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12-04-2008, 08:04 PM   #151
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For all that argue there being a "normal" focal length. There's a misconception I've been sensing in this thread. I just have to say Perspective by definition is not a point in space. It is how shapes appear to shrink & converge with distance. The concept of what is "normal" focal length can not be judged by one point in space, say, the focus point. It is how the size at that point relates to points closer and [much] farther away, and whether they appear larger & [much] smaller respectively at the same rate & magnitude thru the lens as with the naked eye; when the foreground & background view thru the lens provides the exact same rate of size decrease compared to distance thru the lens as with the naked eye, that focal lens is the normal focal length. A wider lens will shrink the distant object more, a longer less will shrink them less. That in -a- nutshell is all there is to perspective provided by focal length, which is independent of whatever medium [or size] you are using to project the light onto. Again, you just see more or less of the FOV based on the size of the sensor (which is what the OP was asking).

I urge you all to put a 1x eyepiece on a zoom if you think a .95x mag factor eyepiece makes that much difference (it does not! understand there's a 5% error and add it in later) and put it to one eye with the other eye open; zoom out an in, move your feet if you need to make your primary subject the same size, however do NOT only pay attention to your subject. Look at the background as well, and observe how its/their size changes compared to your near field foreground object size. When the background & foreground is the same size thru the lens and naked eye... well, I'd just be repeating myself yet again.


Last edited by m8o; 12-04-2008 at 08:18 PM.
12-04-2008, 11:36 PM   #152
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QuoteOriginally posted by m8o Quote
The concept of what is "normal" focal length can not be judged by one point in space, say, the focus point. It is how the size at that point relates to points closer and [much] farther away, and whether they appear larger & [much] smaller respectively at the same rate & magnitude thru the lens as with the naked eye
Please look at the sample pictures posted earlier in this thread by Sean Nelson and tell me if you can ascertain any difference whatsoever between the shots taken with different focal lengths. The relative size of the lamp and the door stayed completely constant as the focal length changed. There is no point anywhere in the pictures that is any different at all. Not the focus point, not the foreground, not the background. The perspective is 100% identical. I don't know how to put this any more plainly, but focal length does *not* affect perspective anywhere in the image. Spending two minutes with a zoom lens repeating Sean's should be enough to convince anyone of this.

QuoteQuote:
when the foreground & background view thru the lens provides the exact same rate of size decrease compared to distance thru the lens as with the naked eye
Making reference to what you see through the viewfinder misses the point. Photography is not about what you see through the viewfinder - it is about what actually gets recorded on the film/sensor. Yes, it is true that for a given viewfinder, a certain focal length will produce the same magnification as the naked eye - we all know this. That has never been under the slightest doubt. But in photography, the magnification through the viewfinder is irrelevant, because the image itself is obviously going to be magnified more.
12-05-2008, 12:18 AM   #153
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It's true that the relative size of objects taken from a particular point doesn't change with focal length. But what m8o is saying has validity because the field of view is different even if the perspective (defined here as lines of sight and relative sizes of objects) isn't.

What I'm getting at is that even though the lines of sight and relative size of objects is the same in all the sample shots I previously posted, the 12mm encompases a lot of other information that isn't present in the 130mm shot. You can see path in the foreground and how it's edges converge into the distance. The inclusion of the extra information gives you more depth cues than you see in the telephoto shot, hence the impression of depth with the wide angle (and conversely, "flatness" with the telephoto).

That difference in "sense of depth" has nothing to do with focal length and everything to do with field of view. You could stitch together who-knows-how-many shots taken at 130mm from the same position and end up with the same picture as was taken with the 12mm, and it would have the same sense of depth. And similarly, as we've seen, you can crop a 12mm shot to the same field of view as a 130mm and end up with the same sense of flatness.

Focal length equates to field of view for a given sensor size, so people tend to use the two interchangeably. But the OP compared 50mm on APS-C to 75mm on full frame - and those are (for our purposes) identical in terms of field of view. So they will give identical pictures in terms of both "lines of sight" and "sense of depth".

What we're lacking here is the terminology to differentiate the concepts of "lines of sight / relative sizes of objects" from "sense of depth imparted by field of view". People use "perspective" to describe both, but they are in fact separate concepts.
12-05-2008, 02:39 AM   #154
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QuoteOriginally posted by roentarre Quote
Still 50mm will have the perspective of the 50mm instead of 75mm
Not if you move further back to get the same field of view that it gave on a film body.

12-05-2008, 03:41 AM   #155
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QuoteOriginally posted by pw-pix Quote
Not if you move further back to get the same field of view that it gave on a film body.
Field of view(=angle of view) doesn't change when you move. Moving back could give almost the same composition but field of view would still be narrower than with 35mm camera. Really important to understand these terms...
12-05-2008, 08:02 AM   #156
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QuoteOriginally posted by Marc Sabatella Quote
Please look at the sample pictures posted earlier in this thread by Sean Nelson and tell me if you can ascertain any difference whatsoever between the shots taken with different focal lengths.
There's definitely some more of "we're talking about different things" in your response. But regarding the above, I'm glad you engaged me with that... It's using two items in close physical proximity to each other when compared to the camera's distance to it. Of course the perspective hasn't visibly changed. The ratio of the door nob distance to the camera is near the same as that from the lamp. I agree with what you are saying about two different lenses with very different focal lengths rendering the same point containing object in close proximity to one another the same. (I'm not sure it it was you who I'd said that earlier that I agreed with pages back when I provided a link to the dpreview page doing the same as the cited example). I return to "we're talking about different things". I have the perfect page that provides a sequence of images that describe what I'm saying. (Can't find it now. edit: found it, it's below) And yes in retrospect, I guess I'm including one moving physically to keep the primary subject the same size, and how that affects the whole FOV, to explain why a 50mm isn't really a 75mm when used on a 1.5x cropped sensor... on our digital, it just provides the same "window" onto the subject you are shooting as a 75mm on the focus point would on film, but not the same perspective on the whole depth of field in relation to the primary subject's size as a 75mm would give you on film. A 75mm on film's "object lines" would not converge as fast going off into the distance, as does the object's sizes & perspective lines that a 50mm on digital's image gives you. Hope I wasn't too confusing there.

re-edit: Perfect! I found the link. Now will you please look at this link, and look how the images with different focal length lenses affects the size of the foreground doggie finger puppet & background spire? (Yes, the article is about how DOF is not affected by focal length. But it does such a great job at also showing the difference in perspective [yes, that is perspective] that different focal lengths provide... leading to that there -is- a "normal" lens. Just repeating myself; not debating anything you'd said in that regard)
-> [ http://www.luminous-landscape.com/tutorials/dof2.shtml ] <-
End. That's my point.

re-re-re-re-etc-edit. And after partaking in about 10 of these threads over my time here, if I ever feel the urge to enter into another one of these threads, I'm going to both start & end with that point, and link!

Last edited by m8o; 12-05-2008 at 09:00 AM. Reason: made major edits & addred the link I wanted
12-05-2008, 10:13 AM   #157
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Wow, the thread has gone on to rehash a lot of things that I thought were properly explained. Some people have brought up good points and then used them to try to prove something that is not the case.

The crux of the confusion seems to be between these two points.

If you use several different focal lengths to photograph the same subject(s) from the same spot, the ratio of the size of one part of the image to another part will remain the same (although the whole size will of course change). I think there are a couple of series of images already posted that prove this.

If you use several different focal lengths from the same spot, there will be perspective distortion in the wide-angle shots and telephoto shots, but none apparent in shots of about "normal" focal length. The same images will show this, if you look at them with this in mind.

People seem to think that these two points are contradictory, and that is where the confusion comes in. Not only are they not contradictory, but the first point is the cause of the second.

Perspective distortion is caused by making things look closer or further away than they actually are. If you stand in a certain spot the ratio of the size of one part of an image to another part at a different distance will be one value. If you move closer or further away from what you are looking at, this ratio will change. If you do not move closer or further away, but only make it appear that you did with a telephoto or wide-angle lens, then the ratio will not change, and the perspective will look "wrong."

When you take a photo of someone close up with a wide-angle lens, the apparent distortion comes not from how close the subject is to the camera, but from the fact that they appear to be further away from the camera than they actually are. Their appearance would seem normal if they seemed as close to the camera as they actually were.

Likewise, when you take a photo of someone (or something) with a telephoto lens, the apparent flatness comes, not from how far away the subject is, but from the fact that they appear to be closer than they are. If they seemed as far away as they actually were, then their appearance would seem normal.

Again, the view from the viewfinder does not correlate to the resulting photo (although it would be nice if it did) because it is affected by different factors than the photo, like how far away the focus screen is from the eye, for example.
12-05-2008, 10:23 AM   #158
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QuoteOriginally posted by m8o Quote
re-edit: Perfect! I found the link. Now will you please look at this link, and look how the images with different focal length lenses affects the size of the foreground doggie finger puppet & background spire? (Yes, the article is about how DOF is not affected by focal length. But it does such a great job at also showing the difference in perspective [yes, that is perspective] that different focal lengths provide... leading to that there -is- a "normal" lens. Just repeating myself; not debating anything you'd said in that regard)
-> [ DOF2 ] <-
End. That's my point.
You do realize the camera was moved for each of the different focal lengths (to keep the mogwai puppet's size constant) ? *That* is what causes the perspective to change between each shot, *not* the focal length difference.

12-05-2008, 11:00 AM   #159
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QuoteOriginally posted by m8o Quote
I have the perfect page that provides a sequence of images that describe what I'm saying. (Can't find it now. edit: found it, it's below) And yes in retrospect, I guess I'm including one moving physically to keep the primary subject the same size, and how that affects the whole FOV, to explain why a 50mm isn't really a 75mm when used on a 1.5x cropped sensor... on our digital, it just provides the same "window" onto the subject you are shooting as a 75mm on the focus point would on film, but not the same perspective on the whole depth of field in relation to the primary subject's size as a 75mm would give you on film. A 75mm on film's "object lines" would not converge as fast going off into the distance, as does the object's sizes & perspective lines that a 50mm on digital's image gives you. Hope I wasn't too confusing there.
Why would that be? I think 50mm f/2 on 1.5x crop sensor would give identical image with 75mm f/3 on full frame. Only difference would be the size. Larger format needs less enlargement if the print size is the same. Therefore the quality is better.



QuoteQuote:
-> [ DOF2 ] <-
He has his definitions wrong. How can he talk about DOF or compare DOF of different lenses without defining acceptable circle of confusion? Maintaining subject size while walking closer to subject and changing lenses to wider, shrinks the background. When it shrinks enough it becomes acceptably sharp and is inside DOF.
12-05-2008, 11:14 AM   #160
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QuoteOriginally posted by m8o Quote
Of course the perspective hasn't visibly changed. The ratio of the door nob distance to the camera is near the same as that from the lamp.
It's got nothing to do with the ratio of the distances between the subject and the background. If you do the same test taking all pictures from the same position with the lamp 1/4 of the distance or 1/8 of the distance, or indeed if you photograph any relatively nearby object against distant mountains (so that the ratio may be 1:100 or 1:1000) you'll see exactly the same thing - the relative sizes of objects stay the same with different focal lengths.

Again, I invite you to check this out using your zoom lens. When you stand in one position and zoom from wide to telephoto, do nearby objects get bigger FASTER than distant ones? No - everything in the image gets bigger AT THE SAME RATE.

The reason there's a difference in the size of nearby vs. far objects in the photos in your link is because the pictures were taken not only with different focal lengths, but also from different distances so as to maintain the same subject size in the viewfinder. It's the CHANGE IN DISTANCES that causes the change in the relative sizes of the objects.
12-05-2008, 11:49 AM   #161
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QuoteOriginally posted by m8o Quote
....a 50mm isn't really a 75mm when used on a 1.5x cropped sensor... on our digital, it just provides the same "window" onto the subject you are shooting as a 75mm on the focus point would on film, but not the same perspective on the whole depth of field in relation to the primary subject's size as a 75mm would give you on film.
OK, so I don't have a FF digital camera and I don't want to bother shooting a roll of film to prove that you're under a misconception here. But I do have a Canon A710 with a much smaller sensor than my K100D Super, so I've put it to use.

Here are two shots, one taken with the K100D at 35mm and the other taken with my A710 (in "widescreen mode so as to get a 3:2 aspect ratio) at 8.3mm. The ratio of focal lengths, about 4x, is about the same as the ratio of the sensor sizes.

A710 at 8.3mm:




K100D Super at 35mm:




Despite a 4X difference in crop factor / focal length, both shots have identical perspective. Similarly, there's no difference between the perspective of a shot taken with a 50mm on APS-C vs. 75mm on full frame.
12-05-2008, 12:55 PM   #162
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Sean's last two posts are dead on. (I'm not saying that his other posts aren't -- I'm just only talking about the last two. )

Just to clarify:

When he mentions the 4x difference in crop factor over focal length, he means that both the numerator and the denominator of the ratio are changed by that much, resulting in an unchanged overall ratio.

Also, changing focal lengths will change depth of field (assuming the aperture setting remains the same), despite what the article at Luminous Landscape that m80 pointed out claimed (using a faulty testing procedure). However, depth of field has nothing to do with linear perspective, which is what we have been talking about in this thread. It is related to accommodation perspective, but that is another subject.
12-05-2008, 12:59 PM   #163
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QuoteOriginally posted by Sean Nelson Quote
The reason there's a difference in the size of nearby vs. far objects in the photos in your link is because the pictures were taken not only with different focal lengths, but also from different distances so as to maintain the same subject size in the viewfinder. It's the CHANGE IN DISTANCES that causes the change in the relative sizes of the objects.
I thank you for your thoughts. I will point out however I did write "And yes in retrospect, I guess I'm including one moving physically to keep the primary subject the same size, and how that affects the whole FOV... etc...". I did so because when you're using a cropped sensor, that's taken into account with the mirrorbox & eye-piece. All you're doing is trying to frame the subject the same with both cameras. But where you have to stand to do so, will factor into that! (as others have echoed too)

All right, let me write it here because I'm obviously incapable of getting my meaning across in the manner I write. I HAVE ALWAYS AGREED STANDING IN THE SAME SPOT AND CROPPING DOES NOT AFFECT PERSPECTIVE ... the link I posted to the dpreview page pages back in this thread says that. I apologize for my mis-conception in how that relates to the the sample images posted by Sean. I was definitely wrong in how I related that to what I was trying to convey, because in those photos, camera position did not change.

But my point in "is a 50mm a 75mm on digital" is about -where- you would stand if you shot 35mm film, vs -where- you would have to stand when you use the lens on a 1.5x cropped digital camera, to fill the frame the same amount with your subject. Imagine that point WITHOUT cropping after the fact to get the final images the same size in the frame. [!] ....And then consider how that changes the background.

CF, it's m8o, not the explosive m80. Thanx for that, and I'll "LIU" ("look it up"). However LL's been very helpful to me making better pictures.

Tokina, did -I- say anything about aperture? You mis-understood me (you said f/2 then f/3 ... is that what you thought about when I said "speed" in regards to object lines converging?). I was talking about the speed the objects lines converge to the infinity point in how they are affected by focal length when holding a primary subject at the same size. The speed the lines converge in the image you capture on either sensor or film over the same distance decrease the longer in focal length you go. If you had a lens that had an infinite focal length, the parallel lines of an infinitely long object would not converge. ...in retrospect, maybe strike that. Is what I'm thinking the effect of cropping an infinitely small point instead?

My involvement in this subject is an exercise in masochism.

Last edited by m8o; 12-05-2008 at 02:01 PM. Reason: final edit? I hope so.
12-05-2008, 01:18 PM   #164
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QuoteOriginally posted by m8o Quote
Tokina, did -I- say anything about aperture? You mis-understood me. I was talking about the speed the objects lines converge to the infinity point in how they are affected by focal length when holding a primary subject at the same size,
They are not affected by focal length. They are affected by perspective. If you compare 50mm on 1.5x crop and 75mm on full frame, you obviously take photos from the same position and they will be indentical.
12-05-2008, 01:25 PM   #165
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QuoteOriginally posted by CFWhitman Quote
Wow, the thread has gone on to rehash a lot of things that I thought were properly explained. Some people have brought up good points and then used them to try to prove something that is not the case.

The crux of the confusion seems to be between these two points.

If you use several different focal lengths to photograph the same subject(s) from the same spot, the ratio of the size of one part of the image to another part will remain the same (although the whole size will of course change). I think there are a couple of series of images already posted that prove this.

If you use several different focal lengths from the same spot, there will be perspective distortion in the wide-angle shots and telephoto shots, but none apparent in shots of about "normal" focal length. The same images will show this, if you look at them with this in mind.

People seem to think that these two points are contradictory, and that is where the confusion comes in. Not only are they not contradictory, but the first point is the cause of the second.

Perspective distortion is caused by making things look closer or further away than they actually are. If you stand in a certain spot the ratio of the size of one part of an image to another part at a different distance will be one value. If you move closer or further away from what you are looking at, this ratio will change. If you do not move closer or further away, but only make it appear that you did with a telephoto or wide-angle lens, then the ratio will not change, and the perspective will look "wrong."

When you take a photo of someone close up with a wide-angle lens, the apparent distortion comes not from how close the subject is to the camera, but from the fact that they appear to be further away from the camera than they actually are. Their appearance would seem normal if they seemed as close to the camera as they actually were.

Likewise, when you take a photo of someone (or something) with a telephoto lens, the apparent flatness comes, not from how far away the subject is, but from the fact that they appear to be closer than they are. If they seemed as far away as they actually were, then their appearance would seem normal.

Again, the view from the viewfinder does not correlate to the resulting photo (although it would be nice if it did) because it is affected by different factors than the photo, like how far away the focus screen is from the eye, for example.
What if I say that perspective distortion of photos is only caused by the viewing distance and the size/shape of the surface they are printed on? Would that be correct too? Wide angle photos should be viewed from closer distance to avoid perspective distortion. And telephotos from further away. I think you assumed standard viewing distance.
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