Focal length and perspective

Discussion in 'Canon EOS' started by george_schafer, Nov 5, 2008.

  1. I'm a student at NAU, and I have a two teachers with conflicting views on crop sensors and their effect on focal
    lengths. One teacher insists that because the sensor is smaller, it works much like going from 35mm to 4x5. So an
    85mm lens is a telephoto on a 35mm frame, though it's quite short on a 4x5 camera.

    The other teacher insists that an 85mm lens is an 85mm lens, regardless. So the perspective from an 85mm lens is
    the same, no matter what.

    This concerns me because I've been using a 50mm lens (80mm on a crop sensor body) as a portrait lens. Now I'm
    concerned that I'm not getting the perspective I want for my portraits.

    So which is it, anybody?
     
  2. Perspective, the size relationship between near and far objects, is goverened by position of the the lens and has nothing to do with focal length.

    What does happen is the angle of view gets smaller as the sensor or film gets smaller.

    Have no fear. The perspective of a formal portrait will be rendered correctly from a distance to the subject of about 5 feet. If you were to move in closer, the nose would start to appear to large in relation to the face and the reverse if you were to back away. There is considerable leaway before things start to look strange. To take in less subject, use a longer local length.

    I would say both your teachers are correct. They just need to explain it better.
     
  3. Perspective is a function of distance to subject and relative distance between camera/subject/background. If you
    stand in the same place and take the same picture, then crop to the same framing, perspective will be the same.
    It isn't really about the focal length or format size necessarily.

    Format size and focal length combine to determine what angle of view a given lens will take in. Once you have
    this angle of view (wide angle/telephoto/whatever) it determines how close you need to stand to your subject to
    frame the shot the way you want. For portraits if you stand too close things start to look distorted. Stand far
    away and things look compressed. The perspective isn't about whether it is an 85mm lens or a 24mm lens or a 150mm
    lens, it is about how you position yourself and your subject. A 150mm lens on an 8x10 camera for headshots would
    have terrible perspective because you'd need to be 6 inches off the person's nose. A 24mm lens on a pocket
    digicam (tiny sensor) could have a nice compressed look for the same headshot because it would back you up away
    from your subject.

    Your 50mm lens on a crop sensor camera has a moderately tight angle of view (telephoto). It forces you to back
    away from your subject slightly, so you get a pleasing perspective.

    In a sense, both teachers are right but aren't explaining the full reasons why things work the way they do.
     
  4. A hunter 50,000 years ago understood that perspective only matters from the vantage point to the subject. Today folks that are vastly more educated that the 50,000 year old man cannot fathom this fact.
     
  5. A duffus 50,000 years ago that wanted to say perspective varies with spear length; bead color; hair color; size of ones wife; either failed with hunting and got eaten; or went into raising crops.Today the modern duffus adds crop factor and focal length besides spear length or number of shoes owned; it a quest to add dumb things that dont change perspective; to increase their duffusity.
     
  6. Just to muddy the water...

    With small frame cameras (Canon 40D, Nikon 300D, etc.) one stands in the same position to keep the same perspective as when using a full frame camera (5D, D3, etc.) but the small frame user picks a shorter lens to to see the same angle of view. In the end only the DOF changes a bit. But in this way the length of the lens, and sensor size interact to maintain the same perspective and field of view.

    Alternatively, you can keep the same lens and step back a bit with the small frame camera, and thus keep the same field of view but change the perspective. So lens length is indirectly related to the subject of perspective.

    Both your teachers are right, but neither is giving the complete answer. But we all do that.
     
  7. Kelly-
    I understand that the focal length doesn't change, however, just as when going down from 35mm to 110 film, or 35mm to 4x5 film, there are "effective focal lengths" that give perspective.

    Rather like a caveman deciding to use a bow & arrow rather than a spear because you don't need to work as hard to kill the same wildebeest. I can't afford an 85mm lens, so if using a 50 on a crop sensor body works, that's what I'm doing.

    Even highly evolved people need points of clarification.
     
  8. Here is an example of a shot taken with full frame camera (Canon 5D) using a 100mm lens. The red box shows approxiamtely the view with the same lens on a 1.6 crop factor camera. As you can see the perspctive does not change but the field of view does change. .However on the crop factor camera the 100mm lens has te same field of view as a 160mm lens on the full frame body.
    00RP3W-85799684.jpg
     
  9. The answers are correct but overcomplicated.
    <p>
    a) Perspective is purely a function of distance camera to subject as stated<p>
    b) If you put a 50mm on a 1.6x camera and a 80mm on a FF camera to take a portrait then they would have the same angle of view so you would stand the same distance from the subject to fill your picture frame with the subject<p>
    Hence<p>
    c) The perspective will be the same.
    <p>
    If you<p>
    d) Put the 80mm lens on the 1.6x camera it would have a narrower angle of view (like that of a 128mm on FF) compared with the same lens on FF and so you would end of standing further away from the subject and hence: <p>
    e) The perspective would be affected (more telephoto flattening of perspective)
     
  10. George; with a cellphone camera most ALL FOLKS are not confused about perspective. They just move the unit to gather the "look" they want. Since the the sensor size and focal lenght is rarely known; these USELESS facts do not muddy folks brains. <BR><BR>Perspective only depends on where you are with respec the subject; for the 50,000 year old hunter; a painter 2000 years ago; or a photographer today.<BR><BR>The sensor/film size and lens focal length determine the angular coverage. <BR><BR>If the caveman; painter, or photographer moves with respect to the subject; the perspective changes. Perspective has nothing to do with the cavemans spear size; painters brush size or photographers lens focal length. A "newbie" thats a lost soul will say the focal length does; and quickly MOVIE the cameras location to prove his/her ill thinking; and equate the lens to making the perspective change; and not the location shift.<BR><BR>Perspective only depends on where you are with respec the subject; for the 50,000 year old hunter; a painter 2000 years ago; or a photographer today.
     
  11. In professional movie/cine work one moves the camera to the place to get the perspective one wants; <BR><BR>THEN one determiones what the angular coverage one needs' for the "scene"; <BR><BR> then one selects the lenses focal length to give one the angular coverage.<BR><BR><BR>There is NO confusion since movie folks know that perspective is only determined by location of the eye/camera with respect to the subject; and angular coverage for a given format is deternmined by the focal length.
     
  12. It's easy enough with digital cameras and zoom lenses to walk out into the backyard, and get a quick hands on demonstration of perspective. First stand in the same spot and shoot the same scene wide, normal, and long. Next keep the focal length the same, but try to duplicate the angles of view from the previous 3 shots by changing distance to subject. If you have multiple format cameras you can experiment with changing that variable too. Lastly compare the photos on the computer; what effects perspective is pretty obvious.
     
  13. "One teacher insists that because the sensor is smaller, it works much like going from 35mm to 4x5. So an 85mm lens is a telephoto on a 35mm frame, though it's quite short on a 4x5 camera.

    The other teacher insists that an 85mm lens is an 85mm lens, regardless. So the perspective from an 85mm lens is the same, no matter what."

    Teacher #1 is talking about field of view. Teacher #2 is talking about magnification.
     
  14. The responses given above are correct and essentially complete. Yet in many cases, we can look at a photographic image and tell immediately if it was taken with a "wide angle" or "telephoto" lens. This perception is often referred to as wide-angle or telephoto "perspective," even by people who know better than to use that term (and frequently in magazine articles it seems). It has to do with the observer's angle of view in looking at the image compared with the angle of view represented in the image.

    Here is an example: Assume you are viewing an 8"x12" print made from an uncropped frame of 35mm film (or a 24mm x 36mm sensor) from a distance of 18". Your diagonal angle of view is ~47 deg. If the image was taken with a 50mm focal length lens (diagonal angle of view ~46 deg - from Canon EF Lens Specifications table) you would think the image looks "normal." If the image was taken with a 16mm lens (angle of view ~108 deg), you would think "wide-angle shot." Similarly, if the image was taken with a 300mm lens (angle of view ~8.25 deg), you would think "telephoto shot." This is the basis for discussions of "lens perspective," which of course is not perspective at all. If you could adjust your viewing distance so your angle of view matched that represented in the image, that wide-angle or telephoto shot would appear more "normal." This has more to do with physiology and psychology than optics!
     
  15. Thanks everybody!

    Colin-
    So theoretically, I could have gotten the same photo in the crop frame if I used a crop sensor with a 62mm lens?
     
  16. ...er...100 / 1.6 = 62.5. Yes, that';s right.
     
  17. George, another point: When photographers fret about their limited choices of focal length, and wonder aloud (often of this forum!) about which lens to purchase next, they are sometimes advised to "zoom with your feet." You now understand that this is simply not possible. Changing your field of view of a scene by moving closer or farther away and changing your field of view by changing lens focal length or cropping are not at all the same thing. And sometimes "zooming with your feet" is simply not feasible, as when you are standing at the edge of the Grand Canyon!
     
  18. Ben,
    Zooming with your feet is something the teachers at NAU try to pound into students as much as they possibly can. It's great advice, and I try to tell people who don't use their 35mm frame in the photo lab.
     
  19. I hope your professors don't regard their positions as mutually exclusive. If they do, time to consider a different academic institution... :)

    It is true that an 85mm lens is an 85mm lens no matter what camera format it is used on. It is also true that the 85mm lens will provide a
    different angle of view on each format.

    You or your professors might also be confusing "perspective" with "angle of view." They are not the same thing.

    Regarding the comment about primes v. zooms...

    In my view, the zooming with your feet notion is curmudgeonly old school advice. If you carefully consider the arguments for this they
    basically come down to... well, not much at all. There is nothing that you can do with a prime in terms of its angle of view coverage that one
    cannot also do with a zoom. If you want to stick to, say, 35mm you don't have to spin the zoom ring.

    It is not true that all photographers simply zoom rather than taking the time to move and compose their shot. This is often held up as a false
    comparison between the users of primes and zooms. It often goes more or less like this: "It is better to use a zoom and carefully consider
    your composition and move around the subject and closer and further from the subject than to just stand in one place and zoom." Well, duh...
    :)

    But using a zoom does not restrict you to standing in one place and spinning the zoom ring. I shoot with both primes and zooms and I move
    around just as much when I'm trying to establish an effective composition with a zoom as with a prime.

    That said, there _are_ times when you have no choice but to change focal length in order to get the right framing of your subject. You can do
    this with a zoom or with a prime - though it is easier and faster in many cases with the zoom.

    On top of this, good composition is about more than just making the subject the right size and well positioned within the two dimensions of
    the frame borders. It is also, significantly, about things like the relative apparent size of subject, foreground, and background objects - which
    will vary depending upon the choice of focal length. Using a zoom is the quickest and most effective for one to get a handle of these
    concepts since you can vary all of the parameters with one lens: move left/right, move up/down, move forward/backward, change focal length
    to alter relative sizes, etc.

    What are the real reasons for selecting a prime over a zoom? In _some_ cases the additional resolution of a prime (esp. at largest apertures)
    may be significant. In more cases the possibility of using a larger aperture (to stop action or diminish the DOF) can be significant advantages.
    Occasionally, the light weight, small size, and simplicity of shooting with a single prime can be a good thing.

    But for the majority of DSLR photographers, and _especially_ for beginners, a zoom is generally a a better choice at first.

    Dan
     
  20. I didn't read anything but Ronald's first answer, but just saying that whatever else was said, Ronald has it exactly right.
     
  21. Here is an example: Assume you are viewing an 8"x12" print made from an uncropped frame of 35mm film (or a 24mm x 36mm sensor) from a distance of 18". Your diagonal angle of view is ~47 deg. If the image was taken with a 50mm focal length lens (diagonal angle of view ~46 deg - from Canon EF Lens Specifications table) you would think the image looks "normal." If the image was taken with a 16mm lens (angle of view ~108 deg), you would think "wide-angle shot." Similarly, if the image was taken with a 300mm lens (angle of view ~8.25 deg), you would think "telephoto shot." This is the basis for discussions of "lens perspective," which of course is not perspective at all. If you could adjust your viewing distance so your angle of view matched that represented in the image, that wide-angle or telephoto shot would appear more "normal." This has more to do with physiology and psychology than optics!
    Ben, this is interesting. I would like to hear more about this. I'm not sure how a persons diagonal angle of view is defined. Is it the case that as you move closer to an image, your diagonal angle of view would increase, or is it the other way around?
     
  22. I found this from canon. Try it and select the "Choosing Lenses for Different Kinds of Photo" then click Part 3 and click 2x on the next. It will show you the "Lens Angle of View" and "Lens Perspective"

    Great learning demo go ses it.

    http://web.canon.jp/imaging/enjoydslr/index.html
     

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