A levels

[D Cox]

The amount of light in an image is determined by three things - the luminance of the scene, the size of the aperture and the angle of view. As you expand the angle of view the amount of light projected onto the image plane increases. This is inconvenient when you have a sensitive medium that requires a given amount of light, since it means that you have to reset the camera as the angle of view changes. Therefore it has become customary to express the aperture diameter in terms of the angle of view when it comes to labelling settings, though we use the focal length of the lens as a proxy for angle of view. So, rather than setting the aperture in millimetres, we set it as a fraction of the focal length. As an example, if we had a 50mm lens and wanted an aperture of 25mm, we would choose a setting of f/2, where 'f' stands for the focal length, in this case 50mm.

This convention has served photography well for many years but with the advent of digital has been somewhat abused by the manufacturers for marketing reasons. In the digital world we have a much greater variety of active frame sizes in the mass market than with film, which was constrained by availability of processing services in a way that digital isn't.

I don't think this is true. Film went all the way from Minox up to 10x8 inches or even larger. There are no large format digital cameras for general photography, and even the so-called "medium format" sensors are barely larger than 35mm.

The manufacturers like to express their focal lengths in equivalents but then fail to do the same for the aperture.

Aren't f numbers an "equivalent" scale ? I haven't seen any current lens with apertures marked in mm or inches.

So, to take the example above, a 35mm camera with a 50mm lens gives an angle of view of 46.8 degrees. A lens described as 'f/2' will have a value of 'f' of 50mm, giving an aperture of 25mm. Now if we compare with a 1/2.4" camera. A lens giving the same angle of view, 46.8 degrees, which would be described by the manufacturer as '50mm equivalent' or quite often just '50mm' will actually have a focal length of 8.9mm, and if it has an aperture of 'f/2' the 'f' is now 8.9mm, so the aperture is actually 4.5mm. This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

Yes, it would be best to stop talking about "equivalent focal lengths". It confuses everyone. The angle of view of a lens should be shown as, for example, 45 degrees on a 36mm wide sensor. (Or use diagonals as for TV screens.) The true focal length should be given too.

This would not be important except for the fact that it is the amount of light admitted that primarily determines image quality, so many consumers are being conned into thinking that they will get higher image quality than they will in practice.

So, while not for a moment expecting this to happen, I would like to propose a different method of expressing aperture which meets the original objectives of expressing by focal length but is not similarly subject to abuse. I propose that instead of expressing aperture in terms of focal length, we do so directly in terms of the angle of view. Instead of an 'f-number' we have an 'a level'. So, for instance in the cases above the 50mm lens used on FF would be said to have an aperture of a/1.9, where a is the angle of view (diagonal, in degrees) whilst the 1/2.4" camera would be said to have an aperture of a/10.4

Two lenses can have the same angle of view but different focal lengths. For instance, a 150mm lens is "normal" for 5x4 and a 45mm lens is "normal" for 24x36mm.

Or, they can have the same focal length but different angles of view. A 150mm lens could be designed as a "normal" lens for 5x4, and another 150mm lens could be designed as a long lens for 24x36mm.

A longer lens spreads the light from a given aperture (for instance 12mm diameter) out over a larger area, so the exposure is less. It is the same as moving a projector further from the screen. The point of f numbers is to have a figure on the lens that gives the same exposure regardless of focal length.

 

[D Cox]

This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

The focal length is generally expressed as "50mm equivalent"

I think that the 'equivalent' frequently gets lost or relegated to the very fine print, so far as the manufacturers are concerned.

which is not the same as "50mm", so its not really all that confusing. I doubt many people expect their P&S to admit the same amount of light as a 35mm FF format body.

I think you would be surprised. There are plenty of discussions here where it is clear that very many have precisely that expectation.

in that case, I suspect your new proposed system would only confuse them even more. Or, we mandate the requirement that anyone buying a non-FF format camera must attend a one-week course on "equivalence" before the purchase can be completed.

Probably only those who buy an M4/3 camera need to attend.

 

[D Cox]

This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

The focal length is generally expressed as "50mm equivalent"

I think that the 'equivalent' frequently gets lost or relegated to the very fine print, so far as the manufacturers are concerned.

which is not the same as "50mm", so its not really all that confusing. I doubt many people expect their P&S to admit the same amount of light as a 35mm FF format body.

I think you would be surprised. There are plenty of discussions here where it is clear that very many have precisely that expectation.

in that case, I suspect your new proposed system would only confuse them even more.

There is no reason that it should.

Or, we mandate the requirement that anyone buying a non-FF format camera must attend a one-week course on "equivalence" before the purchase can be completed.

On the contrary, my system removes the need to know about 'equivalence' entirely.

There was never any need in the first place.

It is a hobby for theologians.

 

[Tulsaghost]

This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

The focal length is generally expressed as "50mm equivalent"

I think that the 'equivalent' frequently gets lost or relegated to the very fine print, so far as the manufacturers are concerned.

which is not the same as "50mm", so its not really all that confusing. I doubt many people expect their P&S to admit the same amount of light as a 35mm FF format body.

I think you would be surprised. There are plenty of discussions here where it is clear that very many have precisely that expectation.

in that case, I suspect your new proposed system would only confuse them even more.

There is no reason that it should.

Or, we mandate the requirement that anyone buying a non-FF format camera must attend a one-week course on "equivalence" before the purchase can be completed.

On the contrary, my system removes the need to know about 'equivalence' entirely.

There was never any need in the first place.

It is a hobby for theologians.

How many photographers can dance on the head of a pin?

Chuck

 

[mosswings]

This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

The focal length is generally expressed as "50mm equivalent"

I think that the 'equivalent' frequently gets lost or relegated to the very fine print, so far as the manufacturers are concerned.

which is not the same as "50mm", so its not really all that confusing. I doubt many people expect their P&S to admit the same amount of light as a 35mm FF format body.

I think you would be surprised. There are plenty of discussions here where it is clear that very many have precisely that expectation.

in that case, I suspect your new proposed system would only confuse them even more.

There is no reason that it should.

Or, we mandate the requirement that anyone buying a non-FF format camera must attend a one-week course on "equivalence" before the purchase can be completed.

On the contrary, my system removes the need to know about 'equivalence' entirely.

There was never any need in the first place.

It is a hobby for theologians.

How many photographers can dance on the head of a pin?

Chuck

Depends on the size of their cameras.

 

[Tulsaghost]

This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

The focal length is generally expressed as "50mm equivalent"

I think that the 'equivalent' frequently gets lost or relegated to the very fine print, so far as the manufacturers are concerned.

which is not the same as "50mm", so its not really all that confusing. I doubt many people expect their P&S to admit the same amount of light as a 35mm FF format body.

I think you would be surprised. There are plenty of discussions here where it is clear that very many have precisely that expectation.

in that case, I suspect your new proposed system would only confuse them even more.

There is no reason that it should.

Or, we mandate the requirement that anyone buying a non-FF format camera must attend a one-week course on "equivalence" before the purchase can be completed.

On the contrary, my system removes the need to know about 'equivalence' entirely.

There was never any need in the first place.

It is a hobby for theologians.

How many photographers can dance on the head of a pin?

Chuck

Depends on the size of their cameras.

So less for FF?

 

[techjedi]

The amount of light in an image is determined by three things - the luminance of the scene, the size of the aperture and the angle of view. As you expand the angle of view the amount of light projected onto the image plane increases. This is inconvenient when you have a sensitive medium that requires a given amount of light, since it means that you have to reset the camera as the angle of view changes. Therefore it has become customary to express the aperture diameter in terms of the angle of view when it comes to labelling settings, though we use the focal length of the lens as a proxy for angle of view. So, rather than setting the aperture in millimetres, we set it as a fraction of the focal length. As an example, if we had a 50mm lens and wanted an aperture of 25mm, we would choose a setting of f/2, where 'f' stands for the focal length, in this case 50mm.

This convention has served photography well for many years but with the advent of digital has been somewhat abused by the manufacturers for marketing reasons. In the digital world we have a much greater variety of active frame sizes in the mass market than with film, which was constrained by availability of processing services in a way that digital isn't. The manufacturers like to express their focal lengths in equivalents but then fail to do the same for the aperture. So, to take the example above, a 35mm camera with a 50mm lens gives an angle of view of 46.8 degrees. A lens described as 'f/2' will have a value of 'f' of 50mm, giving an aperture of 25mm. Now if we compare with a 1/2.4" camera. A lens giving the same angle of view, 46.8 degrees, which would be described by the manufacturer as '50mm equivalent' or quite often just '50mm' will actually have a focal length of 8.9mm, and if it has an aperture of 'f/2' the 'f' is now 8.9mm, so the aperture is actually 4.5mm. This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

This would not be important except for the fact that it is the amount of light admitted that primarily determines image quality, so many consumers are being conned into thinking that they will get higher image quality than they will in practice.

So, while not for a moment expecting this to happen, I would like to propose a different method of expressing aperture which meets the original objectives of expressing by focal length but is not similarly subject to abuse. I propose that instead of expressing aperture in terms of focal length, we do so directly in terms of the angle of view. Instead of an 'f-number' we have an 'a level'. So, for instance in the cases above the 50mm lens used on FF would be said to have an aperture of a/1.9, where a is the angle of view (diagonal, in degrees) whilst the 1/2.4" camera would be said to have an aperture of a/10.4

I have often had similar thoughts, albeit probably from a more naïve experience in photography.

In terms of marketing or correctness, if you were to start labeling lenses by angle of view, what happens to the angle of view for the same lens mounted to different sensors sizes? Is the angle of view unchanged when the crop is applied on a smaller sensor?

 

[Great Bustard]

This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

The focal length is generally expressed as "50mm equivalent"

I think that the 'equivalent' frequently gets lost or relegated to the very fine print, so far as the manufacturers are concerned.

which is not the same as "50mm", so its not really all that confusing. I doubt many people expect their P&S to admit the same amount of light as a 35mm FF format body.

I think you would be surprised. There are plenty of discussions here where it is clear that very many have precisely that expectation.

in that case, I suspect your new proposed system would only confuse them even more.

There is no reason that it should.

Or, we mandate the requirement that anyone buying a non-FF format camera must attend a one-week course on "equivalence" before the purchase can be completed.

On the contrary, my system removes the need to know about 'equivalence' entirely.

There was never any need in the first place.

There was never any need to know that the Sun, rather than the Earth, was the center of the Solar System for 99.9%+ of the population, either.

It is a hobby for theologians.

Denying the facts? Absolutely.

 

[bobn2]

The amount of light in an image is determined by three things - the luminance of the scene, the size of the aperture and the angle of view. As you expand the angle of view the amount of light projected onto the image plane increases. This is inconvenient when you have a sensitive medium that requires a given amount of light, since it means that you have to reset the camera as the angle of view changes. Therefore it has become customary to express the aperture diameter in terms of the angle of view when it comes to labelling settings, though we use the focal length of the lens as a proxy for angle of view. So, rather than setting the aperture in millimetres, we set it as a fraction of the focal length. As an example, if we had a 50mm lens and wanted an aperture of 25mm, we would choose a setting of f/2, where 'f' stands for the focal length, in this case 50mm.

This convention has served photography well for many years but with the advent of digital has been somewhat abused by the manufacturers for marketing reasons. In the digital world we have a much greater variety of active frame sizes in the mass market than with film, which was constrained by availability of processing services in a way that digital isn't.

I don't think this is true. Film went all the way from Minox up to 10x8 inches or even larger. There are no large format digital cameras for general photography, and even the so-called "medium format" sensors are barely larger than 35mm.

I do try to put the necessary qualifiers in. I think that the relevant one here was 'mass-market'. The users of more esoteric film sizes generally did their own processing and were fully cognisant of the relationship between aperture, angle of view and image quality.

The manufacturers like to express their focal lengths in equivalents but then fail to do the same for the aperture.

Aren't f numbers an "equivalent" scale ? I haven't seen any current lens with apertures marked in mm or inches.

It's a 'relative' scale, not an 'equivalent' one. f-numbers are strictly the reciprocal of the relative aperture - as in 'the aperture is f/2.8', the aperture is expressed relative to the focal length ('f'). In this example the 'relative aperture' is 1/2.8 (more commonly written 1:2.8 as you see on the front ring of a lens) and the 'f-number' is 2.8. My proposal is also a 'relative aperture', but the aperture is given relative to the angle of view.

So, to take the example above, a 35mm camera with a 50mm lens gives an angle of view of 46.8 degrees. A lens described as 'f/2' will have a value of 'f' of 50mm, giving an aperture of 25mm. Now if we compare with a 1/2.4" camera. A lens giving the same angle of view, 46.8 degrees, which would be described by the manufacturer as '50mm equivalent' or quite often just '50mm' will actually have a focal length of 8.9mm, and if it has an aperture of 'f/2' the 'f' is now 8.9mm, so the aperture is actually 4.5mm. This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

Yes, it would be best to stop talking about "equivalent focal lengths". It confuses everyone. The angle of view of a lens should be shown as, for example, 45 degrees on a 36mm wide sensor. (Or use diagonals as for TV screens.) The true focal length should be given too.

Yes, my proposal is to move to angles of view and angle-of-view-relative apertures. So for a FF 35mm camera with a 50mm lens with a 25mm aperture, you would quote it as a '46.8° a/1.9' and for a 1/2.3" camera with a 8.9mm lens, you would quote it as a '46.8° a/10.4'. I expect in the unlikely case of this ever being taken seriously, we would use more convenient values, so things would settle on an a/2 and a/11 apertures, with the sequence going in stops pretty much as it does for f-numbers.

I'm not sure why, when this system is in place, anyone would need to know the actual focal length.

This would not be important except for the fact that it is the amount of light admitted that primarily determines image quality, so many consumers are being conned into thinking that they will get higher image quality than they will in practice.

So, while not for a moment expecting this to happen, I would like to propose a different method of expressing aperture which meets the original objectives of expressing by focal length but is not similarly subject to abuse. I propose that instead of expressing aperture in terms of focal length, we do so directly in terms of the angle of view. Instead of an 'f-number' we have an 'a level'. So, for instance in the cases above the 50mm lens used on FF would be said to have an aperture of a/1.9, where a is the angle of view (diagonal, in degrees) whilst the 1/2.4" camera would be said to have an aperture of a/10.4

Two lenses can have the same angle of view but different focal lengths. For instance, a 150mm lens is "normal" for 5x4 and a 45mm lens is "normal" for 24x36mm.

Exactly, and my system conveniently takes account of that. If you mount a lens on a different camera, the AoV and a-number will change, but then they document what the lens does, not what it is.

Or, they can have the same focal length but different angles of view. A 150mm lens could be designed as a "normal" lens for 5x4, and another 150mm lens could be designed as a long lens for 24x36mm.

A longer lens spreads the light from a given aperture (for instance 12mm diameter) out over a larger area, so the exposure is less. It is the same as moving a projector further from the screen. The point of f numbers is to have a figure on the lens that gives the same exposure regardless of focal length.

Yes, you have to stop thinking about 'exposure', an image field concept and instead think about 'insposure', its object field equivalent. The ISO Exposure Index will have to change too, so we will need an Insposure Index (II instead of EI) and hopefully ISO will develop the relevant standard so we can call it 'ISO'. So with a suitable constant one could arrange for an II of 100 on 35mm FF to correspond to an EI of 100. In the example above, if both cameras were engineered in the same way as currently, the FF camera would have an II range of 100 upwards, while the 1/2.3" camera would have an II of 3200 upwards.

 

[bobn2]

The amount of light in an image is determined by three things - the luminance of the scene, the size of the aperture and the angle of view. As you expand the angle of view the amount of light projected onto the image plane increases. This is inconvenient when you have a sensitive medium that requires a given amount of light, since it means that you have to reset the camera as the angle of view changes. Therefore it has become customary to express the aperture diameter in terms of the angle of view when it comes to labelling settings, though we use the focal length of the lens as a proxy for angle of view. So, rather than setting the aperture in millimetres, we set it as a fraction of the focal length. As an example, if we had a 50mm lens and wanted an aperture of 25mm, we would choose a setting of f/2, where 'f' stands for the focal length, in this case 50mm.

This convention has served photography well for many years but with the advent of digital has been somewhat abused by the manufacturers for marketing reasons. In the digital world we have a much greater variety of active frame sizes in the mass market than with film, which was constrained by availability of processing services in a way that digital isn't. The manufacturers like to express their focal lengths in equivalents but then fail to do the same for the aperture. So, to take the example above, a 35mm camera with a 50mm lens gives an angle of view of 46.8 degrees. A lens described as 'f/2' will have a value of 'f' of 50mm, giving an aperture of 25mm. Now if we compare with a 1/2.4" camera. A lens giving the same angle of view, 46.8 degrees, which would be described by the manufacturer as '50mm equivalent' or quite often just '50mm' will actually have a focal length of 8.9mm, and if it has an aperture of 'f/2' the 'f' is now 8.9mm, so the aperture is actually 4.5mm. This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

This would not be important except for the fact that it is the amount of light admitted that primarily determines image quality, so many consumers are being conned into thinking that they will get higher image quality than they will in practice.

So, while not for a moment expecting this to happen, I would like to propose a different method of expressing aperture which meets the original objectives of expressing by focal length but is not similarly subject to abuse. I propose that instead of expressing aperture in terms of focal length, we do so directly in terms of the angle of view. Instead of an 'f-number' we have an 'a level'. So, for instance in the cases above the 50mm lens used on FF would be said to have an aperture of a/1.9, where a is the angle of view (diagonal, in degrees) whilst the 1/2.4" camera would be said to have an aperture of a/10.4

I have often had similar thoughts, albeit probably from a more naïve experience in photography.

I don't think it's naïve, the current system is clearly broken.

In terms of marketing or correctness, if you were to start labeling lenses by angle of view, what happens to the angle of view for the same lens mounted to different sensors sizes? Is the angle of view unchanged when the crop is applied on a smaller sensor?

Obviously not, the angle of view changes as you mount the lens on different crops, but this is what is really happening. Quoting the AoV is preferable to an 'equivalent focal length', IMO.

 

[bobn2]

This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

The focal length is generally expressed as "50mm equivalent"

I think that the 'equivalent' frequently gets lost or relegated to the very fine print, so far as the manufacturers are concerned.

which is not the same as "50mm", so its not really all that confusing. I doubt many people expect their P&S to admit the same amount of light as a 35mm FF format body.

I think you would be surprised. There are plenty of discussions here where it is clear that very many have precisely that expectation.

in that case, I suspect your new proposed system would only confuse them even more.

There is no reason that it should.

Or, we mandate the requirement that anyone buying a non-FF format camera must attend a one-week course on "equivalence" before the purchase can be completed.

On the contrary, my system removes the need to know about 'equivalence' entirely.

There was never any need in the first place.

If that's your opinion then you should enthusiastically support my proposal, which removes the need even to debate equivalence.

It is a hobby for theologians.

And people who'd like to be able to predict what their photo will look like.

 

[bobn2]

This would not be important except for the fact that it is the amount of light admitted that primarily determines image quality,

An extraordinary claim! Reference, please.

One must include the proper qualifier of "all else equal".

I thought that the qualifier 'primarily' was sufficient.

I don't think "primarily" is enough. For example, ignoring DOF considerations, the SIgma 50 / 1.4A on my 6D at f/2.8 likely has "better" IQ than the Canon 50 / 1.4 at f/2, even though the latter puts twice as much light on the sensor for a given exposure time.

OK, I was assuming a definition of 'IQ' orthogonal from lens effect, mainly about noise.

so many consumers are being conned into thinking that they will get higher image quality than they will in practice.

Your concern for cropped sensor consumers is quite touching, but the statement above also applies to fullframe consumers who are also prone to ignoring the implications of equivalence (as you well know from your struggles with them). I suspect that your a-number solution will increase unwarranted smugness on the fullframe side of the ledger as much as it reduces unwarranted optimism on the cropped side.

If one were learning photography from the start, thinking in terms of angle of view and aperture diameter instead of focal length and relative aperture would be the most simple, as well as using LS (lumen · seconds -- total light) instead of EV (1 + log2 (lux/5) -- exposure / time).

The only problem being that the total amount of light for a given aperture diameter changes as the angle of view changes, which is inconvenient if you're setting things manually.

Absolutely. But this can be dealt with easily by various auto exposure modes.

I think that is a cop out. With my system of angle-of-view-relative apertures, you are set fro manual modes too. Really, that has to be the baseline.

That's why we use 'relative aperture', it's aperture relative to the focal length (in fact we tend to use 'f-number' in American/Japanese usage, which is the reciprocal of the relative aperture). My proposal is also a 'relative' aperture, it's just aperture relative to the angle of view, rather than the focal length, so effectively it has factored the sensor size in.

Sure.

However, once one is entrenched in focal length and relative aperture, probably best just to use the Equivalence Ratio (Crop Factor) to relate different formats.

But if one had the chance to start again...

You'd have taken more nude pics of ex-girlfriends? ;-)

I think I took enough. It was the publishing that made them 'ex'.

 

[techjedi]

The amount of light in an image is determined by three things - the luminance of the scene, the size of the aperture and the angle of view. As you expand the angle of view the amount of light projected onto the image plane increases. This is inconvenient when you have a sensitive medium that requires a given amount of light, since it means that you have to reset the camera as the angle of view changes. Therefore it has become customary to express the aperture diameter in terms of the angle of view when it comes to labelling settings, though we use the focal length of the lens as a proxy for angle of view. So, rather than setting the aperture in millimetres, we set it as a fraction of the focal length. As an example, if we had a 50mm lens and wanted an aperture of 25mm, we would choose a setting of f/2, where 'f' stands for the focal length, in this case 50mm.

This convention has served photography well for many years but with the advent of digital has been somewhat abused by the manufacturers for marketing reasons. In the digital world we have a much greater variety of active frame sizes in the mass market than with film, which was constrained by availability of processing services in a way that digital isn't. The manufacturers like to express their focal lengths in equivalents but then fail to do the same for the aperture. So, to take the example above, a 35mm camera with a 50mm lens gives an angle of view of 46.8 degrees. A lens described as 'f/2' will have a value of 'f' of 50mm, giving an aperture of 25mm. Now if we compare with a 1/2.4" camera. A lens giving the same angle of view, 46.8 degrees, which would be described by the manufacturer as '50mm equivalent' or quite often just '50mm' will actually have a focal length of 8.9mm, and if it has an aperture of 'f/2' the 'f' is now 8.9mm, so the aperture is actually 4.5mm. This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

This would not be important except for the fact that it is the amount of light admitted that primarily determines image quality, so many consumers are being conned into thinking that they will get higher image quality than they will in practice.

So, while not for a moment expecting this to happen, I would like to propose a different method of expressing aperture which meets the original objectives of expressing by focal length but is not similarly subject to abuse. I propose that instead of expressing aperture in terms of focal length, we do so directly in terms of the angle of view. Instead of an 'f-number' we have an 'a level'. So, for instance in the cases above the 50mm lens used on FF would be said to have an aperture of a/1.9, where a is the angle of view (diagonal, in degrees) whilst the 1/2.4" camera would be said to have an aperture of a/10.4

I have often had similar thoughts, albeit probably from a more naïve experience in photography.

I don't think it's naïve, the current system is clearly broken.

In terms of marketing or correctness, if you were to start labeling lenses by angle of view, what happens to the angle of view for the same lens mounted to different sensors sizes? Is the angle of view unchanged when the crop is applied on a smaller sensor?

Obviously not, the angle of view changes as you mount the lens on different crops, but this is what is really happening. Quoting the AoV is preferable to an 'equivalent focal length', IMO.

What about some metric of "perspective compression". Wouldn't that be unchanged with he same lens regardless if the image is cropped?

 

[tex]

There's a lot of important information and discussion that is scattered through the forums about this topic over the last ....decade maybe? I've only seen what's on the forums I visit or used to visit: Oly, Sony, Pentax, and Open. There must be tons of info on the Canon and Nikon forums. Easily enough to have a separate forum for all the threads from all the forums....

 

[bobn2]

The amount of light in an image is determined by three things - the luminance of the scene, the size of the aperture and the angle of view. As you expand the angle of view the amount of light projected onto the image plane increases. This is inconvenient when you have a sensitive medium that requires a given amount of light, since it means that you have to reset the camera as the angle of view changes. Therefore it has become customary to express the aperture diameter in terms of the angle of view when it comes to labelling settings, though we use the focal length of the lens as a proxy for angle of view. So, rather than setting the aperture in millimetres, we set it as a fraction of the focal length. As an example, if we had a 50mm lens and wanted an aperture of 25mm, we would choose a setting of f/2, where 'f' stands for the focal length, in this case 50mm.

This convention has served photography well for many years but with the advent of digital has been somewhat abused by the manufacturers for marketing reasons. In the digital world we have a much greater variety of active frame sizes in the mass market than with film, which was constrained by availability of processing services in a way that digital isn't. The manufacturers like to express their focal lengths in equivalents but then fail to do the same for the aperture. So, to take the example above, a 35mm camera with a 50mm lens gives an angle of view of 46.8 degrees. A lens described as 'f/2' will have a value of 'f' of 50mm, giving an aperture of 25mm. Now if we compare with a 1/2.4" camera. A lens giving the same angle of view, 46.8 degrees, which would be described by the manufacturer as '50mm equivalent' or quite often just '50mm' will actually have a focal length of 8.9mm, and if it has an aperture of 'f/2' the 'f' is now 8.9mm, so the aperture is actually 4.5mm. This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

This would not be important except for the fact that it is the amount of light admitted that primarily determines image quality, so many consumers are being conned into thinking that they will get higher image quality than they will in practice.

So, while not for a moment expecting this to happen, I would like to propose a different method of expressing aperture which meets the original objectives of expressing by focal length but is not similarly subject to abuse. I propose that instead of expressing aperture in terms of focal length, we do so directly in terms of the angle of view. Instead of an 'f-number' we have an 'a level'. So, for instance in the cases above the 50mm lens used on FF would be said to have an aperture of a/1.9, where a is the angle of view (diagonal, in degrees) whilst the 1/2.4" camera would be said to have an aperture of a/10.4

I have often had similar thoughts, albeit probably from a more naïve experience in photography.

I don't think it's naïve, the current system is clearly broken.

In terms of marketing or correctness, if you were to start labeling lenses by angle of view, what happens to the angle of view for the same lens mounted to different sensors sizes? Is the angle of view unchanged when the crop is applied on a smaller sensor?

Obviously not, the angle of view changes as you mount the lens on different crops, but this is what is really happening. Quoting the AoV is preferable to an 'equivalent focal length', IMO.

What about some metric of "perspective compression". Wouldn't that be unchanged with he same lens regardless if the image is cropped?

'Perspective compression' goes with angle of view and subject distance.

 

[techjedi]

The amount of light in an image is determined by three things - the luminance of the scene, the size of the aperture and the angle of view. As you expand the angle of view the amount of light projected onto the image plane increases. This is inconvenient when you have a sensitive medium that requires a given amount of light, since it means that you have to reset the camera as the angle of view changes. Therefore it has become customary to express the aperture diameter in terms of the angle of view when it comes to labelling settings, though we use the focal length of the lens as a proxy for angle of view. So, rather than setting the aperture in millimetres, we set it as a fraction of the focal length. As an example, if we had a 50mm lens and wanted an aperture of 25mm, we would choose a setting of f/2, where 'f' stands for the focal length, in this case 50mm.

This convention has served photography well for many years but with the advent of digital has been somewhat abused by the manufacturers for marketing reasons. In the digital world we have a much greater variety of active frame sizes in the mass market than with film, which was constrained by availability of processing services in a way that digital isn't. The manufacturers like to express their focal lengths in equivalents but then fail to do the same for the aperture. So, to take the example above, a 35mm camera with a 50mm lens gives an angle of view of 46.8 degrees. A lens described as 'f/2' will have a value of 'f' of 50mm, giving an aperture of 25mm. Now if we compare with a 1/2.4" camera. A lens giving the same angle of view, 46.8 degrees, which would be described by the manufacturer as '50mm equivalent' or quite often just '50mm' will actually have a focal length of 8.9mm, and if it has an aperture of 'f/2' the 'f' is now 8.9mm, so the aperture is actually 4.5mm. This is clearly confusing, because if the focal length has been expressed as '50mm' then one would expect the aperture to be 25mm, and let in as much light as expected from a 25mm aperture with a 46.8 degree angle of view. In fact, the camera is letting in 1/32nd the amount of light suggested.

This would not be important except for the fact that it is the amount of light admitted that primarily determines image quality, so many consumers are being conned into thinking that they will get higher image quality than they will in practice.

So, while not for a moment expecting this to happen, I would like to propose a different method of expressing aperture which meets the original objectives of expressing by focal length but is not similarly subject to abuse. I propose that instead of expressing aperture in terms of focal length, we do so directly in terms of the angle of view. Instead of an 'f-number' we have an 'a level'. So, for instance in the cases above the 50mm lens used on FF would be said to have an aperture of a/1.9, where a is the angle of view (diagonal, in degrees) whilst the 1/2.4" camera would be said to have an aperture of a/10.4

I have often had similar thoughts, albeit probably from a more naïve experience in photography.

I don't think it's naïve, the current system is clearly broken.

In terms of marketing or correctness, if you were to start labeling lenses by angle of view, what happens to the angle of view for the same lens mounted to different sensors sizes? Is the angle of view unchanged when the crop is applied on a smaller sensor?

Obviously not, the angle of view changes as you mount the lens on different crops, but this is what is really happening. Quoting the AoV is preferable to an 'equivalent focal length', IMO.

What about some metric of "perspective compression". Wouldn't that be unchanged with he same lens regardless if the image is cropped?

'Perspective compression' goes with angle of view and subject distance.

So if I take a cropped view from a lens, the perspective compression actually changes with angle of view?

 

[Joe Pineapples II]

It is rational, straight-forward, and would clear-up a source of much confusion. However, in a capitalist society, ignorance is a valuable commodity, so any such idea would be opposed all the way in the current US or UK. If you want to push the idea, I would start in one of the more enlightened European countries rather than suggesting it on a US-based forum.

J.

 

[fotowbert]

Perhaps using radian , the SI unit of angular measure, instead of degrees, in your proposal would make it more acceptable at least to the scientific community.

• John

 

[Lee Jay]

The amount of light in an image is determined by three things - the luminance of the scene, the size of the aperture and the angle of view.

Uh...you missed the fourth one Bob.

 

[Ysarex]

This would not be important except for the fact that it is the amount of light admitted that primarily determines image quality,

An extraordinary claim! Reference, please.

One must include the proper qualifier of "all else equal".

I thought that the qualifier 'primarily' was sufficient.

I don't think "primarily" is enough. For example, ignoring DOF considerations, the SIgma 50 / 1.4A on my 6D at f/2.8 likely has "better" IQ than the Canon 50 / 1.4 at f/2, even though the latter puts twice as much light on the sensor for a given exposure time.

OK, I was assuming a definition of 'IQ' orthogonal from lens effect, mainly about noise.

All else isn't equal and pragmatically the "amount of light determines image quality" rule sounds good except when you start taking photos and then maybe not so good. For instance: in my retirement I go for walks around the neighborhood multiple times a week. I could carry the FF camera but normally I prefer to carry my 1/1.7 sensor compact. Not just because it's smaller and lighter but also because it's so much better in low light and I often walk into the early evening.

Yep, I just said that. Better say it again: My 1/1.7 sensor compact is better in low light than the big FF gun. My lens on that camera is a 5.2 - 17.2 f/1.4 zoom. Let's just use the wide end and if you like call it 83 degrees AOV. On the FF we'll need a 24mm lens for 83 degrees AOV. Crop factor for my compact is 4.6.

As the light dims I'll drop shutter speed on the compact as low as 1/60 th sec. and usually get away with it keeping the ISO at base (100). Nice thing about that small sensor is that at f/2 I'll have all the DOF I could want taking a photo with the 5.2mm end of the zoom. It's a great lens and with a 12 megapixel sensor and 12 bit ADC I get damn good photos with it.

OK, switch cameras. Using the FF camera I'm going to have to stop the lens down to f/9.5 to get the same DOF I get from my compact at f/2. That's going to force me to raise the ISO on the FF camera to 2300 (base ISO 100). Large sensor cameras are much better at high ISO than small sensor cameras -- that's the rule. But they aren't that much better. I get to capture a full sensor exposure at base ISO with my compact camera and with the FF camera I'd be using what -- less than 1/5 of the sensor's capacity or even less? I don't want or need to do that math. I know from experience in that situation I really am going to get better photo IQ from my compact.