Telephoto Depth of Field

[KKB22]

I've tried to research many many sourcies, even the ASC manual, and not one will tell me why telephoto lenses have less depth of field than wide angle. All the manuals tell me what they do, but I want to know why. What happens optically that creates a shallower depth of field. :blink:

 

 

Cheers

[David Mullen ASC]

Longer focal-length lenses have less depth of field, but a longer lens is not necessarily a telephoto lens if on a larger format camera.

 

A "telephoto" lens means something that produces a very narrow field of view. Now on a small format camera, a relatively short focal length may do this.

 

"Telephoto" lens can make the depth of field seem lower because the background behind the subject looks larger in frame, and thus it is easier to tell that it is not in focus. On a wide-angle lens, the same background looks farther away, which makes it harder to tell that it is soft in focus.

 

But that's not the same thing as focal lengths and why they affect depth of field.

[KKB22]

Longer focal-length lenses have less depth of field, but a longer lens is not necessarily a telephoto lens if on a larger format camera.

 

A "telephoto" lens means something that produces a very narrow field of view. Now on a small format camera, a relatively short focal length may do this.

 

"Telephoto" lens can make the depth of field seem lower because the background behind the subject looks larger in frame, and thus it is easier to tell that it is not in focus. On a wide-angle lens, the same background looks farther away, which makes it harder to tell that it is soft in focus.

 

But that's not the same thing as focal lengths and why they affect depth of field.

<{POST_SNAPBACK}>

 

 

So it is purely the field of view that affects the depth of field? The focal length only determines the field of view?

[Tim J Durham]

I've tried to research many many sourcies, even the ASC manual, and not one will tell me why telephoto lenses have less depth of field than wide angle. All the manuals tell me what they do, but I want to know why. What happens optically that creates a shallower depth of field.  :blink:

Cheers

<{POST_SNAPBACK}>

 

 

http://www.dvinfo.net/articles/optics/dofskinny.php

[David Mullen ASC]

So it is purely the field of view that affects the depth of field? The focal length only determines the field of view?

<{POST_SNAPBACK}>

 

No, that's NOT what I said. Focal length affects depth of field.

 

Format determined field of view for a focal length. Telephoto images make what's in and out of focus more obvious, but that's a slightly different concept than depth of field: Depth of field is the range in which an object is acceptably IN focus. Telephoto lenses make a background that is OUT of focus larger and therefore more obviously soft. But the RANGE in which the in-focus subject can move forward and back and look acceptably sharp is determined by focal length, f-stop, and circle of confusion.

[Ilmari Reitmaa]

Incidentally, and hate to nitpick here, but...

 

Telephoto does not refer to the angle of view of a focal length / format combination but to a particular lens construction characterized by a convex lens element in front of a concave element separated by a space, resulting in a back focal distance shorter than that of a normal construction of the same focal length [see e.g. L. Stroebel: View Camera Technique 7e, Focal Press 1999].

 

So what Mr. Mullen stated applies regardless of the construction of the lens, to which 'telephoto' refers.

 

Just terminology, I know... B)

[Paul Bruening]

Hey,

 

Tim's DVinfo tip is good. David is the one of the most knowledgeable people on this forum. However, I'm not sure we know what you want to know. We just know how lenses behave, not as much about the physics of why. You may have to go to an engineering referrence to find the answers you seek. It has to do with how light is bent and managed. The rules we all know are consistent due to those inherent physics of light and optics.

[Stephen Williams]

Telephoto does not refer to the angle of view of a focal length / format combination but to a particular lens construction characterized by a convex lens element in front of a concave element separated by a space, resulting in a back focal distance shorter than that of a normal construction of the same focal length [see e.g. L. Stroebel: View Camera Technique 7e, Focal Press 1999].

 

<{POST_SNAPBACK}>

 

Hi,

 

Thats correct.

 

Many wide angle lenses are inverse telephoto design, the back focal distance is greater than the focal length, allowing space for a spinning mirror!

 

Stephen

[KKB22]

Hey,

 

Tim's DVinfo tip is good. David is the one of the most knowledgeable people on this forum. However, I'm not sure we know what you want to know. We just know how lenses behave, not as much about the physics of why. You may have to go to an engineering referrence to find the answers you seek. It has to do with how light is bent and managed. The rules we all know are consistent due to those inherent physics of light and optics.

<{POST_SNAPBACK}>

 

I should've been more specific, yes I wanted to know the actual physics of why longer focal length lenses have less depth of field. Like you said, all the cinematography books I research give the characteristics, but not the physics.

 

Thanks anyways, sorry about the mishap.

[Tim J Durham]

I should've been more specific, yes I wanted to know the actual physics of why longer focal length lenses have less depth of field. Like you said, all the cinematography books I research give the characteristics, but not the physics.

 

Thanks anyways, sorry about the mishap.

<{POST_SNAPBACK}>

Hi,

Call this number:

 

++49 7364/20-6175

 

or e-mail:

 

photo@zeiss.de

 

You can contact the Carl Zeiss tech. info people. It sounds like you need information for a paper

for school? I can only speak for myself here but I only need to know "how" a lens performs. Not necessarily "why" it performs the way it performs.

[David Mullen ASC]

There are lots of websites with diagrams explaining depth of field / circles of confusion, etc. I think I saw one on the Edmund Scientific website just the other day. Just do some Yahoo! searching...

 

I use the word "telephoto" to describe a field of view that is narrower than a "normal" lens, because if I say "longer lens" instead, it suggests a long focal length, and on a small format camera, it may actually be a relatively short focal length (although longer than the "normal" focal length for that format.)

 

So for me, "telephoto" describes a visual effect. I don't really care if it's not scientifically accurate... we all know what a telephoto image tends to look like, which is why I use that term.

[Laurent Andrieux]

Stephen is right. The wide angle construction is called a retrofocus.

 

Back to the first question, yes, it's agood thing to consider that - at a given format - a focal length is a viewing angle. It's very easy to show that tan(A/2) = l/2.f where A is the viewing angle, l the format width and f the focal length. It also works for the vertical angle, where you'd take the format's height for l instead of it's width for the horizontal angle.

 

But it's not that simple to consider a long lens to have less DOF than a wide angle, unless you keep all the other parameters the same.

 

I mean that a 50 mm at 10 feet will give you nearly the same DOF than a 25 mm at 5 feet, you see.

 

When you say you consider a long lens has less DOF, it is true if you compare to a shot that is done with all the other parameters left the same. Imagine you have a zoom. You set a shot, look at it at the wide angle, you get a DOF. Then you just zoom in as to get a longer focal length, then you have less DOF.

 

This is because when you use a longer focal length, it is exactly like if you would enlarge the image you had with the wide angle, with a magnifying glass.

 

So the things that were acceptably sharp at the wide angle, then reveal they weren't just as sharp and when you enlarge, they look out of focus.

[Ilmari Reitmaa]

I use the word "telephoto" to describe a field of view that is narrower than a "normal" lens, because if I say "longer lens" instead, it suggests a long focal length, and on a small format camera, it may actually be a relatively short focal length (although longer than the "normal" focal length for that format.)

<{POST_SNAPBACK}>

I don't actually see the problem in saying "longer lens" since it's a relative term in any case (long in comparison to what?); the usual definition is that focal lengths sufficiently larger than the format diagonal are "long", so actually "long lens" does unambiguously describe the visual effect (although to a cinematographer the intuitive absolute frame of reference is probably the 35mm format). Why I however do feel a little uneasy with using "telephoto" to describe the visual effect is because it makes things like "inverse telephoto" or "telephoto macrolens" sound much more mysterious than they actually are.

 

I do realize though that using "telephoto" to describe the visual effect is common practice, I probably use it myself at times :D

[KKB22]

Thanks for all your help everyone, I'm not writing a paper on the physics of lenses, I'm just a Camera Operator/Camera Assistant studying at film school with a very curious mind. For me personally, with anything I study, I need to know why as well as how. Heh. :lol:

 

I'll contact Carl Zeiss.

[Tim J Durham]

Thanks for all your help everyone, I'm not writing a paper on the physics of lenses, I'm just a Camera Operator/Camera Assistant studying at film school with a very curious mind. For me personally, with anything I study, I need to know why as well as how. Heh.  :lol:

 

I'll contact Carl Zeiss.

<{POST_SNAPBACK}>

That's OK, I know lots of guys who seek obscure knowledge just so they can sound impressive when they have a discussion with someone who may be hiring. 'course that's not the reason THEY'D give...

[Laurent Andrieux]

Yep, one could think it works with girls, but I can tell you it usually doesn't :unsure: :wacko:

[Adam Frisch FSF]

Maybe this is the time to reiterate what a 'normal' lens is. A 'normal' lens is a lens who's focal lenghth is twice the diagonal of the image capture device. This is a good guide for gauging lenses relative 'wideness' against each other.

 

In 35mm that means roughly 25mmx2=50mm. Everything significantly less than that is a wide angle and significantly above is a telephoto. The boundaries are blurred, obviously, but as a rough guide it works quite well. For 35mm I'd argue that:

 

0-30mm is a wide angle.

30-75mm is a normal lens.

75-xxxx is a telephoto lens.