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Thread: How do you remember your DoF numbers?

  1. #1
    whited3's Avatar
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    How do you remember your DoF numbers?

    I've got a table of numbers I keep in my bag and a nice little app on my phone, but both are cumbersome to use when I'm out and about with my camera.

    How do you remember DoF numbers for your various lenses?

    Cheers.

  2. #2
    rpcrowe's Avatar
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    Re: How do you remember your DoF numbers?

    I don't memorize them. When I want a wide DOF, I use a small f/stop (large number); when I want a narrow DOF, I use a wide f/stop (small mumber).

    When DOF is critical, I use the DOF preview button to see what the DOF will look like...

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    Have a guess :)

    Re: How do you remember your DoF numbers?

    Pretty much the same as Richard, although I must confess, I've never found the DoF preview button to be much use; usually at higger F-Numbers the image is so dark my eyes can't really resolve what's in or out of focus through a tiny viewvinder anyway.

    After a while it just becomes an experiance thing, with the distance from camera to subject being one variable (the closer you get the more careful you need to be to ensure a good D of F), and the ratio of camera to subject -v- subject to background being the other (the greater the ratio, the more the background will be out of focus).

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    whited3's Avatar
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    Re: How do you remember your DoF numbers?

    Quote Originally Posted by Colin Southern View Post
    .....I've never found the DoF preview button to be much use; usually at higger F-Numbers the image is so dark my eyes can't really resolve what's in or out of focus through a tiny viewvinder anyway.
    That's me too. The eyes aren't as good as they used to be. I found the DoF button on the 500D useless but seems much improved on the 7D.

  5. #5
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    Re: How do you remember your DoF numbers?

    I worked out an approximate formula from formulas given elsewhere:
    H=hyperfocal distance
    f=focal length
    d=distance to point of focus

    then d*[H/(H+d), H/(H-d)]

    but in reality I only use the end point and assume the near point is 1/3 closer than the point of focus.

    Now I also remember the Hyperfocal distances for 17, 35, 50, 70,200mm at a fixed aperture but for some reason I forgot 135mm

    Whatever, H is also approximated by f^2/(A*c) so a lot of approximations going on here.

    A=aperture and c=constant

    What is important to notice is if you double f you get 4 times the hyperfocal and if you half A you double the hyperfocal.

    At 70mm f4 which is the most useful aperture for me, I can remember the hyperfocal is 65 metres.

    So at 140mm I know it is 260 metres @f4 but f8 is 130 metres.

    Supoose the distance to target is 30 metres then

    30*130/(130-30) = 30*1.3 or 39 metres @ f8

    So my guess is 20 metres to 39 metres dof, AND IT IS A GUESS.

    Checking with dofmaster gives 24.5 to 39 metres.

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

    with my 50D

    Once you got the dof, then any focal length at that aperture has the same dof at the same magnification, but with longer lenses the OOF bits are bigger and perspective different.

    It is all a waste of time if you don't know the distance to target though.
    Last edited by arith; 15th August 2011 at 09:47 AM.

  6. #6
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    Re: How do you remember your DoF numbers?

    I agree that the DOF button is better on the 7D than on my previous cameras, especially better than the 350D I once had... BTW I gve it and the 28-135mm IS lens to my son in law who thinks the 350D is the greatest camera in the world. It is simply what he is comparing it to; his wife's keychain camera, she bought at a supermarket check out counter...

  7. #7
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    Re: How do you remember your DoF numbers?

    I don't memorise them, don't think I've ever even bothered with them.

    I want a shallow DoF I open the lens up - I want lots it gets stopped down. If it is really critical and I have the time I'll take a shot and check it 100% on the rear screen.

  8. #8

    Re: How do you remember your DoF numbers?

    How do you remember DoF numbers for your various lenses?
    i must have missed that bit in the manual

    To be honest I have never thought about it. I am not brilliant at judging distances to any accuracy anyway so there is an inherent flaw in my application of DoF calculator. Its more intuition really - I know the principles, yes I can get it wrong when close focusing wide open but I usually take a couple of shots anyway if I an dealing with wafer thin DoF. I sort of accept there must be some reason for wanting to calculate field depth to the nearest couple of decimal places but it simply hasn't occurred in my photography yet. I might be more bought in for the need if someone can give a practical example where it would be critical to the out come.

  9. #9
    Moderator Dave Humphries's Avatar
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    Re: How do you remember your DoF numbers?

    I am with Robin and Colin; usually experience, backed by chimping when necessary

    But I don't often shoot subjects where it is an issue.

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