Focal length "equivalence": a proposal

[StewartR]

We're all familiar with the notion of focal length "equivalence": the idea that a lens of focal length "X" on camera "A" is "equivalent" (in some sense, usually field of view) to a lens of focal length "Y" on camera "B". Given that different cameras can have differently sized sensors, it's pretty much inevitable that we have to grapple with this concept. If I've used, say, a 300mm lens to take a particular photo, I can't tell you whether you would get the same results without knowing what camera you have; and then I might say that the "equivalent" lens for you (i.e. the one that frames the subject the same way) would be, say, 500mm.

As I said, the concept is inevitable. But the way we talk about it isn't, and that's where my proposal comes in.

There is definitely a powerful (but probably unthinking) tendency to define the 35mm sensor with a 3:2 aspect ratio as "normal", and to express everything in terms of this. "Oh, so your Micro 4/3rds lens is 25mm? Well, that's equivalent to 50mm." "200mm on a Nikon DX camera is equivalent to 300mm." And so on.

To my mind this unconscious bias is unhelpful. It perpetuates the myth that "full frame" camera are "normal", and therefore that "cropped sensor" cameras (and, by extension, people using such cameras) are "inferior". It creates and feeds this pointless and expensive process whereby so many people feel they need to "upgrade" to full frame, regardless of the fact that for the majority of them it won't have any practical impact on their photography. And it is clearly not the case that full frame cameras are "normal". For example here's a chart from Thom Hogan [1], showing Nikon worldwide sales from 2007 to 2015. You could pick data from different manufacturers and/or different years, if you like, but you know even before you do it that the results will be broadly similar. The full frame "standard" is anything but "standard".



So I propose that we adopt a different language for talking about focal length equivalence.

The first proper photograph taken with a camera is widely held to be "View from the Window at Le Gras", created by Nicéphore Niépce in 1826 or 1827 [2]. It was made on a "sensor" - actually a pewter plate coasted with bitumen - which measured 20.2 cm x 16.2 cm. It makes obvious sense to me to honour Niépce by adopting this as the "standard" to which other camera systems are compared. This has the clear advantage that no current camera systems use this size, so no current systems would be "normal" - instead, it would reflect the simple reality that cameras come in different sizes and no size is intrinsically superior to any other.

It would need a bit of adjustment in terms of the kinds of numbers we're used to using, but that's OK. The diagonal measurement of Niépce's "sensor" was 259mm so that gives us, for example:

• A Micro 4/3rds camera with a sensor measuring 17.3mm x 13mm has a crop factor of 12.
• A Nikon DX camera with a sensor measuring 24mm x 16mm has a crop factor of 9.
• A "full frame" camera with a sensor measuring 36mm x 24mm has a crop factor of 6.
• A Fuji GFX camera with a sensor measuring 43.8mm x 32.9mm has a crop factor of 4.5.
• A Phase One camera with a sensor measuring 53.7mm x 40.4mm has a crop factor of 4.

That all seems pretty straightforward.

Then we'd need to start thinking about lenses properly in terms of their "equivalent" focal length. There are so many different types of lenses, and we all have our favourites, so I don't think it would be helpful to give too many worked examples. But things would be rather different - for example we'd want to start thinking of an 18-55mm "kit" lens as being "equivalent" to roughly 160mm to 500mm in Niépce standard terms. That does feel a bit weird to us, but it wouldn't to Niépce. However, overall the numbers work out quite nicely:

• 100mm "equivalent" focal length is ultra-wide
• 200mm "equivalent" focal length is wide
• 300mm "equivalent" focal length is normal
• 400mm to 600mm "equivalent" focal length is a short telephoto
• 700mm to 1000mm "equivalent" focal length is a telephoto
• anything longer is a super-telephoto

How much easier could that possibly be?

I think it's time. What do you think?


[1] http://www.dslrbodies.com/newsviews/how-do-fx-and-dx-sales.html
[2] https://en.wikipedia.org/wiki/View_from_the_Window_at_Le_Gras

 

[HoppyUK]

You've just changed one reference standard - currently full-frame 35mm that most people can relate to and have used with film - for another completely meaningless reference of a huge pewter plate some 200 years old that was almost one foot across. Not sure that will catch on

 

[snerkler]

I can't decide whether this is a serious post or not?

 

[Cagey75]

Crop factor of 12 You just made m43 worse than ever

Zack Arias does it best, why crop doesn't matter -

 

[bass_junkie83]

The real question is do you hire out 20.2 cm x 16.2 cm systems so that I can give it a try and see if it is worth the upgrade?

 

[Canon Bob]

Your average healthy hippo has a BMI of 750. If we accept that as normal then we conquered obeisity.

 

[woof woof]

It may work for youngsters but I grew up with film and specifically 35mm film and for whatever reason that's what I always think back to and I don't think it'll matter if I live long enough to have used digital longer. I think I'll always relate back to 35mm film.

I'd agree that for people getting into all this with cameras that aren't 35mm or 35mm equivalent all this equivalence talk is probably next to meaningless and just adds confusion.

 

[LongLensPhotography]

I will respond to this once we change all distance and speed measurements to meters and kilometers in this country

Metric system is demonstrably more relatable and logical. Inches, yards and miles clearly belong in the history landfill.

 

[Harlequin565]

That all seems pretty straightforward.

I'd vote for you! However I suspect we'd be a minority.

This disparity is one of the hardest things to explain to beginning photographers and comes in (in terms of misconceptions) below "full frame is best" but above "you need 300dpi"

I'm just glad that instead of being disappointed in my new 50mm lens, I discovered it has the same field of view as my Fuji 16mm!

 

[woof woof]

I will respond to this once we change all distance and speed measurements to meters and kilometers in this country

Metric system is demonstrably more relatable and logical. Inches, yards and miles clearly belong in the history landfill.

Hmmm.

I don't think so

Measurements based on parts of the human body (oh er) such as thumbs, hands etc are older and would make more sense than the more modern but more artificial, invented and rather more baffling metric system. A metre based upon the earth? meaningless IMO

 

[Nostromo]

I must admit that I've never used ff digital of 35mm film (ok I have used film, but it was a long long time ago and I don't really remember much about it. I just took photos/snaps, sent the film off and got my prints back).
In the last 3 or 4yrs I've taken more of an intrest in photography and have only used APS-C cameras and am so used to what any given focal length looks like on these cameras, I take that fov as normal. The ff equivalent is pointless to me, I can't see that I need to know it (unless someone can enlighten me of course). I did look at going ff a while ago and after looking into lens and what I would need to buy to get the effective reach I get on my longer lenses, I thought better of it. I'm really quite happy using a APS-C.

 

[tijuana taxi]

About as likely to be adopted as Esperanto

 

[DG Phototraining]

If the equivalence is really about Field of View (FoV) then if (BIG IF) anything needs to change then surely adopting that is enough

Just replace 50mm on the lens barrel with the simpler 39.6 degrees Horizontal on a FX sensor and supply a conversion table for other sensor sizes, perhaps etched into the lenshood - simples

Dave

 

[ukaskew]

Then we'd need to start thinking about lenses properly in terms of their "equivalent" focal length. There are so many different types of lenses, and we all have our favourites, so I don't think it would be helpful to give too many worked examples. But things would be rather different - for example we'd want to start thinking of an 18-55mm "kit" lens as being "equivalent" to roughly 160mm to 500mm in Niépce standard terms. That does feel a bit weird to us, but it wouldn't to Niépce. However, overall the numbers work out quite nicely:

• 100mm "equivalent" focal length is ultra-wide
• 200mm "equivalent" focal length is wide
• 300mm "equivalent" focal length is normal
• 400mm to 600mm "equivalent" focal length is a short telephoto
• 700mm to 1000mm "equivalent" focal length is a telephoto
• anything longer is a super-telephoto

How much easier could that possibly be?

I actually like this, a lot. The numbers do all work out quite neatly.

 

[Willo]

Might want to tell Olympus to stop putting 35mm equiv on their boxes and marketing material.

 

[Ed Sutton]

I think there is a problem with 'equivalence' when different aspect ratios are used because (IIRC) the equivalence is based on the diagonal of the sensor/film, which doesn't equate with how we perceive the angle of view - horizontally. A 25mm lens on m4/3 gives a slightly narrower horizontal angle than 50mm on a 35mm sensor. Or it did when I tried a rough and ready comparison.

 

[HoppyUK]

If the equivalence is really about Field of View (FoV) then if (BIG IF) anything needs to change then surely adopting that is enough

Just replace 50mm on the lens barrel with the simpler 39.6 degrees Horizontal on a FX sensor and supply a conversion table for other sensor sizes, perhaps etched into the lenshood - simples

Dave

That's more logical and there was a suggestion years ago that photo magazines should adopt it. On the magazines I worked on, we included field-of-view alongside focal length for a while, but then it got forgotten (and nobody noticed).

A slight problem with field-of-view is that lens manufacturers always quote the angle across the diagonal, from corner to corner rather than side to side. And we'd still need actual focal length and crop factor to work out other things like f/number and depth-of-field.

 

[john.margetts]

If the equivalence is really about Field of View (FoV) then if (BIG IF) anything needs to change then surely adopting that is enough

Just replace 50mm on the lens barrel with the simpler 39.6 degrees Horizontal on a FX sensor and supply a conversion table for other sensor sizes, perhaps etched into the lenshood - simples

Dave

Canon do this. When you buy a Canon lens you get a chart showing horizontal, vertical and diagonal field of view on both full frame and crop frame. It is not Canon's fault if no one pays any attention.

 

[sk66]

I think there is a problem with 'equivalence' when different aspect ratios are used because (IIRC) the equivalence is based on the diagonal of the sensor/film, which doesn't equate with how we perceive the angle of view - horizontally. A 25mm lens on m4/3 gives a slightly narrower horizontal angle than 50mm on a 35mm sensor. Or it did when I tried a rough and ready comparison.

It's ~ a 45* diagonal FOV because both your eyes and a lens see/project a circular FOV. The only way to measure a circular FOV is at the widest point, which equates to the diagonal of a rectangular excerpt.

Edit: if it wasn't a diagonal measure then it would also change with sensor aspect (i.e. 3:2/4:5/6:6).

 

[Ed Sutton]

... both your eyes and a lens see/project a circular FOV.

Only if you are a cyclops.

 

[sk66]

It creates and feeds this pointless and expensive process whereby so many people feel they need to "upgrade" to full frame, regardless of the fact that for the majority of them it won't have any practical impact on their photography.

While I agree that this is often the case... IMO, the differences/benefits really have nothing to do with FL or what is considered a normal lens.

And even if we could find a way to standardize/clarify/simplify on this forum, you're still going to have to battle with the conventions of the rest of the world/web.

 

[sk66]

Only if you are a cyclops.

Primary/binocular vision works by "overlaying" the two images as a single circular image, that's what gives you the ability to perceive depth. Along with monocular visual cues when the distance prevents the binocular effect. Only periphery vision is strictly monocular, and it is not considered part of the "normal"/primary visual FOV.

Edit: periphery vision makes the FOV oblong... but just try to bring something in the periphery into focus w/o moving your eyes; you can't. The FOF(focus) is actually quite narrow, something like 3* IIRC. But the wider primary/normal FOV is still in focus enough to be of high visual value. Periphery vision is only useful for spatial awareness.

 

[gad-westy]

About as likely to be adopted as Esperanto

mi konsentas 100%

 

[Mr Badger]

Snip:

I think it's time. What do you think?

I think time is something that you perhaps had just a little too much of this morning!

 

[HoppyUK]

I will respond to this once we change all distance and speed measurements to meters and kilometers in this country

Metric system is demonstrably more relatable and logical. Inches, yards and miles clearly belong in the history landfill.

What's the time on your metric clock?

 

[Mr Badger]

What's the time on your metric clock?

If it's accurate it should say 7 months to Brexit!

 

[jakeblu]

Hmmm.

I don't think so

Measurements based on parts of the human body (oh er) such as thumbs, hands etc are older and would make more sense than the more modern but more artificial, invented and rather more baffling metric system. A metre based upon the earth? meaningless IMO

I'm sure you've seen this before, but it does pretty much sum up imperial measurements

 

[LongLensPhotography]

What's the time on your metric clock?

Time needs sorting for sure. Second is SI unit and that's where the common sense ends. When you need to do any sort of maths with [day]time it is real headache. That's even before you get into crazy stuff like daylight savings (that's because a day is not 24 hours precisely) and leap year!

If am sure if apple came out with alternative watch everybody would jump on the bandwaggon immediately

 

[sirch]

I just got into 4x5" format but do you think I should upgrade to 8"x6 3/8", I'm feeling that I need that full frame look?

 

[LongLensPhotography]

I just got into 4x5" format but do you think I should upgrade to 8"x6 3/8", I'm feeling that I need that full frame look?

Why not go even bigger?

 

[Mr Badger]

As Britain invented the steam engine, the steam turbine, the electric lightbulb, the telegraph, the telephone, television, the marine chronometer, radar, the glider, the jet engine, the tension-spoked wheel, the pneumatic tyre, the passenger railway, the rifled gun barrel, the military tank, the safety bicycle, the seed drill, the spinning frame, cement, the electric motor, the electronic programmable computer, the hovercraft, waterproof clothing, disc brakes, the electric vacuum cleaner and the photographic negative, to name but a few, all using 'imperial' weights and measures, it doesn't seem to have held us back too much, does it?!

 

[snerkler]

I will respond to this once we change all distance and speed measurements to meters and kilometers in this country

Metric system is demonstrably more relatable and logical. Inches, yards and miles clearly belong in the history landfill.

I'm of the generation that's in-between ie I use both metric and imperial. Measurements I can talk in mm, cm, m, inches and feet but I get lost with yards. Weight I generally talk kg but not with food, I'd order 1/4lb of bacon for example but don't ask me what that is in grams. Weight of people I still work in stones and pounds. Distance I talk miles for large distances but meters for shorter distances. There's really no hope for me

 

[Mr Badger]

I'm of the generation that's in-between ie I use both metric and imperial. Measurements I can talk in mm, cm, m, inches and feet but I get lost with yards. Weight I generally talk kg but not with food, I'd order 1/4lb of bacon for example but don't ask me what that is in grams. Weight of people I still work in stones and pounds. Distance I talk miles for large distances but meters for shorter distances. There's really no hope for me

1/4 lb of bacon? You need at least 6oz to make a decent round of bacon butties! Hope you still drink in pints too.

 

[snerkler]

1/4 lb of bacon? You need at least 6oz to make a decent round of bacon butties! Hope you still drink in pints too.

Sure do

 

[DG Phototraining]

I'm of the generation that's in-between ie I use both metric and imperial. Measurements I can talk in mm, cm, m, inches and feet but I get lost with yards. Weight I generally talk kg but not with food, I'd order 1/4lb of bacon for example but don't ask me what that is in grams. Weight of people I still work in stones and pounds. Distance I talk miles for large distances but meters for shorter distances. There's really no hope for me

I'm quite happy to use either and can fairly easily flick between them in the more 'usual' Imperial measurements

However, I've no idea where we are with acres, furlongs, chains, and I'm totally lost on leagues, rods and cables, let alone minims, gills and scruples !!!

Dave

 

[snerkler]

I'm quite happy to use either and can fairly easily flick between them in the more 'usual' Imperial measurements

However, I've no idea where we are with acres, furlongs, chains, and I'm totally lost on leagues, rods and cables, let alone minims, gills and scruples !!!

Dave

Say what now?

 

[Terrywoodenpic]

if they printed the angle of View, for the intended sensor size, on all lenses as well as the focal length. in time we would all begin to think in terms of angle of view when selecting a lens.

As it is, we all know what to expect if we put an 18-55 on an Aps camera. However very few of us know what that is in terms of included angle of view. even fewer could give numbers for it.

Eventually we would all be able to talk about and instinctively know what lens would be needed for what.
A full frame user and a crop sensor users will still be using different focal length lenses, but would use lenses designated with the same angle of view.
Photographers would soon learn that lenses over 90° produce obvious geometric distortion. and that a 5° angle is an extreme telephoto.
And that is true for every camera and sensor.

 

[snerkler]

if they printed the angle of View, for the intended sensor size, on all lenses as well as the focal length. in time we would all begin to think in terms of angle of view when selecting a lens.

As it is, we all know what to expect if we put an 18-55 on an Aps camera. However very few of us know what that is in terms of included angle of view. even fewer could give numbers for it.

Eventually we would all be able to talk about and instinctively know what lens would be needed for what.
A full frame user and a crop sensor users will still be using different focal length lenses, but would use lenses designated with the same angle of view.
Photographers would soon learn that lenses over 90° produce obvious geometric distortion. and that a 5° angle is an extreme telephoto.
And that is true for every camera and sensor.

FOV would certainly be a better option imo, there's no equivalence then, everything is on a level paying field

 

[Mr Badger]

A full frame user and a crop sensor users will still be using different focal length lenses, but would use lenses designated with the same angle of view.

Your new idea lenses might be marked with a field of view, but they wouldn't visually give the same field of view when fitted to a crop sensor and a full frame camera, would they? So we'd be back to square one again.

I know, let's just do away with crop sensor cameras and make everyone use a proper one (full frame) and then we'd know where we were! Joking aside, I think people soon get the hang of the current 35mm focal length equivalent system, so why reinvent the wheel? Unless of course it's for a tension-spoked one fitted with a pneumatic tyre and measured in inches, or parts thereof!

 

[Nod]

I'm of the generation that's in-between ie I use both metric and imperial. Measurements I can talk in mm, cm, m, inches and feet but I get lost with yards. Weight I generally talk kg but not with food, I'd order 1/4lb of bacon for example but don't ask me what that is in grams. Weight of people I still work in stones and pounds. Distance I talk miles for large distances but meters for shorter distances. There's really no hope for me

I'm the same, although with measurements, I drop in and out. Very small tends to be thought of in thou(sandths of an inch) up to a millimetre then millimetres take me up to an inch or so then a yard (on the golf course) or metre until we hit a mile, although when driving an LHD car, km slide in. Weights depend on which scales I'm using - I use imperial weights on the balance scales but tend to have the digital scales set to grammes. I'm about 18 stone and would have to look at the scales to see what that is in kg! Cider comes in pints (or cans - not sure how much they hold but it fits in a pint glass with a little room to spare.)

For lenses, surely the diagonal of the sensor is approximately the "standard" length and anything smaller is a wide angle and larger is a telephoto. Equivalence only really needs to come into the equation if the same lens is used on different bodies or a user uses 2 or more different sized sensors and needs to think about it.