Some DSLR sensor size questions, little help???

Discussion in 'Beyond the Basics' started by kkamin, Sep 7, 2009.

  1. kkamin

    kkamin TPF Noob!

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    Hello,

    I recently moved from film to digital (Canon 450D) and had some questions about the smaller sensor size.

    1. Since the sensor is smaller it produces a larger depth of field, but do the lenses made for it compensate? Does a 50mm on a 35mm film camera give have roughly the same depth of field as my Canon 450D with a 50mm lens made for it?

    2. If no, how much depth of field am I losing? A subjective answer if fine (e.g. a little, moderate, a lot, etc.)

    3. I love shallow depth of field, am I screwed? I have two lenses currently that only go down to 4.5, but I haven't played with a faster lens yet. I want a 50mm prime that goes to 1.4, would I be happy with the DOF?

    Thanks, everybody!

    :confused:
     
  2. Plato

    Plato TPF Noob!

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    Check this out.
     
  3. Derrel

    Derrel Mr. Rain Cloud

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    You are "gaining" depth of field by moving down in capture size from FF film, which is 864 square millimeters in size, to Canon's APS-C which is 329 square milimeters in size. With the smaller sensor, for each equivalent field of view height and width, your pictures made with the EOS 450D will have *more* depth of field than the same image shot with a 35mm film camera, at the same f/stop. The lenses do not,to use your worlds, compensate.

    Because the sensor is smaller, you will need to use shorter lenses on APS-C, to get the same angle of view as with FF. Shorter lenses have deeper depth of field. If you use the same lens on FF and APS-C, you will need to move the camera farther back with APS-C to get the same field of view. MY favorite example is 8.47 foot tall FOV of man and wife standing. FF, 20 feet distance with FF, with APS-C Canon, 34 feet distance. Both shot with Canon 85mm/1.8. Check the DOF calculator Plato pointed you to.

    Because the film format is smaller on APS-C, and the camera to subject distance is 34 feet, the depth of field on APS-C will be significantly deeper, with much more of an in-focus background on the Rebel shot.

    To get really,truly shallow DOF on APS-C, you need to shoot close, shoot with a wide aperture, and have the background be 'relatively' far away from the camera. Many slower zoom lenses will not allow you to throw the background out of focus to nearly the same degree as with FF. You are not totally 'screwed' with APS-C, but you are more-limited at producing shallow depth of field shots--BUT the flip side is you can get deep depth of field quite easily. The ability to use traditional lenses at traditional distances (24,28,35,50,85,100/105,135,200) to get selective focus in-doors is the main reason so many serious people-photographers have made the transition to FF cameras like the 5D or D700. The 5D is an excellent body for people photography.

    Here is an article I wrote about how the APS-C format affects people photography Derrel's Photography Blog: How The DX Format Impacts People Photography
     
    Last edited: Sep 7, 2009
  4. kkamin

    kkamin TPF Noob!

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    So the light entering the my EOS 18-55 lens is spilling over the boundaries of the sensor considerably--and to use your example, I need to move back to get the same framing as a FF sensor or film camera. And since I am moving back I am increasing my depth of field since when the subject moves farther away, it intrinsically increases the depth of field and using a shorter lens will too...?

    I just want to make sure I get it. So to attain the same framing at the same distance with my APS-C, I need to use a shorter lens, but my depth of field will increase somewhat. So there really is no way to get an identical shot with an APC-C by only moving back (depth of field will increase) or by only using a shorter lens (depth of field will increase), the only way would be to open up the aperture as well as one of those adjustments.

    1. What is a normal lens for APS-C? (50mm being "normal" for FF)

    2. Say 25mm gives me the same framing as a FF 50mm, does the image start to distort though, concaving at the edges? Or is the sensor small enough to avoid noticeable distortions?

    3. I'm confused. An APS-C with a 85mm f/1.8 at 20 feet gives 1.13 ft. of acceptable focus. A FF with identical settings gives 1.78 ft. of acceptable focus. I understand the framing would be different but why wouldn't the amount of acceptable focus be identical?

    Thanks for your answer and link.
     
    Last edited: Sep 7, 2009
  5. Dwig

    Dwig TPF Noob!

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    50mm is not a "normal" lens on 35mm FF; 40-43mm is. The equivalent of a 50mm on FF is approximately 31-32mm on Canon's version of APS-c (1-2mm longer on Nikon's). Normal (FL=diagonal measure of image at sensor/film) is ~40mm on 35mm film and 43mm on FF digital (film in invariably cropped slightly when printed). Normal is 27mm on Canon's APS-c format and 28mm on Nikon's, in round numbers.

    There is nothing about the shorter FL that implies there would be any difference in the optical corrections. The issues of rectilinear distortion (barrel and pincushion) stem largely from the need to cover a wider field. A 25mm made of APS-c doesn't have that problem any more than a 40mm would on full frame.

    Compensate for what ?????
     
  6. KmH

    KmH Helping photographers learn to fish Supporting Member

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    Here is some good sensor size tech info.
     
  7. Dwig

    Dwig TPF Noob!

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    Good reference link, thanks.
     
  8. kkamin

    kkamin TPF Noob!

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    edit: thanks, you're right. sorry, my first reply was contesting your statement. thanks again!
     
    Last edited: Sep 7, 2009

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