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Thread: New Lens - 50mm F1.8

  1. #1

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    New Lens - 50mm F1.8

    Hi you lot
    Me again I am going on my course so will soon stop asking daft questions. But heres todays I am now the proud owner of a 50mm F1.8 lens so is the DoF cos its 50mm or cos its F1.8 I need my course and your help
    rob

  2. #2

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    Re: New Lens

    The aperture (F-number) determines the depth of field, the 50mm is decides how zoomed in the lens is.

    Try Understanding Camera Lenses for it a bit better explained
    Last edited by McQ; 6th September 2009 at 05:33 PM.

  3. #3
    rpcrowe's Avatar
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    DOF (depth of field)

    DOF is controlled by several interrelated factors:

    1. Focal length of the lens

    Given that the other three variables are the same, the lens with a longer focal length will have a more shallow DOF.

    2. F/stop selected

    Given that the other three variables are the same, the larger aperture, smaller f/number, will have a more shallow DOF. (as an example f/2.8 is a larger aperture [smaller f/number] than f/8 and will therefore [other variables being equal] will have a more narrow DOF)

    3. Distance focused on

    Given that the other three variables are the same, when the lens is focused on a closer point, the DOF will be more shallow and if the lens is focused on a more distant point, the DOF will be wider.

    4. Sensor or film size

    The reason that the sensor or film size is considered regarding DOF is due to a factor called "circle of confusion". Basically, a larger film or sensor size will need to be enlarged less to result in a viewable size print so the original image on the film or sensor is not required to be absolutely as sharp as when you need to enlarge an image from a smaller original.

    As an extreme example of the above: If you are shooting using 4x5 film and a 135mm lens focused at 10 meters using f/4; your total DOF will be 4.54 meters. However if you are using a 135mm lens, focused at 10 meters and f/4 aperture on a 1.6x digital sensor; your DOF will be only .82 meters.

    As a less extreme example; if you are using the same focal length, f/stop and distance focused on with a full-frame camera, your DOF will be 1.3 meters as opposed to the .82 meters using the 1.6x crop camera.

    You will sometimes read that a full-frame camera has a more shallow depth of field than a 1.6x camera. This is patently untrue when variables 1-3 are equal. When they are equal, the camera with the largest format will ALWAYS have a wider depth of field.

    However, when shooting with 1.6x and full-frame equipment, variable #1 (focal length) is seldom the same. If you are shooting an image using a 50mm focal length on 1.6x camera and you want to cover an equivalent field of view, you will need to use an 80mm focal length on a full frame camera. So using a 1.6x camera with a 50mm lens, focused at 10 Meters @ f/2.8 will provide a total DOF of 4.48 meters. Using a full frame camera you would need an 80mm lens to cover the same field of view. The DOF would then be 2.68 meters.

    Along the same line, if you are using the same focal length and f/stop on a full-frame camera and on a 1.6x camera and want to cover the same field of view, you will need to back up further from your subject and the extra distance to the point focused on (variable #3) will result in a wider DOF for the 1.6x format.

    Does a full frame format have a more shallow depth of field than a 1.6x format? The answer is YES or NO, depending upon how you interpret the question!

    Here is an excellent depth of field calculator which will allow you to play with the four above variables to see what changes in DOF result when you adjust the values of 1-4.

    http://www.dofmaster.com/dofjs.html
    Last edited by rpcrowe; 25th September 2009 at 05:08 PM.

  4. #4

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    Re: DOF (depth of field)

    Quote Originally Posted by rpcrowe View Post
    DOF is controlled by several interrelated factors:

    4. Sensor or film size

    The reason that the sensor or film size is considered regarding DOF is due to a factor called "circle of confusion". Basically, a larger film or sensor size will need to be enlarged less to result in a viewable size print so the original image on the film or sensor is not required to be absolutely as sharp as when you need to enlarge an image from a smaller original.

    http://www.dofmaster.com/dofjs.html
    I am not very convinced on this point!

    How can sensor size affect DOF? It can change the effective focal length and thus have an impact on the DoF. Otherwise.... i am not sure!

  5. #5
    rpcrowe's Avatar
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    Re: DOF (depth of field)

    Quote Originally Posted by sreelalts View Post
    I am not very convinced on this point!

    How can sensor size affect DOF? It can change the effective focal length and thus have an impact on the DoF. Otherwise.... i am not sure!
    The only place a lens is in perfect focus is the point focused on...

    Any other point on your total subject is actually a circle on the sensor or film plane - called a "circle of confusion". Depth of Field (DOF) is controlled (all other things f/stop, focal length and distance focused on being equal) by how large (or out of focus) that point can be and still be able to provide a reasonably sharp print at a viewable size.

    Obviously, the larger the format, the less the image needs to be enlarged - so the greater the allowable circle of confusion size.

    As an example, the circle of confusion or circle of acceptable focus for a full frame camera is .03mm however, the circle of confusion for a 1.6x format is .019mm - much smaller than for a full-frame format.

    Play with the different formats and other variables on:

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

  6. #6
    Moderator Dave Humphries's Avatar
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    Re: DOF (depth of field)

    I agree that DoF varies with sensor size, but that could be argued to be because the f number does not equate between formats. Just as one uses a crop factor multiplier on focal length, for equivalent DoF between formats, one should also use it on the f number; so f8 on a compact point and shoot with a c/f of 4.7 gives the same DoF as f37.6 (4.7 x 8) on full frame, or f25 on a 1.5 c/f DX camera.

    I'm not disagreeing with what is written above, this is just another way of looking at it.

    Interesting DoF calculator, nice graphics, etc.

  7. #7

    Re: DOF (depth of field)

    Quote Originally Posted by Dave Humphries View Post
    Interesting DoF calculator, nice graphics, etc.
    I like this one. I think it just uses magnification and physical aperture to work it out.

  8. #8
    Moderator Dave Humphries's Avatar
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    Re: DOF (depth of field)

    Hey, yes, that's neat too

    Never seen one for tubes before

  9. #9

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    Re: DOF (depth of field)

    Quote Originally Posted by Andy View Post
    I like this one. I think it just uses magnification and physical aperture to work it out.
    I like the one on my iPhone

    img_0085.png
    Attached Images Attached Images

  10. #10

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    Re: New Lens - 50mm F1.8

    I would like that on my phone will have a loo k for it

  11. #11

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    Re: New Lens - 50mm F1.8

    Quote Originally Posted by bucketman View Post
    I would like that on my phone will have a loo k for it
    It's called "Simple DoF" if that helps.Works well, and has all the usual customisations.

  12. #12

    Re: DOF (depth of field)

    Quote Originally Posted by Dave Humphries View Post
    Hey, yes, that's neat too

    Never seen one for tubes before
    Aye, I spoke to the guy that wrote it to thank him for such a useful calculator after he updated it for the Canon MP-E 65mm and he updated it again for using the MP-E with tubes

    The DoF table that came with the MP-E isn't much use for modern DSLRs:
    New Lens - 50mm F1.8
    The figures that the site gives me seem to be spot on....much less than the above table says!

  13. #13

    Re: DOF (depth of field)

    Quote Originally Posted by Colin Southern View Post
    I like the one on my iPhone
    Certainly looks a lot nicer than an excel spreadsheet on a windows mobile!

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