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Thread: sensor size and DOF

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    Default sensor size and DOF

    Is there a relationship between sensor size and the depth of field given all other parameters constant?

    e.g. a 1/2.5" sensor vs a 2/3" sensor shooting at the same focal length and same f number on the same subject, which one will give a more blurred background?


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    bigger sensor size shallower DOF.
    smaller sensor size deeper DOF.

    that is why compact PnS have almost infinte DOF
    “How fortunate for leaders that men do not think.” - Adolf Hitler

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    Quote Originally Posted by HydroPoP
    Is there a relationship between sensor size and the depth of field given all other parameters constant?

    e.g. a 1/2.5" sensor vs a 2/3" sensor shooting at the same focal length and same f number on the same subject, which one will give a more blurred background?
    Don't look at the 35mm equivalent focal length bullshit. Same true focal length, aperture and subject distance means same depth of field. However, the smaller sensor will be more cropped. If you alter the subject distance to get equivalent field of view, then the smaller sensor will give you higher depth of field.

  4. #4
    vince123123
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    funny, my experience with Medium Format digital backs show that DOF is much lesser than 35mm DSLRs. more background blur even at close distances of subject to B/g

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    i believe it's somewhere discussed before?
    bigger sensor/film require lens of bigger aperture (physical size)? then that affects the DOF/blurness...

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    DOF for a lens will be the same no matter what the sensor size is.
    But because the sensor is smaller you are only taking the image from the centre of the lens. So if you have a shallow DOF on full frame, then a 1.6x sensor would only take the centre part and give you relatively more DOF...ie you get lesser of the blurred background if you have a shallow DOF....



    You can see from above... say the DOF covers the range indicated in blue.... on the full frame you still get a bit of blurred OOF background... but on the 1.6x sensor, these OOF background is lost... given the same focal length, f stop.

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    good work zplus

    for those who cannot process pics and are better at number crunching as proof.

    Use the DOF calculator here
    The calculator uses true lens focal length and not the 35mm equivalent.

    summarise my results:
    constants: focus at 3m using f/5.6
    canon A95:
    focal length: 7.8mm (field of view is 38mm in 35 equivalent)
    DOF : 1.12m to infinity

    canon 350D:
    focal length: 23.5mm (field of view is 38mm in 35 equivalent)
    DOF : 1.9m to 7.13m

    23.5 after 1.6 crop factor will give the same FOV of ~38mm

    is my above calculation correct?
    “How fortunate for leaders that men do not think.” - Adolf Hitler

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    Quote Originally Posted by Zplus
    DOF for a lens will be the same no matter what the sensor size is.
    But because the sensor is smaller you are only taking the image from the centre of the lens. So if you have a shallow DOF on full frame, then a 1.6x sensor would only take the centre part and give you relatively more DOF...ie you get lesser of the blurred background if you have a shallow DOF....



    You can see from above... say the DOF covers the range indicated in blue.... on the full frame you still get a bit of blurred OOF background... but on the 1.6x sensor, these OOF background is lost... given the same focal length, f stop.
    I think you have misunderstood DOF. It has nothing to do with which part of the frame the image is taken, or how much of the background is included.

    DOF refers to the range of subject distance for which in the final image (usually on a 8X10 inch print) any object within this range of distance can be rendered to be sufficiently sharp and cnosidered to be in focus.

    Cannot really explain the whole thing here, but I suggest you read the following before you give advice on this topic...

    http://www.cs.mtu.edu/~shene/DigiCam...-of-field.html
    http://www.wrotniak.net/photo/dof/
    http://www.luminous-landscape.com/tu...ries/dof.shtml
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    Quote Originally Posted by linse
    Don't look at the 35mm equivalent focal length bullshit. Same true focal length, aperture and subject distance means same depth of field. However, the smaller sensor will be more cropped. If you alter the subject distance to get equivalent field of view, then the smaller sensor will give you higher depth of field.
    I think there is some misconception here.

    In calculating DOF the final output size of the image has to be included in the calcuation. Most DOF calcuation formulas assume a standard print size, a standard viewing distance thus a certain circle of confusion (based on resolution of human vision), so it appears that it is only affected by focal length and aperture setting.

    While the circle of confusion (COC) on the image sensor remains the same regardless of sensor size as long as the focal length and aperture is the same, the magnification required to produce the final output image if the same size will be different for different sized sensors.

    Calculating DOF based based on the COC applied to the sensor surface alone is meaningless because we do not view the final image at the size that is the same as the physical size of the sensor.

    To properly calculate DOF one has to first determine the final image size and the viewing distance. Then based on the resolving power of the human eye the circle of confusion on the final image is determined. After that the COC (on final image) is translated to the COC on the sensor, based on the size ratio between the final image size and the sensor size. From there, the sensor COC is used in the formula that takes in also the focal length, aperture and focusing distance to dermine the DOF.

    DOF refers to the range of subject distacne for which objects will be rendered sufficiently sharp in the final output image.

    One good analogy is to imagine viewing an image at different magnification on screen. Imagine you have an image of 2 objects that has slightly diferrent distance from the lens. When the image is taken the focus is set on one of the subjects, so the other is slightly out of focus.

    If you view the image at a small magnification both subjects appears sharp. If you blow up your image more and more then the subject that was not on the plane of focus will start to appear more and more blur. The DOF has decreased as the magnification is increased, on the same image.
    Last edited by roygoh; 22nd April 2005 at 04:09 PM.
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    Quote Originally Posted by vince123123
    funny, my experience with Medium Format digital backs show that DOF is much lesser than 35mm DSLRs. more background blur even at close distances of subject to B/g
    Your observation actually agrees with yanyewkay's generalisation:

    bigger sensor size shallower DOF.
    smaller sensor size deeper DOF.

    Since medium format sensor size is larger then 35mm, so it should have a shallower DOF.
    As complexity rises, precise statements lose meaning and meaningful statements lose precision.

  11. #11
    vince123123
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    so you're agreeing with me that MF has shallower DOF than 35mm? Cos that's what I seem to be getting and it is now beckoning me to enter the world of expensive MF backs LOL (like real)



    Quote Originally Posted by roygoh
    Your observation actually agrees with yanyewkay's generalisation:

    bigger sensor size shallower DOF.
    smaller sensor size deeper DOF.

    Since medium format sensor size is larger then 35mm, so it should have a shallower DOF.

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    Quote Originally Posted by roygoh
    I think you have misunderstood DOF. It has nothing to do with which part of the frame the image is taken, or how much of the background is included.

    DOF refers to the range of subject distance for which in the final image (usually on a 8X10 inch print) any object within this range of distance can be rendered to be sufficiently sharp and cnosidered to be in focus.

    Cannot really explain the whole thing here, but I suggest you read the following before you give advice on this topic...

    http://www.cs.mtu.edu/~shene/DigiCam...-of-field.html
    http://www.wrotniak.net/photo/dof/
    http://www.luminous-landscape.com/tu...ries/dof.shtml
    Let me try again....

    Given the same lens, set at a focal length, aperture and focus distance... the different sensor sizes will result in different crops. The DOF will be the same... but on a smaller sensor... you'd get less of the blurred background becoz its been cropped away... It would APPEAR to have more DOF because more of the image is in focus...

    So while the DOF does not change in each case... the composition is affected by the sensor size.

    Thought the diagram shows that... the DOF is the same in both the full frame and 1.6x crop....

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    Let me try, pls correct me if wrong

    1) shoot a subject with full frame + 50mm lens.
    Subject to fill up 3/4 of the full frame.

    2) shoot the same subject with dslr with eg 1.6x crop + 50mm lens.
    Increase the distant between camera and subject so that the subject to fill up 3/4 of the frame as in (1)


    With the same aperture setting, the DOF will be similar.


    However, the DOF will be deeper if
    3) shoot the same subject with dslr with eg 1.6x crop + 50mm lens.
    Maintain the same distant between camera & subject as in (1), hence the subject is bigger than the frame (hence what we normally say, 50mm becomes 80mm lens).
    photography makes one sees things from all angles.

  14. #14

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    Quote Originally Posted by Zplus
    Let me try again....

    Given the same lens, set at a focal length, aperture and focus distance... the different sensor sizes will result in different crops. The DOF will be the same... but on a smaller sensor... you'd get less of the blurred background becoz its been cropped away... It would APPEAR to have more DOF because more of the image is in focus...

    So while the DOF does not change in each case... the composition is affected by the sensor size.

    Thought the diagram shows that... the DOF is the same in both the full frame and 1.6x crop....
    Not really. DOF is influenced by sensor size as well. Larger sensors will have more apparent DOF. Another variable to consider is pixel pitch which directly affects CoC size which again affects DOF.

    In this case, the sensor size is smaller which implies more DOF for DSLR, yet, the CoC for DSLR is smaller, which implies lesser DOF. The nett effect depends on which factor has a stronger effect.

    The only way to gauge is to check it on a final print.

    Thus DOF is a function of many factors other than focal length and subject distance.

    Edit: The 2nd paragraph shd read "sensor size is smaller which implies less DOF for a given output size." Hence the nett effect is less DOF.
    Last edited by Zerstorer; 22nd April 2005 at 05:48 PM.

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    Quote Originally Posted by denniskee
    Let me try, pls correct me if wrong

    1) shoot a subject with full frame + 50mm lens.
    Subject to fill up 3/4 of the full frame.

    2) shoot the same subject with dslr with eg 1.6x crop + 50mm lens.
    Increase the distant between camera and subject so that the subject to fill up 3/4 of the frame as in (1)


    With the same aperture setting, the DOF will be similar.
    The result would be unknown as there are more variables at work here.

    However, the DOF will be deeper if
    3) shoot the same subject with dslr with eg 1.6x crop + 50mm lens.
    Maintain the same distant between camera & subject as in (1), hence the subject is bigger than the frame (hence what we normally say, 50mm becomes 80mm lens).
    In this case the DOF of the DSLR will actually be less.

    Roygoh has already supplied all the links which explain why it is so. Or an easier way would just be to check online DOF calculators and key in the appropriate values.

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    Senior Member denniskee's Avatar
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    Quote Originally Posted by Zerstorer
    The result would be unknown as there are more variables at work here.


    In this case the DOF of the DSLR will actually be less.

    Roygoh has already supplied all the links which explain why it is so. Or an easier way would just be to check online DOF calculators and key in the appropriate values.
    Thanks Zerstorer, it will be beneficial to us is you can adivce what are the other variables.

    Will go to the links, thanks Roygoh.

    I remember some CSer posted a link of a photog using the photos he took to try explain the effect of smaller sensor size, but I lost it. Anyone else remeber it?
    Last edited by denniskee; 22nd April 2005 at 04:44 PM.
    photography makes one sees things from all angles.

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    Quote Originally Posted by denniskee
    Thanks Zerstorer, it will be beneficial to us is you can adivce what are the other variables.
    1) shoot a subject with full frame + 50mm lens.
    Subject to fill up 3/4 of the full frame.

    2) shoot the same subject with dslr with eg 1.6x crop + 50mm lens.
    Increase the distant between camera and subject so that the subject to fill up 3/4 of the frame as in (1)


    With the same aperture setting, the DOF will be similar.
    Its already stated in Roygoh's post and my post above. Namely:
    1) CoC, a full frame will have a different CoC compared to a 1.6Crop.

    2nd, the distance is different, also the perspective would also be changed since you have changed the distance. Although your subject may be framed the same, there will be more of the background seen in the 1.6 crop.

    You get the same perspective only if you crop down the fullframe down to 1.6 while shooting at the same distance.

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    Quote Originally Posted by Zerstorer
    Not really. DOF is influenced by sensor size as well. Larger sensors will have less apparent DOF. Another variable to consider is pixel pitch which directly affects CoC size which again affects DOF.

    In this case, the sensor size is smaller which implies greater DOF for DSLR, yet, the CoC for DSLR is smaller, which implies lesser DOF. The nett effect depends on which factor has a stronger effect.

    The only way to gauge is to check it on a final print.

    Thus DOF is a function of many factors other than focal length and subject distance.
    Ok... then my understanding must be really wonkers.... hahaha...
    Anwyay, say you have a lens and on a film body you get the following DOF:


    From 1m to 3m the image is sufficiently sharp on film....
    How would replacing the film back with a digital back (be it 35mm DSLR or MF) change the DOF?

  19. #19

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    Quote Originally Posted by Zplus
    Ok... then my understanding must be really wonkers.... hahaha...
    Anwyay, say you have a lens and on a film body you get the following DOF:


    From 1m to 3m the image is sufficiently sharp on film....
    How would replacing the film back with a digital back (be it 35mm DSLR or MF) change the DOF?
    DOF is a relative measure, its not an absolute. DOF depends on final output size as well as the resolving power of the imager.

    Film grain affects its CoC, so does the pixel pitch of a digital back, CoC is normally defined as the largest circled that is appears to be a point when printed out at a pre-defined output size. Hence it is directly affected by the characteristics of the imager.

    Without full details of every characteristic, it is difficult to say for certain whether the DOF will be more or less unless all factors are in agreement, compared to a known set of conditions.

    Basically, DOF is only relevant when you are comparing it at the final output size, without all variables no one can say what the apparent DOF is.

    When one says DOF is sufficient at 3-5m, the same thing does not hold when one is printing at 4R compared to 12R.

  20. #20

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    Short extract from DOFmaster 50mm lens and 10m subject distance at f2.8

    8x10" sheet film:
    Near limit of acceptable sharpness 3.08 m
    Far limit of acceptable sharpness Infinity

    35mm film:
    Near limit of acceptable sharpness 7.48 m
    Far limit of acceptable sharpness 15.1 m

    Canon 20D:
    Near limit of acceptable sharpness 8.24 m
    Far limit of acceptable sharpness 12.7 m

    All these are based on estimates and are accurate only if the values are accurate. Just a numbers of interest nothing more, the ultimate test is your final print.

    Note that there is some contradiction in the numbers, that's dependent on what CoC values they have in their database. Also note that sensor size affects framing, so to get the same framing requires a change in focus distance, which again throws everything off.
    Last edited by Zerstorer; 22nd April 2005 at 05:15 PM.

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