing I know about focal ratio, but it's also a reason why I tend to go with lens of a long focal length rather than a wider lens, even if their apertures are the same.

Click to expand...

People use "aperture" for both "aperture diameter" and "aperture setting", hence the confusion. It is less important what you call it, if it is clear if you need units of length or not to measure it.

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Thank you, your answer seems like the most concise. To anyone wondering, I've become accustomed w/using Focal Ratio because I've been learning to use astronomical optics e.g. telescopes. Even then, telescopes have an aperture value.

Tom Axford said:

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about.

Click to expand...

In photography, "aperture" usually means f-number (also called f-stop), defined as the focal length divided by the entrance pupil diameter.

I can't recall seeing the term "focal ratio". I have no idea what it means. How would you define it?

Click to expand...

It's a common term in astronomy and a reference to the equation, f/#. One subtle difference that comes immediately to mind between f-stop in photography and focal ratio in astronomy, is that the aperture of many astronomical telescopes is the physical diameter of the primary optic while, in photography, the equation, f/#, describes the diameter of a virtual entrance pupil.

Either way, both phrases describe the light-gathering ability of the optic. The telescope aperture or lens virtual entrance pupil determine how much light energy is collected from any subject in the field of view. More formally, they're factors in Entendue.

There are other similarities, such as noise affecting the view at an eyepiece and noise in photo are largely determined by total light gathered.

Bill Ferris said:

Tom Axford said:

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about.

Click to expand...

In photography, "aperture" usually means f-number (also called f-stop), defined as the focal length divided by the entrance pupil diameter.

I can't recall seeing the term "focal ratio". I have no idea what it means. How would you define it?

Click to expand...

One subtle difference that comes immediately to mind between f-stop in photography and focal ratio in astronomy, is that the aperture of many astronomical telescopes is the physical diameter of the primary optic while, in photography, the equation, f/#, describes the diameter of a virtual entrance pupil.

Click to expand...

It's not different at all. It's the same thing. In an astronomical telescope the diameter of the virtual entrance pupil is the diameter of the objective.

Bill Ferris said:

Either way, both phrases describe the light-gathering ability of the optic. The telescope aperture or lens virtual entrance pupil determine how much light energy is collected from any subject in the field of view. More formally, they're factors in Entendue.

There are other similarities, such as noise affecting the view at an eyepiece and noise in photo are largely determined by total light gathered.

Click to expand...

MacM545 said:

Myself being someone who enjoys astrophotography sometimes, it's a great thing I know about focal ratio, but it's also a reason why I tend to go with lens of a long focal length rather than a wider lens, even if their apertures are the same.

Click to expand...

I don't know why you would do that. One chooses the focal length for the field of view, not for the aperture.

Bill Ferris said:

Tom Axford said:

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about.

Click to expand...

In photography, "aperture" usually means f-number (also called f-stop), defined as the focal length divided by the entrance pupil diameter.

I can't recall seeing the term "focal ratio". I have no idea what it means. How would you define it?

Click to expand...

It's a common term in astronomy and a reference to the equation, f/#. One subtle difference that comes immediately to mind between f-stop in photography and focal ratio in astronomy, is that the aperture of many astronomical telescopes is the physical diameter of the primary optic while, in photography, the equation, f/#, describes the diameter of a virtual entrance pupil.

Click to expand...

It is the same thing. The entrance pupil is a virtual image of the opening (aperture) in the optical device. If you look down the telescope, you will see the front element and no diaphragm mechanism or obstructions behind it. The physics is exactly the same.

ThrillaMozilla said:

Bill Ferris said:

Tom Axford said:

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about.

Click to expand...

In photography, "aperture" usually means f-number (also called f-stop), defined as the focal length divided by the entrance pupil diameter.

I can't recall seeing the term "focal ratio". I have no idea what it means. How would you define it?

Click to expand...

One subtle difference that comes immediately to mind between f-stop in photography and focal ratio in astronomy, is that the aperture of many astronomical telescopes is the physical diameter of the primary optic while, in photography, the equation, f/#, describes the diameter of a virtual entrance pupil.

Click to expand...

It's not different at all. It's the same thing. In an astronomical telescope the diameter of the virtual entrance pupil is the diameter of the objective.

Click to expand...

Hence, my choice of the word, "subtle," as a reference to optical systems having the same light-gathering capacity. A difference in the physical aperture sizes would not change that.

ThrillaMozilla said:

ThrillaMozilla said:

Either way, both phrases describe the light-gathering ability of the optic. The telescope aperture or lens virtual entrance pupil determine how much light energy is collected from any subject in the field of view. More formally, they're factors in Entendue.

There are other similarities, such as noise affecting the view at an eyepiece and noise in photo are largely determined by total light gathered.

Click to expand...

Click to expand...

The principles of the f-number system hold true for camera and optical telescope. We are talking “focal ratio”, a value derived by dividing the focal length of lens by the working diameter of optical system. The key word here is “ratio” because a ratio is dimensionless. In other words, we are comparing two or more quantities using a value that is a pure number, the differences in dimension having been canceled out by mathematics used.

As an example, how do the following two optical systems compare as to their light gathering capability? System A 100mm focal length, with a 25mm working diameter compared to system B with a focal length of 300 inches with a working diameter of 75 inches?

Calculate the focal ratios of both: A = 100 ÷ 25 = 4. B = 300 ÷ 75 = 4. The focal ratio (f-number) being pure and dimensionless is f/4 for both systems. The light gathering power is effectively the same for both. We cast out minor accountability like differing transparencies and flare. If higher accuracy is required, we exchange T-stop for f-stop. The f-stop is simply the ratio, the T-stop value is measured using a photometer.

However, there are several key differences between pictorial photography instruments and astronomical instruments. When dealing with focused point sources such as stars, image brightness is not affected by the system’s focal ratio. Focal ratio applies to objects that have area when viewed or imaged. Affected are galaxies, nebula, comets. Stars are exempt as their light rays arrive in bundles of parallel rays.

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about. A 50mm F/1.4 & 100mm F/1.4 for example; the 100mm can let in more let despite the same aperture. It seems to me like people think F/stop is the main deciding factor in light gathering ability. Myself being someone who enjoys astrophotography sometimes, it's a great thing I know about focal ratio, but it's also a reason why I tend to go with lens of a long focal length rather than a wider lens, even if their apertures are the same.

Click to expand...

If, as stated by other people than the OP, the focal ratio is the focal length over the diameter of the objective, the differences between focal ratio and f-stop are not important in most circumstances. With complex lens designs, f-stop is, to me, the preferred metric.

--
https://blog.kasson.com

A Marcus said:

The principles of the f-number system hold true for camera and optical telescope. We are talking “focal ratio”, a value derived by dividing the focal length of lens by the working diameter of optical system. The key word here is “ratio” because a ratio is dimensionless. In other words, we are comparing two or more quantities using a value that is a pure number, the differences in dimension having been canceled out by mathematics used.

As an example, how do the following two optical systems compare as to their light gathering capability? System A 100mm focal length, with a 25mm working diameter compared to system B with a focal length of 300 inches with a working diameter of 75 inches?

Calculate the focal ratios of both: A = 100 ÷ 25 = 4. B = 300 ÷ 75 = 4. The focal ratio (f-number) being pure and dimensionless is f/4 for both systems. The light gathering power is effectively the same for both.

Click to expand...

The optic with the 75-inch working aperture collects more light from any subject within the field of view of both instruments. Within the context of an astronomical telescope and assuming perfect optics, both systems collect and project the same light energy per unit area upon an eye pupil or imaging sensor. However, the 300-inch f/4 optic is collecting light from a subject occupying 9x the area within the FOV and pojecting an image in which the subject occupies 9x the area of that projected by the shorter focal length system.

The net result is that the larger aperture system produces an image - of an emission nebula, for instance - in which the subject appears larger and more detailed. It may even appear more colorful. This is due to the fact that the larger aperture telescope collects more total light from that nebula.

A Marcus said:

We cast out minor accountability like differing transparencies and flare. If higher accuracy is required, we exchange T-stop for f-stop. The f-stop is simply the ratio, the T-stop value is measured using a photometer.

However, there are several key differences between pictorial photography instruments and astronomical instruments. When dealing with focused point sources such as stars, image brightness is not affected by the system’s focal ratio. Focal ratio applies to objects that have area when viewed or imaged. Affected are galaxies, nebula, comets. Stars are exempt as their light rays arrive in bundles of parallel rays.

Click to expand...

However, aperture does affect both point sources and extended objects. No optical telescope used for visual observing presents an image of any celestial object in which that object has a greater apparent surface brightness (i.e. light energy per unit area) than that object has when observed with the naked eye.

However, increased aperture and the greater total light energy collected from the object make stars appear brighter in the eyepiece and allow low contrast extended objects and details to be discerned.

A Marcus said:

The principles of the f-number system hold true for camera and optical telescope. We are talking “focal ratio”, a value derived by dividing the focal length of lens by the working diameter of optical system. The key word here is “ratio” because a ratio is dimensionless. In other words, we are comparing two or more quantities using a value that is a pure number, the differences in dimension having been canceled out by mathematics used.

As an example, how do the following two optical systems compare as to their light gathering capability? System A 100mm focal length, with a 25mm working diameter compared to system B with a focal length of 300 inches with a working diameter of 75 inches?

Calculate the focal ratios of both: A = 100 ÷ 25 = 4. B = 300 ÷ 75 = 4. The focal ratio (f-number) being pure and dimensionless is f/4 for both systems. The light gathering power is effectively the same for both.

Click to expand...

It depends on how one quantifies it. The total amount of light gathered from a given nebula over the same exposure time will be different, while luminous exposure at a pixel level will be the same.

A Marcus said:

We cast out minor accountability like differing transparencies and flare. If higher accuracy is required, we exchange T-stop for f-stop. The f-stop is simply the ratio, the T-stop value is measured using a photometer.

However, there are several key differences between pictorial photography instruments and astronomical instruments. When dealing with focused point sources such as stars, image brightness is not affected by the system’s focal ratio. Focal ratio applies to objects that have area when viewed or imaged. Affected are galaxies, nebula, comets. Stars are exempt as their light rays arrive in bundles of parallel rays.

Click to expand...

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about. A 50mm F/1.4 & 100mm F/1.4 for example; the 100mm can let in more let despite the same aperture. It seems to me like people think F/stop is the main deciding factor in light gathering ability. Myself being someone who enjoys astrophotography sometimes, it's a great thing I know about focal ratio, but it's also a reason why I tend to go with lens of a long focal length rather than a wider lens, even if their apertures are the same.

Click to expand...

But the 50/1.4 'collects' the light from a bigger area.

jiberlin said:

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about. A 50mm F/1.4 & 100mm F/1.4 for example; the 100mm can let in more let despite the same aperture. It seems to me like people think F/stop is the main deciding factor in light gathering ability. Myself being someone who enjoys astrophotography sometimes, it's a great thing I know about focal ratio, but it's also a reason why I tend to go with lens of a long focal length rather than a wider lens, even if their apertures are the same.

Click to expand...

But the 50/1.4 'collects' the light from a bigger area.

Click to expand...

The 50mm f/1.4 lens has a (50mm/1.4=35.71mm) 36mm diameter entrance pupil. The 100mm f/1.4 lens has a 71mm diameter entrance pupil. The longer lens, used at the same distance in the same light with the same settings, collects 4x as much light from any subject in the frame of both lenses.

Both lenses project the same exposure (light energy per unit area upon the plate, film negative, or sensor) but the longer lens with the larger entrance pupil projects significantly more light from any person, bird or tree in the frame.

jiberlin said:

MacM545 said:

We usually talk about aperture (or should I use the term F/stop?) & T stop, but focal ratio is rarely talked about. A 50mm F/1.4 & 100mm F/1.4 for example; the 100mm can let in more let despite the same aperture. It seems to me like people think F/stop is the main deciding factor in light gathering ability. Myself being someone who enjoys astrophotography sometimes, it's a great thing I know about focal ratio, but it's also a reason why I tend to go with lens of a long focal length rather than a wider lens, even if their apertures are the same.

Click to expand...

But the 50/1.4 'collects' the light from a bigger area.

Click to expand...

It does not. The area from which light is collected depends entirely on how the lens is used (the angle of view being used). A 100mm lens on a large format camera may use a much larger angle of view than a 50mm lens on Micro Four Thirds, for example.