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Thread: focal length

  1. #21

    Default Re: focal length

    Quote Originally Posted by LittleWolf View Post
    Referring the statements of reader "night86mare" in the discussion on "focal length", we would like to clarify the following.

    I'm not aware that I called anyone here names. I think my points have been spelled out pretty clearly. If you don't afford my assurances some credibility, then I don't see how you would believe anything I would write further on, and any discussion would be futile.

    You're free to believe what you want. You can even believe in fantasy focal lengths if that floats your boat. I don't depend on convincing you of anything. I am just as free to not waste my time to force-feed a basic scientific concept to someone who refuses to accept it.

    We thank the writer for his feedback and his interest in science education. We'd further like to assure the general public that we remain committed to scientific outreach for those who are willing to listen.
    I totally agree with you in this thread.

    People who don't understand will claim that their 200mm lens on their Olympus DSLR is 400mm and therefore longer reach than the 200mm on a full frame DSLR.

    But if the person using 200mm lens on the full frame shooting at the same distance and then do a crop in post-processing, he can easily get "300mm", "400mm", 600mm, "800mm" or even "1600mm" simply by cropping appropriately.

    People who really understand it know that the 35mm equivalent refers to Field of View and not focal length.

    Unfortunately or fortunately for manufacturers, most consumers do not understand and threfore erroneously think in terms of focal length. That is why we see many people say : "hey, my Olympus DSLR or 1.5x crop camera is better than full frame for birding because it has a longer reach" or some Prosumer camera users making the mistake that their camera has longer reach than a DSLR with a 200mm lens when the actual focal lenght of their prosumer cameras are only with 70-88mm lens..

    Due to this unfortunate common practice of stating focal length in 35mm format equivalent, there is now widespread misconception that 35mm format equivalent refers to focal length instead of Field of view, resulting in many people making wrong conclusion and assumption about the focal lengths of their cameras and thus the actual magnification factor and their cameras' ability to capture details.

  2. #22

    Default Re: focal length

    Quote Originally Posted by Clockunder View Post
    I totally agree with you in this thread.

    People who don't understand will claim that their 200mm lens on their Olympus DSLR is 400mm and therefore longer reach than the 200mm on a full frame DSLR.

    But if the person using 200mm lens on the full frame shooting at the same distance and then do a crop in post-processing, he can easily get "300mm", "400mm", 600mm, "800mm" or even "1600mm" simply by cropping appropriately.

    People who really understand it know that the 35mm equivalent refers to Field of View and not focal length.

    Unfortunately or fortunately for manufacturers, most consumers do not understand and threfore erroneously think in terms of focal length. That is why we see many people say : "hey, my Olympus DSLR or 1.5x crop camera is better than full frame for birding because it has a longer reach" or some Prosumer camera users making the mistake that their camera has longer reach than a DSLR with a 200mm lens when the actual focal lenght of their prosumer cameras are only with 70-88mm lens..

    Due to this unfortunate common practice of stating focal length in 35mm format equivalent, there is now widespread misconception that 35mm format equivalent refers to focal length instead of Field of view, resulting in many people making wrong conclusion and assumption about the focal lengths of their cameras and thus the actual magnification factor and their cameras' ability to capture details.
    now this one i don't get, there seems to be something new that i'm not aware of - so are you saying that 400mm in 35mm terms

    can give you different things on different cameras?

    i thought it is based on sensor size? so are you saying that 200mm lens on say, 1.3x crop factor giving 260mm is not NECESSARILY the same as 130mm on 2x crop factor dslr?

    if you are, then this is interesting

    if not then you seem equally confused, because what i understood was that the answer to my own question was that IT WOULD BE THE SAME, hrm. unless you're talking about RESOLVING DETAIL, in which case i have no idea why you think full frame sensor will resolve more detail than 1.5x crop factor sensor, i thought there are almost uncountable factors in this, like lens quality, etc

    what the ts was originally talking about was
    1) why manufacturers don't put everything in 35mm terms

    and i don't really get what littlewolf is talking about, i suppose he's saying the same thing as you, that x mm in 35mm terms may not necessarily mean the same thing all the time - are you sure of that? i was not aware of that.
    Last edited by night86mare; 18th October 2007 at 03:09 AM.

  3. #23

    Default Re: focal length

    Quote Originally Posted by night86mare View Post
    now this one i don't get, there seems to be something new that i'm not aware of - so are you saying that 400mm in 35mm terms

    can give you different things on different cameras?

    i thought it is based on sensor size? so are you saying that 200mm lens on say, 1.3x crop factor giving 260mm is not NECESSARILY the same as 130mm on 2x crop factor dslr?

    if you are, then this is interesting

    if not then you seem equally confused, because what i understood was that the answer to my own question was that IT WOULD BE THE SAME, hrm. unless you're talking about RESOLVING DETAIL, in which case i have no idea why you think full frame sensor will resolve more detail than 1.5x crop factor sensor, i thought there are almost uncountable factors in this, like lens quality, etc

    what the ts was originally talking about was
    1) why manufacturers don't put everything in 35mm terms

    and i don't really get what littlewolf is talking about, i suppose he's saying the same thing as you, that x mm in 35mm terms may not necessarily mean the same thing all the time - are you sure of that? i was not aware of that.
    Frankly, it's no surpise to me that you have so much misconceptions and failure to comprehend what others said.

    If you don't understand, my sincere advice to you is : be the type of people which I talked about in the Fujifilm crop factor thread : read more and understand, then you will be able to understand what we’re saying here and thus have much fewer misconceptions. Instead of always just read bindly, think harder too about what others are saying to avoid not only failing to comprehend but also mis-interpreting.

    As a result of your failure to comprehend what others are saying, you're often not corresponding to others. The other party often has to explain in details not only what they’re saying but also to impart some fundamental photography knowledge.

    Again, just like in the Fujifilm thread, I do not have all the time to explain everything and to educate.

    (p.s. I may have to explain the difference between effects of real panning and effects of artificial panning from post-processing).
    Last edited by Clockunder; 18th October 2007 at 03:45 AM.

  4. #24

    Default Re: focal length

    hurhur

    look at these two sites

    35mm/kmount1.5xcrop calculator calculating field of view from focal length
    site on image sensor size and field of view

    i still don't see any difference between
    FIELD OF VIEW
    and focal length in 35mm terms, they are used interchangeably

    i have been searching, just in case there is some magical secret that only clockunder and littlewolf know
    but there is no magical secret
    just a lot of rubbish and waste of my precious time

    the ts reason for asking is to provide a common ground for comparison, i.e. what you see is what you get
    if i say 75mm lens on full frame (1x crop factor, just for discussion), versus 50mm lens on 1.5x crop factor, IT IS THE SAME THING
    that's what he's saying

    going into semnatics is just really a waste of anyone's time - like i've said before, why do you bloody need to know that some lens has x focal length?
    what i want to know is what i will see
    i'm sure it matters to the all-professional and jazz like that - but hurhur, i sincerely doubt that either are professionals; looks like argument for the sake of argument
    and profession of superior knowledge, when it is all technical jargon
    Last edited by night86mare; 18th October 2007 at 06:17 AM.

  5. #25
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    Default Re: focal length

    When a comparison is to be made, it must be made apple to apple. I agree that the effective focal length is not an actual representation of the physical focal length of the lens itself. But by using the EFL, it gives us the convenience of knowing the FOV, doesn't it?

    For example, if I tell you that a lens is 7mm, while another has a focal length of 100mm, can anyone tell me what is the FOV of these two lenses? Without the sensor size or the film size, it is impossible to tell what is the perspective that the lens is giving, right?

    How about if I say that the effective focal length of a lens @ 35mm format is 28mm, while the other lens has an EFL of 50mm @35mm format?

    I assure you that many of us do know that the effective focal length of the lens is not the real physical focal length of the lens, and that we know that the effective focal lens only tell us the FOV, rather than that the focal length of a lens, when used on a different crop sensor, will have its real focal length changed. However, like when we are comparing the two light bulbs, you can never say that a 6V light bulb is brighter than a 10V light bulb. Instead, you need to compare the power that is output by the light bulb.

  6. #26

    Default Re: focal length

    "35mm equivalent" is a term that is used as a reference for sensors that are built in different size. You will get a different focul length for the same lens in some sense. 35mm is referring to the size of conventional 135 film, which often call a full frame. When someone told you that the lens is 18-200mm in 35mm equivalent, they are telling you that you will get a 18-200 focal length for your image when the lens is mounted on a full frame (or 35mm) camera. But the so call effective focal length will be different when you mount the same lens on a camera of a different sensor size.

    Often, when we are discussing on a lens at a 35mm equivalent, we are referring to the focal length, not the field of view, because you are referring to the lens, not your sensor. But I do agree that the difference in so call effective focul length is affected by the field of view, but more accurately, it's the size of sensor. Please note that some lens brouchure also show the FOV in term of angle to explain the power rating. the brouchure also corelated FOV to the focul length.

    as far as I know, the power of zoom is provided in PNS camera generally for commercial reasons, and i believe so. for general consumer, you not have to educate them of the photography terms, which is very confusing sometime. it is easier to tell a 50 year old lady how is the zoom power like 3x, 5x, 10x etc, then telling her the actual focul length of the lens.

    the power of zoom generally refers to the magnification factor of what you will get in your image. actually, the 3x zoom in one PNS camera may not be the same as the 3x zoom of another. the power of zoom can be simply derived by the longest focal length of that camera divided by the widest focal length (in 35mm sense). For example, one 3x camera can have a focal length of 35-105mm while the other camera can also have a 3x zoom but with the focal length of 28-84mm (in 35mm sense).

    if i have said anything wrong, would appreciate the bros to correct me. thanks.
    Last edited by Draccoyap; 19th October 2007 at 05:17 PM.

  7. #27

    Default Re: focal length

    just to add-on to what i said earlier. a focal length provided in a lens is fix to that lens. it is a rating for the lens design. I don't think it has much to do with the FOV. FOV is only meaningful when the lens is mounted on a camera. but not matter which camera you mounted it on, you will not change the focal length of that lens. of course, when you say "the effective focal length you will get when it is mounted on a 1.6x crop camera", you are referring to the "actual" focal length you will get, not the FOV. Whan you are referring to what you will get in an image, you are relating it to FOV.

    BTW, I do not think that FOV and FL can be used interchangably. they are referring to different thing. when you are referring to angle of view to the sensor, you use FOV. when when you are referring to length you use FL. In real life, we use FL more than FOV. I seldom hear people saying that X lens will give you Y degree of FOV on Z camera.
    Last edited by Draccoyap; 19th October 2007 at 04:15 PM.

  8. #28

    Thumbs up Re: focal length


    Hmm.. I'm still confuse with focal length even after reading this thread. 1 question. My Canon S5 IS with 1/2.5" (5.8 x 4.3 mm) CCD sensor, is actually 6 to 72 mm, giving 36 to 432 mm in a 35 mm equivalent sensor (12x optical zoom).
    Compare to an entry level DSLR, etc 1000D, 450D, what are the lens that are nearest zoom to 432mm (35mm equivalent)?
    S5 IS aperture is f2.7 to f8.0
    Last edited by mutant; 11th August 2008 at 10:08 AM. Reason: add info

  9. #29

    Default Re: focal length

    I think there's only a few lenses for dSLRs that can rival the zooming capabilites of prosumers..

    something like the newly announced Tamron 18-270?

  10. #30
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    Default Re: focal length

    Quote Originally Posted by mutant View Post

    Hmm.. I'm still confuse with focal length even after reading this thread. 1 question. My Canon S5 IS with 1/2.5" (5.8 x 4.3 mm) CCD sensor, is actually 6 to 72 mm, giving 36 to 432 mm in a 35 mm equivalent sensor (12x optical zoom).
    Compare to an entry level DSLR, etc 1000D, 450D, what are the lens that are nearest zoom to 432mm (35mm equivalent)?
    S5 IS aperture is f2.7 to f8.0
    to get (35mm equiv) of 432mm on a 1000D and 450D (1.6x crop factor), just use 432/1.6 = 270mm. for canon 1d series, crop factor is 1.3x, so its 432/1.3 = 330mm. but most of the 300mm lens are about f5.6 for dSLR

  11. #31

    Default Re: focal length

    Thanks for the replies. Pricing wise, roughly how much for DSLR zoom lens that reach 270mm for original low-end (etc Canon / Nikon) and 3rd party low-end (etc Sigma / Tamron)? The aperture range roughly?
    Last edited by mutant; 12th August 2008 at 10:08 AM.

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