Focal length 35mm equivalent, but not F-stop?

Started Jul 24, 2014 | Discussions
Great Bustard Forum Pro • Posts: 44,163
Re: Simple solution:

Bmoon wrote:

Great Bustard wrote:

Bmoon wrote:

Great Bustard wrote:

First and foremost, the lens should be stamped with it's *actual* focal length and f-ratio only. Secondly, if equivalents to another format are given, then the format must be named, and the equivalents spelled out. Let's use the Pentax MX-1 you gave as an example.

The lens is a 6-24 / 1.8-2.5, should be labeled as such, and it is. In terms of FF, it is equivalent to a 28-112 / 8.4-11.7. The way this should be written is:

The Pentax MX-1 has a 6-24 / 1.5-2.5 lens, which is equivalent to a 28-112 / 8.4-11.7 lens on FF, in terms of diagonal angle of view, depth of field, diffraction, and the total amount of light projected on the sensor for a given shutter speed.

Done. Simple, correct, and no confusion.

That would be too simple

I know, right?

I feel that perception of a product plays a role in what they acknowledge, something about having a 400mm F2.8 that you can fit in your pocket has some appeal.

Are you implying that marketing has more to do with the labeling than the facts?  You're treading on dangerous ground here, buddy! 

Beachcomber Joe
Beachcomber Joe Senior Member • Posts: 1,256
Re: Incorrect
1

Lee Jay wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

Well, then it's always been thought of wrong. f-stop measures light intensity (illuminance) not light gathering ability (total light). Yes, they go together if the sensor size is constant.

That is certainly true if that is the way you choose to define the two terms.  I happen to be a photographer so, like most photographers, my definitions are based on real world and may be less specific than those in a physics lab.  In my world light gathering ability is understood to be the ability to put a certain amount of light on a specific area.  What you choose to call illuminance.

This is why we refer to lenses with wide apertures as fast, not shallow. That ability does not magically change with format.

Yes, it does. A bright light on a small surface means a relatively small amount of total light compared with a dimmer light over a much larger surface. Total light controls image quality (noise) and noise controls how fast your shutter speed can go.

You may continue to play with, among other things, semantics.  The light gathering ability of a photographic lens is measured by the amount of light per unit of area.  Your definition of image quality being the amount of noise is equally out of touch with the real world of photography.

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eyeswideshut Regular Member • Posts: 333
Re: Focal length 35mm equivalent, but not F-stop?

Lee Jay wrote:

eyeswideshut wrote:

Lee Jay wrote:

eyeswideshut wrote:

Lee Jay wrote:

John Sheehy wrote:

Chikoo wrote:

All camera manufacturer when they publish technical specification of their camera lens publish the focal length and it's equivalent in 35mm. But the same is not done for F-stop. Why?

f-numbers are ratios of real focal length divided by physical aperture size, and have nothing to do sensor sizes.

If manufacturers used the "35mm equivalent focal lengths" in the ratio, then they'd also have to scale the ISOs also to make exposure triangles work. Do you want that kind of a mess?

Absolutely I do.

Lee, no one is stopping you from engaging in all those calculations. Get down and dirty with the math!

Why is multiplying focal length by crop factor easy and obvious while multiplying f-stop by crop factor is "getting down and dirty with the math"?

Read on.

For the great unwashed though, focal length is what it has always been: shorthand for angle of view. It's been that way for ever and ever and ever. And that is all that manufacturers are implying when they give equivalent focal lengths. Only when you ignore that simple semantic fact can you meaningfully go off on the equivalist tangent.

What?

Equivalence includes focal length equivalence as well as f-stop equivalence. In fact, what it's saying is that you shouldn't use one without the other.

Says who? Great Bustard? You see, the reason I call this exercise 'equivalism' rather than 'a paradigm of equivalence' is because it has taken on all the trappings of a religion with converts, neophites, disciples and masters. If you had read and engaged what I said in a simple four line paragraph you could have saved yourself all the rest:

f-stop = focal length / aperture

f-stop (equivalent) = focal length (equivalent) / aperture

Another way to put it is simple algebra - you have to do the same thing to both sides of the equation.

f-stop = focal length / aperture

multiply both sides by crop factor

f-stop * crop factor = focal length * crop factor / aperture

recognize that focal length * crop factor is focal length (equivalent) and you have:

f-stop * crop factor = focal length (equivalent) / aperture

now, rename the left side:

f-stop (equivalent) = focal length (equivalent) / aperture

See, once you understand (and respect) that most people use equivalent focal length as shorthand for angle of view and have done so for the longest time, you will understand that there is no need to bring in the entire equivalist model every time someone says 'this lens is the equivalent of a 50mm lens on ff'. If that person had said it is a 45° AOV lens you would no longer want to quibble, right?

Depends. If they specify aperture then, in that case, they should do so the way we do it with telescopes - aperture diameter. I have an 11" telescope. Hardly anyone would call it a 2,800mm f/10 lens.

If you want to specify actual aperture, rather than f-stop, then all this goes away. Same with angle of view though in that case you're including sensor size.

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Lee Jay

I take it you see my point then? A manufacturer - as well as any run of the mill photographer - using fl as short hand for angle of view simply need not worry about all the ramifications of creating equivalent images. And the beauty of f-stops is that they are already expressed as a ratio.

Only if manufacturers were to suggest that say a bridge camera with a small sensor and some wild super zoom could create equivalent images to some significantly larger sensored camera would they actually mislead. But the fact is they don't. Equivalists simply impute that nasty intention - usually out of a samaritan impulse to save a newby from dire straits

Regarding telescopes I have no experience with them. 11" pretty nearly equals 2800mm so I don't know which you call it - but it seems to be the same thing. Equally, I don't know what typical apertures are on telescopes, but if it is indeed f/10 then a diameter of 2800/10 = 280mm = 2,8 cm should follow.

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eyeswideshut

eyeswideshut Regular Member • Posts: 333
Re: Focal length 35mm equivalent, but not F-stop?
2

Chikoo wrote:

Beachcomber Joe wrote:

Because it is not necessary.

Says who?

The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field. This is why we refer to lenses with wide apertures as fast, not shallow. That ability does not magically change with format. In the heyday of film and until recently in digital, photographers were ISO restricted in low light conditions. The difference of a stop of light gathering ability was a significant reason to pay the premium in cost, weight and size for a fast lens. That is how most fast lenses have been marketed and the primary reason those lenses are purchased. In many cases the shallower depth of field of the faster lens was looked upon as a negative.

That was then. You got to change with the times.

Says who?

Aren't you the guy who so innocently posted the original question? Seems it was quite loaded with a preconceived answer after all, eh?

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eyeswideshut

Beachcomber Joe
Beachcomber Joe Senior Member • Posts: 1,256
Re: Focal length 35mm equivalent, but not F-stop?

Chikoo wrote:

Beachcomber Joe wrote:

Because it is not necessary.

Says who?

The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field. This is why we refer to lenses with wide apertures as fast, not shallow. That ability does not magically change with format. In the heyday of film and until recently in digital, photographers were ISO restricted in low light conditions. The difference of a stop of light gathering ability was a significant reason to pay the premium in cost, weight and size for a fast lens. That is how most fast lenses have been marketed and the primary reason those lenses are purchased. In many cases the shallower depth of field of the faster lens was looked upon as a negative.

That was then. You got to change with the times.

Why?  Large format shooters have managed for years without f/stop equivalents.  FF and APS-C don't need them.  The only people to whom it seems to matter are MFT users.  They are, they tell us, so much smarter and cleverer than the rest of us.  That being the case I'm sure they have an app to figure it out themselves and don't need published figures.

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Lee Jay Forum Pro • Posts: 55,399
Re: Focal length 35mm equivalent, but not F-stop?
2

eyeswideshut wrote:

I take it you see my point then? A manufacturer - as well as any run of the mill photographer - using fl as short hand for angle of view simply need not worry about all the ramifications of creating equivalent images.

That depends.  If you're like me and commonly shoot with many different formats, sometimes all at the same time, it's a good concept to keep handy especially if you want a consistent "look" (noise, mostly) from several cameras shot at the same time in the same conditions but with different sensor sizes.

And the beauty of f-stops is that they are already expressed as a ratio.

Which is relatively meaningless now, since we have in-camera metering, the ability to adjust ISO real-time, and more people using more available formats.  When cameras were manual and everyone used 35mm film or just one medium format, keeping f-stops in mind was much more important than it is now.

Only if manufacturers were to suggest that say a bridge camera with a small sensor and some wild super zoom could create equivalent images to some significantly larger sensored camera would they actually mislead.

That's exactly what they do when they say stuff like "25-600mm f/2.8 lens".

But the fact is they don't.

Yes, they do.

Equivalists simply impute that nasty intention - usually out of a samaritan impulse to save a newby from dire straits

I've had people tell me straight up that their 400mm/2.8 lens on their hyperzoom was just as good as a $10,000 400/2.8 at only $400 and with a camera included.  Of course, their hyperzoom doesn't have a 400mm/2.8, it has a 72mm/2.8.

Regarding telescopes I have no experience with them. 11" pretty nearly equals 2800mm

280mm

so I don't know which you call it - but it seems to be the same thing.

No, one is focal length, one is aperture.

Equally, I don't know what typical apertures are on telescopes, but if it is indeed f/10 then a diameter of 2800/10 = 280mm = 2,8 cm should follow.

28cm.

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Lee Jay

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Beachcomber Joe
Beachcomber Joe Senior Member • Posts: 1,256
Re: Focal length 35mm equivalent, but not F-stop?

Allan Olesen wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

The equivalent f-stop also describes the light gathering ability.

You need effective lens area to gather light. A 50 mm f/2 on full frame has much more area than a 12.5 mm f/2 on 4x crop. So the small lens cannot gather as much light.

In fact, a 50 mm f/8 on full frame will have the same effective lens area as a 12.5 mm f/2 on 4x crop.

This is why we refer to lenses with wide apertures as fast, not shallow.

"Fast" is very vague in comparisons between different sensor sizes. You can always get a fast shutter speed by increasing the ISO. You don't want to do this because of noise. So the only relevant comparison is at ISOs which produce the same amount of noise. Usually, a modern FF camera will have the same amount of noise at a 16x higher ISO than a modern 4x crop camera.

See how the numbers compare?

The difference between ISOs with the same noise (4 stops) is the same as the difference between equivalent f-stops (4 stops).

So at the same shutter speed, you can expect the FF camera at f/8 and ISO 1600 to have the same noise as the 4x crop camera at f/2 and ISO 100.

So equivalent f-stops tells us much, much more about a camera's low light abilities.

And it tells us much, much more about how fast the lens really is: Which shutter speed can we get with a given amount of noise?

Comparing real f-stops across different sensor sizes is a waste of time as it tells us nothing about the low light abilities.

(If you do a comparison between real cameras, you may find that the numbers are 1/2 stops off because small sensors of the same generation are usually slightly more efficient than large sensors. But this 1/2 stop error is nothing in comparison to the 3.5 stop error you would make if you just looked at the real f-stops.)

Sadly, this simple truth is rejected by large groups of forum members which appear to either...:

  • ...feel threatened on their photographic knowledge.
  • ...or feel threatened on their small sensor cameras.

So they make these kneejerk reactions in the same way as a creationist being told about Darwin.

Yes, your post certainly proves that noise is an issue.

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Great Bustard Forum Pro • Posts: 44,163
Let's correct it, then.
2

Beachcomber Joe wrote:

Lee Jay wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

Well, then it's always been thought of wrong. f-stop measures light intensity (illuminance) not light gathering ability (total light). Yes, they go together if the sensor size is constant.

That is certainly true if that is the way you choose to define the two terms. I happen to be a photographer so, like most photographers, my definitions are based on real world and may be less specific than those in a physics lab. In my world light gathering ability is understood to be the ability to put a certain amount of light on a specific area. What you choose to call illuminance.

The f-ratio is an *indicator* of light gathering ability, but the same f-ratio on different formats does not gather the same amount of light.

That is, f/2 gathers the same amount of light for a given scene luminance, shutter speed, and lens transmission regardless of focal length *on a given format*, but it does not gather the same amount of light as f/2 does on a different format.

This is why we refer to lenses with wide apertures as fast, not shallow. That ability does not magically change with format.

Yes, it does. A bright light on a small surface means a relatively small amount of total light compared with a dimmer light over a much larger surface. Total light controls image quality (noise) and noise controls how fast your shutter speed can go.

You may continue to play with, among other things, semantics. The light gathering ability of a photographic lens is measured by the amount of light per unit of area.

Sure. But the same light per unit of area, in terms of the visual properties of the photo, has different effects on different formats.

Your definition of image quality being the amount of noise is equally out of touch with the real world of photography.

Noise is merely one aspect of IQ:

While this section is concerned solely with IQ, it is important to note that IQ is but one attribute of a camera system.

But what, exactly, is IQ, and what does it have to do with the "success" of a photo? The first step in defining "IQ" is to make the distinction between "image quality" and a "quality image". Many would take it as a given that if we have two photos of the same scene with the same composition then, all else equal as well, the photo with "higher IQ" would be "more successful". That is, the photo with "higher IQ", for example, would place better in a photo competition, would be more likely to sell, would sell for a higher price, etc. For sure, this may certainly be true for a large number of photos, such as a landscape photo displayed at a huge size. But it is important to acknowledge that there is a class of photography where image quality, as opposed to a quality image, is all but irrelevant (please see these outstanding photos, for example).

Furthermore, while one system may yield "higher IQ" than another, those differences may not be large enough to make any significant difference in the appeal of the photo, depending on the QT (quality threshold) of the viewer, the scene itself, the size at which the photo is displayed, and how closely it is scrutinized (see here for an interesting example of this point), and the processing applied to the photo. In other words, it is not merely whether System A has "higher IQ" than System B, but under what conditions it has higher IQ (and, indeed, which has "higher IQ" may flip-flop, depending on the conditions), and if the IQ is "enough higher" to make any significant difference.

For some photographers, IQ may be the most important aspect of photography. For others, it may play no role at all or simply be an added plus. But it is time well spent to reflect on just how important IQ is to our own photography, given that IQ is, at best, merely a means to achieving a quality image, and, at worst, completely irrelevant to the photo.

tko Forum Pro • Posts: 13,066
because they don't want you to know
1

Multiplying focal length isn't a bad thing. A 1000MM equivalent lens can be very useful.

But do you think they want to know that aperture gets multiplied as well? All of the sudden that F2.8 lens you thought was really fast is really only F10 equivalent?

Of course not! Because people might figure out that a true F2.8 FF lens is actually a pretty good deal compared to a smaller format equivalent F10 lens. And the public would know the lenses are smaller because they do less, not because of any magic dust.

What advertise your shortcomings? It's all about marketing.

eyeswideshut Regular Member • Posts: 333
Re: Focal length 35mm equivalent, but not F-stop?

Lee Jay wrote:

eyeswideshut wrote:

I take it you see my point then? A manufacturer - as well as any run of the mill photographer - using fl as short hand for angle of view simply need not worry about all the ramifications of creating equivalent images.

That depends. If you're like me and commonly shoot with many different formats, sometimes all at the same time, it's a good concept to keep handy especially if you want a consistent "look" (noise, mostly) from several cameras shot at the same time in the same conditions but with different sensor sizes.

it doesn't depend at all. Sometimes equivalism has it's uses, sometimes it is useless. I shoot from Pentax Q to Pentacon Six. But never do I put down one camera to pick up the other and wonder what will it take to create another equivalent image of the one I just took. On the other hand I know exactly that the Q is not my first choice for classic head/shoulder portraits. IF I wanted to know the precise relationship (as opposed to a quick and approximate seat of the pants judgment) equivalism is just the ticket. And that is ALL the model is good for. No more - No less. Recognizing that fl is used as short hand for aov simply sidesteps the issues of equivalism. And that is not deceptive but simply the way it has been done forever.

And the beauty of f-stops is that they are already expressed as a ratio.

Which is relatively meaningless now, since we have in-camera metering, the ability to adjust ISO real-time, and more people using more available formats. When cameras were manual and everyone used 35mm film or just one medium format, keeping f-stops in mind was much more important than it is now.

Only if manufacturers were to suggest that say a bridge camera with a small sensor and some wild super zoom could create equivalent images to some significantly larger sensored camera would they actually mislead.

That's exactly what they do when they say stuff like "25-600mm f/2.8 lens".

But the fact is they don't.

Yes, they do.

No they don't. You know full well what I mean when I say 'equivalent image' - you are after all quite familiar with great bustard's article. Need I provide the link

Equivalists simply impute that nasty intention - usually out of a samaritan impulse to save a newby from dire straits

I've had people tell me straight up that their 400mm/2.8 lens on their hyperzoom was just as good as a $10,000 400/2.8 at only $400 and with a camera included. Of course, their hyperzoom doesn't have a 400mm/2.8, it has a 72mm/2.8.

Fanboys of any ilk are a nuissance. Bridgefanboys are no exception.

Regarding telescopes I have no experience with them. 11" pretty nearly equals 2800mm

280mm

Of course - getting late here.

so I don't know which you call it - but it seems to be the same thing.

No, one is focal length, one is aperture.

Equally, I don't know what typical apertures are on telescopes, but if it is indeed f/10 then a diameter of 2800/10 = 280mm = 2,8 cm should follow.

28cm.

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Lee Jay

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eyeswideshut

Gerry Winterbourne Forum Pro • Posts: 16,779
What ordinary people want
4

Chikoo wrote:

All camera manufacturer when they publish technical specification of their camera lens publish the focal length and it's equivalent in 35mm. But the same is not done for F-stop. Why?

Ordinary people just want to take photos. Most of them know absolutely nothing about photography and nothing relating to 35mm cameras means anything to them. Quite a few, though, have some former experience with 35mm, or access to people with such experience.

I'll start with APS-C because that was the first widely used sensor size. In use an APS-C DSLR was, and is, very similar to an SLR , with one major difference - the crop factor. I was caught out by this (my fault for insufficient research but caught I was). Focal length has various effects but the one that most obviously affects users who know about 35mm photography is the FOV so stating FLs in 35mm equivalent as well as the actual FL is a simple courtesy to buyers.

The same thing applies to users of other sensor sizes and camera types: either FL means nothing to them or they have some knowledge about 35mm FL/FOV. So that information is useful. But anything beyond is superfluous: you only have to read the Beginners forum here to know how few newcomers understand DOF as a concept, so what earthly use to them is data on equivalency?

On the other hand, anyone who knows - and cares - about such things is able to work them out so the information isn't needed.

Ditto the noise equivalency mentioned elsewhere in the thread: get into that and you should also be asking why camera makers don't publish data on the noise performance of all cameras. Even at the same sensor size, why don't we see noise data to allow us to compare a D700 with a 6D with ...? Because without that it's pointless to look at a P&S noise performance v a specific other camera.

So what makers give is a compromise between completeness and confusion.

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_______________________________________
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Chikoo
OP Chikoo Senior Member • Posts: 1,630
Re: Focal length 35mm equivalent, but not F-stop?

eyeswideshut wrote:

Chikoo wrote:

Beachcomber Joe wrote:

Because it is not necessary.

Says who?

The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field. This is why we refer to lenses with wide apertures as fast, not shallow. That ability does not magically change with format. In the heyday of film and until recently in digital, photographers were ISO restricted in low light conditions. The difference of a stop of light gathering ability was a significant reason to pay the premium in cost, weight and size for a fast lens. That is how most fast lenses have been marketed and the primary reason those lenses are purchased. In many cases the shallower depth of field of the faster lens was looked upon as a negative.

That was then. You got to change with the times.

Says who?

Aren't you the guy who so innocently posted the original question? Seems it was quite loaded with a preconceived answer after all, eh?

Beachcomber comes along and state "Because it is not necessary". Even an innocent person like me would be ruffled by such forceful statements.

Chikoo
OP Chikoo Senior Member • Posts: 1,630
Re: Focal length 35mm equivalent, but not F-stop?

Beachcomber Joe wrote:

Chikoo wrote:

Beachcomber Joe wrote:

Because it is not necessary.

Says who?

The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field. This is why we refer to lenses with wide apertures as fast, not shallow. That ability does not magically change with format. In the heyday of film and until recently in digital, photographers were ISO restricted in low light conditions. The difference of a stop of light gathering ability was a significant reason to pay the premium in cost, weight and size for a fast lens. That is how most fast lenses have been marketed and the primary reason those lenses are purchased. In many cases the shallower depth of field of the faster lens was looked upon as a negative.

That was then. You got to change with the times.

Why? Large format shooters have managed for years without f/stop equivalents. FF and APS-C don't need them. The only people to whom it seems to matter are MFT users. They are, they tell us, so much smarter and cleverer than the rest of us. That being the case I'm sure they have an app to figure it out themselves and don't need published figures.

Maybe they never needed them because practice told them which lens works best with their camera, and then they stick to it. Another reason why it is difficult for those to change systems because they are do not know how it will work with other cameras/formats.

Now if this was made simpler, it would be simpler for photographers to switch systems without being a first grade mathematician first.

Chikoo
OP Chikoo Senior Member • Posts: 1,630
Re: What ordinary people want

Gerry Winterbourne wrote:

Chikoo wrote:

All camera manufacturer when they publish technical specification of their camera lens publish the focal length and it's equivalent in 35mm. But the same is not done for F-stop. Why?

Ordinary people just want to take photos. Most of them know absolutely nothing about photography and nothing relating to 35mm cameras means anything to them. Quite a few, though, have some former experience with 35mm, or access to people with such experience.

I'll start with APS-C because that was the first widely used sensor size. In use an APS-C DSLR was, and is, very similar to an SLR , with one major difference - the crop factor. I was caught out by this (my fault for insufficient research but caught I was). Focal length has various effects but the one that most obviously affects users who know about 35mm photography is the FOV so stating FLs in 35mm equivalent as well as the actual FL is a simple courtesy to buyers.

The same thing applies to users of other sensor sizes and camera types: either FL means nothing to them or they have some knowledge about 35mm FL/FOV. So that information is useful. But anything beyond is superfluous: you only have to read the Beginners forum here to know how few newcomers understand DOF as a concept, so what earthly use to them is data on equivalency?

On the other hand, anyone who knows - and cares - about such things is able to work them out so the information isn't needed.

Ditto the noise equivalency mentioned elsewhere in the thread: get into that and you should also be asking why camera makers don't publish data on the noise performance of all cameras. Even at the same sensor size, why don't we see noise data to allow us to compare a D700 with a 6D with ...? Because without that it's pointless to look at a P&S noise performance v a specific other camera.

So what makers give is a compromise between completeness and confusion.

A person who has grown up using 35mm camera, and loves the way his f2.8 lens provide a nice bokeh, and sees the new camera in the store which is smaller and lighter, and boasts the same focal length and zoom range + F-stop, will be more than happy to purchase that camera. Only to find out that it is not the same. He is not getting the same quality of picture as his 35mm f2.8 lens gave him for the last 30 years.

That is deceptive to say the least.

This is like stating the hp of a car with a 4 cylinder engine as a ratio of power to weight and sell it saying it is the same or better than the large v8 he currently drives.

Pixel Pooper Veteran Member • Posts: 3,259
Re: Focal length 35mm equivalent, but not F-stop?

Lee Jay wrote:

John Sheehy wrote:

Chikoo wrote:

All camera manufacturer when they publish technical specification of their camera lens publish the focal length and it's equivalent in 35mm. But the same is not done for F-stop. Why?

f-numbers are ratios of real focal length divided by physical aperture size, and have nothing to do sensor sizes.

If manufacturers used the "35mm equivalent focal lengths" in the ratio, then they'd also have to scale the ISOs also to make exposure triangles work. Do you want that kind of a mess?

Absolutely I do.

This got me thinking, what if cameras did use equivalent apertures, not just in spec sheets, but for exposure settings as well? Would it be more or less confusing for everyday shooting? I'm not sure I know the answer but it could be an interesting discussion and I think it deserves its own thread, so I started this one.

Great Bustard Forum Pro • Posts: 44,163
Re: Focal length 35mm equivalent, but not F-stop?

Pixel Pooper wrote:

Lee Jay wrote:

John Sheehy wrote:

Chikoo wrote:

All camera manufacturer when they publish technical specification of their camera lens publish the focal length and it's equivalent in 35mm. But the same is not done for F-stop. Why?

f-numbers are ratios of real focal length divided by physical aperture size, and have nothing to do sensor sizes.

If manufacturers used the "35mm equivalent focal lengths" in the ratio, then they'd also have to scale the ISOs also to make exposure triangles work. Do you want that kind of a mess?

Absolutely I do.

This got me thinking, what if cameras did use equivalent apertures, not just in spec sheets, but for exposure settings as well? Would it be more or less confusing for everyday shooting? I'm not sure I know the answer but it could be an interesting discussion and I think it deserves its own thread, so I started this one.

Not a fan. Actual focal lengths and actual apertures, whether that be the relative aperture (f-ratio) or virtual aperture (entrance pupil) diameter.

JBurnett Contributing Member • Posts: 835
A Camera Club Perspective
3

Two years ago I moved across country and joined a new camera club with about 50 members. Here are some observations:

  • In a club of 50, about 40 members use APS-C sensored DSLRs
  • I would estimate that 35 of them purchased their APS-C cameras with the kit lens, and had no concept of equivalencies at the time. 
  • Even after some educational sessions within the club, probably only 10 could tell you what their APS-C lens is "equivalent to" in terms of FF focal length. The Nikon users are more likely to know than the Canon users, simply because it's easier to multiply by 1.5 instead of 1.6. 
  • Maybe 10 understand that there is a difference in depth of field between their camera and cameras with larger or smaller sensors. Only 3 would know where to go to calculate that equivalence (or would ever care to do so).
  • Maybe 25 understand (through personal experience or education within the club) about "noise" and that it is related to ISO and sensor size.
  • Perhaps 2 or 3 would understand any discussion about "light gathering" with regards to sensor size.
  • Most members understand that if a light meter says f/4 at 1/60 at ISO 200 on one camera, they could use the same settings on a different camera. Perhaps 5 still have no idea about the relationships between aperture, shutter speed and ISO. We're working on it, but some may never care and are happier using scene modes. 
  • Only one person in the club really cares about equivalencies, because he shoots both full-frame and APS-C bodies at the same time.
  • The majority of time I've been drawn into a discussion about equivalencies, it has been a newer member who wants to move from a P&S to a DSLR or mirrorless. Equivalent focal lengths to what they see with their existing camera makes sense to them. Most of the time, the remainder of "equivalency" talk would cause them to glaze over and shut down. After all, they just want to know what "kit" to buy.
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(unknown member) Veteran Member • Posts: 6,680
Re: Focal length 35mm equivalent, but not F-stop?

Allan Olesen wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

The equivalent f-stop also describes the light gathering ability.

You need effective lens area to gather light. A 50 mm f/2 on full frame has much more area than a 12.5 mm f/2 on 4x crop. So the small lens cannot gather as much light.

This is what is known as wrong.

Any lens will gather the same amount of light at any given aperture. A 50mm f2 lens on a 35mm format will gather exactly the same amount of light as a 12.5mm lens on a micro four thirds lens if both are set to the same f/2 aperture.

Peoiple really need to learn what they are talking about before they start to spread the sort of misinformation that is quoted above..

In fact, a 50 mm f/8 on full frame will have the same effective lens area as a 12.5 mm f/2 on 4x crop.

This is why we refer to lenses with wide apertures as fast, not shallow.

"Fast" is very vague in comparisons between different sensor sizes. You can always get a fast shutter speed by increasing the ISO. You don't want to do this because of noise. So the only relevant comparison is at ISOs which produce the same amount of noise. Usually, a modern FF camera will have the same amount of noise at a 16x higher ISO than a modern 4x crop camera.

See how the numbers compare?

The difference between ISOs with the same noise (4 stops) is the same as the difference between equivalent f-stops (4 stops).

So at the same shutter speed, you can expect the FF camera at f/8 and ISO 1600 to have the same noise as the 4x crop camera at f/2 and ISO 100.

So equivalent f-stops tells us much, much more about a camera's low light abilities.

And it tells us much, much more about how fast the lens really is: Which shutter speed can we get with a given amount of noise?

Comparing real f-stops across different sensor sizes is a waste of time as it tells us nothing about the low light abilities.

(If you do a comparison between real cameras, you may find that the numbers are 1/2 stops off because small sensors of the same generation are usually slightly more efficient than large sensors. But this 1/2 stop error is nothing in comparison to the 3.5 stop error you would make if you just looked at the real f-stops.)

Sadly, this simple truth is rejected by large groups of forum members which appear to either...:

  • ...feel threatened on their photographic knowledge.
  • ...or feel threatened on their small sensor cameras.

And some, it seems, spread bad information because they are ignorant. You are one of those people.

So they make these kneejerk reactions in the same way as a creationist being told about Darwin.

You should stop jerking your knees and educate yourself before you make a total fool of yourself.

Great Bustard Forum Pro • Posts: 44,163
Whoops!
4

Wheatfield7 wrote:

Allan Olesen wrote:

Beachcomber Joe wrote:

Because it is not necessary. The f stop of lenses has always been marketed and thought of as an indicator of its light gathering ability, not its depth of field.

The equivalent f-stop also describes the light gathering ability.

You need effective lens area to gather light. A 50 mm f/2 on full frame has much more area than a 12.5 mm f/2 on 4x crop. So the small lens cannot gather as much light.

This is what is known as wrong.

Um, it is absolutely correct.

Any lens will gather the same amount of light at any given aperture. A 50mm f2 lens on a 35mm format will gather exactly the same amount of light as a 12.5mm lens on a micro four thirds lens if both are set to the same f/2 aperture.

Incorrect.

Peoiple really need to learn what they are talking about before they start to spread the sort of misinformation that is quoted above..

Yes, they do.  The fact of the matter is that 16x as much light will fall on the sensor for a given perspective, framing, and shutter speed with 50mm f/2 on FF as will fall on the sensor at 12.5mm f/2 on 4x.

In fact, a 50 mm f/8 on full frame will have the same effective lens area as a 12.5 mm f/2 on 4x crop.

This is why we refer to lenses with wide apertures as fast, not shallow.

"Fast" is very vague in comparisons between different sensor sizes. You can always get a fast shutter speed by increasing the ISO. You don't want to do this because of noise. So the only relevant comparison is at ISOs which produce the same amount of noise. Usually, a modern FF camera will have the same amount of noise at a 16x higher ISO than a modern 4x crop camera.

See how the numbers compare?

The difference between ISOs with the same noise (4 stops) is the same as the difference between equivalent f-stops (4 stops).

So at the same shutter speed, you can expect the FF camera at f/8 and ISO 1600 to have the same noise as the 4x crop camera at f/2 and ISO 100.

So equivalent f-stops tells us much, much more about a camera's low light abilities.

And it tells us much, much more about how fast the lens really is: Which shutter speed can we get with a given amount of noise?

Comparing real f-stops across different sensor sizes is a waste of time as it tells us nothing about the low light abilities.

(If you do a comparison between real cameras, you may find that the numbers are 1/2 stops off because small sensors of the same generation are usually slightly more efficient than large sensors. But this 1/2 stop error is nothing in comparison to the 3.5 stop error you would make if you just looked at the real f-stops.)

Sadly, this simple truth is rejected by large groups of forum members which appear to either...:

  • ...feel threatened on their photographic knowledge.
  • ...or feel threatened on their small sensor cameras.

And some, it seems, spread bad information because they are ignorant. You are one of those people.

As it turns out, you got that exactly backwards.

So they make these kneejerk reactions in the same way as a creationist being told about Darwin.

You should stop jerking your knees and educate yourself before you make a total fool of yourself.

Doctor, heal thyself.

mostlyboringphotog Veteran Member • Posts: 9,049
Re: What ordinary people want

Chikoo wrote:

Gerry Winterbourne wrote:

Chikoo wrote:

All camera manufacturer when they publish technical specification of their camera lens publish the focal length and it's equivalent in 35mm. But the same is not done for F-stop. Why?

Ordinary people just want to take photos. Most of them know absolutely nothing about photography and nothing relating to 35mm cameras means anything to them. Quite a few, though, have some former experience with 35mm, or access to people with such experience.

I'll start with APS-C because that was the first widely used sensor size. In use an APS-C DSLR was, and is, very similar to an SLR , with one major difference - the crop factor. I was caught out by this (my fault for insufficient research but caught I was). Focal length has various effects but the one that most obviously affects users who know about 35mm photography is the FOV so stating FLs in 35mm equivalent as well as the actual FL is a simple courtesy to buyers.

The same thing applies to users of other sensor sizes and camera types: either FL means nothing to them or they have some knowledge about 35mm FL/FOV. So that information is useful. But anything beyond is superfluous: you only have to read the Beginners forum here to know how few newcomers understand DOF as a concept, so what earthly use to them is data on equivalency?

On the other hand, anyone who knows - and cares - about such things is able to work them out so the information isn't needed.

Ditto the noise equivalency mentioned elsewhere in the thread: get into that and you should also be asking why camera makers don't publish data on the noise performance of all cameras. Even at the same sensor size, why don't we see noise data to allow us to compare a D700 with a 6D with ...? Because without that it's pointless to look at a P&S noise performance v a specific other camera.

So what makers give is a compromise between completeness and confusion.

A person who has grown up using 35mm camera, and loves the way his f2.8 lens provide a nice bokeh, and sees the new camera in the store which is smaller and lighter, and boasts the same focal length and zoom range + F-stop, will be more than happy to purchase that camera. Only to find out that it is not the same. He is not getting the same quality of picture as his 35mm f2.8 lens gave him for the last 30 years.

That is deceptive to say the least.

This is like stating the hp of a car with a 4 cylinder engine as a ratio of power to weight and sell it saying it is the same or better than the large v8 he currently drives.

This was true even when "I, Roman" ruled the world - that's why they warned "Caveat Emptor".

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