Field of view and rendering space

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Matt Pritchard

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Field of view and rendering space

PostTue Jul 30, 2013 11:45 pm

I do not currently own a BMCC, but I am seriously considering purchasing one at some point. I am aware of the various wide angle lens solutions for the camera (including speed boosters) but I have been wondering how you select your lenses for shots when compressing space or stretching space is as important as field of view. On the EF model, using a 21mm CP .2 will give you around 50mm field of view, but it cannot mimic the "normal" quality when rendering space, correct? Has anyone taken this into consideration? Space is equally important to field of view in my opinion.

If I buy a BMCC, I'll definitely go for the m4/3 version with a speed booster, but I am going to rent an EF camera for a short film this fall. Anyone have any advice on lensing this thing?
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Dmitry Kitsov

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Re: Field of view and rendering space

PostWed Jul 31, 2013 1:46 am

Matt Pritchard wrote:I do not currently own a BMCC, but I am seriously considering purchasing one at some point. I am aware of the various wide angle lens solutions for the camera (including speed boosters) but I have been wondering how you select your lenses for shots when compressing space or stretching space is as important as field of view. On the EF model, using a 21mm CP .2 will give you around 50mm field of view, but it cannot mimic the "normal" quality when rendering space, correct? Has anyone taken this into consideration? Space is equally important to field of view in my opinion.

If I buy a BMCC, I'll definitely go for the m4/3 version with a speed booster, but I am going to rent an EF camera for a short film this fall. Anyone have any advice on lensing this thing?


What do you mean by "rendering space"? Do you mean perspective?
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Matt Pritchard

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Re: Field of view and rendering space

PostWed Jul 31, 2013 2:22 am

I may not be using the correct terminology, but I am referring to the effect that wide angle and telephoto lenses have. Longer lenses tend to crush the distance between the foreground and background, and wide angle lenses tend to exaggerate the distance. to my mind, crop factor shouldn't actually affect that quality of the lens as you are basically just punching in on the frame. Am I off base here?
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Erik Swan

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Re: Field of view and rendering space

PostWed Jul 31, 2013 2:41 am

Matt Pritchard wrote:I may not be using the correct terminology, but I am referring to the effect that wide angle and telephoto lenses have. Longer lenses tend to crush the distance between the foreground and background, and wide angle lenses tend to exaggerate the distance. to my mind, crop factor shouldn't actually affect that quality of the lens as you are basically just punching in on the frame. Am I off base here?

As long as the lens is a rectilinear lens (i.e. not fisheye), FOV and perspective are directly related. Cropping a wide-angle lens (assuming the crop is centered) is the exact same as using a longer-focal length lens from the same point in space.

You might be thinking of the effect of taking an image of a subject using a wider focal length lens close up, and using a longer focal length lens farther away. In this case, the subject you are focusing on can appear to be the same size in the frame, but the distance between it and other objects will feel much greater with the wider focal length lens. This is best demonstrated in what's known as the
.

In other words, for rectilinear lenses, perspective is only defined by where you are standing and where you are pointing the lens. The focal length/field of view of a lens only affects how much of the theoretical view you see.
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Matt Pritchard

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Re: Field of view and rendering space

PostWed Jul 31, 2013 2:43 am

Thank you so much. That answers my question completely.
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adamroberts

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Re: Field of view and rendering space

PostWed Jul 31, 2013 5:47 am

The compression of the foreground to background is a property of the focal length.

For example: If you put a 50mm on a 5D and a 35mm on a 7D the FOV will be similar but the "look" will not be the same as the compression of the foreground to background will be greater on the 5D.

This same effect is present with the BMCC compared to S35.

Using the SpeedBooster addresses this issue as it scales the image circle down by 0.71. In effect making the image look like it was captures on a larger sensors.

So rather than using a 35mm you would use a 50mm to achieve the same FOV and you gain the compression of foreground to background that is in the 50mm focal length.
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Erik Swan

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Re: Field of view and rendering space

PostWed Jul 31, 2013 9:03 am

adamroberts wrote:The compression of the foreground to background is a property of the focal length.

For example: If you put a 50mm on a 5D and a 35mm on a 7D the FOV will be similar but the "look" will not be the same as the compression of the foreground to background will be greater on the 5D.

Sorry, but this is incorrect. The compression of the foreground to background (known as perspective distortion or, more precisely, axial magnification) is a property of the position of the observer (in our case, a camera) in space, and which direction that observer (camera) is pointing.

The "look" will be exactly the same assuming the FOV is the same and you are shooting from the same spot.

This isn't even necessarily related to lenses, but to image projection and 3-point perspective in general (of which lenses are one method).

If you don't believe me and have a 3D rendering program available that can simulate physical cameras, you can setup a scene and try it. I did, and as you can see, cropping an image is identical to using a different focal length lens from the same point in space:

(click to make big - sensor size of this "camera" is a full frame, ~36mm horizontal)
Image

Wikipedia explains it best (my emphasis):
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 farther 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. Conversely, if all three lenses are used from distances such that the face fills the field, the wide-angle will be used from closer, making the nose larger, and the telephoto will be used from farther, making the nose smaller.

There may be minor differences between a crop and an equivalent focal length in the real-world due to vignetting, lens distortions, etc., but they're going to be minor (and won't affect perspective distortion), and for this discussion we're assuming ideal lenses.

The whole thing may seem counter-intuitive but it's true, it's just math. :D

Source: https://en.wikipedia.org/wiki/Perspective_distortion_(photography)
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Dmitry Kitsov

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Re: Field of view and rendering space

PostWed Jul 31, 2013 5:58 pm

Eric is correct.
What's funnier than that is that depth of field is also always the same at the same f-stop. And this is so regardless of the focal length of the lens. What changes is the apparent/perceived dof, which is a function of the sensor size and resolution and wether footage is cropped or not.
That's right both 11mm and 180 mm lenses have the same depth of filed at f/4.
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Mac Jaeger

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Re: Field of view and rendering space

PostWed Jul 31, 2013 6:00 pm

To the photographer it may seem like the depth of a scene (don't know a professional english term for it, sorry) is depending on the lenses focus length, but this only because you have to move your cameras position to frame the subject of your shot the same way. What we perceive as depth of scene is actually the relation of distance to the nearest and farest objects in the scene, measured from the lens.

Imagine two people standing 4m apart. With a wide lens you might stand 1m in front of person A, then person B is five times that distance away (5m). With a normal lens you might stand 4m in front of person A, then person B is only two times that distance away (8m). And if you take a tele lens and have to stand 16m in front of person A, then person B will only be 20% further away, or 1.2 times. Those distance relations translate directly into proportions of height and width of objects / persons in our shots, and from these proportions the viewer deduces distances in the scene (among other depth clues, as any stereographer will tell you).

It is essential to comprehend that the experience of depth is only related to the position of your camera when recording, yet not to the lens used. You shouldn't let the lens tell you where to position the camera, but you should decide which lens (or at least which focal length) to use after you chose your camera position.
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Dmitry Kitsov

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Re: Field of view and rendering space

PostWed Jul 31, 2013 6:04 pm

Mac Jaeger wrote:To the photographer it may seem like the depth of a scene (don't know a professional english term for it, sorry) is depending on the lenses focus length, but this only because you have to move your cameras position to frame the subject of your shot the same way. What we perceive as depth of scene is actually the relation of distance to the nearest and farest objects in the scene, measured from the lens.

Imagine two people standing 4m apart. With a wide lens you might stand 1m in front of person A, then person B is five times that distance away (5m). With a normal lens you might stand 4m in front of person A, then person B is only two times that distance away (8m). And if you take a tele lens and have to stand 16m in front of person A, then person B will only be 20% further away, or 1.2 times. Those distance relations translate directly into proportions of height and width of objects / persons in our shots, and from these proportions the viewer deduces distances in the scene (among other depth clues, as any stereographer will tell you).

It is essential to comprehend that the experience of depth is only related to the position of your camera when recording, yet not to the lens used. You shouldn't let the lens tell you where to position the camera, but you should decide which lens (or at least which focal length) to use after you chose your camera position.

But even if you do not move and just swap lenses from the same position, it will seem as if 24 mm has more dof/sharper then 50mm. This is because more image matter is jammed into your picture because of the perspective of the 24mm is wider then that of the 50mm. This makes angular distances between image elements smaller, which makes it appear sharper to us.
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Mac Jaeger

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Re: Field of view and rendering space

PostWed Jul 31, 2013 6:19 pm

Dmitry Kitsov wrote:But even if you do not move and just swap lenses from the same position, it will seem as if 24 mm has more dof/sharper then 50mm. This is because more image matter is jammed into your picture because of the perspective of the 24mm is wider then that of the 50mm. This makes angular distances between image elements smaller, which makes it appear sharper to us.

This is also true. DOF is an other depth indicator that helps viewers to determine how far away things are in relation, because we are used to our own eyes DOF. Fast Lenses that create a shorter DOF make the scene look deeper, separating foreground from background. There is quite a number of additional depth indicators, like contrast/saturation/shading, visibility/obstraction, or even speed of lateral motion, just to name a few. All depth indicators work together to let the viewer experience "depth" in a flat image. In two dimensions you are relatively free to combine several effects to achieve and enhance this experience. It's different once you start working in 3D, but that's another story... (which i'm going to explore once i'm satisfied with my first pocket cc; eventually i plan to build a stereo pair if the usual syncing techniques via lanc proof to work here as well).
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Erik Swan

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Re: Field of view and rendering space

PostWed Jul 31, 2013 7:24 pm

Dmitry Kitsov wrote:Eric is correct.
What's funnier than that is that depth of field is also always the same at the same f-stop. And this is so regardless of the focal length of the lens. What changes is the apparent/perceived dof, which is a function of the sensor size and resolution and wether footage is cropped or not.
That's right both 11mm and 180 mm lenses have the same depth of filed at f/4.

Actually, this is incorrect as well. If you look at the renderings I posted above, you can see that true DOF actually does increase with shorter focal lengths, not just perceived DOF (look at the amount of blurring in the equivalent crop for each focal length). What you stated is only true if you aren't taking the photos from the same spot, but instead moving towards the subject so they are roughly the same size in the frame (so basically the opposite of the situation we just discussed).

(from here):
If the subject occupies the same fraction of the image (constant magnification) for both a telephoto and a wide angle lens, the total depth of field is virtually* constant with focal length!

On the other hand, when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field (even though the pictures will show something entirely different). This is more representative of everyday use, but is an effect due to higher magnification, not focal length.


If you use a DOF calculator like this one, and hold aperture and focus distance constant (and sensor size, obviously), you will see that DOF does indeed change with focal length.

I can't exactly say why this happens; DOF is actually a property of the lens system and is more complicated than simple projection math.

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