# Formulae

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

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## Are you familiar with this formula?

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1. ### 10988495

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. ### BeBu Lamar

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. ### John Seaman

What a load of tosh. Really.

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4. ### Bill Bowes

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. ### 10988495

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. ### 10988495

It is a proportionality coefficient, a ratio of some kind that would enable you to compare different focal lengths ...

7. ### 10988495

The Yes or No, just to see if you know the equation or not.

8. ### BeBu Lamar

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
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9. ### William MichaelModeratorStaff 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.

William

10. ### William MichaelModeratorStaff Member

Correct: the formula in context would greatly assist repsondents.

WW

11. ### rodeo_joe|1

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 PeriMetairie, LA

Hmm... I get E=MC2

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13. ### rodeo_joe|1

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. ### Bill Bowes

Another banana for the Deep Thinker.

15. ### Karim Ghantous

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. ### rodeo_joe|1

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. ### John Seaman

All I can get out of it is three fifths of five eighths of not much at all.

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18. ### 10988495

It is in Portuguese but I see what I can do.

19. ### 10988495

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