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Thread: Focal length variation???

  1. #1
    Hans's Avatar
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    Focal length variation???

    I've searched around and found nothing about this surprising discovery I made today.

    I'm not very technically orientated and am at a loss to explain it and hoping some of the experience here might shine a light on the findings.

    I'm off to Nepal at months end and was looking at light weight backpacking kit to take with me.

    I ended up purcahsing an E series 100mm f2.8 to go with my 20mm (30mmFOV) and 35mm (50mm FOV) lenses (total weight for three lenses is 600g - a lot lighter than the more versatile Nikon 12-24 and 80-200 AF-S I contemplated lugging around which exceeds 2KG!)

    The 100mm E series is a very nice little package with sharp images at 2.8, nice contrast and deep colours and not any bigger than my 50mm f1.4D (though lighter).

    I was comparing it to my 18-105mm VR kit lens with the 18-105 at 105mm FL shooting on two separate D90 bodies and discovered that the FOV on the E series was noticably tighter than the 18-105 at 105mm when shooting a subject at about 1m away??? I had to move the sensor plane of the camera more than a foot to get the same FOV on the LCD screen. The 105 looked more like an 85mm than 105mm (seriously!). Even when the subject is at a good distance the FOV with the 100mm is still tighter, but less noticable.

    The 18-105mm doesn't seem to suffer focus breathing very much so that is probably not part of the issue.

    Could it be that the 18-105 is not really 105mm FL or could the 100mm be that much longer than 100mm FL (bearing in mind it was designed for Film bodies)?

    I don't have access to home computer so can't post pics.

    With the sensor planes being the same distance from the subject you could expect a similar FOV despite the phisical length of the lens, right?

    Thanks for any thoughts,

  2. #2
    Moderator GrumpyDiver's Avatar
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    Manfred Mueller

    Re: Focal length variation???

    Congratulations - you have figured out the lens maker's dirty little secret. The focal length marked on the lens is only "correct" when your focal point is set to infinity. It gets shorter as you get closer to your subject. Nikon's excellent and highly regarded pro lens; the f/2.8 70-200mm was criticized when it first came out because the 200mm setting when focused to the minimum focus distance was only 135mm.

  3. #3
    William W's Avatar
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    Re: Focal length variation???

    Quote Originally Posted by Hans View Post
    Could it be that the 18-105 is not really 105mm FL or could the 100mm be that much longer than 100mm FL (bearing in mind it was designed for Film bodies)?
    Just so I am clear, what I understand your question to mean is: you have two lenses, an 18 to 105 zoom and a 100 prime, and you keep the camera and subject in the same positions and swap the lenses and set the zoom to approximate FL = 100mm and here is a SUSTANTIAL difference in the FoV of the two images you make.

    If this is the case then all you have discovered is that the Focal Length Nomenclature (especially on Zoom Lenses is an indicative and NOT an absolute. If your 18 to 105 is in reality about a 19.2 to 98.3mm, then that would not be uncommon.

    ***

    Quote Originally Posted by Hans View Post
    The 18-105mm doesn't seem to suffer focus breathing very much so that is probably not part of the issue.
    What is "focus breathing" - I have not seen that term before- Is it "Zoom Creep" which means the Zoom will move without much effort because the friction is not great enough to lock it at the FL set by the user?

    WW

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    Hans's Avatar
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    Re: Focal length variation???

    Manfred and Bill, well I never new that. Especially about the 70-200mm!
    That would be it then, and the issue is related to the zoom lens.

    Bill, I actually just had them on two separate bodies but yes, shot from the same point and at teh same subject to camera distance.

    RE: Focus breathing http://www.youtube.com/watch?v=bIrJUFtYxiQ

    Thanks again guys!

  5. #5
    Moderator GrumpyDiver's Avatar
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    Re: Focal length variation???

    Focus breathing is something completely different; it the size change seen in objects in the image when changing the focus point in the image. This is something that usually is not something photographers worry about too much; unless perhaps they are doing macro work and want to do some image stacking. On the other hand, videographers are very concerned about this happening when the focal point is changed during a shot.

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    William W's Avatar
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    William (call me Bill)

    Re: Focal length variation???

    Ah! Thank you both.
    I knew, I understand that – just the phraseology that I use, is different.

    We (in cinematography here) refer to that anomaly simply as “Breathing”.
    For example, we would say: “When we 'Pull Focus' in the ‘Opening Shot' – the 'Final Shot' will have to account for 'Breathing'.”

    WW

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    Re: Focal length variation???

    Quote Originally Posted by GrumpyDiver View Post
    The focal length marked on the lens is only "correct" when your focal point is set to infinity.
    I often wondered about that recently as a result of situations encountered while making photos in my makeshift studio and never got around to asking about it. Thank you!

  8. #8
    Moderator GrumpyDiver's Avatar
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    Re: Focal length variation???

    Quote Originally Posted by William W View Post
    Ah! Thank you both.
    I knew, I understand that – just the phraseology that I use, is different.

    We (in cinematography here) refer to that anomaly simply as “Breathing”.
    For example, we would say: “When we 'Pull Focus' in the ‘Opening Shot' – the 'Final Shot' will have to account for 'Breathing'.”

    WW
    I sometimes wonder if this is why there has been a move to high quality prime lenses in the film and video business as HD has rolled out. Cooke, Zeiss, Leica, Red, etc. are all in this business and a good set of primes will set you back several hundred thousand. Primes are sharper and have less of a tendency to breath.

  9. #9
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    Re: Focal length variation???

    Focus breathing is indeed very much apparent when taking multiple macro images for stacking, particularly when close to minimum working distance. One of the stacking programs I used to use in the past would sometimes developed streaks near the edges, i think for this reason. Zerene, which I have used for several years, has never had a problem with this. However, you do have to take it into account when framing the image--you have to remember to frame using the tightest FOV you will have in the stack.

  10. #10
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    Re: Focal length variation???

    Quote Originally Posted by GrumpyDiver View Post
    Focus breathing is something completely different; it the size change seen in objects in the image when changing the focus point in the image. This is something that usually is not something photographers worry about too much; unless perhaps they are doing macro work and want to do some image stacking.
    Oh yes, and it's a real problem that results in parallax shifting and is the bane of macro stackers.

    Thanks for reminding me Manfred. I'm doing less stacking than I used to because of it - I'm starting to take single shots with more DOF which of course results in messier BG's - which I attempt to fix in LR using the adjustment brush. Good old focus breathing.

    Glenn

  11. #11
    DanK's Avatar
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    Re: Focal length variation???

    Glenn,

    What are you using for stacking? I have found that Zerene does not give me the parallax-related problems around the edges that the Combine programs sometimes gave me. The only problems related to this that I still encounter arise when there is an edge that is a substantial distance (perpendicular to the sensor) from what is behind it. In these cases, parallax problems create a halo, and the difference in perspective between the image with the forward object in focus and the back object in focus is sometimes too large to fix. When it is modest, however, you can often fix it with Zerene's retouching tool, painting from the original images onto the stacked image. for example, the image I'll post below--not one of my best--has quite a bit of retouching because it is so deep.

    Dan

    Focal length variation???

  12. #12
    William W's Avatar
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    Re: Focal length variation???

    Quote Originally Posted by DanK View Post
    . . .Zerene, which I have used for several years, has never had a problem . . .
    Thank you.

    WW

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