Lens lengths

[SpikeK6]

Ok just been thinking tonight whilst looking through the forum of different makes of lens, Canon Nikon Panasonic Sony etc, and who or what decides on the focal length of a lens.

OK you get zooms from say 10-22 17-55 70-200 50- 500 and then there are the primes 50 85 100 35 etc tec

So where or how do the numbers get made up????

 

[Phil V]

They're not 'made up' they're measured.

 

[SpikeK6]

Phew that was hard reading lol!!!

By made up I meant who decided on what length a zoom should be made too. what makes a manufacturer deice that a wide angle zoom will be 10-22 instead of 10-19 say??

 

[woof woof]

Well, the manufacturers want to make money so they make stuff they think us lot will want to buy.

Also, the technical specifications will be a balance between quality, ease of manufacture and what will sell / money to be made

I guess...

 

[big soft moose]

a lot of lenses fro crop sensor cameras are based on equivalent FoV for a 35mm SLR , so 10-20 for instance was a wide angle for crop cameras, because the standard wide angle for 35mm was 16-35

as to the question of who decided on the lengths originally

a) they arent milimetre perfect anyway - so your 10-20 might be 9-19 or 11-21 and you wouldnt really notice so its always going to be an estimation

and

b) 50mm (or thereabouts) is , on 35mm, roughly what your eye sees , so a lot of early lenses would have been in multiples or fractions of that ie 100mm = 2x, 400mm = 8x, 25mm (actually usually 24 or 28)= 0.5 and so on

why certain numbers were picked though I'm not sure - probably to do with the maths of actualy making an individual lens

 

[HoppyUK]

It's basically related to the field of view that photographers find most useful.

That then translates to a focal length, as appropriate to the format. We've come to use focal lengths as short-hand for of field of view, and because 35mm film was almost universal for a very long time, some of those numbers have stuck. Hence the common reference to '35mm equivalent' focal lengths.

 

[Gazamonk]

Hence the common reference to '35mm equivalent' focal lengths.

I often wonder how long that will go on for, I get the feeling it will always be used. Some things just don't cross over into the public psyche. It may be a British thing lol. We never did go fully metric did we ? youngsters often ask how long is an inch/foot/yard as they have no reference point because its all meters etc in schools now, yet they understand and still measure speed in MPH.

Being an old fart myself I still think of lenses in 35mm terms and I'm not going to change now. So thank goodness all the kind reviewers and magazines still kindly quote these figures for us.

Sorry not relevant to the OP just a random observation.

 

[HoppyUK]

I often wonder how long that will go on for, I get the feeling it will always be used. Some things just don't cross over into the public psyche. It may be a British thing lol. We never did go fully metric did we ? youngsters often ask how long is an inch/foot/yard as they have no reference point because its all meters etc in schools now, yet they understand and still measure speed in MPH.

Being an old fart myself I still think of lenses in 35mm terms and I'm not going to change now. So thank goodness all the kind reviewers and magazines still kindly quote these figures for us.

Sorry not relevant to the OP just a random observation.

Yes, I think you're right, it will always be used. Now that there are so many different sensor formats out there, we need some form of constant reference and the old 35mm film or full-frame as it's now known, is a good as any.

I've often thought it would be better if we referered to say a 50mm lens as a 40 degrees field of view lens and then we wouldn't have this problem and newcomers wouldn't get so confused with crop factors and stuff, but we don't so there it is.

Photography is full of quirks and historical anomolies, as much as any other 150 years old topic. The most confusing one is f/stops

 

[Gazamonk]

The most confusing one is f/stops

You got that right, I'm sure many of us have tried explaining the f-stop and half f-stop incremental system to a novice and looking at that blank vacant look you get.

"Why is it numbered like that ? its very confusing? Why is the stop between 2 and 4, 2.8 and not 3 ? Why does it go from 4 to 5.6 then 8 ?"

It just does, now shut up and pay attention. LOL

 

[HoppyUK]

You got that right, I'm sure many of us have tried explaining the f-stop and half f-stop incremental system to a novice and looking at that blank vacant look you get.

"Why is it numbered like that ? its very confusing? Why is the stop between 2 and 4, 2.8 and not 3 ? Why does it go from 4 to 5.6 then 8 ?"

It just does, now shut up and pay attention. LOL

Why is it called an f/stop? And if that's true, then what has it got to do with shutter speeds? And why are bigger numbers smaller sizes? ;gag:

 

[Jayst84]

f/stops are pretty simple maths though, explain it once to them and they should be fine..

 

[Nod]

f stops are indeed simple maths but try explaining the square root of 2 to someone whose maths is shall we say rudimentary! And at larger numbers, for the sake of convenience, the numbers are rounded down - f/22 should be f/22.8, so closer to f/23. Maybe we should start using EV rather than f/ and /s?

Now that FF is getting more popular, the reference to 35mm equivalence is almost certainly here to stay. Even if we were to start using angles of view rather than focal lengths as the measurement of width, a conversion factor would need to be used for crop sensors - not sure what the conversion rate would be for a 1.5x focal length crop when stated as an angle of view (and it's too early for my maths head to work it out!). At least some manufacturers are kind enough to use that 1.5x crop factor - far easier to just add half the FL to get the 35mm equivalent than to add half then add a tenth (of the original) for a 1.6x crop!

Then there's the matter of compact sensors, many of which have their sensor size listed as (say) 1 1/3rd". I know there are tables converting that to a more usual xmm x xmm but off the top of most people's heads?

AND the angle of view - I've seen it quoted as across the diagonal, along the long side and along the short side (or both). If we're going to change the system, we need to have it standardised so the consumer can tell at a glance what they're getting!

 

[Simon photo]

Why is it called an f/stop? And if that's true, then what has it got to do with shutter speeds? And why are bigger numbers smaller sizes? ;gag:

Its focal length divided by aperture diameter/iris.
Focal length being film/sensor plane to nodal point (usually the diaphragm) of lens
Called an F stop as it physically "stops" amounts of light passing in fractions. Not originally a blade diaphragm, waterhouse stops, aperture wheels, wooden blocks, they've all been used for the same purpose and with the same results.

 

[HoppyUK]

f stops are indeed simple maths but try explaining the square root of 2 to someone whose maths is shall we say rudimentary! And at larger numbers, for the sake of convenience, the numbers are rounded down - f/22 should be f/22.8, so closer to f/23. Maybe we should start using EV rather than f/ and /s?

Now that FF is getting more popular, the reference to 35mm equivalence is almost certainly here to stay. Even if we were to start using angles of view rather than focal lengths as the measurement of width, a conversion factor would need to be used for crop sensors - not sure what the conversion rate would be for a 1.5x focal length crop when stated as an angle of view (and it's too early for my maths head to work it out!). At least some manufacturers are kind enough to use that 1.5x crop factor - far easier to just add half the FL to get the 35mm equivalent than to add half then add a tenth (of the original) for a 1.6x crop!

Then there's the matter of compact sensors, many of which have their sensor size listed as (say) 1 1/3rd". I know there are tables converting that to a more usual xmm x xmm but off the top of most people's heads?

AND the angle of view - I've seen it quoted as across the diagonal, along the long side and along the short side (or both). If we're going to change the system, we need to have it standardised so the consumer can tell at a glance what they're getting!

That's the whole point, we wouldn't need a conversion factor.

Its focal length divided by aperture diameter/iris.

Then it should be called the f/ratio (which sometimes, it is).

Focal length being film/sensor plane to nodal point (usually the diaphragm) of lens

Not the diaphragm, but from the rear nodal point, and that is often actually outside the physical lens.

Called an F stop as it physically "stops" amounts of light passing in fractions. Not originally a blade diaphragm, waterhouse stops, aperture wheels, wooden blocks, they've all been used for the same purpose and with the same results.

Yes, originally Waterhouse Stops, but that meaning has now morphed into referring to any halving or doubling of exposure, even though 'stops' never originally referred to shutter speeds or sensitivity (ISO).

Just goes to make the point really, that it really doesn't matter. You just have to learn it.

 

[Nod]

That's the whole point, we wouldn't need a conversion factor.

Sorry to disagree but we would! The angle of view on a crop sensor is narrower than that on a FF sensor, hence the crop factor when described in 35mm EFL terms. Of course, manufacturers could quote the angles of view for all relevant sensor types, just as they could for focal lengths in terms of 35mm equivalence.

Thank the gods I am lucky enough to use FF and not worry about crop factors any more, although both the Fujis I'm using have the lens markings in 35mm EFLs, EXIF reports the true FLs of the lenses, leaving me with the maths to do!

 

[HoppyUK]

Sorry to disagree but we would! The angle of view on a crop sensor is narrower than that on a FF sensor, hence the crop factor when described in 35mm EFL terms. Of course, manufacturers could quote the angles of view for all relevant sensor types, just as they could for focal lengths in terms of 35mm equivalence.

Thank the gods I am lucky enough to use FF and not worry about crop factors any more, although both the Fujis I'm using have the lens markings in 35mm EFLs, EXIF reports the true FLs of the lenses, leaving me with the maths to do!

The idea would be, say, a 50mm lens on full-frame would be known as a '40 AoV' lens (40 degrees angle of view, from side to side). So would a 31mm lens on 1.6x crop, so would an 18.5mm lens on a Nikon 1. All 40 AoV.

 

[Gazamonk]

f/stops are pretty simple maths though, explain it once to them and they should be fine..

Really ? It may be simple maths to you but not to the average novice photographer and simple maths doesn't help when fielding questions like

"So, how come the hole thingy is bigger at f2 and smaller at f22,"

Or

" I know your telling me about depth of field but I'm not photographing a field ".



Now go back and read all the subsequent posts , explanations and opinions since you wrote yours and read them as if you were a complete novice who knew nothing about f-stops, shutter speeds, focal distance and the depth of dirt in a field !


How's your simple maths working out for you now. :bang: :shrug:





I'm only being funny by the way don't take the above personally

 

[Nod]

The idea would be, say, a 50mm lens on full-frame would be known as a '40 AoV' lens (40 degrees angle of view, from side to side). So would a 31mm lens on 1.6x crop, so would an 18.5mm lens on a Nikon 1. All 40 AoV.

Problem being that FF lenses can be used on both those other formats, so a conversion factor is needed. (OK, the crop on a Nikon APS-C is 1.5x rather than 1.6 but you know what I mean!)

 

[HoppyUK]

Problem being that FF lenses can be used on both those other formats, so a conversion factor is needed. (OK, the crop on a Nikon APS-C is 1.5x rather than 1.6 but you know what I mean!)

Oh right, sorry, talking at cross-purposes.

Yes, that's at the heart of it, so we might as well stick with focal lengths and crop factors, with some reference to full-frame equivalents for comparsison. No easy answers, and it's not going to change anyway!

 

[DemiLion]

Really ? It may be simple maths to you but not to the average novice photographer and simple maths doesn't help when fielding questions like

"So, how come the hole thingy is bigger at f2 and smaller at f22,"

Or

" I know your telling me about depth of field but I'm not photographing a field ".



Now go back and read all the subsequent posts , explanations and opinions since you wrote yours and read them as if you were a complete novice who knew nothing about f-stops, shutter speeds, focal distance and the depth of dirt in a field !

How's your simple maths working out for you now. :bang: :shrug:

I'm only being funny by the way don't take the above personally

The maths looks strange because you've left out one very important mathematical symbol (which a lot of people do) namely '/'.

An F-stop is focal length(f) divided by a proportion of that length(x) or {f/x} or {f x 1/x}. On other words it's a fraction, which means that as the denominator (x) gets bigger, the total value (or aperture) gets smaller.

The maths is fairly simple, after all a quarter is smaller than a half, the ability to comprehend it is down to the
explanation!

 

[Nod]

Very true Hoppy, very true! The important thing is that a user of whatever format can visualise what AoV a given focal length will give him/her - or, at the very least, having a pretty good idea as to which zoom will be suitable!

One of the reasons I was so happy to be able to afford my D700 was that at last I could use my lenses on digital without needing to do any maths to decide which to use!

 

[Simon photo]

I used the simple formula of focal length divided by aperture for a lens i built to go on my homemade 4x5 camera. And it works.

 

[HoppyUK]

I used the simple formula of focal length divided by aperture for a lens i built to go on my homemade 4x5 camera. And it works.

Yes, focal length and f/ratio calcs works for combos like that, but it all goes wrong with telephoto and inverted-telephoto (retrofocus) lenses like wide-angles on DSLRs.

Take a Canon 10-22mm where the lens mount is 44mm from the sensor. The rear nodal point never moves from inside the camera. And aperture sizes are often very diffferent to the f/ratio, like a 600mm f/4 super-tele where it should be 150mm diameter, but is actually a lot smaller than that, positioned in the narrow part of the lens near the camera.

 

[DemiLion]

I used the simple formula of focal length divided by aperture for a lens i built to go on my homemade 4x5 camera. And it works.

The one slight problem with that is your use of the word aperture!

Aperture = focal length / ratio of focal length

 

[Gazamonk]

The maths looks strange because you've left out one very important mathematical symbol (which a lot of people do) namely '/'.

An F-stop is focal length(f) divided by a proportion of that length(x) or {f/x} or {f x 1/x}. On other words it's a fraction, which means that as the denominator (x) gets bigger, the total value (or aperture) gets smaller.

The maths is fairly simple, after all a quarter is smaller than a half, the ability to comprehend it is down to the
explanation!

What ? Eh ?

Did you read my post or is this reply attached to the wrong quote. I was taking the p**s in a non offensive humorous way about the maths being thrown about in the replies and you reply with a totally unrelated issue about a missing notation symbol, which incidentally I didn't leave out because it was never intended to be in. Pedantry and jokes don't mix. Lol

I don't think you got the intent of that one.

 

[DemiLion]

What ? Eh ?

Did you read my post or is this reply attached to the wrong quote. I was taking the p**s in a non offensive humorous way about the maths being thrown about in the replies and you reply with a totally unrelated issue about a missing notation symbol, which incidentally I didn't leave out because it was never intended to be in. Pedantry and jokes don't mix. Lol

I don't think you got the intent of that one.

Didn't come across like that, even with the last line thrown in.

Btw, the point I was making isn't unrelated to what you were saying - it's ta the core of it and it's got nothing to do with pedantry.

 

[Gazamonk]

Lol the 'pedantry and jokes' bit was aimed at myself Mark, not you. This is getting all mixed up and misread and the last line wasn't 'thrown in' . I still don't think you read it in the vain it was meant. I was simply making a humorous retort to Jayst84's comment so accuracy in mathematical operators and notation was not required.

 

[SpikeK6]

Cor blimey this thread has moved along since I started it

Ask a simple question and it went off on another mathematical term "tangent"

So after reading all that it comes down to "they just are that length" because that's what they have always been. Makes sense about what the eye sees and they made a lens to suit that and go from there.

As for all the talk on f/stops and mathematical equations

 

[HoppyUK]

Cor blimey this thread has moved along since I started it

Ask a simple question and it went off on another mathematical term "tangent"

So after reading all that it comes down to "they just are that length" because that's what they have always been. Makes sense about what the eye sees and they made a lens to suit that and go from there.

As for all the talk on f/stops and mathematical equations

They are 'that length' because that's the field of view photographers want. That's as true today as it ever was.

And it's not got much to do with human vision. See how many words either side of this one you can actually read without moving your eyes? That's a tiny angle of view, like a couple of degrees, but at the same time you're 'aware' of anything up to 180 degrees to the sides.

 

[Simon photo]

The one slight problem with that is your use of the word aperture!

Aperture = focal length / ratio of focal length

Well of course, im no optical engineer and my maths leaves a lot to be desired. And yeah i see what you mean. Should have said focal length/size of the hole

 

[SpikeK6]

They are 'that length' because that's the field of view photographers want. That's as true today as it ever was.

And it's not got much to do with human vision. See how many words either side of this one you can actually read without moving your eyes? That's a tiny angle of view, like a couple of degrees, but at the same time you're 'aware' of anything up to 180 degrees to the sides.

I see what your saying but I do not think I am explaining myself very well.

See is this works. for get F stops

85mm prime
70-200
50-500
24-105

They all have 85mm in them somewhere for a set field of vision but why 70-200, etc. what makes a manufacturer decide one day he was going to make a 70-200 or any length for that matter.

 

[DemiLion]

Lol the 'pedantry and jokes' bit was aimed at myself Mark, not you. This is getting all mixed up and misread and the last line wasn't 'thrown in' . I still don't think you read it in the vain it was meant. I was simply making a humorous retort to Jayst84's comment so accuracy in mathematical operators and notation was not required.

Fair point!

 

[Nod]

Relatively short zoom ratios are easier to make with wide apertures than long ratios, hence the 24-70 and 70-200 f/2.8 options that are so popular. They're also relatively light and manageable for the user. It's possible to make a huge range fast zoom but not easy, cheap or light! (Look for the huge Sigma - can't remember the F/L range but it's a mahoooosive lump!)

Not sure why the ranges are what they are (thinking the Nikon trinity of 14-24, 24-70 and 70-200) other than history and what people they've consulted want. In the old days, there were few zooms available and even fewer decent ones, all we had were primes. My bag was (IIRC, all those years ago...) a 24mm, 50mm, 85mm and a 135mm. I lusted after but could never afford a Vivitar Series One 70-210 zoom but would still have needed the shorter primes. I expect there was a shorter zoom available but I don't remember it having the attraction or reputation of the 70-210.

 

[HoppyUK]

I see what your saying but I do not think I am explaining myself very well.

See is this works. for get F stops

85mm prime
70-200
50-500
24-105

They all have 85mm in them somewhere for a set field of vision but why 70-200, etc. what makes a manufacturer decide one day he was going to make a 70-200 or any length for that matter.

Ultimately, the customer decides. Manufactuers will make whatever they can make most profit from.

Their skill comes in finding the best blend of focal length range, max aperture, size, weight, build quality, performance and cost. And they're all in conflict.

 

[weybourne]

I used the simple formula of focal length divided by aperture for a lens i built to go on my homemade 4x5 camera. And it works.

The one slight problem with that is your use of the word aperture!

Aperture = focal length / ratio of focal length

Now that's confused me! I understand aperture to mean 'hole' or opening' (i.e. the hole in the diaphragm) and the 'f-ratio' (focal length divided by aperture) to be the comparative measure by which we can compare the amount of light that different lenses (with particular focal lengths and hole-sizes) can let through. Where does 'Aperture = focal length / ratio of focal length' come from and what does it mean?

 

[HoppyUK]

Now that's confused me! I understand aperture to mean 'hole' or opening' (i.e. the hole in the diaphragm) and the 'f-ratio' (focal length divided by aperture) to be the comparative measure by which we can compare the amount of light that different lenses (with particular focal lengths and hole-sizes) can let through. Where does 'Aperture = focal length / ratio of focal length' come from and what does it mean?

It's just not very clearly expressed.

The f/number is the focal length divided by the diameter of the aperture, expressed as a ratio. Eg, 50mm lens with an aperture 25mm across is f/2.

That's the theory, but in practise with some lenses (eg 600mm f/4) it's not the phyisical diameter of the aperture but the 'effective' aperture as seen through the lens, and they're not always the same thing. Sorry to complicate!

 

[weybourne]

It's just not very clearly expressed.

The f/number is the focal length divided by the diameter of the aperture, expressed as a ratio. Eg, 50mm lens with an aperture 25mm across is f/2.

That's the theory, but in practise with some lenses (eg 600mm f/4) it's not the phyisical diameter of the aperture but the 'effective' aperture as seen through the lens, and they're not always the same thing. Sorry to complicate!

It's beginning to make sense now So an array of lens elements is capable not only of putting the focal point in a different position to the effective focal length (e.g. making the lens body physically shorter for a telephoto lens) but also of magnifying the aperture - making it an effective diameter wider than the lens body. So the two components in working out the f-ratio for a modern complex lens are both likely to be virtual and not actual dimensions. I was thinking in terms of only the focal length being 'virtual' :bang:

 

[HoppyUK]

It's beginning to make sense now So an array of lens elements is capable not only of putting the focal point in a different position to the effective focal length (e.g. making the lens body physically shorter for a telephoto lens) but also of magnifying the aperture - making it an effective diameter wider than the lens body. So the two components in working out the f-ratio for a modern complex lens are both likely to be virtual and not actual dimensions. I was thinking in terms of only the focal length being 'virtual' :bang:

You're probably over-complicating it. Really all you need to know is that the f/number is a ratio and there is a direct relationship between it's size and focal length, that affects exposure. It means that longer lenses need much bigger apertures to maintain exposure level, hence the huge size of big telephotos, but how far beyond that you want to take it is optional.

You don't necessarily need to know that T/stops and f/numbers are a bit different, ie actual light Transmission relative to theory, or that both focal length and transmission only hold good at infinity focus. Internal focusing often reduces focal length, sometimes by as much as 20-30%, but not always; and when you get down to 1:1 with macro, you've lost two stops, eg f/2.8 becomes T/5.6 (inverse square law). Teleconverters also lose light for the same reason. TTL metering takes care of all these things.

 

[weybourne]

You're probably over-complicating it. Really all you need to know is that the f/number is a ratio and there is a direct relationship between it's size and focal length, that affects exposure. It means that longer lenses need much bigger apertures to maintain exposure level, hence the huge size of big telephotos, but how far beyond that you want to take it is optional.

You don't necessarily need to know that T/stops and f/numbers are a bit different, ie actual light Transmission relative to theory, or that both focal length and transmission only hold good at infinity focus. Internal focusing often reduces focal length, sometimes by as much as 20-30%, but not always; and when you get down to 1:1 with macro, you've lost two stops, eg f/2.8 becomes T/5.6 (inverse square law). Teleconverters also lose light for the same reason. TTL metering takes care of all these things.

Nah - not over-complicating it but gaining clarification. I'm fully aware of the stuff in your first paragraph above - it's the "optional" bit beyond that, and the way that modern 'complex' lenses behave/affect in relation to the basic 'f-ratio = focal length/aperture diameter' that I needed the clarification on. T-stops? I'll just accept that no lens is 100% efficient

 

[HoppyUK]

Nah - not over-complicating it but gaining clarification. I'm fully aware of the stuff in your first paragraph above - it's the "optional" bit beyond that, and the way that modern 'complex' lenses behave/affect in relation to the basic 'f-ratio = focal length/aperture diameter' that I needed the clarification on. T-stops? I'll just accept that no lens is 100% efficient

Haha, yes, I'm with you. I just enjoy collecting knowledge, and often it's useful for my work, but if you delve into the finer optical workings of a modern multi-element zoom you'll find all sorts of anomolies, most of which the manufacturers would rather we didn't know about But do they work? That's the important thing.

T/stops is an old cine/movie industry thing, going back to the time when they used complex zooms that absorbed a lot of light in the glass. Lens coatings were poor back in the day, and no glass ever transmits 100% of the light hitting it.

Video lenses are often still marked with T/stops, like f/2.8 and T/3.5 - 2/3rds of a stop different, though I'm not sure why they need to know this information any more than still shooters these days. Lens manufacturers are against it though, as it would immediately show that the f/2.8 lens you bought specifically for extra low light capability, actually didn't gain as much as you thought. Then there's vignetting to consider, distortion is also an effective focal length change (v slight) and so it goes on