This puzzling business of "35mm lens equivalent"

[Slobodan Blagojevic]

... It is nice to be in the presence of such intellect...

You are welcome, thewiseguywitha645d.

But you evaded my question: would your friend have any idea what they are talking about if they told him the lens is 6mm? I sincerely doubt. The plethora of different nail sized-sensor dimensions in digital compacts makes it even more difficult to visualize what a 6mm lens is.

Btw, why would you perceive what I said as an insult? If, in your own words, your friend did not have a clue what they are talking about, that makes him, by definition, ignorant (at least in the field under discussion). Nothing wrong with that, there are thousands of fields where I am an absolute ignoramus as well and I take no offense when someone points it out.

[Slobodan Blagojevic]

... The point is that a 35mm reference is archaic. It is no longer a dominant format where people can identify with it.

Which one is it then? APS-C? If so, is it:

1.3x (Canon)
1.52x (Nikon, Fuji, Sony)
1.53x (Pentax, some)
1.54x (Pentax, some)
1.6x (Canon)
1.7x (Sigma)
1.85x (Canon G1X)

[theguywitha645d]

And that's the point. You made a statement and delivered a proof for it.
To argue your proof it would be wise to show an example, too. And I can't deliver it right now. Not because it is impossible, but it has to be done properly.

The point of view is not without pitfalls when discussing perspective. SCNR, this thread is born for bad puns.

Ciao, Walter

Walter, are your saying that the example image I posted shows a change in perspective because of the crop, yet you are going to show an example which will show that the image, while showing a change in perspective, is not showing a change in perspective?

You may find most photographer have never studied perspective. So why they would be an authority. There is nothing in my argument that has any pitfalls as it is a pretty standard stuff about perspective.

[mouse]

But I provided the proof.

Unfortunately your proof is unconvincing.  The ratio of the vertical dimensions of any two objects is the same in both images, and would remain the same at any enlargement or crop.  What am I missing?

[theguywitha645d]

Walter, let me give you a hand. Here is a standard definition of perspective from a standard reference: The Focal Encyclopedia of Photography, Third Edition, page 548-55:

PERSPECTIVE Perspective refers to the appearance of depth when a three-dimensional object or scene is represented in a two-dimensional image, such as a photograph, or when the subject is viewed directly.

If you are simply going to shoot something from a fixed distance with different focal lengths and show the image is the same, then I can save you time. I wouldn't bother--that really does not explain perspective enough and is not my point. But go back and reread my post.

[Walter Schulz]

Would it be tolerable for you to wait for what I like to show you and discuss this afterwards rather than to discuss your opinion about what you think I might have to show?

Your proof might lead to the conclusion that size matters. It does but (playing the devil's advocate) in consequence you may take any painting and resize it. And then I will take your proof to prove that the perspektive of the painting has changed.
And I suppose this is the point which vexed "mouse" a lot.

EDIT: ops, overlap. I wrote before reading your last post. But I really have to go to bed now.

Ciao, Walter

[theguywitha645d]

Unfortunately your proof is unconvincing.  The ratio of the vertical dimensions of any two objects is the same in both images, and would remain the same at any enlargement or crop.  What am I missing?

You are missing that perspective is not defined by image size ratios. Perspective is not a comparative quality decided on some king of absolute frame or specific objects in the frame. Perspective is the global appearance of depth in an image.

In linear perspective, the rate lines converge define the degree of perspective. This is pretty well known. The lines do not converge at the same rate and so the appearance of depth of the image changes

[theguywitha645d]

Which one is it then? APS-C? If so, is it:

1.3x (Canon)
1.52x (Nikon, Fuji, Sony)
1.53x (Pentax, some)
1.54x (Pentax, some)
1.6x (Canon)
1.7x (Sigma)
1.85x (Canon G1X)

What happened to the old fashioned why of describing angle of view in degrees? For a photographer, in an earlier post I showed how to calculate angular magnification.

I won't give you a hard time about calling my friend names, but the crop factor thing has lots of people confused. Does the aperture change kind of thing.

[theguywitha645d]

Would it be tolerable for you to wait for what I like to show you and discuss this afterwards rather than to discuss your opinion about what you think I might have to show?

Your proof might lead to the conclusion that size matters. It does but (playing the devil's advocate) in consequence you may take any painting and resize it. And then I will take your proof to prove that the perspektive of the painting has changed.
And I suppose this is the point which vexed "mouse" a lot.

EDIT: ops, overlap. I wrote before reading your last post. But I really have to go to bed now.

Ciao, Walter

Walter, I am quite happy to discuss perspective without you doing a lot of work. If it is necessary, we can then talk about examples.

Sure, I can take any image, taken at any distance, with any focal length and match the radial lines of linear perspective. But there is a common frame, standard viewing distance which is the distance equal to the diagonal of the display area--I think Michael will be happy if I don't post huge images. So no matter what size I display and image, the standard viewer will see the same image. This is why both original and cropped image are the same size. (No one prints to an absolute frame depending on format, object distance, focal length, etc. People make 8x10s or 16x20s or whatever regardless of how the image was taken.)

(Yes, viewing distance will alter perspective as well, but lets take one thing at a time. But my basic point does not alter that.)

[feppe]

I am meeting more and more people who have no experience with 35mm. There was a compact camera being sold in Japan with the sales point that is was a 28mm wide. I asked a friend what they thought that meant. They had no idea. I really don't see the 35mm reference becoming anymore than a curiosity in the future.

You (and others) keep bringing up that point. While people with no experience with 35mm format don't know what 50mm equivalent is, it is nevertheless useful shorthand to compare. We can't continue dumbing things down to the lowest common denominator.

Even for those who don't what 35mm equivalent means, it's a matter of explaining 50mm+ equivalent is tele, 50mm- is wide. I guarantee that's much easier than trying to explain different crop factors and how to compare the focal lengths of J1, E-PL3 and NEX-5 to a novice about to buy his first non-P&S.

[mouse]

In linear perspective, the rate lines converge define the degree of perspective. This is pretty well known. The lines do not converge at the same rate and so the appearance of depth of the image changes

I still fail to see how any amount of cropping (with or without subsequent enlargement) can change the rate at which lines converge.

[Slobodan Blagojevic]

... continue dumbing things down...

Ooopps! You did it again! Insulted his friend, that is

[theguywitha645d]

I still fail to see how any amount of cropping (with or without subsequent enlargement) can change the rate at which lines converge.

Did you see the image I posted? The lines following the linear features in the image do not converge at the same rate for the uncropped image compared to the cropped image. Without the enlargement, the radial lines would be the same, but because we crop and enlarge, the rate this lines converge changes.

[theguywitha645d]

You (and others) keep bringing up that point. While people with no experience with 35mm format don't know what 50mm equivalent is, it is nevertheless useful shorthand to compare. We can't continue dumbing things down to the lowest common denominator.

Even for those who don't what 35mm equivalent means, it's a matter of explaining 50mm+ equivalent is tele, 50mm- is wide. I guarantee that's much easier than trying to explain different crop factors and how to compare the focal lengths of J1, E-PL3 and NEX-5 to a novice about to buy his first non-P&S.

Actually, 43mm+ is tele and 43mm- is wide. I guess this 35mm equivalency is not really very good.

Some would say the whole crop factor thing is dumbing down.

[Ray]

I tend to agree with guywitha645d on this issue of perspective. There certainly is a lot of confusion about the relationship between focal length of lens, shooting distance and perspective distortion.

Most people who use a camera have probably experienced the effect of perspective distortion when taking a portrait from really close up using a wide-angle lens. The nose appears too large in relation to the rest of the face.

The question thus arises when using a cropped-format camera with a 50mm lens versus a full-frame camera with an 80mm lens from the same position, will the wider lens on the cropped format cause some degree of perspective distortion which is not apparent with the 80mm lens on the full-frame camera.

The simple answer is no, provided apertures are adjusted on each lens to ensure equal DoF.

Unfortunately, some photographers, perhaps to demonstrate how expert they are, go overboard on this issue and make absurd claims that perspective has nothing to do with focal length of lens and everything to do with shooting distance. This is clearly nonsense.

One of the first things that most photographers learn is that wide-angle lenses tend to enlarge the foreground in relation to the background, and that telephoto lenses tend to compress the foreground in relation to the background. The effect is very obvious, and that effect clearly represents a change in perspective resulting from the focal length of lens, despite the shooting distance being the same.

However, the counter argument generally goes something like this. If you crop the wide angle shot so it has the same field of view as the telephoto shot, then the perspective will look the same, if both shots were taken from the same position.

Now this statement is also perfectly true (setting aside resolution and DoF limitations), but what has been overlooked is the fact that cropping a shot taken with a wide-angle lens turns it into a less-wide lens, or even a telephoto lens, 'effectively'.

Well, who would have thought that! Taking shots of the same scene from the same position using the same 'effective' focal length of lenses results in the same perspective. How profound! 

The lesson here is that changing the field of view by cropping, whether such cropping is performed in-camera or through post-processing, increases the 'effective' focal length of the lens, whatever lens is used.

Considering the plethora of different camera formats now on the market, knowing the 35mm format focal length equivalent is very useful. It's not necessary to have used a 35mm camera in order to understand that 14mm equivalent is really wide and that 600mm equivalent is really long, or that 24mm equivalent is wider than 28mm equivalent.

[mouse]

One of the first things that most photographers learn is that wide-angle lenses tend to enlarge the foreground in relation to the background, and that telephoto lenses tend to compress the foreground in relation to the background. The effect is very obvious, and that effect clearly represents a change in perspective resulting from the focal length of lens, despite the shooting distance being the same.

However, the counter argument generally goes something like this. If you crop the wide angle shot so it has the same field of view as the telephoto shot, then the perspective will look the same, if both shots were taken from the same position.

Now this statement is also perfectly true (setting aside resolution and DoF limitations), but what has been overlooked is the fact that cropping a shot taken with a wide-angle lens turns it into a less-wide lens, or even a telephoto lens, 'effectively'.

How exactly do you reconcile these two statements?  Are you saying that by cropping the image from a wide angle lens you then can reproduce the "compression of the foreground in relation to the background" which would be produced by a telephoto lens?

From Wikipedia, perhaps not the ultimate authority but usually pretty accurate:

Note that perspective distortion is caused by distance, not by the lens per se – two shots of the same scene from the same distance will exhibit identical perspective distortion, regardless of lens used. However, since wide-angle lenses have a wider field of view, they are generally used from closer, while telephoto lenses have a narrower field of view and are generally used from further away. For example, if standing at a distance so that a normal lens captures someone's face, a shot with a wide-angle lens or telephoto lens from the same distance will have exactly the same perspective on the face, though the wide-angle lens may fit the entire body into the shot, while the telephoto lens captures only the nose. However, crops of these three images with the same coverage will yield the same perspective distortion – the nose will look the same in all three.

[uaiomex]

I am too late for this discussion but I have to say that I agree with those that believe that still there's no better parameter for comparing the zillion different formats we have now than the old 135/35/24X36/FF. Even medium format back manufacturers refer sometimes to the 24X36 (as in being a 0.60 x crop for just an example).

I would gladly and immediately adopt a better one if someone comes with it.

Perhaps something based on the diagonal of the format. A 1X lens in the case of 35 format has to be a 43mm lens. A 0.5-2-0 zoom lens would always be a zoom lens that goes from half the diagonal to twice the diagonal. In the case of 24X36, it's a 21-86mm zoom lens. For 4/3 it translates to a 10.8-43.2mm zoom lens.  Is this of some help?

How about just declaring the actual angle of view in degrees and forgetting completely the FL? "104-57 zoom lens". Or it could be used in reverse. First the coverage at the tele position. "57-104 zoom lens"
For 24X36, do you know which zoom lens I'm referring to?

Just my 2 cents. Please don't feel forced to adopt any of these two just yet.  
Eduardo

 

[Ray]

How exactly do you reconcile these two statements?  Are you saying that by cropping the image from a wide angle lens you then can reproduce the "compression of the foreground in relation to the background" which would be produced by a telephoto lens?

From Wikipedia, perhaps not the ultimate authority but usually pretty accurate:
 

The question you should ask is 'which foreground' in 'which image'? When the wide-angle shot is cropped to the same FoV as the telephoto shot, you are not necessarily compressing the foreground in the wide-angle shot but perhaps displaying the distant background which is already compressed. If you were to crop the foreground in the wide-angle shot, you would possibly exclude the distant background, or at least part of it, so that there would be no sense, or less sense, of an enlarged foreground in relation to the background. Size is relative. Big is big only in relation to something smaller.

The Wikipedia article sounds a bit confused to me. It is not sensible to compare issues of perspective in essentially different images of different FoV and therefore different compositions. Imagine if we were to compare the DoF in two images taken with different focal lengths of lens from the same position using the same f stop, then crop one or both images to exclude any apparent differences in DoF for the purpose of claiming that DoF is the same. Would you think that sensible? I know that situation is not strictly analagous because it would not work for all areas cropped. However, my point is, the composition of an image is important in order to create a specific sense of perspective, or DoF.

To turn a wide angle lens into a telephoto lens through cropping in order to claim that focal length has no bearing on perspective sounds like chicanery to me. A big nose in relation to a small face looks different to that same nose cropped, in relation to nothing but a plain background.

[mouse]

Perspective in an image depends solely on the distance from the subject, and is uninfluenced by lens focal length or the field of view,  the size of the sensor/negative, or what portion of the captured image is used to produce the final print, or the size of such print.

A reasonable sampling of internet discussions on this subject makes it apparent that this concept is a hard sell.  All manner of reasoning and rationalization have been invoked to show that this should not be so. Nevertheless I have yet to read anything that has convinced me that I, along with the vast majority of much wiser authorities, are wrong.

When one looks at a scene photographed from the same position, with both a short and long focal length lens, and subsequently printed at the same size, it is abundantly clear that the images are vastly different.  And part of that difference is that the wider angle view contains much additional content which may serve to convey what one may legitimately describe as a greater feeling of depth.  But perspective, properly defined, does not depend on added content nor upon a feeling of depth.  The ultimate proof of this is a crop of a wide angle shot to show only the area captured by the longer lens will have precisely the same perspective (distortion) measured by any parameters you care to apply.

[EduPerez]

Perspective in an image depends solely on the distance from the subject, and is uninfluenced by lens focal length or the field of view,  the size of the sensor/negative, or what portion of the captured image is used to produce the final print, or the size of such print.

A reasonable sampling of internet discussions on this subject makes it apparent that this concept is a hard sell.  All manner of reasoning and rationalization have been invoked to show that this should not be so. Nevertheless I have yet to read anything that has convinced me that I, along with the vast majority of much wiser authorities, are wrong.

When one looks at a scene photographed from the same position, with both a short and long focal length lens, and subsequently printed at the same size, it is abundantly clear that the images are vastly different.  And part of that difference is that the wider angle view contains much additional content which may serve to convey what one may legitimately describe as a greater feeling of depth.  But perspective, properly defined, does not depend on added content nor upon a feeling of depth.  The ultimate proof of this is a crop of a wide angle shot to show only the area captured by the longer lens will have precisely the same perspective (distortion) measured by any parameters you care to apply.

Absolutely.

Problem comes because people tends to use different lenses in different situations: wides when near the subject, and teles when far from the subject; and they tend to associate the different perspective (caused by the different distances) to the difference in focal length. For example, if I am making a headshot, and I want to fill the frame with the subject, I will move forwards and backwards as I change the focal length; and thus I will be changing the perspective, with my feet.