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Thread: DOF (DSLR v.s SLR)

  1. #1

    Default DOF (DSLR v.s SLR)

    Apologize if this question has been asked before. I do not own a digital DLR yet but am curious over this DOF differences. See http://www.photo.net/learn/optics/dofdigital/ for a technical explanation.

    Techinal aside, how much more difficult it is to take a picture with background blur with digital SLR compared to a film SLR? I mean, when you take a portrait with a 90mm lens, you will most likely set the aperture around f/4 and focus on the eyes. If you own a DSLR with the same view, do you need to go to f/1.8 to get the same effect? From the article, it seems to relate to the lens' hyperfocal distance .... anyone with both digital and film SLR cares to comment on this?

  2. #2
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    For your specific example above - let's say your focus point is 2m away for 35mm film, 90mm at f/4.

    To get the same field of view in digital- assume you are using an APS size sensor, 1.5x FOV crop - and same DOF, you need 60mm at f2.8.

    Look here for a DOF calculator for different formats
    http://www.nikonians.org/html/resour...perfocal2.html

  3. #3

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    One more rather silly question about this 1.6x cropping factor on digital SLR ....

    A standard 60mm lens on 35mm film SLR will give the angle of view of 96mm on digital SLR.

    Assuming you stand 2m from your subject and take a photo. Would taking a photo with digital SLR with 60mm lens and film SLR with 96mm lens at the same aperture give you exactly the same result?

  4. #4

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    Eh.. Took me quite a while to get this in my head but i'll give it a shot here:

    1. DOF is affected by the aperture that you use. Example: F2.8 at 20mm is actually the same as F2.8 at 50mm. The perspective will be different but the depth of field is exactly the same.
    2. Perspective is affected by the focal length of your lens (and of course any distortion associated with it, ie fisheye lenses). If you shoot the same subject with it filling in the same space in your frame at 20mm and 50mm, you will find the following: At 20mm, you will see more of the background behind the subject due to the greater angle of view. At 50mm, you will see less of the background behind the subject due to the smaller angle of view.

    3. Cropping factor. Now the very interesting question, is a 60mm lens on a DSLR with a 1.6x cropping factor the same as a 96mm lens on a SLR? Yes and No. The 60mm lens on the DSLR will give the same perspective as the 60mm lens on the SLR, but it's just that from this frame, it is cropped smaller (the crop factor). It's just like taking a pic on the 60mm lens on the SLR, scanning it into photoshop and then cropping it to the size of the DSLR frame size. Not exactly the same as a 96mm on SLR.

    Hope this helps.

  5. #5
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    Quote Originally Posted by Denosha
    Eh.. Took me quite a while to get this in my head but i'll give it a shot here:

    1. DOF is affected by the aperture that you use. Example: F2.8 at 20mm is actually the same as F2.8 at 50mm. The perspective will be different but the depth of field is exactly the same.
    Sorry, but I don't think that is correct. DOF is affected by focal length, aperture, distance to focus point and Circle of Confusion.

    So assuming distance to focus point and CoC is the same - f/2.8 at 20mm will have very different DOF compared to f/2.8 at 50mm. For example, for 35mm film, and distance to focus point is 2m. f/2.8 at 20mm will give you 0.73m DOF, while f/2.8 at 50mm will give you 0.22m DOF

  6. #6

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    Ok, may be I give one scenario:

    Imagine you are with your friend shooting lions in Tanzania. You have a 35mm film SLR with 300mm lens and your friend has a 35mm digital SLR (not a full-frame sensor so with 1.6x cropping factor). Assuming you and your friends are at the same spot, now you shoot the lion using your 300mm lens at f/5.6. Now you friend does not have a 300mm lens because he paid too much for the digital SLR :-) so he uses his 55-200 consumer zoom lens to shoot the same lion at 300/1.6= 187.5mm at the same aperture. Assuming the same composition and quality of the lenses, does the 2 photos look the same i.e. the lion appears to be same size, same DOF ???!!!!

  7. #7

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    Quote Originally Posted by jitkiat
    Ok, may be I give one scenario:

    Imagine you are with your friend shooting lions in Tanzania. You have a 35mm film SLR with 300mm lens and your friend has a 35mm digital SLR (not a full-frame sensor so with 1.6x cropping factor). Assuming you and your friends are at the same spot, now you shoot the lion using your 300mm lens at f/5.6. Now you friend does not have a 300mm lens because he paid too much for the digital SLR :-) so he uses his 55-200 consumer zoom lens to shoot the same lion at 300/1.6= 187.5mm at the same aperture. Assuming the same composition and quality of the lenses, does the 2 photos look the same i.e. the lion appears to be same size, same DOF ???!!!!

    nope, because he uses the focal length of 187.5mm while you use the focal length of 300mm, so there will not be the same DOF.

    The 1.6 crop is just what it says, crop out!

    if you take the same lens 50mm 1.8 on a DSLR and SLR, the perspective may be different because of the 1.6 crop but using the same aperture, you get the same DOF.

  8. #8

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    same theory for shutter speeds,

    most ppl have a common rule,without any antishake, IS,VR your shutter speed should follow its focal length e.g if you shoot at 50mm your acceptable handhold shutter speed should be 1/50 (1/60) and above to avoid handshake.
    1/200 for 200mm.

    Now what if you put your 50mm on your 1.6 crop DSLR? Is your minimum handholding shutter speed now 1/80 or still 1/50? One salesman told me i should follow 1/80, is he correct?

  9. #9

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    Another question, what is the exposure latitude of a digital SLR? How does it compare to film?

  10. #10

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    Quote Originally Posted by jitkiat
    Another question, what is the exposure latitude of a digital SLR? How does it compare to film?

    which DSLR? Which Film?

  11. #11

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    Gooseberry: What i meant is if the subject size is the same on the frame, as long as your aperture is the same, the focal length used will not affect your DOF.

    This article better illustrates what i mean.

    Belle&Sebastain: "if you take the same lens 50mm 1.8 on a DSLR and SLR, the perspective may be different because of the 1.6 crop but using the same aperture, you get the same DOF." <-- Precisely. But only if you physically move closer with your DSLR to fill the frame with the subject (to the point where is the same size as the SLR one). If you leave the 2 cameras at the same spot where your subject just nicely fills the SLR's frame, your DSLR would give you a "chopped-off" image.

  12. #12

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    Quote Originally Posted by Denosha
    Gooseberry: What i meant is if the subject size is the same on the frame, as long as your aperture is the same, the focal length used will not affect your DOF.

    This article better illustrates what i mean.

    Belle&Sebastain: "if you take the same lens 50mm 1.8 on a DSLR and SLR, the perspective may be different because of the 1.6 crop but using the same aperture, you get the same DOF." <-- Precisely. But only if you physically move closer with your DSLR to fill the frame with the subject (to the point where is the same size as the SLR one). If you leave the 2 cameras at the same spot where your subject just nicely fills the SLR's frame, your DSLR would give you a "chopped-off" image.

    we are not talking about the perspective now, only depth of field. The question is will you get the same depth of field not perspective. you quoted what i say right? dun confuse me.

    anyone knows a 1.6 crop will give you a different perspective, but ppl are confuse as will it affect your DOF. the answer is no for DOF!

  13. #13

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    Quote Originally Posted by Belle&Sebastain
    Now what if you put your 50mm on your 1.6 crop DSLR? Is your minimum handholding shutter speed now 1/80 or still 1/50? One salesman told me i should follow 1/80, is he correct?
    Umm....I follow 1/80 myself. So am i right?

  14. #14

    Default Exposure Latitude

    Quote Originally Posted by Belle&Sebastain
    which DSLR? Which Film?
    Say EOS300D vs Fuji Sensia 100

  15. #15

    Default Quote from Photo.net

    Quote Originally Posted by Belle&Sebastain
    nope, because he uses the focal length of 187.5mm while you use the focal length of 300mm, so there will not be the same DOF.

    The 1.6 crop is just what it says, crop out!

    if you take the same lens 50mm 1.8 on a DSLR and SLR, the perspective may be different because of the 1.6 crop but using the same aperture, you get the same DOF.
    Quoted from photo.net ...

    1. For an equivalent field of view, the EOS 10D has at least 1.6x MORE depth of field than a 35mm film camera would have - when the focus distance is significantly less then the hyperfocal distance (but the 35mm format need a lens with 1.6x the focal length to give the same view).

    2. Using the same lens on a EOS 10D and a 35mm film body, the 10D image has 1.6x LESS depth of field than the 35mm image would have (but they would be different images of course since the field of view would be different)

    3. If you use the same lens on a EOS 10D and a 35mm film body and crop the 35mm image to give the same view as the digital image, the depth of field is IDENTICAL

    4. If you use the same lens on an EOS 10D and a 35mm film body, then shoot from different distances so that the view is the same, the 10D image will have 1.6x MORE DOF then the film image.

    5. Close to the hyperfocal distance, the EOS 10D has a much more than 1.6x the DOF of a 35mm film camera. The hyperfocal distance of the EOS 10D is 1.6x less than that of a 35mm film camera.

  16. #16

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    Quote Originally Posted by 2100
    Umm....I follow 1/80 myself. So am i right?
    i handhold it usually at 1/30 to 1/60 without any problems of blurness.

    so if you own a 200mm f2.8, you will shoot it at at least 1/320 right? then will the shutter speed limit you in your shooting style?

    For your info, i dun believe in the salesman, my train of thoughts is that the image is been physical cropped by the sensor (smaller than film), not that your physical sensor focal plane has been moved further back so you need to increase the shutter speed to compensate. i may be wrong but that what i believe in and so far i had no problems with sharpness to 200mm.

  17. #17

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    Quote Originally Posted by jitkiat
    Say EOS300D vs Fuji Sensia 100
    i'm sorry, not sure of the difference for these two above.

  18. #18

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    Quote Originally Posted by Belle&Sebastain
    i handhold it usually at 1/30 to 1/60 without any problems of blurness.

    so if you own a 200mm f2.8, you will shoot it at at least 1/320 right? then will the shutter speed limit you in your shooting style?

    For your info, i dun believe in the salesman, my train of thoughts is that the image is been physical cropped by the sensor (smaller than film), not that your physical sensor focal plane has been moved further back so you need to increase the shutter speed to compensate. i may be wrong but that what i believe in and so far i had no problems with sharpness to 200mm.
    Yeah, compared to p&s cams, SLRs I can definitely handhold in a better way. This is actually academic, of course. But theorectically, i tend to believe that it is safer to go with the "1/80" rule, esp if you have good optics and shooting at the optimum apertures of the glass.

    My thought is that say if you shoot at "1/50", it looks a bit at 4R size, or say 50% in PS. Blow it up to S8R or 100% in PS, you can detect appreciable blur. But at 25% web size, swee swee man. Same for this kind of sensor cropping lor. I don't find it limiting in shooting style, to get lower speeds so that you can gain in other areas (eg ISO), i use monopods/tripods.

  19. #19

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    Ok, I think gooseberry answer is quite practical. If you shoot at 90mm f4 on film SLR, you must then shoot at 60mm f2.8 to get about the same view and DOF assuming the cropping factor is 1.5-1.6. So, I am done with this issue about DOF of DSLR vs film SLR. I am opening a new thread on discussion on exposure latitude. I am also interested to know the correct answer to the shutter speed issue... may be someone can open a new thread to continue the discussion on shutter speed.

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