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DX 17-55 true 17mm?

Discussion in 'General Technical Discussion' started by Electromen, Sep 2, 2005.

  1. The 17-55mm DX is designed for the DX format of digital cameras. Does this mean it is a true 17mm or does the 1.5 crop factor stiil exist.
    Just wondering.
  2. Good question Greg, I have always thought that it did not have a crop factor but will admit that I am not sure. Looking forward to the answer.
  3. obelix


    Mar 17, 2005
    Fremont, CA, USA
    Crop factor still exists. The focal length is 17mm, but the FOV equivalent to 35mm would be 17x1.5
  4. mf44


    Jun 4, 2005
    NJ & MD
    The DX designation means that, yes, it is for the digital bodies. It doesn't however mean that the lengths have been corrected for crop factor. Therefore, 17mm would behave like ~26mm on a 35mm film SLR. The 1.5 crop factor still exists.
  5. Not that I disagree with what you have to say about the crop factor your explanation still leaves me wanting. When you say the lens was made for digital bodies, what makes it so. Again, my understanding is that it focus the light onto a smaller area than the 35mm film size. This means that they can use smaller and lighter internal elements in the manufacture of the lens. As an example I believe it will not be suitable for a full frame digital camera and if that is true then your explanation about the lens being made for digital cameras is not correct. Am I wrong? As to the crop factor, you may be correct but again, I am not sure.
    Last edited by a moderator: Sep 2, 2005
  6. marsh


    Aug 11, 2005
    Tennessee, USA
    The 17-55 is supposed to be the digital photographer's 28-70. Its effective length (after crop) is 25.5 to 82.5--pretty close to the magical Beast with AFS and fixed F2.8 aperture.
  7. obelix


    Mar 17, 2005
    Fremont, CA, USA

    Made for digital:

    1) Smaller imaging circle of digital sensors.
    2) Possible changes in the way light hits the sensor. [Please see the flash animation in http://www.fourthirds.org, very informative].

    The crop factor exists on a Digital SLR on any sensor smaller than full frame, no matter what lens you use.

    I think the 17-55 can be used without any vignetting on a full frame DSLR / film SLR from 24-55mm.

    Nothing prevents you from mounting the 17-55 on a full frame DSLR, it would vignette, thats about it.

  8. nfoto

    nfoto Guest

    Of course 17 mm is 17 mm. Lenses cannot think by themselves so they have no idea about what the recording surface behind it might be. For all the lens knows, it could be 8x10" or 24x36 mm or DX. The lens is plain stupid and doesn't care anyway. So it cannot change its inherent focal length at will. Neither can it change its DOF properties. It is just too stupid to achieve such a feat.

    Another fact is that any lens has a "crop" factor. Always. It just so happens that for some recording format, based upon tradition, the factor is unity, elsewhere it is not. Thus, we are entitled to call an 85 mm lens as being a "crop" factor 2 lens on the traditional 24x36 mm (it is the normal lens for 6x6 cm medium format), or having "crop" factor 3 for DX format, or "crop" factor 0.75 for the 4x5". All of these factors are of course nonsense and utterly useless information.

    Since we *should* focus on angle of view instead of the silly "crop" factor, learn that any lens with a focal length (approximately) equal to the diagonal of the format gives 45 degrees field of view. This is what we call the "normal" for the given format. If you double the focal length, the angle of view is cut into half, if you make the focal length half of the "normal", the angle of view is doubled. This is simple math and based upon this, you can easily calculate the angle of view for any lens on any format. Or if your math capacity is impaired, just look into the viewfinder. The answer is the same.

    The rationale behind "DX" or "digital" lenses has nothing to do with the size of their image circle as such, although Nikon DX lenses are designed specifically for the smaller "DX" sized sensor. They are so-called "telecentric" designs, meaning they send virtually parallel rays onto the imager. The digital chip thinks this is a very nice way to be treated, since the angle of incidence will be virtually normal (approx. 90 degrees), and accordingly it responds with better image quality, almost no geometric vignetting into the corners of the frame, and much less CA as well - the optical equivalent to a heartfelt "thank you".

    A pecularity with DX lenses is that their image circle is biggest at infinity focus and is reduced when they focus closer (exactly opposite of non-telecentric lenses). So even if a DX lens can fill the entire frame of a 24x36 camera at infinity, it may not do so at close range. Many of the zoom DX lenses from Nikon can be used on "full-frame" cameras (for which the DX lenses will have "crop" factor less than unity, viz. 0.67) for some part of their focal range, but not all, for example the 18-70 DX has its largest projected image circle at its 50 mm setting but still vignettes on a 24x36 mm camera.
    Last edited by a moderator: Sep 2, 2005
  9. Thanks Bjorn, that makes sense to me.
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