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Thread: CMOS sensors and depth of field

  1. #1

    CMOS sensors and depth of field

    We know that aperture and focal length have much to do with DOF, but what about sensor type and size? Is their any evidence that a CMOS sensor (canon 5D for example) will producer a shallower DOF than a CCD sensor. ALL this comes in to play with video camera's and SLR video capabilities.

    Thanks in advance!

  2. #2
    arith's Avatar
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    Re: CMOS sensors and depth of field

    Canon 5D f1 50mm dof=0.2 metres when focused at 1 metre
    Canon 50D f1 50mm dof=0.1 metre when focused at 1 metre.


    http://www.dofmaster.com/dofjs.html

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    Re: CMOS sensors and depth of field

    A Canon 5D is a full frame sensor and the 50D is an APS-C sized sensor. The size difference of the sensors will make the difference in DOF not the sensor type.

    Both the 5D and the 50D use CMOS sensors. If I remember correctly ALL of Canons DSLR's use CMOS sensors, going back to the D30. I do know the Canon EOS 1D used a CCD sensor but don't remember which came first the 1D or the D30.

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    arith's Avatar
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    Re: CMOS sensors and depth of field

    Nikon D3000 has an apparently different sensor type to Nikon D5000, but they are both the same size and have the same dof. I was only showing the difference is caused by size.

  5. #5

    Re: CMOS sensors and depth of field

    I fear to rush in here but...

    A true 50mm lens at a given aperture should have the same DOF regardless of sensor size. The change in DOF occurs because the photographer using a smaller sensor must back away from the subject to get the same field of vision. Thus the DOF will be larger with a smaller sensor.

    In my circles, we use a rule of thumb: close down one stop if you are using a full frame sensor.

    Dofmaster.com is factoring in "effective" focal length.

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    Re: CMOS sensors and depth of field

    Quote Originally Posted by MarkSalo View Post
    A true 50mm lens at a given aperture should have the same DOF regardless of sensor size. The change in DOF occurs because the photographer using a smaller sensor must back away from the subject to get the same field of vision. Thus the DOF will be larger with a smaller sensor.
    The sensor size, or rather the "circle of confusion" of that sensor size, is one of the four parameters that must be figured in to ascertain DOF.

    IN THE FOLLOWING CALCULATIONS, WE ARE ALWAYS TALKING ABOUT THE TRUE FOCAL LENGTH, NOT EQUIVALENT FOCAL LENGTH. A 50mm lens is 50mm despite being used on 1.6x, full frame or even 6cm square formats.

    The four parameters controlling DOF are:

    1. focal length with which you are shooting
    2. f/stop you are using
    3. distance focused on
    PLUS
    4. THE CIRCLE OF CONFUSION (which is determined by the format size you are shooting)

    I have always hated the term "Circle of Confusion" but, it is an accepted term in our complicated photo-speak. Here is a full explanation of the theory of Circle of Confusion http://en.wikipedia.org/wiki/Circle_of_confusion

    Circle of confusion of a 1.6x sensor: .019mm
    Circle of confusion of a full frame sensor: .03 mm

    The smaller sensor needs a sharper image (smaller circle of confusion) because it will be enlarged to a greater degree than the larger sensor when producing a product of the same size...

    DEPTH OF FIELD IS A TWO EDGED SWORD AND CAN BE CONSIDERED IN TWO DIFFERENT WAYS

    First is when parameters 1-3 are the same and only the sensor size (Circle Of Confusion) changes:

    Shooting with a 50mm lens at approximately 3 meters using f/4

    1.6x sensor: .54m DOF
    Full frame sensor: .87m DOF - the full frame sensor has a wider DOF

    BUT WAIT - WE ALWAYS HEAR THAT FULL FRAME SENSORS PROVIDE A MORE NARROW DOF THAN DO CROP CAMERAS! IS EVERYONE WRONG...

    No! Because will will probably not shoot from the same distance using the different size sensors. When you shoot at 3 meters with the 50mm lens and full frame sensor, you will be shooting at about 5 meters to frame your subject in the same way with a 1.6x crop camera and the 50mm lens.

    Full frame sensor: .87m DOF
    1.6x crop sensor:1 .54m DOF - the full frame sensor has a more narrow DOF

    Thus, the full frame sensor has both a wider and a more narrow DOF than a 1.6x sensor; depending on the distance you are shooting and how you consider which is wider and which is more narrow....

    Addendum:

    If you are shooting with a 1.6x sensor camera and 50mm lens at f/4 from 3 meters you will need about an 80mm lens to frame your subject from that distance using a full frame camera:

    1.6x sensor 50mm lens at 3 meters: DOF .54 meters
    full frame sensor 80mm lens at 3 meters: DOF .33 meters

    Thus for all practical purposes, the full frame sensor can be considered to have a more narrow DOF than the crop sensor.
    Last edited by rpcrowe; 21st February 2011 at 05:12 PM.

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    Re: CMOS sensors and depth of field

    Quote Originally Posted by MarkSalo View Post
    I fear to rush in here but...
    Your fears are well justified

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    Re: CMOS sensors and depth of field

    Quote Originally Posted by rpcrowe View Post
    If you are shooting with a 1.6x sensor camera and 50mm lens at f/4 from 3 meters you will need about an 80mm lens to frame your subject from that distance using a full frame camera:
    Thus for all practical purposes, the full frame sensor can be considered to have a more narrow DOF than the crop sensor.

    Practical example using an 85mm lens on a 20D and a 135mm lens on a 5D at the same shooting distance:



    CMOS sensors and depth of field

    WW

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