"focal Length" or not
  • The Hair Bear
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    "focal Length" or not

    by The Hair Bear » Sun Jun 16, 2013 7:19 pm

    Hi ya all. I am worried what I am about to ask is a very silly question but I have tramped over you tube and even looked on Wikipedia and I am still confused. What the hell is the Focal Length (FL) of a lens mean? I have a 14mm prime lens. What is 14mm. What part of the lens is 14 mm???? I have a 24-70...24-70 from what. Every time I look up on You tube, I am getting explanations about the effect of FL on the image but I cant seem to find what it actually means. It cant be distance to the sensor because my sensor sits more than 14mmm back from the connecting ring of the camera. I dont really know why I want to know this but I just do. I also have a nikkor Micro 60mm lens. Dont understand what the relevance of the 60mm is...why is it micro. as previously said...so many questions....such little time Does anyone have a link that will explain this or is this a "Scott" question. I promise I will stop being obnoxious very soon.

    I had a similar dilemma with DOF. Everywhere said wide aperture, shallow DOF and showed images of this...but it took me tens if not hundreds of hours finding out why. I know...I need a life but I am finding photography so bloody interesting that I want it to occupy my spare time...finally given up the booze so I need to keep occupied. Whilst there is definitely less calories in Photography, at this point, I cant say is is cheaper and Im not sure which activity (drinking or photography) is more taxing on the brain cells!!!!

    regards to all

    darren
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    Re: "focal Length" or not

    by Simsy » Sun Jun 16, 2013 7:37 pm

  • The Hair Bear
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    Re: "focal Length" or not

    by The Hair Bear » Sun Jun 16, 2013 9:49 pm

    Thanks Simsy. Had a quick look. Looks like a good site. (I found it a bit technical for my tiny brain but I will perservere.

    cheers
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    Re: "focal Length" or not

    by Doug » Sun Jun 16, 2013 10:22 pm

    As I understand focal length it is the point inside the lens where the image crosses and becomes inverted, like the illustration in my avatar. (your view finder pentaprism flips the image back the right way)
    This picture shows a pin hole camera and it happens at the pinhole, so the focal length is from the film plane to the pin hole.
    The point where it happens on your 14mm lens is 14mm in front of the digital sensor and with your 24-70 zoom it moves as you zoom.
    Most likely if you measure 14mm in front of the sensor you would reach a point in fresh air, the shape of the lens elements determines where that point will happen.
    This is all a bit simplistic and I don't really understand much of the technicalities.

    Focal length is a spec that comes with the lens and its only practical use is to associate it though familiarity of use with what it will look like though the view finder.
    A better spec that does actually physically describe what we are interested in and directly relate what it will look like though the view finder is "Angle of view" but you won't find this printed on the lens, on the box it came in, or in general descriptions of a lens.
    Because of this, if I said I had a 47° angle of view lens not many would relate to what that would look like though the view finder even though it describes it. If I said it was a 50mm lens (which has a 47° AOV) then though owning a 35mm film camera at some stage that came standard with a 50mm lens, then what that looks like though the view finder would be familiar.

    If you put a 50mm lens on a crop frame camera then what that 50mm describes does not change, but what you see through the view finder does.
    In the case of a Canon camera you get people describing it as an 80mm equivalent lens (50x 1.6 crop factor =80mm) when put on a crop frame camera, because the result though the view finder looks like a 80mm lens on a full frame camera.

    If the spec "Angle of view" was the norm however the lens on a full frame would be a 47° AOV and on a crop frame it would be a 30° AOV.

    So focal length does not mean a lot in practice unless you are a lens designer, its just the only spec per lens in wide circulation that can be made to tie in if only indirectly with what we expect to see when we choose to attach that lens over another in the camera bag.
  • The Hair Bear
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    Re: "focal Length" or not

    by The Hair Bear » Sun Jun 16, 2013 10:57 pm

    Thankyou very much Doug. I understand most (not all) of what you are saying. I am doing a course at the moment and I will ask the tutor on Wednesday and put the results up if I think they will be of any benefit to anyone. I think you have made a valid point (how I interpreted it anyway) that the physics are not important, it is how the particular lens responds that matters. I think I am starting to miss my point and shoot!!! not really.

    cheers

    darren
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    Re: "focal Length" or not

    by Doug » Mon Jun 17, 2013 12:56 am

    I think it is commendable to inquire and not just take things on face value, it does help to figure things out with a deeper understanding of what is going on.

    Photography has a long legacy and has always been a bit geeky, if you dig deep enough you will find some formula that explains the values.
  • The Hair Bear
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    Re: "focal Length" or not

    by The Hair Bear » Mon Jun 17, 2013 7:58 pm

    cheers

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