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Thread: Focal length vs focal distance

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    Banner1976's Avatar
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    Focal length vs focal distance

    Hello. I'm a beginner in digital photography, so if someone could explain to me a difference between focal length and focal distance. Please, use the simplest words!

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    Moderator Dave Humphries's Avatar
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    Re: Focal length vs focal distance

    Hi there,

    Welcome to the CiC forums, do drop by the Intro thread and tell us a bit about yourself if you get a chance.

    I'll start with my simple idea and someone else will be along shortly with a more accurate and possibly technical answer.

    Assuming we are talking at a practical camera and photographic level, rather than of the pure physics of light and lens, and keeping it simple, I would say;
    Focal Length means how much the image is apparently magnified (or not) when you take a picture. For example a "telephoto" lens will produce a camera image like looking through binoculars. Where as a "wide angle" will produce an image that shows more than you would see with the naked eye. In 35mm film camera terms a focal length of 50mm is considered to equate to what the naked eye sees. In theory, anything with a longer focal length will be telephoto, lets say 200mm. Going the other way, a 28mm will be a wide angle lens.

    Focal distance means something quite specific at a technical level, but keeping it simple, I'd suggest you think of it as just being the distance between the camera and the subject you have focussed on. However, this isn't really that good a definition, so others views will differ, but it is going to be difficult to be more accurate without getting technical.

    Have you tried the tutorial here?

    Hope that helps,
    Last edited by Colin Southern; 30th September 2009 at 07:44 AM. Reason: Tweaking Dave's "Accent" ;)

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    Re: Focal length vs focal distance

    Hi there and welcome.

    Focal length is the length in mm between the front part of the lens and the sensor or film at the back of the camera that is required to get the image in focus. The longer the focal length then the further the light will have to travel between passing the front of the lens and hitting the sensor. This affects the field of view because it is narrower. Try this test. Sit down and look directly ahead. Take mental note of what you can see, especially to the sides. Your focal length is from your iris/pupil area to the retina at the back of your eye - let's say 2 inches. Now curl your fists pretending you are using binoculars and place them to your eyes and pretend there is a lens glass at the end of your hands. Your focal length now is from the outer edge of your hands (the glass) to the retina at the back of your eye, let's say 6 inches. Your field of view has narrowed because light from the sides is blocked by the 'lens' which is your hands, but you can still see objects directly ahead of you.

    You might find this short video very useful (it's also quite amusing) http://www.youtube.com/watch?v=CIFNdfjem18
    Last edited by Colin Southern; 30th September 2009 at 07:45 AM.

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    rpcrowe's Avatar
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    Telephoto as opposed to long focus lenses

    Quote Originally Posted by carregwen View Post
    Hi there and welcome.

    Focal length is the length in mm between the front part of the lens and the sensor or film at the back of the camera that is required to get the image in focus.
    These days, we generally call all lenses of long focal length: "telephoto", and generally, all long focal length lenses of today are of telephoto construction. However, the correct definition of the term, "telephoto" is:

    "In photography and cinematography, a telephoto lens is a specific construction of a long focal length photographic lens in which the physical length of the lens is shorter than the focal length. ..."
    en.wikipedia.org/wiki/Telephoto_lens

    A long focus lens is the correct term that is used for a lens in which the physical focus distance is equal to the focal length. There are few, if any, long focus lenses produced in modern times but, they were once very popular.

    The telephoto lens through complicated optics, utilizes a lens to film or sensor distance shorter than the focal length of the lens. One good example of a telephoto lens to film/sensor distance being shorter than the actual focal length of the lens is the 400mm f/5.6L lens from Canon. The total physical length of this 400mm focal length lens is only 256.5mm; ergo the distance between the lens elements and the film plane/sensor cannot be 400mm.

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    Administrator Colin Southern's Avatar
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    Re: Focal length vs focal distance

    Quote Originally Posted by Banner1976 View Post
    Hello. I'm a beginner in digital photography, so if someone could explain to me a difference between focal length and focal distance. Please, use the simplest words!
    I'll comment fom a different angle.

    In terms of focal length, the bigger the number, the closer things will appear.

    In terms of local length -v- focal distance, don't worry about it!

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    Re: Focal length vs focal distance

    Thank you for all posts. They are very helpful for me. Step by step i will try to bring all of yours advices into effect. It takes time of course. Currently I'm working on Depth of field. So I think more questions will come out as I get deeper. Now I know where to look for help. Thank you again!
    Last edited by Banner1976; 1st October 2009 at 02:50 AM.

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    Administrator Colin Southern's Avatar
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    Re: Focal length vs focal distance

    Quote Originally Posted by Banner1976 View Post
    Thank you for all posts. They are very helpful for me. Step by step i will try to bring all of yours advices into effect. It takes time of course. Currently I'm working on Depth of field. So I think more questions will come out as I get deeper. Now I know where to look for help. Thank you again!
    DoF is an easy one.

    The smaller the F-Stop numbers, the smaller the DoF, and the smaller the distance to the subject the smaller the DoF. Easy eh?

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    Re: Focal length vs focal distance

    Quote Originally Posted by Banner1976 View Post
    Thank you for all posts. They are very helpful for me. Step by step i will try to bring all of yours advices into effect. It takes time of course. Currently I'm working on Depth of field. So I think more questions will come out as I get deeper. Now I know where to look for help. Thank you again!
    Have a look at this image I just posted. It gives a good example of depth of field. No clear signal

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    Re: Focal length vs focal distance

    Quote Originally Posted by Colin Southern View Post
    DoF is an easy one.

    The smaller the F-Stop numbers, the smaller the DoF, and the smaller the distance to the subject the smaller the DoF. Easy eh?
    Still a little bit confusing, but it's getting clearer. I have to practice. Thanks!!

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    Re: Focal length vs focal distance

    Focal length in the photographic lingo represents the zoom . Larger the number, more is the zoom or more closer to the subject. Smaller the focal length, you are zoomed our or get a wider field of view.

    STOP here if you are interested in understanding your camera and take wonderful pictures. (as Collin told earlier)

    If you are little more curious to know the science behind this, read on
    Now, in the technical terms, focal length is an optical property of a lens. It defines the distance from the lens at which a set of "parallel" rays will converge.
    This thread discuss in detail Focal length - does this change when a camera focuses?

    Focal distance is the distance from the lens to the image sensor as defined in this link http://www.visionsystem.com/support/...l_distance.php

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    Re: Focal length vs focal distance

    Quote Originally Posted by Dave Humphries View Post
    Hi there,

    Welcome to the CiC forums, do drop by the Intro thread and tell us a bit about yourself if you get a chance.

    I'll start with my simple idea and someone else will be along shortly with a more accurate and possibly technical answer.

    Assuming we are talking at a practical camera and photographic level, rather than of the pure physics of light and lens, and keeping it simple, I would say;
    Focal Length means how much the image is apparently magnified (or not) when you take a picture. For example a "telephoto" lens will produce a camera image like looking through binoculars. Where as a "wide angle" will produce an image that shows more than you would see with the naked eye. In 35mm film camera terms a focal length of 50mm is considered to equate to what the naked eye sees. In theory, anything with a longer focal length will be telephoto, lets say 200mm. Going the other way, a 28mm will be a wide angle lens.

    Focal distance means something quite specific at a technical level, but keeping it simple, I'd suggest you think of it as just being the distance between the camera and the subject you have focussed on. However, this isn't really that good a definition, so others views will differ, but it is going to be difficult to be more accurate without getting technical.

    Have you tried the tutorial here?

    Hope that helps,
    Looking this question up myself I would like to add a few comments for clarification,

    1) focal length does not "mean how much the figure is magnified". It is related to how much. A subtle difference. Better to say the focal length determines the magnification.

    2) Going straight into telephoto lens complicates the issue since variable focal length is a feature of all ZOOM lenses and not necessarily all TELEPHOTO lenses.

    I hope I'm not being picky and these additions help the beginner. Subtle inconsistencies in vocabulary greatly hinders the nOOb's uptake of camera jargon and understanding : as I have recently found out when I realised something like 90% of "pros" commenting in threads didnt appear to know the difference between a zoom and a telephoto.

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    Administrator Colin Southern's Avatar
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    Re: Focal length vs focal distance

    Quote Originally Posted by snapacious View Post
    Looking this question up myself I would like to add a few comments for clarification,

    1) focal length does not "mean how much the figure is magnified". It is related to how much. A subtle difference. Better to say the focal length determines the magnification.

    2) Going straight into telephoto lens complicates the issue since variable focal length is a feature of all ZOOM lenses and not necessarily all TELEPHOTO lenses.

    I hope I'm not being picky and these additions help the beginner. Subtle inconsistencies in vocabulary greatly hinders the nOOb's uptake of camera jargon and understanding : as I have recently found out when I realised something like 90% of "pros" commenting in threads didnt appear to know the difference between a zoom and a telephoto.
    And 90% of "noobs" to the site can't spot a thread that died 3.5 years ago

    But welcome anyway - great to have you with us

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    Re: Focal length vs focal distance

    Keeping it simple, focal length has to do with the mm numbers on your lens. Just change the mm numbers on your lens by turning it and you can easily see the differences.

    Focal distance has to do with focusing on your subject, referring to the distance the subject is from the camera. Why is it important? Read about hyperfocal distances. It is important to know that to get everything in focus, you do not have to focus on the object that is farthest away from your camera. Do not focus on infinity, focus on an object that is closer. That is what you have to learn about concerning focal distance.

    http://www.cambridgeincolour.com/tut...l-distance.htm

    Another thing concerning your lens and distance is: the closest to a subject you can be with a particular lens and still have the subject be in focus:

    http://imaging.nikon.com/history/basics/19/04.htm

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    Administrator Colin Southern's Avatar
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    Re: Focal length vs focal distance

    Quote Originally Posted by rambler4466 View Post
    Keeping it simple, focal length has to do with the mm numbers on your lens. Just change the mm numbers on your lens by turning it and you can easily see the differences.

    Focal distance has to do with focusing on your subject, referring to the distance the subject is from the camera. Why is it important? Read about hyperfocal distances. It is important to know that to get everything in focus, you do not have to focus on the object that is farthest away from your camera. Do not focus on infinity, focus on an object that is closer. That is what you have to learn about concerning focal distance.

    http://www.cambridgeincolour.com/tut...l-distance.htm

    Another thing concerning your lens and distance is: the closest to a subject you can be with a particular lens and still have the subject be in focus:

    http://imaging.nikon.com/history/basics/19/04.htm
    Hi Frank,

    Thread is 3.5 years old - OP will be long gone.

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