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Author Topic: This puzzling business of "35mm lens equivalent"  (Read 16316 times)
Gary Brown
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« Reply #20 on: January 16, 2012, 05:08:15 PM »
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It seems a possible paradox to me that the perspective is unchanged when the field of view is changed.

Cropping a photo doesn't change perspective, and changing the field of view (without moving the camera) likewise doesn't change perspective — it's equivalent to cropping.
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Walter Schulz
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« Reply #21 on: January 16, 2012, 05:30:57 PM »
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Is a 4/3 with a 24mm lens merely a crop of the center of the same scene shot with a 24mm lens on a FF?

Putting aside diffraction, resolution, noise: Yes, if both are placed at the same spot.

Is it the "same" as a 48mm shot on a FF?

Going further on and putting aside the different sensor ratio, depth of field, background separation, too: Yes, if both cameras are placed at the same spot.

Or is there a further consideration?
If depth of field, background separation have to be considered: Yes, there is.
Because that's the part where it differs.

A photo is a two-dimensional rendition of a three dimensional event (OK, let's leave M-theory and General Relativity out of it Wink).  as a Mercator projection introduces linear distortions of the relative sizes of the continents, is there another factor attributable to product? It seems a possible paradox to me that the perspective is unchanged when the field of view is changed.

Take a look around in the room and fix a point of your choice. Relax and let the picture sink in. Now take a piece of paper and cut a hole in it. Look at the very point as before and move the paper back and forth. Perspective remains the same. (You have to look through the hole in the paper, of course)
Now move yourself nearer to the point or farther away. Perspective changes.

Further reading: Feininger, take a look at the book store or library.

Ciao, Walter

PS: If it comes to ultra wide focal length there will be strange perspective effects when 3-dimensional objects are involved. But this is a very different story not to be mixed up with this discussion.


« Last Edit: January 16, 2012, 05:42:43 PM by Walter Schulz » Logged
Slobodan Blagojevic
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« Reply #22 on: January 16, 2012, 06:06:22 PM »
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Alternatively, you might want to read a 50-page treatise on the subject.
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Slobodan

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theguywitha645d
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« Reply #23 on: January 16, 2012, 06:47:32 PM »
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Boy, this is a topic.

The ratio of image sizes of two or more objects in a frame is directly proportional to the object distances. This however does not describe perspective, or only in comparing image from the same format.

Perspective, the apparent depth in the 2-D image, is a little more difficult. Given the same field of view and object distance, the perspective will be the same. This is the idea of equivalent focal length.

However, cropping or changing the focal length while maintain the object distance will change perspective. This is because the image to reach its final display size will be enlarged proportionally more. You can look at to two ways, either the foreground and background are not the same in the uncropped and cropped image meaning the ratio of object distances to the foreground and background are different in each image and so have different perspective, or the infinity points (used in linear perspective) are displaced giving different perspective in each image. So a 50mm lens on a 35mm camera will create an uncropped image with a different perspective as an uncropped image made with 50mm lens on an APS camera. Don't believe me? On the left is an uncropped image, to the right is a crop taken from the image and enlarged to the same size. The red line clearly show perspective has changed. (Yes, if I reduced the crop in size it will match the set on in the uncropped image, but we do not scale images to some absolute scale based on FoV and format size--magnification is the bit folks forget.)

BTW, while I am sure folks will be shocked, but this is not new nor radical. This is how it has always worked, but folks have been confusing the ratio of image size to the ratio of object distance as a rule about perspective for a very long time when there is not a direct link.
« Last Edit: January 16, 2012, 06:50:59 PM by theguywitha645d » Logged
Ray
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« Reply #24 on: January 16, 2012, 10:08:42 PM »
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Perhaps this issue can be more clearly understood if we start from the beginning.

As I understand, all cameras without exception are cropped-format cameras. If the camera didn't crop the image projected by the lens, one would see a circular image that is progressively dimmer towards the circumference to the point where it would be near black.

The difference between what is called a "full frame 35mm" camera and what is termed a "cropped format" camera is that the cropped-format camera simply crops the image projected by the lens to a greater degree. As a consequence the field of view is narrower but everything within that narrower field of view is the same in images from both cameras, provided pixel quality and pixel density is the same and provided the shots were taken with the same lens from the same position.

A good example of this could be demonstrated by comparing the Canon 5DMkII with the earlier cropped-formats Canon 20D & 30D. Because pixel density and pixel quality is approximately the same for all 3 cameras, the so-called 'cropped formats' 20D and 30D provide no IQ advantage over the 5D2 in any circumstance.

In other words, the usual advantages of the so-called 'cropped format' in terms of longer "effective" focal length and better image quality towards the edges and corners can be achieved with the 5D2 by simply cropping the 5D2 image in post processing to the same field of view as the 20D shot, having used the same lens from the same position with all cameras.

Hope this has clarified the issue.
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Slobodan Blagojevic
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« Reply #25 on: January 16, 2012, 10:44:27 PM »
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... the so-called 'cropped formats' 20D and 30D provide no IQ advantage over the 5D2 in any circumstance...

Your sentence above, when properly read, claims the opposite of what you were trying to say: that the cropped formats do have IQ advantage over FF cameras of the same generation.
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Slobodan

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Ray
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« Reply #26 on: January 17, 2012, 04:29:16 AM »
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Your sentence above, when properly read, claims the opposite of what you were trying to say: that the cropped formats do have IQ advantage over FF cameras of the same generation.

Not true! The opposite of what I'm trying to say would only be apparent from an improper reading of what I wrote.

What I wrote was that the cropped format usually has certain advantages in respect of 'effective' increase in focal length and sharper edges to the frame. This is true because the pixel density of the 'so-called' cropped formats always lead. The pixel densities of the so-called full frame formats eventually catch up with those of much earlier so-called cropped format models, as is the case with my examples of the 5D2 compared with the 20D.

In other words, the usual advantages of the so-called cropped formats, compared with the lesser-cropped full frame formats, are entirely due to the greater pixel densities of the cropped format. Without such greater pixel density, there's no IQ advantage in any respect, using the same lenses of course.

The current 18mp Canon 7D and 60D have far greater pixel density than the 5D2, so the usual advantages would apply.
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EduPerez
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« Reply #27 on: January 17, 2012, 05:41:23 AM »
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Is a 4/3 with a 24mm lens merely a crop of the center of the same scene shot with a 24mm lens on a FF?

Yes, exactly that.
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Rob C
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« Reply #28 on: January 17, 2012, 06:43:00 AM »
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Rob, prepare to have your mind blown. It takes pro-grade pictures *wink wink nudge nudge*.


It's blown!

Rob C
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Tim Gray
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« Reply #29 on: January 17, 2012, 08:15:00 AM »
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The problem is that with such a variety of formats, you can't tell if a 24mm lens is "wide" or not.  Yes, 24mm is 24mm - but the real question is how wide (or how long) is it, and the focal length alone is not sufficient to answer that.  Talking about 35mm equivalent at least eliminates most of the uncertainty. 
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theguywitha645d
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« Reply #30 on: January 17, 2012, 08:52:34 AM »
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The problem is that with such a variety of formats, you can't tell if a 24mm lens is "wide" or not.  Yes, 24mm is 24mm - but the real question is how wide (or how long) is it, and the focal length alone is not sufficient to answer that.  Talking about 35mm equivalent at least eliminates most of the uncertainty. 

Unless you have never used 35mm...

And there is confusion as the normal on 35mm is 43mm, not 50mm that many people assume.

I find the easiest way is dividing the focal length by the diagonal of the format. The product is the magnification factor. I used all kinds of formats and I never had to convert them to another format I did not use to understand its FoV. You really should be working in the format you are using.
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schrodingerscat
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« Reply #31 on: January 17, 2012, 02:44:03 PM »
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Back in the darkest days of film, Pop Photography, or Modern, ran an article that explained it quite well. They put a wide angle lens on a camera and then put the image through a series of crops up to a 200mm equivalent. They also took a shot with a 200mm telephoto...same camera, location, and subject. When placed side by side, both the 200mm full frame and wide cropped to 200mm were both identical in depth of field and perspective.

The use of 35mm equivalents gives the manufacturers a common reference point that most people with even a rudimentary knowledge of photography can understand. Most point and shoots are labeled with their actual focal lengths, and without a reference point there's no way to translate that to real world.
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theguywitha645d
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« Reply #32 on: January 17, 2012, 02:53:20 PM »
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The use of 35mm equivalents gives the manufacturers a common reference point that most people with even a rudimentary knowledge of photography can understand. Most point and shoots are labeled with their actual focal lengths, and without a reference point there's no way to translate that to real world.

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.
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Slobodan Blagojevic
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« Reply #33 on: January 17, 2012, 02:57:48 PM »
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... sales point that is was a 28mm wide. I asked a friend what they thought that meant. They had no idea...

And they would if you told them it is, say, a 6mm lens!? Using ignoramuses as a yardstick!? Oh, my... Sad
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Slobodan

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theguywitha645d
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« Reply #34 on: January 17, 2012, 03:31:50 PM »
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And they would if you told them it is, say, a 6mm lens!? Using ignoramuses as a yardstick!? Oh, my... Sad

Thank you for insulting my friends. It is nice to be in the presence of such intellect.

The point is that a 35mm reference is archaic. It is no longer a dominant format where people can identify with it.
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Walter Schulz
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« Reply #35 on: January 17, 2012, 03:47:25 PM »
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Sorry, but Slobodan has a point. He may be rude, but your argument is invalid and that's exaktly what he has pointed out.
Your friends ignorance doesn't validate anything.
The majority of people I know don't know anything about planet orbits. But this isn't a valid argument to defend or deny Keppler's laws.

Ciao, Walter
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theguywitha645d
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« Reply #36 on: January 17, 2012, 03:51:02 PM »
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And the ignorance that comes from not being able to work in a format and have to convert to another to understand it, that is something to write home about?
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mouse
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« Reply #37 on: January 17, 2012, 03:52:08 PM »
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However, cropping or changing the focal length while maintain the object distance will change perspective. This is because the image to reach its final display size will be enlarged proportionally more. .....

If you believe that then your definition of perspective is much different than mine and that of most photographers.
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theguywitha645d
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« Reply #38 on: January 17, 2012, 03:55:32 PM »
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If you believe that then your definition of perspective is much different than mine and that of most photographers.

But I provided the proof.
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Walter Schulz
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« Reply #39 on: January 17, 2012, 04:12:06 PM »
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But I provided the proof.

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
« Last Edit: January 17, 2012, 04:15:41 PM by Walter Schulz » Logged
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