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  • FOV - 'crop factor' explained.

    This crop factor 'thing' seems (I might be wrong..) to cause some confusion and mis-information. Here is an explanation that might clear some of it up. FOV, or Field of View is a more appropriate term.

    Below is a photo. Suppose I have an 80-200mm lens, and I put that lens on a 35mm Nikon film camera. I take a shot at full extension (meaning at 200mm), and what you see below is the 35mm size. Now, I take the same lens, put it on my D100 and take a shot of the exact same position. The D100 will only capture the image highlighted below in the photo--it will not cover the same area of the 35mm camera, at the same 200mm focal length. The difference is the image circle and field of view. Now here is where the magnification factor comes in, and this is also where many photographers are benefiting from the change in FOV or 'crop'.



    The D100's CCD creates an effect that looks like I actually used a longer lens. I haven't as I used the same lens, I just changed the Field of View, and the image circle. Because it's just a "crop" of the much larger 35mm's field of view (FOV), and then viewed at the same size as the 35mm's, It appears that a longer lens was used.

    Below is a comparison of a standard 35mm's image circle (1), and within that (2) a D100's image circle. You can see the comparison in size between a standard 35mm frame, and the D100's.



    This of course is almost exactly the same on the Canon digital slrs.


    Jeff



    photos from the Nikon Lounge.

  • #2
    Nice explanation! So basically, the crop factor is caused by the D100 CCD's 1.5X Field of View Crop. So let's say when you put a 300mm lens on a D100 you do NOT get a 480mm(?) lens it is still a 300mm lens. What has changed is the format of the camera from 36x24mm film to a 22.7x15mm(?) chip.

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    • #3
      What has changed is what gets recorded. Just look at the bird picture.

      Comment


      • #4
        Jeff here is one for you. Often I hear people say, use a wider angle lense to increase the depth of field. Now while this looks like it increased the depth of field, it actually doesn't.. Now I know the answer why but do you want to give that one a shot.

        And Colin, no you can not answer

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        • #5
          To actually increase the DOF?, or just give the appearance of increased depth.

          An example:

          A 20mm @2.8 @10' the near focus distance = 6'1 far = 27'5
          A 50mm @2.8 @10' the near focus distance = 9'1 far = 11'1
          A 100mm @2.8 @10' the near focus distance = 9'9 far = 10'3

          The depth of field is greater for short focal lengths than for long ones.
          Depth of field increases with the subject distance.

          Is that what your talking about?

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          • #6
            Not quite
            If you had a 16mm lense. The near point and the far point of focus is identical to that of a 10mm or 200mm lense.

            In actual fact, the Dof Field remains the same on all lensesbut angle of view is all that changes.

            I can't quote the specifics, it is all mathematical and I would need to follow that up but it's a point that a lot of photographers get wrong.

            Now I hope I haven't gotten this wrong. I'll need to find my books now

            :

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            • #7
              I'll need to find my books now

              Hmm yes, perhaps

              The precise point of focus for any lens has no depth - there is one plane (no pun intended!) at which light is resolved to the finest point possible.

              Moving forward and back from that plane, the points start to take on a circular nature - they become discs. However, the human eye can only perceive these as discs rather than points after they reach a certain size - this figure is usually pegged at .03mm (when viewed on an 8x10 print at a distance of around 35cm) - anything smaller will be perceived as a point, anything larger as a disc.

              For anyone interested in the math, see

              http://www.dof.pcraft.com/dof.html

              If you work it through you'll find that altering the value of the focal length and/or aperture does indeed change the range of the front and rear limits of DoF.

              Glenn - I think you were thinking of the often said, but incorrect definition that says "everything within the DoF is sharp, everything else blurry". Really the Dof is about a region of acceptable sharpness.

              Cheers,

              Colin
              ________________________________________
              A member of AirTeamImages

              Comment


              • #8
                True to a certain degree. I'll find my notes when I can.

                However, what you quoted sounds to me like the Circle of Confusion that relates to 35mm, which relates to the evenness mount of light that falls on the subject (Being in this case 8x10 paper) I don't recall it all so I may be heading off here but from memory the CofC for 35mm is .3 Medium Format is .5 and large format 1.

                I stand beside my comments but my terminolgy may be incorrect

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                • #9
                  Yes Glenn in this you are correct - the circle of confusion strictly relates to the viewed print - and this is important becuase DOF is not fixed - it depends as well on how the image is cropped/enlargled. Those DOF scales you find on lenses are simply a guide based on a full fame enlargement to 8x10 - and the values are different for different formats.

                  In fact DoF is inversly proportional to the size of the format - which tallies with your COC figures ... (except I think you're a decimal place out - eyes getting old )

                  Cheers,

                  Colin
                  ________________________________________
                  A member of AirTeamImages

                  Comment


                  • #10
                    Well done Jeff.
                    The info is not new to me but you made it more descriptive.
                    greetings,
                    Philip
                    ______________

                    Comment


                    • #11
                      Is it just me or is Glenn and Colin's discussion going way, way, way over your head too?
                      Garry Lewis

                      Air Team Images - www.airteamimages.com
                      Air Traffic Controller - Toronto ACC (West Low)

                      https://flic.kr/ps/AAWk8

                      Comment


                      • #12
                        Originally posted by atco
                        Is it just me or is Glenn and Colin's discussion going way, way, way over your head too?
                        Indeed Garry
                        Maybe I should learn not to be so lazy and skip repies
                        greetings,
                        Philip
                        ______________

                        Comment


                        • #13
                          Sorry colin, I see your point and not my point so could I know make that 1 a .1

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