Results 1 to 7 of 7

Thread: Effect of Zooming on Focal Length

  1. #1

    Effect of Zooming on Focal Length

    I have a camera with f/3.5-5.9 specification.

    What I understand from this specification is

    1. The diameter of the aperture does not change. This keeps the ratio between aperture diameter and focal length the varying- resulting in the varying f/stop when focal length is changed.
    2. Zooming involves changing focal length only.
    3. Focusing involves both changing focal length and f/stop value (i.e. aperture) to get the sharp picture range (DoF) in the desired area

    QUESTIONS

    1. Is the above understanding correct?
    2. Is there any other way to change focal length other than zooming?

  2. #2
    tomdinning's Avatar
    Join Date
    Feb 2013
    Location
    Darwin Australia
    Posts
    188
    Real Name
    tom dinning

    Re: Effect of Zooming on Focal Length

    Quote Originally Posted by Lijo View Post
    I have a camera with f/3.5-5.9 specification.

    What I understand from this specification is –

    1. The diameter of the aperture does not change. This keeps the ratio between aperture diameter and focal length the varying- resulting in the varying f/stop when focal length is changed.
    2. Zooming involves changing focal length only.
    3. Focusing involves both changing focal length and f/stop value (i.e. aperture) to get the sharp picture range (DoF) in the desired area

    QUESTIONS

    1. Is the above understanding correct?
    2. Is there any other way to change focal length other than zooming?
    Some of it is, Ligo. Focusing doesn't involve a change in focal length. Once you have selected the focal length you will need to focus using the focal ring of the lens or allow th camera to do it. You can focus first and then change the focal length but not all zoom lenses keep accurate focus through their full range so if you zoom, check you focus afterwards.
    DoF is also affected by distance from the subject as well as aperture and focal length. If you shift any of these three things you will alter you DoF. Mind you, you probably won't notice if you're a km from the subject and take two steps forward but if you're up close and personal a few cm can make the difference.
    I'm sure others will add to this tale. I'm having dinner so I'll leave it to them.

    Pasta with chicken, pumpkin and mushrooms in a garlic, camenburg and cream sauce, garnished with avocado.

  3. #3

    Re: Effect of Zooming on Focal Length

    Quote Originally Posted by tomdinning View Post
    you will need to focus using the focal ring of the lens or allow th camera to do it.
    What is the exposure parameter that get changed when I do focusing?

  4. #4
    tomdinning's Avatar
    Join Date
    Feb 2013
    Location
    Darwin Australia
    Posts
    188
    Real Name
    tom dinning

    Re: Effect of Zooming on Focal Length

    Quote Originally Posted by Lijo View Post
    What is the exposure parameter that get changed when I do focusing?
    None that you'd notice unless you're using a macro lens. Because the lens elements are pushed away from the sensor as you get close to your subject can be quite extreme in some macro lenses and since there is an inverse square relationship between light intensity and distance there might be an apparent drop off in aperture when using a macro at close range.
    My Nikon 200mm macro, for example is f4 when used at moderate distances but jumps to f5.6 when I get down close and dirty. This doesn't happen with my Nikon 70mm f2.8. Different lenses have different designs as well and some designs have overcome this problem but you do pay for it. The Nikon 105 f2.8 seems to have reduced this effect a bit.

  5. #5
    RustBeltRaw's Avatar
    Join Date
    Jun 2012
    Location
    Detroit, Michigan
    Posts
    1,009
    Real Name
    Lex

    Re: Effect of Zooming on Focal Length

    Quote Originally Posted by Lijo View Post
    1. The diameter of the aperture does not change. This keeps the ratio between aperture diameter and focal length the varying- resulting in the varying f/stop when focal length is changed.
    Correct. Common on entry-level lenses.

    Quote Originally Posted by Lijo
    2. Zooming involves changing focal length only.
    Probably correct for your lens. But I believe constant-aperture lenses must adjust their aperture mechanisms while zooming.

    Quote Originally Posted by Lijo
    3. Focusing involves both changing focal length and f/stop value (i.e. aperture) to get the sharp picture range (DoF) in the desired area
    Correct, and I believe you've got it, but your wording is a little imprecise. Lens aperture and focal length determine depth of field. "Focusing," as it's traditionally understood, is the act of placing the depth of field in the desired area. The two settings usually aren't adjusted at the same time, but they both affect the image's focus.

    On a nerdy technical note, if you're focusing in Live View mode with many digital cameras, the lens aperture will change (usually stopping wide open) while it focuses. I believe this feature is designed to narrow the DoF and bring maximum light to the sensor, both of which would help with autofocus accuracy.

    Quote Originally Posted by Lijo
    1. Is the above understanding correct?
    As far as I can tell, yes.

    Quote Originally Posted by Lijo
    2. Is there any other way to change focal length other than zooming?
    Assuming you can't swap lenses, the only ways you can change focal length are moving the sensor/film plane relative to the lens (further from lens equals more zoom), or decrease the size of the sensor/film plane (smaller equals more zoom in final image). Some high-end Nikon DSLRs can do the latter by using only a small portion of their sensor to collect the final image. Effectively a digital zoom feature, which amounts to cropping since the sensor isn't actually using the same number of pixels to capture the final image. The former can be accomplished with teleconverters or extension tubes, but there are disadvantages.

    Is there a specific project prompting these questions, or are they general?

  6. #6
    Administrator Manfred M's Avatar
    Join Date
    Mar 2012
    Location
    Ottawa, Canada
    Posts
    13,014
    Real Name
    Manfred Mueller

    Re: Effect of Zooming on Focal Length

    Quote Originally Posted by Lijo View Post
    Is there any other way to change focal length other than zooming?
    Yes, it is called walking around and changing your perspective; an essential technique for good photography. The one downside that a zoom lens has is that we tend to plant our feet firmly to the ground and recompose the shot using the zoom feature of the lens. It is often far more productive to move around and shoot the subject matter from a different position and angle of view.

  7. #7

    Join Date
    Apr 2012
    Location
    Dunedin New Zealand
    Posts
    2,697
    Real Name
    J stands for John

    Re: Effect of Zooming on Focal Length

    My feeling is that one changes the focal length of a lens by adding a Close-Up lens or simply focusing which when you have a fixed iris the f/stop value changes. Though you could say that by focusing you are are in fact zooming I am left with just adding the CU lens [ or a negative lens ]
    ZWYF is the abbreviation for Manfred's contribution Not that I blame the zoom lens but rather modern life.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •