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Thread: F stop adjustments in macro photography.

  1. #1
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    F stop adjustments in macro photography.

    I am currently doing a project with macro imagies for city and guilds level 1 photography coarse.

    I am having trouble working out the f stop adjustment equation table.

    We were given a example table in a class at college and I'm having difficulty in understanding what it actually means.

    Can some one point me in the right direction on what the f stop adjustments are.

    The table shows -

    Exposure factor 1.5 f stop adjustment is 0.5

    Just don't know how to work out these numbers.

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    Moderator Dave Humphries's Avatar
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    Re: F stop adjustments in macro photography.

    Hi Miffy,

    Have a read of this and see if things makes more sense.
    http://www.cambridgeincolour.com/tut...a-exposure.htm

    Welcome to the CiC forums from ...

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    Re: F stop adjustments in macro photography.

    I think the question was about how macro extensions change the effective f-stop, although I don't exactly recognize the terminology used in the class. If so, a better tutorial would be http://www.cambridgeincolour.com/tut...cro-lenses.htm. FWIW

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    William W's Avatar
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    Re: F stop adjustments in macro photography.

    Quote Originally Posted by Miffy View Post
    . . .City and Guilds level 1 photography [course].
    I am having trouble working out the f stop adjustment equation table.
    We were given a example table in a class at college and I'm having difficulty in understanding what it actually means.
    Can some one point me in the right direction on what the f stop adjustments are.

    The table shows -
    Exposure factor 1.5 f stop adjustment is 0.5

    Just don't know how to work out these numbers.

    Exposure Factor:
    When used as a Technical Term for Macro Photography refers the factor for the increased exposure required to account for the LESSER AMOUNT of light reaching the sensor or film DUE TO increasing the distance between the LENS and the FILM PLANE by the use of EXTENSION TUBES and / or BELLOWS.

    The Exposure Factor is calculated by the following formulae:
    EF = (M + 1)^2
    Where M = Magnification

    ***

    Maybe we also need to backtrack a bit –
    Magnification

    is a factor of the lens’s FOCAL LENGTH and the EXTENSION LENGTH (measure in the same increments) and is express by this formulae:

    M = E/F

    Where:
    F = Lens’s Focal Length
    E = The Lens Extension (for example the length of the extension tube)

    So for examples:
    • if we have a 50mm lens and we use a 50mm Extension Tube, the MAGNIFICATION will be: 1
    • if we have a 100mm lens and a 50mm Extension Tube the MAGNIFICATION will be: 0.5



    ***


    The Effective ʄ/Stop

    Increasing the magnification reduces the amount of light reaching the image (as mentioned above and this is what the question is all about) this changes what is correctly termed the Effective ʄ/Stop of the Lens.

    It is important to note, that when using cameras with a TTL metering system (Through The Lens metering system) the camera’s light meter doesn’t care about all of this and will just measure the light arriving at the sensor (well actually at the light meter sensor, usually).

    BUT - I assume that this examination is using MANUAL LENSES and MANUAL CAMERAS and that the Course is designed to ensure the Students can calculate from First Principles the additional Exposure required when making Macro Images. (HooRah for that!)


    ***


    If we know the Exposure Factor, then we can convert that to an ʄ/Stop Adjustment and this I assume is the simple table, which you have been provided.

    The Exposure Factor is converted to an ʄ/Stop Adjustment by this formulae:

    fa = logEF / log2

    Where:

    EF = Exposure Factor
    fa = Adjustment in ʄ/Stop

    Here is a table providing Exposure Factor and corresponding Adjustemnt in Stops (Rounded to ONE decimal place):

    EF1.00 - then Adjust 0.0 Stops
    EF1.25 - then Adjust 0.3 Stops
    EF1.50 - then Adjust 0.6 Stops
    EF1.75 - then Adjust 0.8 Stops
    EF2.00 - then Adjust 1.0 Stops
    EF2.25 - then Adjust 1.2 Stops
    EF2.50 - then Adjust 1.3 Stops
    EF2.75 - then Adjust 1.5 Stops
    EF3.00 - then Adjust 1.6 Stops
    EF3.25 - then Adjust 1.7 Stops
    EF3.50 - then Adjust 1.8 Stops
    EF3.75 - then Adjust 1.9 Stops
    EF4.00 - then Adjust 2.0 Stops

    I assume this is something like the table you have been given?

    You’ll notice that for an exposure factor of 1.5, the more correct lens adjustment is actually ⅔ Stop and that should be used rather than ˝Stop, if your lens has ⅓Stop Increments.

    (Maybe you should or should not tell your teacher this - who knows how your teacher will react?)

    However - whatever you do with this knowledge - I suggest you should ensure that you understand Logarithms and how to use them in the formulae above: such that you can make any calculation yourself if you are given an exam problem later to solve.

    WW

    Welcome to CiC.
    Last edited by William W; 13th December 2012 at 02:22 AM. Reason: corrected typo

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    Re: F stop adjustments in macro photography.

    Quote Originally Posted by William W View Post
    for example the length of the extension tube)

    So for examples:
    • if we have a 50mm lens and we use a 50mm Extension Tube, the MAGNIFICATION will be: 1
    • if we have a 100mm lens and a 50mm Extension Tube the MAGNIFICATION will be: 0.5
    Just to clarify something (to make sure I also understand correctly), this magnification is added to the native magnification of the lens, correct? So if the 100mm lens in question has a native magnification of 1x, adding a 50mm Extension Tube will result a 1.5x total magnification?

    Thanks Bill, that really helped me a lot.

  6. #6
    William W's Avatar
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    Re: F stop adjustments in macro photography.

    Quote Originally Posted by Andrew76 View Post
    Just to clarify something (to make sure I also understand correctly), this magnification is added to the native magnification of the lens, correct? So if the 100mm lens in question has a native magnification of 1x, adding a 50mm Extension Tube will result a 1.5x total magnification?

    Hi, how are you?

    Very good question - and the answer is:
    "Yes".

    ***

    But it is a really good question, because it reveals that I have been a naughty boy and I did NOT answer the first question with absolute intense accuracy . . .

    (that was actually intentional, I was keeping my little example very simple - but having now this follow up question I feel it best to clear this up just in case there are any pedant teachers out there who wish to render me six of the best) –

    So let’s correct what I wrote-
    I was keeping my example really simple when I wrote -

    “So for examples:
    •if we have a 50mm lens and we use a 50mm Extension Tube, the MAGNIFICATION will be: 1
    •if we have a 100mm lens and a 50mm Extension Tube the MAGNIFICATION will be: 0.5”



    Every lens has its own MAXIMUM MAGNIFICATION - and this is indeed added to the magnification which is a result of the Extension Tubes or Bellows.

    For example:

    The MAXIMUM MAGNIFICATION at the lens's MINIMUM FOCUS DISTANCE of my Canon EF 50F/1.4 lens is x0.15.

    So, therefore, if I plonk 50mm of tube on it I will indeed have x1.15 Magnification (at the Lens's Min.FD).

    ***

    I assume you are referring to adding Tubes to a MACRO lens – for example the EF100F/2.8 Macro?

    WW
    Last edited by William W; 13th December 2012 at 03:14 AM.

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    Re: F stop adjustments in macro photography.

    Quote Originally Posted by William W View Post
    Hi, how are you?
    I'm very good thank you, and you?


    Quote Originally Posted by William W View Post
    I assume you are referring to adding Tubes to a MACRO lens – for example the EF100F/2.8 Macro?
    That is the lens that I was referring to in that last post, but just as an example for clarification, not for any specific purpose. So when you gave another example using your EF 50mm, that made it all clear!!


    Thanks again!! Believe it or not, the mud is slowly starting to settle at the bottom, and the water is becoming clearer on a daily basis!!

  8. #8
    William W's Avatar
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    Re: F stop adjustments in macro photography.

    Yes, I too am well thank you.

    Keep swimming . . . and keep searching . . . no one has ever found all the answers.

    Crikey pulling those formulae from the back of my brain really did my head in . . .
    So did making that table – I haven’t used a scientific calculator in years, thank the Lord it had batteries in it – I am a bit slow with my old Slide Rule . . . and I can’t find my book of Log Tables anywhere!

    CU round,
    Bill

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