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Thread: MATH / DATA DISCREPANCY

  1. #1
    Bill McCrystyn
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    Default MATH / DATA DISCREPANCY

    I have a D300 200-400VR 1.7X. My math sezzz 1022mm - but my data continues to say 975mm???? :( Which is it O' Wizards
    Last edited by Bill McCrystyn; 03-02-2009 at 09:37 AM.

  2. #2
    Axel Hildebrandt
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    I'm not sure I understand. Since 400x1.7 is 680mm I assume you take the crop factor into consideration, right?

  3. #3
    Bill McCrystyn
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    That's right Alex. 400 x .5 crop= 600 (which my data often shows as 650) x .7 TC = 1022. Something is wacky. I don't have this with the 70-200.

  4. #4
    Robert Amoruso
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    975/400 = 2.4375

    2.4375/1.5 = 1.625 (close to 1.7 TC)

    2.4375/1.7 = 1.433 (close to 1.5 crop factor)

    I am not trying to make sense of it, just trying to work out the arithmetic.

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    Bill- My Canons do not report the crop factor effect in EXIF data, which they shouldn't. They report only the effective (real) focal length which in your case is 1.7 x 200-400 = 340-680 mm. The crop factor effect on focal length is illusionary because the actual size of objects in your field of view as projected on the sensor is the same, regardless of the crop factor, all other things being equal. What a crop body does is exactly that, crop out a border around your subject, giving the appearance that the subject is closer than it really is.

  6. #6
    Axel Hildebrandt
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    It could be a software issue, if the D300 has a 1.5x crop sensor, then 600x1.7 would result in 1020mm. If it were a 1.6x crop sensor, it would be 640x1.7, which is 1088mm. I usually just say camera type and actual focus length and if attached, what kind of TC since the crop sensor doesn't change the focal length, just the field of view.

  7. #7
    Bill McCrystyn
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    John I appreciate what your saying but in PS CS3 my Nikon D300 reports in EXIF as "focal lenght in 35mm film". For example my 70-200 reads as focal lenght 340mm (with 1.7X) - as 35mm film = 510mm assuming that's how I shoot it.
    Last edited by Bill McCrystyn; 03-02-2009 at 11:25 AM.

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    That's strange Bill. My CS3 (Mac) reports (effective) Focal Length and then below correctly IDs the lens and if a tc is being used. Can't see anywhere "Focal length in 35mm film" and was not aware that this is an EXIF datum. Which CS3 File property category is it in (Focal Length in my CS3 is in Camera Data 1).

  9. #9
    Bill McCrystyn
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    I just realized it is making a simular error in reporting the 70-200 factor also looking at my numbers. It should, all being equal, 200 x .5 crop = 300 NOT 340??. I'm going to double check my data when I get back home. I had to go and open my popsicle stand this morning and now am away from my CS3. I'm in the twilight zone. I'll get back to you later with mo info.

  10. #10
    Lance Peters
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    HI John - good point in regards to the crop factor - subject is the same size on a 1.6 and full frame.
    Which is how I have always understood it to be - so what I do not understand is.......

    The very often quoted - I need the DX for the extra reach... when it is nnot giving any extra reach -- all it is doing is compressing the background more via the crop.

    Am I wrong in my understanding or do most people just not get it????

  11. #11
    Axel Hildebrandt
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    Quote Originally Posted by Lance Peters View Post
    HI John - good point in regards to the crop factor - subject is the same size on a 1.6 and full frame.
    Which is how I have always understood it to be - so what I do not understand is.......

    The very often quoted - I need the DX for the extra reach... when it is nnot giving any extra reach -- all it is doing is compressing the background more via the crop.

    Am I wrong in my understanding or do most people just not get it????
    I think what people mean is that if you have 10MP full frame and 10MP 1.x crop at the same distance and focal length the latter gives you more 'reach' when you look at the image at 100%.

  12. #12
    Bill McCrystyn
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    Just quickly as I check my whacky data I have to mention Lance, is you also get to drop using the outside edge of the glass and use the center prime cut, to use a beef analogy. That's always the best, eh?

    I'm working on it John - I couldn't resist that. :D It's like you say Lance, It's not what you see, but how you see it.
    Last edited by Bill McCrystyn; 03-02-2009 at 07:25 PM.

  13. #13
    Bill McCrystyn
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    To answer your question John I have found it in: FILE/FILE INFO/ADVANCED/EXIF Properties/FOCAL LENGHT IN 35mm FILM.

    It would appear that the program can not calculate it's equivalency precisly as different "crop factors" run the gamut. It is close but no cigar. It is "of interest" and gets close. I was concerned my lens was off until I noticed my 70-200 was reading wrong on the low side also. As Axel said "it could be a software issue." Thanks guys - false alarm.

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    Interesting Bill. I have Canon and run CS3 on a Mac, and do not have FOCAL LENGTH IN 35MM FILM at that location. It's either the camera make or the CS3 version. Seems you got to the bottom of it though.

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    Hi Bill,
    When you buy a lans, of say 200 mm focal length, it is a low probability that it is actually 200 mm focal length. Manufacturing tolerances in lens grinding result is small variations in actual focal length. It may be that a lens is measured and the actual focal length put in the chip in the lens. If so (and I have no idea if manufacturers actually do this), then you may be seeing the true focal length. I seem to remember some odd focal lengths reported on zoom lenses when I thought I was at the limits of the lens. For example, if your TC was 1.65x instead of 1.7, you could get slightly different numbers when the lens without the TC would give the same focal length as the published lens value.

  16. #16
    Bill McCrystyn
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    Hmm, That's easy to check. If the software was calculating correctly then the lens would spec out or at least give the same comparitive variance without the TC. Thanks Roger, I'll run a test.

  17. #17
    Bill McCrystyn
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    Preliminary conclusion seems to be the software converter/calculation combo is the culprit. 200-400 set at 400 reads focal lenght 400 / EFL 600. With the 1.7X set at 400 it reads focal lenght 650 / EFL 975. OOPS :( Long on the bottom - way short the mark on top?

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