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Thread: Naked eye versus zoom, magnification, focal distance, crop factor & visor view

  1. #1
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    Naked eye versus zoom, magnification, focal distance, crop factor & visor view

    I read a lot of tutorials, I thought I understood then make a very simple experiment and I'm am lost... Please help me understand this.

    Facts:
    1. First I owned a point and shoot Minolta Z1 with 10x optical zoom. On the lens is written "38 - 380 mm (Equiv.135)".
    2. After a few years bought a DSLR Nikon D90 with Nikkor 18-105 mm lens kit.
    3. Taking some wildlife pictures in the delta of Danube river, I observed that the maximum magnification of Minolta Z1 was bigger than the maximum magnification of D90 w Nikkor 18-105 mm.

    The first questions for me was how big the magnification of D90 w. Nikkor 18-105 is (eventually compared with Minolta Z1)? So I red a lot of tutorials including the "Understanding Camera Lenses" which I found it to be a great tutorial for me. A big THANK YOU to the author/s.

    Let's make some small calculation:
    Crop factor for D90 is 1.5
    35 mm equivalent for Nikkor 18-105 = 1.5 * 18...105 and the result is 27-157.5 mm
    A "normal" focal lenght is 50 mm so the maximum magnification should be 157.5/50 = 3.15 x

    The theory was great, now let's make some practical tests. According to theory a 50 mm (35 mm equiv) should give me a "normal" view. For this case 50 mm (35 mm equiv) means 50/1.5 = 33 mm. I set my lens to 33 mm, look into the wievfinder then compare the image in the viewfinder with the naked eye image...

    The objects in the wievfinder are smaller and the distances looks longer then the naked eye view. Trying to find a match betwwen naked eye and the viewfinder i found out that i should set my lens somewere to 60 mm, not 33 as expected.

    NOW I'M LOST.

    Can anybody please explain me what I am doing wrong, or what is the explanation for this?
    Why the perspective of the viewfinder looks different that the naked eye perspective on the calculated focal length of 33 mm (equivalent 50 mm for classic film)?

  2. #2

    Join Date
    Nov 2009
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    Provence, France
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    Remco

    Re: Naked eye versus zoom, magnification, focal distance, crop factor & visor view

    Quote Originally Posted by RoGeorge View Post
    I read a lot of tutorials, I thought I understood then make a very simple experiment and I'm am lost... Please help me understand this.

    Facts:
    1. First I owned a point and shoot Minolta Z1 with 10x optical zoom. On the lens is written "38 - 380 mm (Equiv.135)".
    2. After a few years bought a DSLR Nikon D90 with Nikkor 18-105 mm lens kit.
    3. Taking some wildlife pictures in the delta of Danube river, I observed that the maximum magnification of Minolta Z1 was bigger than the maximum magnification of D90 w Nikkor 18-105 mm.
    Logical: crop factor for the Nikon is 1.5 so your 18-105 mm lens would be 27-157mm in 35 mm equivalent, i.e. about half the maximum of your Z1 (where they engraved the 35mm equivalent values on the lens)

    <snipped>
    The theory was great, now let's make some practical tests. According to theory a 50 mm (35 mm equiv) should give me a "normal" view. For this case 50 mm (35 mm equiv) means 50/1.5 = 33 mm. I set my lens to 33 mm, look into the wievfinder then compare the image in the viewfinder with the naked eye image...

    The objects in the wievfinder are smaller and the distances looks longer then the naked eye view. Trying to find a match betwwen naked eye and the viewfinder i found out that i should set my lens somewere to 60 mm, not 33 as expected.

    NOW I'M LOST.

    Can anybody please explain me what I am doing wrong, or what is the explanation for this?
    Why the perspective of the viewfinder looks different that the naked eye perspective on the calculated focal length of 33 mm (equivalent 50 mm for classic film)?
    The perspective (relative size of near and far objects) doesn't change by changing focal length*. What changes is the angle of view, and the 50mm (35 mm equivalent) is supposed to have the same angle of view as the human eye (about 45 iirc). As for the image in the viewfinder, that could be reduced in size, so you'd get a smaller image than what your eyes show you. If you zoom in farther (to 60 mm in your case) the image will have the same size as the naked eye image, but you'll have less in the image then you see with the naked eye: bottom, top and sides will be cropped as if you look through a window.

    An extra complication is that your eyes tend to focus on the centre of the image, so if I try to estimate what focal length to use on a given shot, I tend to underestimate by half (e.g. to get a bird at 3m, I need about a 400 mm, just looking I'd have guessed 135 mm should get me something reasonable... I'm getting better at it now )

    *You can test this by taking two shots from the same position, one at wide angle, one at tele position. You can then crop the wide angle image to give you the same image as you got from the tele shot (at a lower resolution ofc.)

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