Common photography myths

Common photography myths (9)

Myth #8: Different focal lengths create different perspectives

September 5th, 2009 - 08:41:36 PM:

People can be very sloppy with terminology. I've noticed that when people say “perspective” they very often mean something else, eg. “field of view”, and that many are unaware of what “perspective” actually is. This happens even to people who should know better: Taken literally from a lens catalog of a camera maker:

[…] Wideangle lenses offer an increased depth-of-field perspective not possible with the human eye. […]

What the hell is a “depth-of-field perspective”!? They probably meant “field of view”.

Perspective has nothing to do with focal length. Perspective describes how a 3D scene is projected onto a 2D canvas, eg. film, digital sensor or even your retina. Perspective describes how the objects in the 3D scene appear in the 2D image, which objects are visible and which are obscured by other objects, how big they appear in relation to each other. Focal length only influences the field of view, ie. how much of the entire scene appears in the image. Focal length does not change the relative sizes of objects in the image. Perspective is only influenced by the relative position of the objects in the scene and by the position of the canvas (viewer, camera). If you want one object in the foreground appear much bigger than another object farther away, it doesn't help to change focal length. The only way to accomplish that is to get closer to the object in the foreground. If you want both to appear as being about the same size, the only way to accomplish that is to get farther away from both objects.

Perspective is only influenced by your position, and field of view is only influenced by focal length.

You often hear things like “wide-angle perspective”. There is no such thing. People mean either “wide-angle field of view” or “close-up perspective” here. You also often hear “zooming with your feet”. This doesn't make any sense at all. Zooming changes focal length only (and therefore field of view), and walking changes perspective only. You can't replace one with the other.

So why is it still that with wide-angle photos the objects in the foreground look quite large, and with tele-photo shots the scene looks “compressed”? Doesn't this contradict what I just said? No, it's just a coincidence. With wide-angle lenses you can focus quite closely, and if you do get close to foreground objects, they will appear large. The wide-angle lens just allows you to also capture some of the background of the scene. With tele-photo lenses, you often shoot objects that are far away. It's that “being far away” that makes the scene look compressed, not the tele-photo lens. The lens only lets you capture the “right” crop of the entire scene, blocking out the foreground and concentrating on the faraway objects.

In many lens catalogs and books and on many web sites you can see a series of photos of the same scene taken with different lenses. With some of these the perspective is indeed different in different shots. Again, that's just a coincidence. What they don't tell you is that the photographer walked back to capture a foreground object at constant size while using ever longer lenses. But it's this “walking back” that caused the change in perspective, not the longer lenses.

Still not convinced? Here are a few examples:


Image 1: Shot taken with 20 mm focal length	Image 2: Crop from image 1	Image 3: Shot taken with 100 mm focal length

If focal length had any influence on perspective, shouldn't image 3 look radically different than image 2? But except for sharpness, they're identical. So why do they look the same? It's because they were taken from the same position.

Another example:


Image 4: Shot taken with 20 mm focal length	Image 5: Shot taken with 20 mm focal length

The background is almost identical in both shots, but in image 5 the foreground is much more pronounced. Both were taken with the same wide-angle lens. Why is that? It's because image 5 was taken closer to the stump in the foreground and from a lower angle. It's not the wide-angle lens that pronounces the foreground, it's the shooting position.

Categories: Articles • Archive of all entries

Readers' comments

#1: Comment posted by Steve Jones on November 8th, 2008 - 03:52:48 PM:

I think Myth #8 as documented is misleading. There is indeed a difference in perspective if you use two different focal length lenses to frame similar scenes. I say similar - it's simply impossible to frame a 3D scene identically with lenses of different focal lengths due to perspective differences; you can only frame a 2D object with a plane parallel to that of the sensor using two different focal lengths. However, people often have the option of picturing something like a building with a wide-angle lens or from a greater distance using a longer lens. If the focal length is markedly different, then so will be the perspective.

One of the most obvious examples of this is with portraiture where there is a radical difference between taking two identically framed photos with different focal lengths (and a common sensor size). In general, short focal length lenses will exaggerate features and the longer focal length lens will tend to flatten them. It's for that reason that moderate telephoto lenses such at 85mm or 135mm (of 35mm FF) are often used for such purposes - the effect is considered to be more flattering. Frame the same person with a 28mm lens and the perspective will be very different indeed.

So perspective is a combination of the distance from the scene, and the focal length used. For similarly framed photographs then the perspective will be differ between lenses of different focal lengths.

It is this effect that people are referring to when they state that focal length changes perspective - it's the view of a "similarly framed" object with different focal lengths which necessarily involves changing the distance too. So I'd characterise this as a "part myth".

Michael Hohner answers:

I can only repeat what I wrote above: the change in perspective that you see when you frame a subject identically with different lenses is not caused by changing focal length, but only by changing the distance to the subject. I think it is important to emphasize on this, because if you don't know which cause has which effect and only work with some vague idea of it, you're less likely to make a conscious decision about which lens and distance to choose to get the effect you want. I admit that when sometimes the myth is stated as titled above, it is only for the purpose of simplification, but I suspect just as often it is stated as such out of ignorance.

#2: Comment posted by John Leonhard on July 7th, 2009 - 08:40:10 AM:
The best explanation of focal length vs. perspective that I have come across. Thanks, you're a good teacher, you are particularly articulate and not confusing. (have you other pages of explanatory information that one can read?)

Michael Hohner answers:
Check the menu to the left. This is all there is currently.

#3: Comment posted by Tristan Grimmer on May 27th, 2011 - 09:13:39 PM:

Regarding Myth 8 again. I really think that what it being described here as a myth is incorrect.

"the change in perspective that you see when you frame a subject identically with different lenses is not caused by changing focal length, but only by changing the distance to the subject."

If a subject is framed identically with a large FOV lens, the ratio of its size to a further-away object will be much larger than with a smaller FOV lens. Changing 'the distance to the subject' is simply what needs to be done to achieve 'identical framing'. It is the FOV/focal-length that very directly affects 'perspective'.

In 3D computer graphics we use a set of transformations to get things into 2D picture form:
v' = PVMv where v' is the final 2D position (like in a final photograph) of a point 'v' on an object in the world. Basically, and reading backwards, positions in the world (Mv) are transformed by a view (V). The view is a function of the camera's position/orientation only. After that the positions are in 'eye space' (camera-space). Then we tranform the point by a projection (P)... the projection transformation represents the 'lens' in a real camera. The projection may be parallel (eg orthogonal like CAD drawings, or isometric), or it may be a perspective transformation. To create the perspective transformation you generally need a FOV angle (horiz, vert, or diag), an aspect-ratio, and near and far clipping distances. The focal-distance,film-width/height directly converts to a FOV/aspect/near-plane-dist. The point here is, the focal distance directly affects the perspective matrix... and that's all... it does NOT affect the view transformation at all. In fact, I would go as far as saying: "The primary variable that affects perspective is the focal distance."

Your example with the cropped pictures is very misleading... the 'cropping' is really another (scaling/clipping) transformation and can't be discounted. In fact, that's exactly what widening the FOV does, it pushes more content into the same space, so to undo it you needed to do your scaling/cropping! Again, the point here is that with the 20mm focal distance (larger FOV) I can make the relative sizes of any two objects I choose in the scene be much more different than with the 100mm lens... yes I need to walk around so I can frame, but I, with the 20mm, will be able to achieve a bigger difference than you with the 100mm... and I don't care where you walk or position yourself. In fact, as the focal distance increases, the projection becomes less and less perspective and more and more parallel.

I will concede that this may be a bit of a problem in naming. To many of us, and certainly mathematically, the word 'perspective' is the effect of things closer being bigger than things far away (but there may be some other artistic or common usage I am unaware of). The other extreme is a 'parallel' projection where distance has no bearing on final size. This is why, for example, taking a portrait is better done with a smaller FOV... you don't get the big noses... less perspective, more parallel... having to walk around (or back in this case) is the 'side-effect'... it's what you need to do to frame your shot.

--tristan

Michael Hohner answers:

In your 3D CGI example, the angle of view does not change perspective at all. It merely restricts what parts of the scene will be visible in the rendering. The size relationships of the objects in the scene do not change when you change angle of view. What does influence perspective is the position of the (virtual) camera in the scene. And that's exactly what also happens in the real world: The lens determines only angle of view, and the position of the camera in the scene determines only perspective.

I know that cropping changes the field of view, that not misleading. It's the whole point of the example. It shows that by changing lenses you only select which parts of the scene are visible in the picture. It does not change the size relationships and the apparent relative position of the objects in the scene (i.e. “perspective”) unless you also change the position of the camera.

I think you're confusing terminology a lot. When you say “FOV” (and most confusingly, “FOV angle”), you sometimes mean “angle of view”, and sometimes “field of view”. These are two very different things and should never be confused. I'm also not sure what you mean with “focal distance”. Maybe simply “distance”. “Focal length”, in any case, is something different. There's also no such thing as “less perspective” or “more perspective”. “Perspective” is not a quantity.

I suggest that you look around with a small frame on your hand. Holding the frame close to your eye corresponds to a short focal length (wide angle of view), holding it at arm's length corresponds to a long focal length (narrow angle of view). When you don't move your eye, but only move the frame in your hand, what's changing? Only what you see inside the frame vs. outside the frame. The size and position of the objects in the frame relative to each other (i.e. perspective) does not change.

#4: Comment posted by Tristan Grimmer on May 28th, 2011 - 01:12:52 AM:

Hmm... interesting. I can say that I agree 100% with what the end effects are as you describe them... but not necessarily the terminology. I don't think I'm 'confusing' terminology but rather that different fields use terminology differently. I must also admit here I'm in the CG field, not photography. FOV for us IS an angle... it is either the horizontal, vertical, or diagonal angle formed by the 'view frustum' sides, the frustum is often represented as a 4x4 'Perspective Projection' matrix. Unlike cameras that often exhibit some sort of 'lens distortion', the sides of our frustum are perfectly flat, and straight lines always remain straight (unlike a real lens).

Also interestingly, I would use the terms 'Angle of View' and 'FOV' interchangeably (and, not that Wikipedia is the end-all and be-all, but it also agrees: "In photography, angle of view describes the angular extent of a given scene that is imaged by a camera. It is used interchangeably with the more general term field of view.")

I'm curious how you (or photographers in general) define them differently... as I suspect Wikipedia may be wrong in this instance.

I also treat length and distance as synonyms... so when I say focal distance I mean what you call focal length. Even in optics, Front focal length (FFL) and Front focal distance (FFD) are synonyms.

I suppose the crux is, CG does NOT define perspective as the size and position of the objects in the frame relative to each other...that's the 'view' transformation that does that. Rather, perspective is a type of projection, one that compresses space more the further away you get... it defines a 'frustum' with non-parallel sides.

I think of perspective as the degree to which parallel lines in the real world end up being NOT parallel in the end photograph.... or in artistic terms, the amount of 'foreshortening'.

Anyways, thanks for your response! Still curious about your take on 'Angle of View' :)

Michael Hohner answers:

Let me clarify the terminology. Imagine the view from the camera being a pyramid in space, with the tip at the camera and the base directed towards the scene (actually, it should be a cone, not a pyramid, but the rectangular recording medium crops the cone to a pyramid). With this visualisation:

Angle between the sides of the pyramid: angle of view
Distance between tip of the pyramid and the base: distance, or if you choose to place the focus of the lens there, also focus distance
Size of the base of the pyramid: field of view
In general, the plane of the base, uncropped: field
(while we're at it, flatness of the base, as it may slightly bend in or out: field curvature)

The focal length in photographic terms is not directly visible here. It's a property of the lens and merely translates to an angle of view.

As you see, changing lenses or zooming only changes the angles between the sides of the pyramid, i.e. angle of view, but not how objects at the base look like in the picture. Walking back and forth (and possibly refocusing) only changes the height of the pyramid and therefore also the size of the base (field of view), but not the angles.

#5: Comment posted by Tristan on May 28th, 2011 - 08:55:20 AM:

Oh yes... to be clear, when I say "more perspective" and "less perspective" I probably should have said "more perspective distortion" and "less perspective distortion". A good example of this can be found here:

http://en.wikipedia.org/wiki/Perspective_distortion_%28photography%29

The cube on the right goes from a high level of perspective distortion to none at all for the final isometric projection.

Again, the claim that "In your 3D CGI example, the angle of view does not change perspective at all" is completely false in the field of CG. The perspective matrix (P) is completely defined by an FOV angle, an aspect ratio, and a near and far clip distance:

http://www.opengl.org/sdk/docs/man/xhtml/gluPerspective.xml

In that particular implementation the fov angle (in radians) is the vertical one... although it could be rewritten to either take a diagonal or horizontal fov angle.

#6: Comment posted by Martin on May 30th, 2011 - 06:40:12 PM:

I think I understand the disagreement that you two are having. Michael's statement is that for a fixed target, shot from a fixed location, perspective is compeletly independent of focal length. Tristan, I think you're trying to say that the shorter focal length allows the maximum amount of perspective distortion, which is also true, but requires you get closer to the target in order to take advantage of this option. Again, for a fixed shooting distance, the amount of perspective distortion is unchanged. Please note, that the example you gave from wikipedia is doing exactly that: the camera is moving away from the target as the focal length is being changed (look at the number of grid squares between the base of the cube and the base of the frame)

#7: Comment posted by jurgen on August 22nd, 2011 - 02:53:48 PM:

the explanation done in myth 8 is 10 % correct to my opinion. Even computer graphics apply the pinhole camera model. First Object coordinates are transformed from world to Camera coordinates, and than to image and pixel coordinates using a perspective projection and a camera matrix. F, the focal length defines the position of the image plane. Changing the focal length only shift this projection plane , a bigger focal length will allow you to project more FOV on the ccd,
but ratios stay the same. Which is of course not the case if you change the camera's position. It fits nicely because I applied the camera model to make simulated images and the explanation here helped me understand. Nice thanks

Michael Hohner answers:
See the discussion above. Focal length in photography does not define the position of the image plane.

#8: Comment posted by Jay S on January 31st, 2012 - 02:37:55 AM:

Thanks for a great write-up!

It took me a while to understand the entire concept behind Myth #8, but I do see that you are completely correct with what you say. Ironically, there are two ways to look at it!

The way I see it, perspective is the trajectories that light takes from the scene to the camera sensor... these lines do not change at all when zooming (which is essentially has the same effect as cropping), but they do change when the camera is moved.

However, the point of view many take is that to get the same frame (in terms of foreground) at a different focal length, the camera must be moved. It's the moving that causes the perspective change, though, and the zooming that resizes the foreground/subjects of the frame.

Thanks, and cheers from Canada!

#9: Comment posted by Sage on April 28th, 2012 - 09:09:37 AM:

@ Tristan Grimmer,

I wouldn't compare 3D programs to life/photography. There's a lot of "cheating" going on under the hood in regards to perspective and field of view (and lighting and reflections, etc.).

From my understanding, changing the Field of View is akin to positioning the lens so that the image projected on the sensor is larger; i.e. being magnified onto the sensor.

Changing the perspective is getting actually closer to the subject so that it simply fills more of the sensor.

The camera doesn't see the subject differently at different FOV's, it's just a way of selecting a specific area of the scene or a broad area, call it pre-cropping.

When you move closer to the subject the camera does see it differently, relative distance between elements of the subject change. If I'm 25 feet away from you, your ears may only be 5 inches further away than your nose. That 5 inch difference is infinitesimal compared to the overall distance of you and I, only 1.6%. Because of that, they will pretty much look relatively undistorted and in the right position. I can now use my 300mm zoom lens to capture a portrait of your head.

Now lets say I switch to a much wider lens, maybe an 18mm wide angle lens. If I want to fill the frame with your head, I now have to get way closer to you because my field of view is much larger now. Lets say I have to get 1.5 feet away in order to fill the frame with your head. Now that 5 inch difference between your nose and ears is significant, it's 27.7% of the entire distance between you and I. The relative size of your nose to your ears is much different because of the new perspective.

The actual figures are estimated but the logic behind them should be valid.

#10: Comment posted by Perspective on May 17th, 2012 - 12:05:44 AM:

The issue with Number 8 is the idea that the camera determines perspective and not the viewer; that perspective is the location of viewpoint.

But If you asked 100 people if image 1 and 3 (from the first example) were different perspectives, everyone would say yes. Because from the viewer's point of view of perspectives are different (there is more stuff in image 1 not in image 3. This is why they call it a wide-angle perspective vs a tele-photo.

There are really two types of perspective going on here.
1) The distance of subjects to the film plane and their relationships to another (this does not changed with focal length)
2) The way the viewer sees the scene in relation to the space it fills on the frame. (this does change with focal length).

Michael Hohner answers:
But #2 is not “perspective”, it's “cropping” or “composition”. That's the whole point.

#11: Comment posted by vangelis on September 4th, 2014 - 04:34:01 PM:
We are in 2014 and people still believes in myth 8.