Isn't a normal lens always a normal lens.

Discussion in 'Beyond the Basics' started by deudeu, Apr 30, 2008.

  1. deudeu

    deudeu TPF Noob!

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    There might be a bunch of people that are going to disagree with me on this one but I just want to make sure that what I think i understand about the physics of lenses is right.

    So in the 35mm era (long long time ago... i am kidding) the 50mm lenses were considered "normal". What normal meant was that it reproduced what the human eye could see. Now with the crop factor, people have a different definition of what a normal lens is depending on the size of the sensor. For exemple, if you have a x1.5 crop factor people will consider a 33mm to be "normal".

    I think that by normal people back then were talking about the perception that the lens was giving of distances, right? What i mean by that is that when you shoot a wide angle lens, the distance between the subject and the background will look greater than in real life, if you shoot a telephoto the background will look closer than in real life, but if you shoot a normal lens, it will look normal. So how does the crop factor affect that? I mean, capturing a smaller portion of that same image is not going to affect the perception the lens renders of distances.

    I know people always say "compared to a 35mm camera" when they talk about crop factors. But how is that relevant when you talk to people like me who started getting serious about photography only with digital.

    When i have my 50mm on my camera and i look into the viewfinder, the distances look quite similar to what i see in real life!

    Now, little question on the side, i red somewhere that the "REAL" normal lens was the 43mm. IS this true?
     
  2. usayit

    usayit No longer a newbie, moving up!

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    Yes... you are basically correct. "Normal" focal length will depend on the format.

    Since DSLRs with cropped sensors are seen as an "adaptation" of the traditional 35mm format, people like to think in terms of crop factor to help translate between full frame 35mm and cropped sensors. This is especially helpful since people are generally accustomed to the 35mm format.

    For medium format users, the "normal" lens is going to be a longer focal length. If you were an exclusive medium format shooter and never picked up a 35mm camera in your life, the so called x1.5 crop factor really doesn't make sense to you.


    The biggest misconception is crop factor is a magnification of a focal length. It is not.. it is a crop of the viewing field. There is no variable in the focal length formula for format. 50mm is 50mm no matter what format.
     
  3. Garbz

    Garbz No longer a newbie, moving up!

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    Exactly. Putting a 30mm on a APS sensors does not make it a 50mm. The perspective distortion is still that of a 30mm, only the field of view will be equivalent to that of a 50mm lens.

    /EDIT: Wrong
     
  4. Alex_B

    Alex_B No longer a newbie, moving up!

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    ??? no.

    30 ish on aps will give you the same as 50 ish on 35mm film. field of view is the same, hence you will have the same distance to the subject, hence the perspective is identical.

    assuming ideal lenses. of course the 30mm lens could have less or more lens distortion that the 50mm, but that depends on the specific lens, not on its focal length.



    the only thing which changes will be the DOF
     
  5. Alex_B

    Alex_B No longer a newbie, moving up!

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    But if you shoot with a 50mm lens on a "crop factor" camera, then you will step back to get the same composition (e.g. the same framing of the subject) compared to shooting with said lens on a 35mm film camera.

    This will alter what you call perception of distances since you alter the perspective.

    if however, you had used a shorter lens, corresponding to the crop factor, you would shoot from the same distance, hence get the same perspective and the same "perception of distances".

    "normal lenses" will be quite long when you shoot large format ;)
     
  6. Sark

    Sark TPF Noob!

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    True, using the same lens the field of view changes with different sized sensors, however, the focal length, or equivalent focal length, has no effect on perspective. A lens is just an optical cropping device. How much of the scene it crops depends on its field of view, and therefore, its focal length (or equivalent focal length). Lens to subject distance is the only factor effecting perspective. The only exception to this are those cameras with moveable film or lens planes, and even then it is only an apparent change in perspective.

    Incidentally, early cameras often included a 45mm as a standard lens, although if I remember rightly, the human eye has a field of view of about 38mm. Of course we have two eyes and see in 3D, so an exact comparison is not really possible. 50mm is now the accepted standard for 35mm SLR's & DSLR's.

    Sark
     
  7. deudeu

    deudeu TPF Noob!

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    I understand that to catch the same subject in your frame, you will have to step back. It's just about the distance between the subject and the background and how it compares to what the human eye sees.
     
  8. deudeu

    deudeu TPF Noob!

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    Wouldn't the field of view of the human eye change from one person to the other. I feel like even a 28mm on 35mm camera doesn't catch all of my field of view but i would have to make a real comparison without actually moving my eyes.

    Also, as far as the perspective is concerned, what lens is equivalent to the human eye.

    I personnally feel like having something that respect the perspective that the human eye has would be quite useful, especially when doing portraits and full body shots. For exemple if your subject is laying down in front of you, using a wide angle would make him/her look more skinny, right?

    If you want him/her to look as close to real life as possible, what's the focal length to use?
     
  9. Helen B

    Helen B TPF Noob!

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    As has already been mentioned, perspective is primarily a property of where the lens or eye is located, not the focal length or format.

    Similarity of perspective in a picture can be obtained when the way the image is viewed matches the way the image was made.

    There is a general notion that the comfortable, natural distance to view a print is at a distance approximately equal to the print diagonal. This does not apply for very small or very large prints.

    By geometrical similarity, this means that if the image was made with a lens that has a focal length roughly equal to the diagonal of the sensor or film, perspective will be maintained if that image is then viewed at a distance equal to the diagonal of the viewed image. That’s where 43 mm comes from – it is the diagonal of full-frame 35 mm film (24x36).

    It’s easy to imagine if you think of large format 10x8 photographs. If you take a 10x8 photograph with a 300 mm lens, the view will be exactly equivalent to holding a 10x8 frame 300 mm from one eye. If you made a contact print from that, and held it 300 mm from your eye, the actual view and the print would be similar in terms of perspective. This applies for all formats, if everything is scaled in proportion.

    Perspective distortion occurs when the print is viewed at a distance that is out of proportion to the focal length of lens that was used to make the image. 'Wide angle distortion' disappears if you view the image from a close enough distance.

    Best,
    Helen
     
  10. deudeu

    deudeu TPF Noob!

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    I have red this one about 4 times and i still can't make any sense out of it.

    Are you telling me that the way we see a picture will change depending on how far we are from the picture? First, this seams counter intuitive, but i tried, and i can't say that it works.

    Another way to think about it would be when you think about movies. There are two main ways to get closer to the subject in movies (which are just moving pictures and for which i feel like optics are pretty similar as in photography). Either they do a traveling towards their subject, which doesn't affect the perspective, or they do a zoom in on the subject, in which case it looks like the background gradually gets closer to the subject.

    So, what i mean by "wider focal length stretches perspective" is pretty well explained on this web site on the first picture.
    http://www.facebook.com/photo.php?pid=30202785&id=182400329&ref=mf

    Here we can see very well how, though the same things are in the picture, the picture shot with the wide angle make the far away door look smaller (and thus further). So the perspective is different than what the human eye perceives. This is true, no matter how far away you are from the image you have captured and how large the print is. Or at least this is what i seam to observe when looking at a picture and walking towards it then away from it like an idiot, as i have been doing for the last 5 minutes.

    So my question is, what focal length should i use to get exactly what the human eye perceive, and why would it change on an APS-C sensor.

    I am pretty sure that the crop factor can't affect that in any way.
     
  11. Helen B

    Helen B TPF Noob!

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    In a way, that is exactly the effect that I am describing. In the first case that you mention neither the viewing positiion nor the focal length is changing, so the persepctive relationship stays the same. In the second case the relationship between the focal length and the viewing position is changing.

    Movies are a good example, because the viewing distance is controlled. Movie theaters were designed so that the 'correct' (intended) perspective is obtained in the middle rows of seats. Typically a 100 mm projection lens is used for projecting 35 mm. This means that the target location (the middle row) will see natural perspective if a 100/2 = 50 mm lens was used on the camera - and that happens to be what is considered a 'normal' lens for 35 mm movie cameras even though the frame is half that of 35 mm still cameras. If you were to sit next to the projector, natural perspective would be obtained when the camera and the projector used lenses of the same focal length.

    We disagree on that one - in the respect that viewing distance does not affect apparent perspective. There are a number of optical illusions that use this property - the image looks distorted when viewed from a normal position, but when viewed from one particular location it looks natural and undistorted.

    Lets' go back to the 10x8 analogy. A 'normal' lens is a 300 mm. If we take a picture with a 100 mm lens, the picture is framed as if we have placed the 10x8 frame 100 mm from our eye. A contact print placed 100 mm from our eye will have an exact 1:1 correspondence with the real scene in terms of perspective - the size relationship between the foreground and background will be identical to that in the original scene if we stood at the place the picture was taken at. It has to be that way: that is a simple optical property of rectilinear lenses.

    If we look at the same contact print at the 'normal' distance of 300 mm the perspective will look 'wide' because we have changed the relationship between the taking condition and the viewing condition.

    There are other ways of explaining the same phenomenon - the next one in line is a mental exercise that involves cropping a photograph:

    Now suppose that I take an image on my 10x8 camera with a hypothetical 50 mm lens. That would produce a picture that would look like one taken with about an 8 mm lens on 35 mm (this is a mental exercise). It would look very very wide when viewed at a normal viewing distance. Now I cut out a 24x36 section from the centre. That is exactly what a 50 mm lens on full frame 35 mm would take - a view that is considered normal. Blow that section up to about 10x8 (ie a 6x enlargement, allowing for the difference in aspect ratio) and it will be exactly like a 10x8 made from a 50 mm lens on a 35 mm camera - ie 'normal' (quality issues aside) when viewed at 300 mm.

    The final step is to go back to the full 10x8 neg, and blow it up to the same degree (6x) as the small section was blown up, and view it at the same distance - it will be huge, and the perspective will be normal when viewed at 300 mm. For sure it will have a very different feel, but that is because of the width of the view, not because of the different perspective.

    The focal length that gives normal perspective depends on format and on viewing condition.

    Best,
    Helen
     
  12. deudeu

    deudeu TPF Noob!

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    So if i am understanding everything right, when using a 50mm "normal" lens on a APS-C camera, you would need to make prints 1.5 times bigger in order to get the "normal" perspective (if the viewing distance remains the same).
    But, that would also mean that a 33mm lens on an APS-C sensor would give the exact same perspective (if we keep the same viewing conditions) as a 50mm of a 35mm camera.
    So as Alex B was saying the only thing that would change would be the depth of field. As far as this is concerned, the smaller sensor cameras would tend to have more depth of field because they are using smaller focal length.

    This was a bit of a headacke but i think that i got it.

    Thank you very much helen for your patience and fine explanations!
     

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