Our discussion forums are a great resource for people looking to learn about photography, and it's therefore not surprising that certain topics are raised time after time. In many cases they are answered quickly and correctly, but certain topics are still subject to a great deal of confusion. One of these is the deceptively simple question of what the word 'standard' means in relation to lenses. It appears that there are certain misconceptions associated with this topic which reappear time after time, so in a bid to put the record straight, here's my attempt at an (almost) definitive answer.
At the simplest level, a standard lens is defined as one which produces images with a natural-looking perspective (the word 'normal' is often used synonymously in this context). This concept is perhaps best illustrated with regard to what it's not, that is to lenses which clearly don't meet this criterion. A telephoto lens, for example, renders distant objects larger in the frame, and has the effect of compressing the apparent spatial relationship between objects. A wideangle does precisely the opposite; in squeezing more content into the image, objects appear smaller and more distant. It's in the happy medium between these two extremes that the standard lens lies; the apparent sizes and spatial relationships between image elements appear natural, and much as they did in real life. So the question is, how de we define which lenses show these characteristics?
The answer lies in considering how we view images, and is perhaps easiest understood with respect to prints. Everything hinges on the fact that (with the exception of small prints) the most comfortable viewing distance is approximately equal to the print diagonal. For example, a 12" x 16" print has a diagonal of about 20", and it turns out that most viewers will choose to look at it from about 20". This allows them to take in the whole of the picture, while still being able to see fine detail.
So how does this relate to the focal length of a 'standard' lens? To understand this, let's conduct a thought experiment in which we take a picture, make a print on the spot, then see how it compares to the scene in front of us. Simple geometry (using the concept of similar triangles) states that a print viewed from a distance equal to it's diagonal will exactly match the subject in perspective when the focal length of the lens that was used to make the picture is equal to the diagonal of the sensor.
When we use this approach to calculate the focal lengths of 'theoretically correct' standard lenses for various sensor sizes, the results are slightly surprising. Full frame works out as 43mm, 1.5x APS-C as 29mm, and Four Thirds, 21mm. This flies in the face of received wisdom (and convention), which suggests 50mm, 35mm and 25mm respectively. How did that happen?
The answer goes all the way back to the earliest 35mm cameras, such as Oscar Barnack's Leica. For practical reasons of lens design as much as anything else, these were often fitted with 5cm lenses, and for various reasons this became a convention which firmly stuck. Oddly enough, the 35mm format counts as something of an outlier in this regard; the various 'medium' formats (such as 6 x 4.5cm) all stuck stubbornly to the 'frame diagonal' convention. Still even for 35mm there were honourable exceptions; many fixed-lens rangefinders (the progenitors of modern compacts) used lenses in the 40-45mm range, and Pentax famously launched its 'Limited' range of premium primes with a 43mm standard lens, which is still on sale today. And while it may sound like there's little difference between 43mm and 50mm, in reality it's the same as that between 28mm and 35mm, or 85mm and 100mm. There is, in fact, a fairly well-developed school of thought (which I must admit I personally subscribe to) that for a standard lens, that 40mm region is 'about right'.
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The Olympus 35RC ca. 1971, with its 42mm F2.8 standard lens |
Now that we've established what a standard lens really is, we can dispense with some of the misconceptions which continually reappear. A standard lens is not one which makes object appear the same size in the camera's viewfinder as they appear from the same position with the naked a eye; this is a function of both focal length and viewfinder magnification (indeed with a 50mm lens focused to infinity, it's used as the definition of viewfinder magnification). And a standard lens is not - quite - a lens with the same angle of view as the human eye; peripheral vision extends well beyond the angle of view occupied by the print at that 'comfortable' viewing distance.
So there we have it, the definition of a standard lens. Of course the punchline (and there always has to be one) follows on from my previous blog post, in which I decried the lack fast 'portrait' primes for APS-C, with the much-touted 50mm lens being a bit short for this role. Well to be honest, I wish manufacturers wouldn't build 50mm lenses for full frame either, but instead make their standard primes in that 43mm region instead (likewise rather than 35mm for Nikon's new DX prime, I'd have preferred 28mm). But at least for once I know this is an eccentricity, and I'm firmly in the minority in this regard.
I remember with fondness the Zuiko 40mm f2 for this reason, a lovely natural perspective.
To make matters worse, most modern 50mm lenses for FF are about 52 or 53 mm in fact. A nice thing about the sigma 50 1.4 is that I calculate it must really be around 47mm. Not that much different, but noticable.
Posted by: David Braddon-Mitchell | 21 Mar 2009 04:20:55
The diagonal of a 24x36 mm exposure is 43.266 mm. The explanation you provide says that 43.266 mm would then be the focal length of a 'Standard' lens for 35 mm film and 'Full Frame' dSLRs. Such a lens would provide an angle of view of 45.177°. If we use a 35 mm film or 'Full Frame' dSLR exposure to create a 12x16 inch print, we would need to crop the exposure to 24x32 mm in order to fit the aspect ratio of the print. When we crop off the outer 4 mm of the exposure, we narrow the angle of view down to 40.589°.
But if we start with an exposure that is 24x32 mm, the focal length of its 'Standard' lens would be 40 mm, and that lens would provide an angle of view of 43.602°. That would project an image that has a different perspective from an image projected from an angle of view of 40.589°, from the same vantage point.
As much as I would like there to be a method of determining the focal length of a 'Standard' lens with a mathematical certainty, I don't think this one stands up to much scrutiny. If the act of cropping alters the determination of what is, and what is not, a 'Standard' lens, and since most images are cropped from their original aspect ratios anyway, what's the point of seeking out a 'Standard' lens for an exposure format anyway. Shouldn't we be searching for a 'Standard' lens that is appropriate for our prints?
I'm not arguing that a lens with a 50 mm focal length is the 'Standard' lens for 35 mm film and 'Full Frame' dSLRs. I'm just saying that this explanation doesn't have much of a mathematical basis.
Posted by: TCav | 21 Mar 2009 12:48:31
The above comment makes the commonly found error of assuming that a change in focal length alters perspective. All images taken from the same vantage point will have exactly the same perspective irrespective of focal length. Enlarge the centre of a short focal length shot and it will exactly match that of a long focal length shot. The confusion comes because with a short focal length lens one tends to get closer to the subject to fill the frame and 'viewpoint' does affect perspective. In fact 'viewpoint' is the only factor that affects perspective.
Posted by: Martin Winson | 21 Mar 2009 18:54:39
Thanks to the above contributor. After I posted my comment, I realized that the phrase "from the same vantage point" was wrong, and should not have been included. Otherwise, the rest of my comment is ok.
And, obviously, I agree that vantage point does affect perspective. My point is that, for the same frame, two different lenses could be called 'Standard' using the method described by the author, and each of them would produce unique perspectives. Therefore, neither of them can truly be called 'Standard'.
A 'Standard' is an object that represents a fixed quantity against which other objects are compared. A gauge block is a standard that is used to calibrate micrometers. There are no 'Standard' lenses. Unless, you're talking about 'Standard Equipment', as in 'Not Optional', like the stuff that comes on a car that you don't have to pay extra for. In that case, a 'Standard Lens' would be the one that comes with the camera, though that term seems to have been replaced by 'Kit Lens'.
What we're discussing here is a 'Normal' lens, which is a lens that reproduces a normal, or natural-looking perspective, which is an angle of view of about 40-45°. That, btw, has nothing to do with the length of a diagonal across the image sensor. Any apparent correlation between the focal length of a 'Normal' lens and the diagonal measurement of an image sensor is coincidental. As evidence, I offer 16:9 image sensors. The Panasonic Lumix TZ50 has an image sensor that, at the aspect ratio of 16:9, is wider than at other aspect ratios, yet a diagonal measurement of the image sensor for each of its aspect ratios is similar enough to call 7.7 mm the 'Normal' focal length for its lens. Yet no one is likely to call a 16:9 image "a natural-looking perspective". Conversely, the Panasonic Lumix FZ28 has an image sensor roughly the same size, yet the horizontal resolution is constant regardless of the aspect ratio selected. So, obviously, the diagonal measurement of the area of the image sensor in use would vary with the aspect ratio selected. The focal length of the 'Normal' lens would be different for each aspect ratio. But, once again, no one is likely to call a 16:9 image "a natural-looking perspective", regardless of the focal length of the lens used to obtain it.
Can we agree that "a natural-looking perspective" is sufficiently ethereal that trying to determine its precise value mathematically is an objective unworthy of our collective effort, and that the method described by the author is flawed?
Posted by: TCav | 21 Mar 2009 23:13:11
A standard lens is whatever lens the manufacturer considers standard for a camera. By this token, for APS-C bodies, an 18-55mm f/4 or thereabouts is the new standard.
A normal lens is one such as if you print a photo and view it at a normal viewing distance, it exactly matches reality. Thus, the focal length of a normal lens cannot be specified independently of print size and your normal print viewing distance.
Posted by: Fazal Majid | 22 Mar 2009 08:26:24
Two points:
Perhaps it would have been more appropriate for me to use the word 'normal' rather than 'standard', but the two terms are used near-interchangeably. And the definition I've used for standard is placed transparently at the start of the article.
Beyond that, there's only one premise, that the preferred viewing distance is about equal to the print diagonal. You may disagree with that if you please, but the deviation from it as a rule is not extreme.
From then on in, the argument is simple trigonometry - unarguable in any way. If you change the format of the camera (for example by arbitrarily decided you have to crop every frame) then the results come out different. That's the whole point, it's why full frame and APS-C give different answers after all.
Posted by: Andy Westlake | 22 Mar 2009 11:36:48
Two things. First, Andy, this is slightly incorrect:
"And while it may sound like there's little difference between 43mm and 50mm, in reality it's the same as that between 28mm and 35mm, or 85mm and 100mm."
43:50 as 30:35 and 86:100.
This part in someone's comment is much more of an issue though:
"All images taken from the same vantage point will have exactly the same perspective irrespective of focal length. Enlarge the centre of a short focal length shot and it will exactly match that of a long focal length shot."
I don't know why I keep seeing this recently, but it is flat out wrong. You can test it yourself easily enough. Take a lens with a decent zoom range. Center on something with good angles/perspective at wide angle, and take the shot. Zoom all the way in and take the shot again, and then try to line that shot up shrunk on top of the wide shot: it won't work, the perspective will be different.
Posted by: hoppa | 22 Mar 2009 17:06:12
I'll join you in the minority. Actually, I did enjoy shooting a 50mm on film, but prefer 28 to 35 on APS-C and really liked the 40mm lens on my old HiMatic.
Re: Hoppa - the perspective cannot be different. Maybe distortions keep the images from lining up.
Posted by: Dennis | 23 Mar 2009 02:10:53
@hoppa:
By definition, perspective depends only on your position relative to the subject - it's literally your 'point of view'.
So why doesn't your own test work? Chances are you're running into the bane of experimentalists everywhere - the problem of implicit assumptions in the initial premise which aren't being met. I can think of several possibilities that might be to blame:
1) As mentioned by Dennis, geometric distortion. If you're using a 'standard' zoom (covering wideangle to telephoto), then the longer focal length shot will likely suffer from pincushion distortion, whereas the centre crop from the wider shot will be essentially undistorted. The longer the zoom range of the lens, the bigger the problem is likely to be.
2) Actual focal lengths vs marked. If you're scaling the images according to the numbers marked on the lens, this will give rise to small errors, as the actual focal lengths won't be quite the same.
3) Subject distance. The test really should be conducted with subjects a long distance away, for two reasons. Firstly, the angle of view of the lens changes on focusing, so this will complicate your scaling. Secondly, if you're using a lens with an extending front element, then zooming in can indeed visibly change the perspective for closeup shots; the lens is effectively looking at the subject from a slightly closer position.
So how might we get the test to work? First of all, use a distant subject. Secondly, use either well-corrected primes with minimal distortion, or correct geometric distortion in software before comparing the images. And finally, don't rely on scaling according to marked focal length, but be prepared to make further adjustments.
The take-home message is that even the simplest tests tend to be more complex than you'd think.
Posted by: Andy Westlake | 23 Mar 2009 09:58:01
Some years ago I took a series of photos of a street scene from just one place but with various different focal length lenses. I then got people to take the prints and go and stand where they thought the picture was taken from. The idea was that the most representative photo of the scene - and therefore I suppose the lens that is closest to 'standard' - would be the one that took people closest to the actual place where the camera was. The answer was also a lens with a focal length around 40mm on a 35mm camera. At the time this was to determine what focal length would be best for presenting a view of a scene to a court - the received wisdom was 50mm at the time.
Posted by: Roger | 23 Mar 2009 10:08:17
Please switch from justified type to left-align type. Your text is needlessly difficult to read.
Thanks.
Posted by: Kickstand | 24 Mar 2009 17:12:33
If I were to stitch pictures together to make a panorama using a standard lens (a 28mm on an APS-c), would it still produce a naturally looking persepctive like that from a singel frame?
Posted by: Michael | 24 Mar 2009 19:42:10
If it's a circular panorama, the perspective is going to be natural no matter how you stitched it.
Posted by: Fazal Majid | 24 Mar 2009 22:25:09
Andy,
We have become somewhat accustomed to the slightly "cropped" or "tunnel vision" look of the 35 mm format shot through a 50 mm lens. Is there any sense that the slightly longer-than-diagonal standard lens created this convention, or was it a natural result of the "miniature" format and its typically smaller printing/display sizes?
Posted by: oms | 25 Mar 2009 16:50:24
Andy wote: "Beyond that, there's only one premise, that the preferred viewing distance is about equal to the print diagonal. You may disagree with that if you please, but the deviation from it as a rule is not extreme."
That would put the preferred viewing distance of a 4x6 print at about 7 inches. NOBODY DOES THAT!
That would put the preferred viewing distance of an 8x10 print at about 13 inches. That is about 50-75% too short, but something else I have noticed is that people will hold an 8x10 print further away than a 10x8 print.
What does that do to your premise?
Posted by: TCav | 25 Mar 2009 18:37:37
Опутеть как интересно, во задвигаете. Класс!
Posted by: illeft | 25 Mar 2009 20:31:46
I've read a number of articles that reference this 'Normal' viewing distance, but none actually reference how this 'Normal' viewing distance was determined. Did somebody actually hang a large image on a wall where observers would be free to position themselves anywhere they wanted while looking at it, and after watching several observers, the average viewing distance was calculated and found to be the diagonal of the image? I would think that this would be an amazing coincidence. Perhaps this was actually done, but nobody references it that I can find.
But let's presume that it was. That would mean that half the observers positioned themselves closer than the average viewing distance, and half positioned themselves further away. So half the people would have been less than 25 inches from a 16x20 image, some probably considerably closer. That strikes me as particularly odd.
But that would also imply that the 'normal' viewing distance for the Mona Lisa, which is 77x53 cm, would be 93.5 cm. I don't actually know how far people are kept from the Mona Lisa, but surely it's further away than the length of the arm of an average adult male.
This premise has always struck me as odd, and no amount of trig piled on top of it will make it any less odd. You can't honestly say that something is 'Normal' unless it really is normal, and it would be simple enough to figure out if it is normal, but if someone did it, then why hasn't anyone referenced it.
Posted by: TCav | 26 Mar 2009 01:13:58
The calculations are very interesting - thanks for taking the trouble. However, the viewing distance assumption is inevitably an approximation, and one that is wide enough to include 50mm comfortably as an approximation to 43mm.
The bottom line is subjective and personal, and here I agree completely with Andy. I love those standard rangefinder lenses of 40mm or so from the 1960's and 70's. It's a great length to work with, and to me feels neutral, neither wide nor long.
I'm cheered by Panasonic's plan (as far as I know) for their first micro 4/3 prime to be a 20mm (equivalent to 40mm).
Another definition of 'standard' or 'normal' (alongside 'matching human visual field' and 'supplied routinely with the camera') might be 'the focal length for which the best and smallest lens can be designed' - which will be different for different systems, and would (I think) be wider for rangefinders than SLRs, and wider for micro 4/3 than for other current systems - so perhaps it's ultimately down to the practicalities of lens design.
Posted by: Matt | 26 Mar 2009 10:47:42
If the focal length of a theoretically correct standard lens for full frame works out as 43mm, then with that figure lying exactly between 35mm and 50mm, is 35mm just as close to the 'standard' ideal as a 50mm lens?
Posted by: Rob | 26 Mar 2009 12:10:43
No definition of the term "Standard" would apply to a lens, except that it is the lens that comes free with the camera. Clearly, that has nothing to do with it's focal length or angle of view. The fact that it might be easier, faster, or cheaper to make, should not grant it the title "Standard" as you use it here. Suppose a lens of one focal length is easier to make for one manufacturer, but a lens of a different focal length is cheaper to make for another. Which focal length would be entitled to use the moniker of "Standard"?
Likewise, "Normal." Something is "Normal" if it is average. You can't call something "Normal" if you haven't determined what "Normal" is, and you can't do that until you've calculated a Mean, a Median and/or a Mode. To my knowledge, no one has done that. And even if they did, I would want to see the data.
These are words that are already in wide use in the English Language, with widely accepted definitions. When you apply either of them to a lens, they bring with them the established definitions. People will think that the definition applies to the lens. If I used either of the terms we're discussing to describe a particular dog, the owners of other dogs would certainly object.
An arguement could also be made for the term "Blanket Lens" since such a lens would COVER most photographic requirements.
But I would want to see the data on that as well.
Posted by: TCav | 26 Mar 2009 19:22:13
Хорошо пишете. Надеюсь, когда-нибудь увижу нечто подобное и на своем блоге…
Posted by: aliecealom | 28 Mar 2009 00:59:31
You're not alone in your preference, Andy - I would have loved a fast 28mm Nikkor for DX. I noticed when looking at photo metadata that I always ended up shooting at 27-30mm on DX when trying to achieve a perspective that appeared invisible. That chimes exactly with your observations and suggestions. I'm currently using a 28mm Sigma f/1.8 DG EX Macro (!) as a normal lens on DX - it's the third 28mm lens I've tried and the first that delivers optical performance I'm really happy with. It was an eBay bargain. Shame about the bulk and the weight but it's a price well worth paying.
Excellent article - thanks.
Posted by: Bahi | 28 Mar 2009 03:51:48
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Posted by: Houcrara | 28 Mar 2009 15:53:02
I agree with TCav and Martin Winson. Andy Wetlake's concession on "normal" instead of "standard" cements the deal.
My very first 35 mm camera came with a fixed 43mm f/1.8 lens. My subsequent fixed lens cameras have 48mm, 50mm, 55mm, and 40mm. The "normal" lenses of my SLRs are 50mm, 52mm, 55mm, & 58mm. Max f/stops of those lenses ranges from 2.0 to 1.2.
For practical reason, I consider all those lenses "normal" and provided as "standard" lens by the camera manufacturer, hence, all the other lenses (wide angle, zooms, telephotos, macros) are not normal nor standard issues.
Posted by: ecube | 28 Mar 2009 17:20:48
Hello! Very Interesting post! Thank you for such interesting resource! PS: Sorry for my bad english, I'v just started to learn this language ;) See you! Your, Raiul Baztepo
Posted by: RaiulBaztepo | 29 Mar 2009 00:14:36
My thoughts are
Working off of 35mm full frame.
Standard lens= 35mm (not e many compacts with this or a 38mm lens), rather less 40mm lenses around.
Normal lens= 50mm
Posted by: Nicky Jones | 30 Mar 2009 15:56:41
Standard lense? What exactly is that and how did it get standardized? For me, the standard lense is the one I use the most and on my Fuji 6000, thats anywhere from 28-300mm (35 mm format) +2X when needed. My fellow photgos and I have debated this for years and the only conclusion I can reach is "standard is what fl you use the most." That's simplicity and - in that - beauty.
Posted by: Raynman | 30 Mar 2009 21:37:06
I first got into 35mm SLR photography 33 years ago, using a razor-sharp 50mm/f1.8 lens, wandering around the streets of Paris where I then lived, shooting a range of subjects and using my feet to change shooting distance, angle of view, but always recording "what the eye saw".
I recently bought a 35mm f/2 AFD Nikkor to put on my D80 in the hope of rediscovering that purity of photography, away from today's range of bulky, slow zooms with arguably inferior optical quality.
I have not been disappointed. Indeed, I have rediscovered the joys of taking my time, composing better images, thinking more about what I was seeing in the viewfinder, all the while recording what the eye sees and how it sees it.
My passion for photography has returned, thanks to the often maligned, humble "standard" lens.
Go try it for yourselves.
Posted by: Ken | 30 Mar 2009 22:52:04
All in the eyes of the beholder. I completely agree with Andy that the "normal" is what most people prefer. For me it's very obvious, just as in Bahi's comment above: when some view strikes me as a "perfect picture" and I start playing with a zoom, most often I stop at 42-43 mm on the full frame, or about 28 on APC format. As to the human eye, it catches to some degree (detects motion, really) in almost 180 degrees view - so what? It's a question of what our brain prefers to see, all the math and optics is irrelevant.
Posted by: Maxim | 31 Mar 2009 19:08:35
Why not spend the time & energy analyzing something important like true ISO, and the fact that digital market treats the concept with disdain?
Posted by: BokehMan | 1 Apr 2009 01:53:26
Regarding the perspective issue: the perspective will be the same for two different lenses IF THE OPTICAL CENTERS ARE AT THE SAME LOCATION, not the camera! Hence the different perspectives when using a zoom lens at different focals without adjusting the camera position, as the optical center shifts when zooming.
Posted by: bzarokian | 1 Apr 2009 12:24:48
thank you for these useful information
omar
My Digital Camera
Posted by: omar | 2 Apr 2009 02:43:23
A standard lens is one that most closely matches the photographer's normal eyesight. So, how long is a piece of string? sort of thang.
IF both our eyes align to give us stereoscopic vision, that field of view that aids our survival is rather wider than the designation of the diagonal of the image in-camera, but this depends on the focus of our attention.
The way we see is like a zoom lens. We get a general view, proceed when it is safe, move and aim or concentrate our sight at the object of our attention.
I used a 50mm on a Leica having no means to buy other lenses. It's a straitjacket, not a discipline.
Zooms are where its at.
The zoom is essential to maintain the point of view that caused you to raise your camera to the eye in the first place, and a standard zoom needs to be 24-120mm on 35mm film equivalent really.
Posted by: peter harrap | 2 Apr 2009 15:27:53
I always felt that 50mm was a slight telephoto in 35mm film days and was never sure why. Now I know, thanks! I also found 35mm disappointing when I wanted a wide perspective. I had a Leica Minilux for a while. I think that lens was about 40mm, from memory and I was amazed how flexible that focal length was. Interesting.
Thanks for the information, much appreciated.
Posted by: Don | 2 Apr 2009 22:17:54
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Posted by: steessyembobe | 4 Apr 2009 00:49:08
TCav regarding your disparaging comments on the viewing distance of 6x4 prints, did you not read this from the author before you jumped in to post? :- "(with the exception of small prints)"
By the way, you are in the wrong here with your stance on this subject. Completely.
As said it is simple trigonometry. So long as you prescribe to the notion prints should be viewed at the optimum distance then that is all there is too it.
Your post where you suggest some will view closer or further away to try and dismiss this notion doesn't hold water.
Posted by: David | 4 Apr 2009 16:33:47
There is no such thing as "normal" or "standard" lens from optics or human physiology point of view.
Human eye is an 23mm/f 2.8 lens with a 6MP "sensor"behind it.
The "diagonal" viewing distance notion comes from the 19th century definition of "acceptable" print sharpness/resolution and then defined circle of confusion.
The viewing disatnce is different for HD or computer displays or cinema screens.
Perspective is not lens property, it's pure geometry. Lens focal length effect on objects relative sizes is a phenomenon called "perspective distortion".
Amazing how many false notions are perpetrated over the net.
Posted by: nugat | 4 Apr 2009 17:38:21
Like Andy mentions in his article, the standard/normal terminology has been with us for the better part of a century.
Andy didn't invent the term, so it's odd that people here are trying to get him to retract his comments on what standard means.
Next thing, you guys are going to be arguing that "camera" is a nonsensical word, because cameras are not rooms! (which is how the word originated.)
Posted by: Derek R | 5 Apr 2009 02:38:03
как сказал один очень умный человек которого мы все хорошо знаем ) В сей мир едва ли снова попадём, Своих друзей вторично не найдём. Лови же миг! Ведь он не повторится, Как ты и сам не повторишься в нём
Posted by: keebra | 5 Apr 2009 09:22:55
Unfortunately the author's definition of standard/normal lens is a tautology (butter is substance that tastes buttery) similarily to the one from wikipedia.
WIKI
"In photography and cinematography a normal lens is a lens that generates images that generally look "natural" to a human observer under normal viewing conditions, as compared with lenses with longer or shorter focal lengths."
"Normal is what looks natural under normal conditions."
AUTHOR
"So how does this relate to the focal length of a 'standard' lens? To understand this, let's conduct a thought experiment in which we take a picture, make a print on the spot, then see how it compares to the scene in front of us. Simple geometry (using the concept of similar triangles) states that a print viewed from a distance equal to it's diagonal will exactly match the subject in perspective when the focal length of the lens that was used to make the picture is equal to the diagonal of the sensor".
Author's definition is additionally based on syllogistic fallacy, because "perspective" is not a property of lens.
Visual/natural perspective is how the human eye sees objects in a 3D world. It depends solely on the objects dimensions and the eye's distance from them.
Artificial projection p./graphical p. is how this 3D worldview in represented on a 2D plane. Once flat, the representation does not change "perspective" (relative sizes of objects) depending on the viewing distance.
Also how this "thought experiment" accounts for the "crop factor"?
A 42mm (FF) lens put on a say micro4/3 sensor will have 2x crop factor. But surprising to many, the "perspective" of the images on the sensor (their relative sizes) will not change at all. 2x crop factor from 42mm to 84mm does not introduce by itself a 'telephoto" perspective distortion. (Such distortion is actually introduced by the position of the image plane).
So is "perspective" tied to the focal length and sensor size or not?
Posted by: nugat | 6 Apr 2009 10:14:44
A "normal" lens to me is a lens that sees the same way my eyes do. By looking through the viewfinder with my right eye and keeping my left eye open, I can then adjust the focal length of the zoom lens until my right eye sees exactly what my left eye sees (except for a slight difference in perspective) and the two sight pictures are in sync. This occurs at around 55mm on my Canon Xsi and at 50mm on my 35mm film camera.
Make sure your right eye sight picture is in focus. You'll know when you are in sync when you get a good 3-D effect.
Posted by: Henderpete | 6 Apr 2009 16:26:46
Above is another total misunderstanding that doesn't take into account the viefinders' various magnification multipliers.
Posted by: nugat | 6 Apr 2009 16:41:46
The author defined nothing! The terminology was invented 75 years ago, if not earlier! He is just restating what is common knowledge among photographers!
Posted by: Derek R | 6 Apr 2009 18:57:42
Sources? Who defined?
Whoever did, the definition is logically "empty of meaning" (Wittgenstein).
Posted by: nugat | 6 Apr 2009 19:04:19
Common knowledge that the Earth is flat persisted for longer than 75 years.
Posted by: nugat | 6 Apr 2009 19:07:05
If you want to be the Christopher Columbus of lens terminology, be my guest.
However the rest of the world will continue using the term "normal" or "standard" for a lens that has a focal length equal to the sensor diagonal.
Posted by: Derek R | 6 Apr 2009 19:32:57
That is fine by me. Just don't make up any pseudophilosophy behind it.
PS.
Funnily the film world claims that "normal lens" has the focal length equal to DOUBLE film frame/sensor diagonal.
Posted by: nugat | 6 Apr 2009 19:59:48
PS. It was Copernicus, not Columbus.
Posted by: nugat | 6 Apr 2009 20:00:34
I mean I'd rather discover that the Earth revolves around the Sun than discover a world to send all ignorant people to...
(joke, joke, I love America)
Posted by: nugat | 6 Apr 2009 20:13:59
While I was reading this excellent thread, I realized that I was positioning my face a certain distance from my monitor in an very unconscious way. I immediately froze, picked up a ruler close at hand and measured - 17 inches. The diagonal of my monitor, 17 inches (behind the technology curve, I know). I think the "diagonal = viewing distance" assumption is pretty strong!
Posted by: Brad Finney | 6 Apr 2009 23:40:26