Spatial Compression/Distortion DX LENES

Discussion in 'Beyond the Basics' started by Richard, Sep 20, 2007.

  1. Richard

    Richard TPF Noob!

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    Ok I understand that the 50mm is considered a standard lens. There is no spatial distortion or spatial compression at this focal length, expect if I am using a DSLR with a crop factor. So now my 50mm is a 75mm and is in telephoto length. My question is if I have a DX lens (digital lens) on my DSLR with a crop factor of 1.5 would my lens at 50mm be a standard lens or is it still considered a 75mm which has spatial compression?
     
  2. (Ghastly) Krueger

    (Ghastly) Krueger TPF Noob!

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    Been wondering about the same.

    Here is an article on this regard. I understand that the distortion will not change, but the DOF will (to get the same framing).

    Hope it helps. Bear in mind that I'm like your self... two steps from a newbie ;)
     
  3. Alex_B

    Alex_B No longer a newbie, moving up!

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    with the 50mm on a sensor smaller than a full 35mm sensor you will have to move backwards from the subject to make it fill the frame the way it would do on a 35mm sensor. hence you change your perspective and the image becomes more "compressed" ...
     
  4. Steph

    Steph No longer a newbie, moving up!

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    A lens is considered standard when its focal lenght is roughly the same as the diagonal of the film/sensor. Anything above is a telephoto and anything under a wide angle. On an APS-c sensor (22.2x14.8mm for a Canon camera) the diagonal is around 27mm, which makes the 50mm a short telephoto and not a standard lens.
     
  5. Garbz

    Garbz No longer a newbie, moving up!

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    To answer the question directly the DX designation means nothing to the focal length. It just means the lens casts a smaller image circle and will not expose an entire 35mm frame. You still get the 1.5 crop factor equivalent which is why you often see non-DX lenses with the 24-105mm range and DX lenses with the 18-70mm.

    The benefits of DX is it allows manufactures to make a smaller lighter lens.
     
  6. Richard

    Richard TPF Noob!

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    Thanks Garbz, I started to come to that same conclusion about DX lens.

    I'm still curious if I put a 35mm lens on my D40 camera (which would give me an angle of view similar to a 50mm on a 35mm format camera) would the 35mm be considered the standard lens and will I get any spatial distortion ?
     
  7. Alex_B

    Alex_B No longer a newbie, moving up!

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    i still wonder what you mean by spatial distortion.

    but did you read lostprophet's and my posts, that should explain, that a 35mm lens on a smaller sensor gives you the same perspective as a longer lens on a larger sensor... hence it would be closer to the "standard" lens then.
     
  8. Richard

    Richard TPF Noob!

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    Spatial Compression
    Take a 50mm and take a picture, now take 200mm lens and shoot the same picture with the same composition (you will have to move back to recompose the shot). When you compare the images you will see compression. For example lets say you shot a head shot and a block down the street there is a telephone pole. When you compare the images you will notice that in the 200mm shot the pole looks a lot closer to your subject. Spatial compression happens with anything above 50mm, you may not notice a big difference with a focal of 55 but it's there.

    Spatial Distortion
    Take a 50mm and take a picture, now take it with a 17mm lens, make sure to have the same composition (you will have to move closer). When you compare the images you will see distortion. For example lets say you shot a head shot (shoulders and head) when you go to the 17mm and compose the same shot you will get that very common funny face distortion, that you usually see on greeting cards with dogs(HUGE nose and little eyes). Spatial Distortion happens with anything below 50mm focal length.

    This is why the 50mm is considered a normal/standard lens. There is no spatial compression/distortion with that length.

    See what is confusing to me is the 35mm lens is really 35mm from the sensor to the focal plane of the lens, but because of the crop factor it gives me a field of view of about 50mm on a 35mm camera. I know that the crop factor effects the field of view, but does it also effect spatial distortion/compression, because a 50mm lens on any camera is always 50mm from the focal point to the film plane/sensor.

    I suppose I could answer my own question if I could find time to shoot some pictures, and compare them. If I do find time I will post some examples. I'm just so busy and thought someone may know the answer already.
     
  9. Alex_B

    Alex_B No longer a newbie, moving up!

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    OK, did not know that by distortion you mean the opposite of compression . The effect you discribe is purely an effect of perspective (e.g. how far you are from the subject) .. while the focal length defines your angle of view (for a given sensor size). as you correctly stated, to fill your frame with the main subject in the same manner, you have to move closer or further depending on your focal length, and hence your distance and perspective change.

    This is the nose being closer to you and hence appearing larger than the eyes. the closer you get to the face (and you have to get pretty close with a 17mm lens to fill the frame), the larger the difference between eyes and nose will become.

    If you take a 17mm lens and a 300 mm lens and take an image of a face from the same distance, then relative sizes between nose and eyes are the same for both lenses.



    yes and yes.

    since what you call distortion/compression depends only on the field of view (or better: the resulting distance you will chose to the subject), the answer is yes.
     
  10. Richard

    Richard TPF Noob!

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    So what your saying is that if I shoot a face real close with a wide angle lens to get that funny face and then pulled out my 50mm prime shot at the same distance that I would get the same distortion. I know I will have a nose only in the shot instead of a face, but the same distortion will be there... hmm I will see if I can test this, because from how my teacher explained it, that wouldn't make sense.
     
  11. Garbz

    Garbz No longer a newbie, moving up!

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    What you are describing is a function of angle of view and distance only. It doesn't matter if you convert the numbers the only thing that matters is the final effect. A 33.3mm lens on a 1.5crop factor camera will produce identical perspective distortion spatial distortion image compression or whatever you want to call it.

    The only thing that changes is how out of focus things appear at a given aperture. I.e. a 50mm f/1.8 will produce a narrower DOF on a film camera than a 33.3mm f/1.8 on a 1.5x crop. P&S cameras are notorious for not being able to produce shallow depth of field.
     
  12. Richard

    Richard TPF Noob!

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    So garbz just to make it clear a 33.3mm lens on a 1.5 crop is now the standard lens?

    When I say standard lens I am talking about a the focal length that does not have distortion or compression. My teacher made that very clear the a 50mm lens has NEITHER. This is the ONLY focal length that doesn't have one or the other.

    I'm starting to wonder maybe I have the defintion of spatial distortion wrong. I've recorded the lecture so I'm going to listen to it again. I know spatial compression is correct.
     

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