Discussion in 'Beginner Questions' started by 10988495, Apr 15, 2020.


Are you familiar with this formula?

  1. Yes

    0 vote(s)
  2. No

    1 vote(s)
  1. I was wondering through some books and I found the following formula.

    A 50mm lens focus an objet 3m away.
    A 25mm lens focus the same object 1,5m away.
    Same opening f/11
    d = distance
    A = opening
    f = focal length

    Pc(alpha) = (A*dˆ2) / fˆ2

    Pc - Proportionality.

    Is the formula valid?
  2. Please elaborate on what is Pc means? The size of the subject?
    Why do you want to make the vote Yes/No?
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  3. What a load of tosh. Really.
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  4. With what ever lens you have on the camera, fill your viewfinder with what your eye & brain want. Yes you need some knowledge of materials, but your eye balls / brain can not be put into a box. Aloha, Bill
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  5. Despite the fact that I found it on an old physics book, I can't seem to validate it, not in a physics sense of photography sense.
  6. It is a proportionality coefficient, a ratio of some kind that would enable you to compare different focal lengths ...
  7. The Yes or No, just to see if you know the equation or not.
  8. If you plug the numbers into the formula it gives you the result about 4 and 2 so what does that mean to you?
    If you can scan at least the page of the book it would help.
    Last edited: Apr 16, 2020
    William Michael likes this.
  9. William Michael

    William Michael Moderator Staff Member

    Moderator Note

    That was not what you originally wrote. You wrote, "Is this formula OK?"

    I have edited the question to reflect your intention.

  10. William Michael

    William Michael Moderator Staff Member

    Correct: the formula in context would greatly assist repsondents.

  11. Think of a number; square it.
    Think of another number.

    Multiply the two numbers you dreamed up. Now divide them by 4 and the square of another random number.

    And shouldn't the distance and focal length be measured in the same units? Metres or millimetres - choose one or the other.
  12. Vincent Peri

    Vincent Peri Metairie, LA

    Hmm... I get E=MC2
    rodeo_joe|1 likes this.
  13. McTwo? I remember them. An ancient Scottish clan that lived fast and died hard. Or did they just dye the plaidy? (That's pronounced to rhyme with 'tidy' BTW.)
  14. Another banana for the Deep Thinker.
  15. I haven't heard of it before. However, I think I understand why it exists. You can fill the frame with the same subject, but with different focal lengths, and the amount of visible background will change in proportion to the subject. Am I correct?
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  16. Nope! At some point a simple ratio will break down, because there's a constant of 2 focal lengths to add into the formula for magnification versus distance.

    And the subject-background ratio can never remain constant with a change in distance. It's called 'perspective'.
    Last edited: Apr 17, 2020
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  17. All I can get out of it is three fifths of five eighths of not much at all.
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  18. It is in Portuguese but I see what I can do.
  19. Relations between depth of field and Aperture, focal distance and distance from object. Depth of Field (profundidade de campo in portuguese).
    PC1/PC2=a1/a2 ----- PC1/PC2=((d1)ˆ2/(d2)ˆ2) ----- PC1/PC2=((f2)ˆ2/(f1)ˆ2) -> PC(alpha)= (a.dˆ2)/fˆ2
  20. Ah ha! So it's a depth of field formula? If I remember right, you need a blur circle diameter (circle of confusion) to do depth of field, so this doesn't look quite right, or I don't see how it's in a useful form. If you can describe what you want to calculate, and from what inputs, any number of us can provide the correct formulas.
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