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Thread: Debunking the DSLR FLM Myth and Misconceptions

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    ClubSNAP Admin Darren's Avatar
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    Exclamation Debunking the DSLR FLM Myth and Misconceptions

    With all the confusion on FLM (Focal Length Magnification) caused by the smaller sized half-frame CCD/CMOS imagers currently found in the respective DSLRs, here is an actual, real-life test to totally debunk and clarify all those myths and misconceptions about DOF, FOV etc. etc.

    Test setup:-
    Cameras used - Nikon F5 and Nikon D1x
    Lenses used - Nikkor AIS 50mm f/1.4 and Nikkor AF 105mm f/2DC
    Camera(s) setup on tripod at fixed location.
    Manual focusing used, with confirmation via AF using center AF sensor on the Leatherman Wave (front object).
    Film scanned using Coolscan IV and cropped to approx. DSLR image size.

    Using 50mm f/1.4 lens at f/2:-



    Using 50mm f/1.4 lens at f/5.6:-


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    Using 105mm f/2DC lens at f/2:-



    Using 105mm f/2DC lens at f/5.6:-


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    Which is which? Did you also do a test with the subject distance maintained?

    Regards
    CK

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    Originally posted by ckiang
    Which is which? Did you also do a test with the subject distance maintained?
    The top image is the FILM version and the bottom image is the DSLR. Will add some text to the image to differentiate.

    Subject distance is maintained for all shots - eg. the tripod was not moved, just changed the camera.

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    Originally posted by Darren
    The top image is the FILM version and the bottom image is the DSLR. Will add some text to the image to differentiate.

    Subject distance is maintained for all shots - eg. the tripod was not moved, just changed the camera.
    Strange, if you did not move, how did you maintain subject size?

    Regards
    CK

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    Originally posted by ckiang
    Strange, if you did not move, how did you maintain subject size?
    The film was scanned and then cropped to approximate the image taken by the DSLR.

    Here are the original images (downsized by 50% otherwise files will be huge) if you want to do your own comparisons. Warning! files sizes vary between 400KB to 1.2MB

    50mm f/2 Film Scan

    50mm f/5.6 Film Scan

    105mm f/2 Film Scan

    105mm f/5.6 Film Scan

    50mm f/2 Digital

    50mm f/5.6 Digital

    105mm f/2 Digital

    105mm f/5.6 Digital

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    After posting the examples, I need to clarify that this test is supposed to show that the DOF between a DSLR and film SLR remains the same given the same subject distance.

    What is being done in effect is "magnifying" the center portions of the film scans to show the same DOF (also equivalent to limiting the FOV) effects.

    Thus if you were to mount the same lens on a film SLR and then on a DSLR, the image that you see on the DSLR will appear to be smaller (magnified?) when in reality, all the DSLR is doing is that the sides of the image normally shown by the film SLR is "cropped" away.

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    When you are comparing for DOF, you have to compare the images at the same size, you cannot compare a larger one to a smaller one (which you are doing by cropping the film image).

    Please see Canon EOS FAQ Q27
    http://bobatkins.com/photography/eos...llany.html#q27

    or

    http://www.outsight.com/hyperfocal.html


    Here are two images of the same size, and you can clearly see that the film image has more DOF.

    Film 50mm f2


    DSLR 50mm f2
    Last edited by reflecx; 15th December 2002 at 04:44 PM.

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    Originally posted by reflecx
    Here are two images of the same size, and you can clearly see that the film image has more DOF.
    sorry, don't quite see where you can infer that the smaller image has more DOF cos the backgrounds of both the pictures are equally blurred and without definition.

    Also, because of the smaller image size of the film scan as shown by you, there is a perception of higher DOF mainly because of the shrinking. But I stand by my point that the DOF remains the same for both images.

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    Originally posted by Darren
    sorry, don't quite see where you can infer that the smaller image has more DOF cos the backgrounds of both the pictures are equally blurred and without definition.

    Also, because of the smaller image size of the film scan as shown by you, there is a perception of higher DOF mainly because of the shrinking. But I stand by my point that the DOF remains the same for both images.
    reflectx has it right. You must compare both uncropped images. What you are doing now is essentially some form of "digital zoom" on the film scan.

    If you really want to compare, what would be needed will be say, a 50mm on the DSLR and a 75mm on the full frame. In this case, if my theory is right, the DoF will then be the same.

    Regards
    CK

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    Originally posted by Darren
    sorry, don't quite see where you can infer that the smaller image has more DOF cos the backgrounds of both the pictures are equally blurred and without definition.
    The lettering on the book is clearly sharper on the film image, and assuming the focus is on the front object, that means that it has more DOF.

    Also, because of the smaller image size of the film scan as shown by you, there is a perception of higher DOF mainly because of the shrinking.
    When you want to compare DOF in 2 images, both images must use the same viewing distance, same image size, and the same circle of confusion. What you have done is use different image sizes.

    Or if you're still not convinced, please use the DOF calculator here

    http://dfleming.ameranet.com/dofjs.html

    or

    http://www.silverlight.co.uk/resources/dof_calc.html

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    Originally posted by ckiang


    reflectx has it right. You must compare both uncropped images. What you are doing now is essentially some form of "digital zoom" on the film scan.

    If you really want to compare, what would be needed will be say, a 50mm on the DSLR and a 75mm on the full frame. In this case, if my theory is right, the DoF will then be the same.

    Regards
    CK

    CK, your theory is incorrect, the equivalent is about 62mm on the FF. Checkout the equations I posted in the other thread to see why it's not a simple formula. Use the online DOF calculators for ease of calculation.

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    Originally posted by reflecx



    CK, your theory is incorrect, the equivalent is about 62mm on the FF. Checkout the equations I posted in the other thread to see why it's not a simple formula. Use the online DOF calculators for ease of calculation.
    Assuming a 1.5x FLM, isn't the equivalent focal length of 50mm on the HF 75mm?

    Using the DoF calculator, and selecting "APS" in place of the DSLR, 50mm lens at f/2, subject distance 2m gives DoF of 0.156m.

    Using 35mm format, 50mm lens @ f/2, subject distance 2m gives DoF of 0.206m.

    So like you said, the film (full frame) system has a larger DoF.

    Can't find a 75mm there, neither can I find a 62mm, so I tried 100mm on the APS/HF, which should then roughly be 150mm on FF.

    HF/APS : 100/2 @ 2m : 0.038m
    FF : 100/2 @ 2m : 0.05m
    FF : 150/2 @ 2m : 0.022m

    Not QUITE the same, but close enough since the APS and 1.5x HF isn't exactly the same size (but close)?

    Regards
    CK

  14. #14

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    After following this thread and the other similar thread, the main confusion lies with the Circle of Confusion, because CoC affects the DOF.

    If the CoC of a 35mm format and a HLF DSLR format are the same, the DOF produced with the same lens of a constant focal length (eg 50mm) and a constant aperture (eg f2) will be the same.

    Originally posted by ckiang

    If you really want to compare, what would be needed will be say, a 50mm on the DSLR and a 75mm on the full frame. In this case, if my theory is right, the DoF will then be the same.
    CK, however, we should not conclude that the the DoF is the same for a 50mm on the DSLR and a 75mm on the full frame. As even amongst the different DSLR, the sensor sizes are different as well. The difference is the CoC, not the FLM.

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    Originally posted by Bean
    After following this thread and the other similar thread, the main confusion lies with the Circle of Confusion, because CoC affects the DOF.

    If the CoC of a 35mm format and a HLF DSLR format are the same, the DOF produced with the same lens of a constant focal length (eg 50mm) and a constant aperture (eg f2) will be the same.

    CK, however, we should not conclude that the the DoF is the same for a 50mm on the DSLR and a 75mm on the full frame. As even amongst the different DSLR, the sensor sizes are different as well. The difference is the CoC, not the FLM.
    The CoC of full frame 35mm and 1.5x DSLRs are DIFFERENT. You cannot have 2 different image sizes with different CoC. And since CoC ultimately affects DoF, the DoF of HF SLR and FF SLR will be different even if lens/aperture remains the same.

    Use the DoF calculator and you'll see what I mean. And in my example, I was assuming a DSLR with a 1.5x FLM for ease of calculation.

    Regards
    CK

  16. #16

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    Originally posted by ckiang
    The CoC of full frame 35mm and 1.5x DSLRs are DIFFERENT. You cannot have 2 different image sizes with different CoC. And since CoC ultimately affects DoF, the DoF of HF SLR and FF SLR will be different even if lens/aperture remains the same.
    Oh yah, forgot to add that the CoC of both format are different.

    CK, what I am trying to highlight is that besides the usual variables like the focal length, aperture and the subject distance, Circle of Confusion also affects the DoF. Different systems have different CoC, thus the different DoF, even if the focal length, aperture and the subject distance remain constant.

    However, I feel that you should not conclude that the the DoF is the same for a 50mm on the DSLR (with a 1.5x FLM) and a 75mm on the full frame, as the FLM does not come into the formula. IMO, this theory of yours will only increase the confusion and mislead the real variables behind DoF.

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    [WAIL]

    but CoC has nothing to do with the film plane...
    it's the problem of the lens, unless in mounting the ccd they shifted the distance...



    so if u compare uncropped SLR shot vs DSLR picture, the DSLR picture MUST HAVE the SAME subject size, i.e. the toy must be as big.. to measure DoF. otherwise I can blow up my 2 mp picture and say it ahs incredible dof...

    [/WAIL]

    I but am confused by why u r confused....
    "I'm... dreaming... of a wide... angle~
    Just like the ones I used to know~"

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    from the shots I can say that there is a little diff in DOF but could be human error
    maybe might have been better if there was lengthwise shot down a ruler or something...


    OT:hi darren what's that new terry book? Night Watch? Discworld or chg universe?
    "I'm... dreaming... of a wide... angle~
    Just like the ones I used to know~"

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    tralala, was reading again and again... understand why some academics can go crazy over some issue and fight wars and stuff...

    so film grain is bigger therefore CoC has a higher tolerance, i.e. can be bigger.
    ccd "grain" is much finer and therefore a DOT vs a CIRCLE is easier to differentiate.
    (might not be correct or close) the lens' CoC (the "grain"f the lens, aka lens resolution) shd be usually much higher. ie 10x or 100x sharper vs ccd or film grain. herein lies the dispute. if lens is much better than film then my argument fails, becos then the CCD's CoC is *really* smaller.
    i.e. hence u can really tell what was in focus vs what was CoCed (CoCed means within DoF, and thus also covers hyperfocal ["looks sharp enough to me" == hyperfocal/DoF]).

    therefore, as such, assuming the paper printed is super sharp/fine then there IS a diff in DoF, becos the ccd can resolve smaller circles as circles and not as dots.

    therein lies the new battle LOL: if I scan into PC and scale it to something practical, both DoF becomes equal becos the new CoC limit is now the screen pixel, which I think is much coarser than film grain or ccd grain. (extreme example: represent a whale and an elephant with 1 pixel. both are grey right? can tell diff?)

    can everyone get this?

    So my revised conclusion is that practically, by the time the photo reaches us in presentable format, it is essentially the SAME.
    But, if u can print the full theoretical image 100%, either via big big enlargements or sci-fi then the DoF is different, as according to the formula, which takes the CoC of the receptor but assumes the presentor media is 1:1.

    Hence I do apologize for insisting that DoF is ALWAYS the same. becos like gravity, in humongous scales there will be the discernible difference. (yeah all physics)
    "I'm... dreaming... of a wide... angle~
    Just like the ones I used to know~"

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    Here we go again.

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