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Author Topic: This puzzling business of "35mm lens equivalent"  (Read 38424 times)

Slobodan Blagojevic

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Re: This puzzling business of "35mm lens equivalent"
« Reply #80 on: January 18, 2012, 10:50:24 am »

How can one argue that a, say, 30-degree angle is not a 30-degree angle just because he measures it enlarged, is beyond me. You really do not need to measure it, it is based on the principles of geometry from several thousand years ago. A 30-degree angle is a 30-degree angle is a 30-degree angle.

However, if one insists on measuring it, then he should be aware that any difference in the measured result can only be possible due to the precision of the measuring device and/or the operator.

theguywitha645d

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Re: This puzzling business of "35mm lens equivalent"
« Reply #81 on: January 18, 2012, 11:34:54 am »

No, folks my mistake. The angle does not change.

The vanishing point does. The object distances from foreground to background do. The image shows a change in perspective. Just keep me away from architectural drawing.
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EduPerez

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Re: This puzzling business of "35mm lens equivalent"
« Reply #82 on: January 18, 2012, 12:09:37 pm »

You are not sure?? You think you might need a wide-angle lens, but stitching images with a moderate telephoto lens with a macro facility might be a sensible alternative?

Sensible!? No, not at all: it would be stupid; theoretically possible, but completely absurd in practice.

Who's moving the camera? I thought we were talking about perspective from the same shooting position using different focal lengths of lenses. If you crop the wide-angle shot to the same FoV as the telephoto shot, you've effectively increased the focal length of that wide-angle lens. If you stitch images to increase the FoV, you've effectively reduced the focal length of the system and created a wide-angle lens equivalent, or at least a wider-angle lens equivalent.

If your point is that the actual and nominated focal length marked on the lens has in itself no fixed and unchangeable bearing on the perspective of a processed image, from a given shooting position, if one can use various strategies to either effectively increase or reduce focal length, such as applying cropping or stitching a number of images together, then I agree completely.

I am not very fond of this "effective focal length" concept, but I think we agree on the basics.
Sorry if my post was confusing.
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Fips

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Re: This puzzling business of "35mm lens equivalent"
« Reply #83 on: January 18, 2012, 12:44:29 pm »

Quote
Sensible!? No, not at all: it would be stupid; theoretically possible, but completely absurd in practice.

Tell that to Mr. Hockney  :D
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Slobodan Blagojevic

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Re: This puzzling business of "35mm lens equivalent"
« Reply #84 on: January 18, 2012, 12:49:52 pm »

...If your point is that the actual and nominated focal length marked on the lens has in itself no fixed and unchangeable bearing on the perspective of a processed image, from a given shooting position, if one can use various strategies to either effectively increase or reduce focal length, such as applying cropping or stitching a number of images together, then I agree completely....

So, let me rephrase your statement based on the bold parts (i.e., just removing the rhetorical fluffiness):

"I agree ... focal length... has... no... bearing on the perspective... if... applying cropping..."

At the same time, thewiseguywitha645d said:

Quote
The [cropped] image shows a change in perspective.

Could you two reconcile the opposing statements before you turn against the rest of us again? I mean, I am used to you two contradicting yourselves, but contradicting yourselves and each other simultaneously is a bit too much for me ;)



theguywitha645d

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Re: This puzzling business of "35mm lens equivalent"
« Reply #85 on: January 18, 2012, 01:07:31 pm »

So I did some more research and it appears Walter is right about the change in perspective being a viewing distance issue.

From Materials and Processes of Photography, Stroebel et al, page 158-59

Quote
4.10 Viewing Distance

It would seem that the distance at which we view photographs should have no effect on linear perspective, since a 2:1 size ratio of images of objects at different distances will not be altered by changes in viewing distance. In practice, however, changes in viewing distance can have a significant effect on the perspective, provided the photograph has good depth clues…those containing dominant objects in the foreground and background or receding parallel lines can change dramatically...

…The correct viewing distance for a 12-inch lens for a contact print of an 8x10-inch negative exposed with a 12-inch focal length lens is 12 inches. Since we tend to view photographs from a distance about equal to the diagonal, the perspective would appear normal to most viewers. If the 12-inch lens were replaced with a 6-inch focal length lens, the print would have to be viewed from 6-inches from the perspective to appear normal. When the print is viewed from the comfortable distance of 12-inches, the perspective will appear too strong. Conversely, the perspective in a photograph made with a 24-inch focal length lens would appear too weak when viewed from 12 inches. It is fortunate that people tend to view photographs from standardized distances rather than adjusting the viewing distance to make perspective appear normal, for that would deprive photographers of one of their most useful techniques for making dramatic and effective photographs.

So I just have it in the wrong end of the process. But this is what is happening when we crop and enlarge or change focal length.

Walter, I hope you have not gone to any work over this.
« Last Edit: January 18, 2012, 01:14:14 pm by theguywitha645d »
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Eric Myrvaagnes

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Re: This puzzling business of "35mm lens equivalent"
« Reply #86 on: January 18, 2012, 01:19:42 pm »

Expressing indignation is not an argument. And the fact you dismiss the example image shows you don't understand geometry, let alone perspective.
I don't argue about facts, especially mathematical ones. I have a Ph. D. in mathematics. Do you?

Eric
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Eric Myrvaagnes

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Re: This puzzling business of "35mm lens equivalent"
« Reply #87 on: January 18, 2012, 01:20:54 pm »

Agreed.  I think part of the problem is that a few people here are arguing different points without realising it.

Jim
I think you are right about that.

Eric
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Eric Myrvaagnes

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Re: This puzzling business of "35mm lens equivalent"
« Reply #88 on: January 18, 2012, 01:21:30 pm »

This discussion reminds me of an old joke: a guy enters a highway in the opposite direction. Police spotted him and broadcasts a warning: "Attention all drivers... There is one idiot driving in the opposite direction." The guy heard the warning as well and says to himself: "Ha! Just one!? Look how many there are!"
;D
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theguywitha645d

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Re: This puzzling business of "35mm lens equivalent"
« Reply #89 on: January 18, 2012, 01:34:31 pm »

I don't argue about facts, especially mathematical ones. I have a Ph. D. in mathematics. Do you?

Eric

No, and I have put my mea culpa in the thread and so I apologize.

But I do have a science degree in imaging and have been trying to find a solution to this problem that has been nagging me. Particularly in regards to perspective drawing, and the issues in drafting and photography are different enough to make it tricky.

One thing I do know is our perception of perspective in a cropped image is different from the uncropped. And perception and vision is not my field, although I have assisted a number of researchers in making illusions--I am doing some stuff on reverse perspective at the moment. However, I have just posted the solution to the problem above.
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theguywitha645d

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Re: This puzzling business of "35mm lens equivalent"
« Reply #90 on: January 18, 2012, 01:41:27 pm »

Could you two reconcile the opposing statements before you turn against the rest of us again? I mean, I am used to you two contradicting yourselves, but contradicting yourselves and each other simultaneously is a bit too much for me ;)

Perhaps you just need to put in a bit of effort. The cynic is an easy position. Beside, if you knew the change in perspective was a viewing distance issue, why did you not tell us?
« Last Edit: January 18, 2012, 01:45:23 pm by theguywitha645d »
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Slobodan Blagojevic

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Re: This puzzling business of "35mm lens equivalent"
« Reply #91 on: January 18, 2012, 01:52:19 pm »

Perhaps you just need to put in a bit of effort. The cynic is an easy position. Beside, if you knew the change in perspective was a viewing distance issue, why did you not tell us?

I think you need another mea culpa:

Not correct. As Walter pointed out, it has nothing to do with sensor size or focal length, but everything to do with the distance from the object.

jalcocer

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Re: This puzzling business of "35mm lens equivalent"
« Reply #92 on: January 18, 2012, 01:57:33 pm »

I've been reading this thread and can't help feeling some what confuse. I use aps-c and micro 4/3 (don't think ever justify a FF body).

the image seen by the camera, is a cropped version of the same seen in a full frame with the same lens? or does it correspond to the image viewed in a ff with a bigger focal lenght?? (ie. 20mm m4/3 and 40mm on ff).

thanks
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theguywitha645d

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Re: This puzzling business of "35mm lens equivalent"
« Reply #93 on: January 18, 2012, 01:58:14 pm »

I think you need another mea culpa:



You are not Walter and what you are quoting is not what I am talking about. Walter eluded to print size, he has yet to expound on it...
« Last Edit: January 18, 2012, 02:01:55 pm by theguywitha645d »
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theguywitha645d

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Re: This puzzling business of "35mm lens equivalent"
« Reply #94 on: January 18, 2012, 02:11:38 pm »

I've been reading this thread and can't help feeling some what confuse. I use aps-c and micro 4/3 (don't think ever justify a FF body).

the image seen by the camera, is a cropped version of the same seen in a full frame with the same lens? or does it correspond to the image viewed in a ff with a bigger focal lenght?? (ie. 20mm m4/3 and 40mm on ff).

thanks

Don't mind us. Just work in your format with the lenses you have. None of this is really something you need to know to take pictures.

What we are talking about, at least that is what I think we are talking about, is if you put a 40mm lens on a FF and a m4/3, will those images have the same perspective. The relative size of the objects will stay the same, but will the perspective stay the same? Naturally, the field of view (or angle of view) will be different for each camera.

The answer, which is posted above, is the perspective will change depending on the viewing distance. If both images are printed to the same size and viewed from the same distance, the perspective will appear weaker in the m4/3 image as it goes through more magnification resulting from a different "correct" viewing distance.
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Slobodan Blagojevic

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Re: This puzzling business of "35mm lens equivalent"
« Reply #95 on: January 18, 2012, 02:11:57 pm »

You are not Walter and what you are quoting is not what I am talking about. Walter eluded to print size, he has yet to expound on it...

Jesus, thewiseguywitha645d, you do not give up, do you? Walter did not "allude" [sic] to print size, but quite directly and explicitly to perspective. The same thing I was talking about. As for what you are talking about, I am not sure anymore even you know.

But you got one thing right in this debate: I am not Walter ;)

theguywitha645d

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Re: This puzzling business of "35mm lens equivalent"
« Reply #96 on: January 18, 2012, 02:15:18 pm »

Jesus, thewiseguywitha645d, you do not give up, do you? Walter did not "allude" [sic] to print size, but quite directly and explicitly to perspective. The same thing I was talking about. As for what you are talking about, I am not sure anymore even you know.

But you got one thing right in this debate: I am not Walter ;)

Slobadanto, why are you even taking part in this thread? Surely your charms would be better suited elsewhere.
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jalcocer

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Re: This puzzling business of "35mm lens equivalent"
« Reply #97 on: January 18, 2012, 03:22:30 pm »

Don't mind us. Just work in your format with the lenses you have. None of this is really something you need to know to take pictures.

What we are talking about, at least that is what I think we are talking about, is if you put a 40mm lens on a FF and a m4/3, will those images have the same perspective. The relative size of the objects will stay the same, but will the perspective stay the same? Naturally, the field of view (or angle of view) will be different for each camera.

The answer, which is posted above, is the perspective will change depending on the viewing distance. If both images are printed to the same size and viewed from the same distance, the perspective will appear weaker in the m4/3 image as it goes through more magnification resulting from a different "correct" viewing distance.

Maybe you are right, is better just to keep taking pictures and don't think about that, either way you end up framing the picture the way you want it and backing off the necesary distance to make it happen (or zooming out).

thanks
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AlfSollund

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Re: This puzzling business of "35mm lens equivalent"
« Reply #98 on: January 18, 2012, 03:41:06 pm »

Mouse is 100% correct. Why anyone would spend any efforts in trying to contradict facts is beyond me.

As to "35mm lens equivalent"; this is just a useful yardstick, a reference with roots in photographic history.
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feppe

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Re: This puzzling business of "35mm lens equivalent"
« Reply #99 on: January 18, 2012, 03:55:23 pm »

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

You're being pedantic. While I believe that is indeed the technically correct definition, 50mm is universally considered a normal lens in 35mm format. My point remains either way.

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

I'm sure. Nevertheless, it is unreasonable to expect anyone to know the crop factors of all the different formats, from mirrorless cameras from Samsung, Sony, Olympus/Panasonic and Nikon, to SLRs from Sony, Canon and Nikon, to Medium Format cameras from Leica, Phase, Pentax. And I haven't even mentioned the endless P&S and phone camera formats.
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