# Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
Re: Equivalent focal length for MFT lenses
1

s_grins wrote:

Detail Man wrote:

Detail Man wrote:

s_grins wrote:

KenBalbari wrote:

rrr_hhh wrote:

As for the rest, I hate these equivalence debates and won't go further. You have been caught of guard stating that the F stands for focal length while it isn't and you are now trying to rationalize this, using the term aperture in a way most photographers won't use it, unless they are into optics and lens design. Just admit you have goofed with that minor thing,  like I admitted myself above.

But all I was saying is that the f in f/6.7 stands for focal length.  That is correct.

Ken, please stop it.You're doing a great disservice to those whom you wnt to support.

F/5.7 is not a focal lens, it is aperture.

Nobody here thinks that aperture is a diameter of the opening in iris (please wikipedia this word for yourself- iris).

Stop Ballsh^tting and begin to learn things you do not know.

The f-number N is given by:

N = f / D

where f is the focal length, and D is the diameter of the entrance pupil (often called the aperture). It is customary to write f-numbers preceded by f/, which forms a mathematical expression of the entrance pupil diameter in terms of f (a symbol denoting the focal length), and the f-number. For example, if a lens's focal length is 10 mm and its entrance pupil diameter is 5 mm, the f-number is 2 and the aperture size would be expressed as f/2.

I am surprised by just how much Wikipedia does not appear to contribute to the forum ...

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s_grins wrote:

Thank you for the snip

I thought it appropriate to point out that what Ken stated (regarding the symbolism of the letter "f" in the expression "f/") is correct. You are most welcome for the Wikipedia reference, my friend.

"The confusion seems to come from you believing that F-number is the same thing as aperture.  Clearly it isn't.  Did you read the wikipedia article you linked?  The F-number is the ratio of the focal length to aperture diameter.  When I say aperture here, I am always refering to the actual aperture diameter."

There is absolutely nothing incorrect in Ken's statement quoted above. It is refreshing to see people use the term "aperture" to refer (only) to the physical diameter of a pupil (the entrance pupil to be exact), as opposed to the all too common vague and misleading use of the term "aperture" to refer to F-Number (F-Ratio, F-Stop), which we all seem to agree (?) equals the Focal Length divided by the Virtual Aperture Diameter (Entrance Pupil Diameter).

If there is a problem with the accuracy of anything that Ken stated, I do not see one. Please advise.

DM ...

There is a problem with accuracy.

I would say that this is not a matter of "accuracy". Instead, this is a matter of people expressing themselves coherently. You have been made aware that, in fact, Ken is absolutely correct in everything that he has stated ("accurate") regarding notational conventions as well as derivations.

If you also refer aperture to actual aperture dia. , please do it this way:

A = 6.7 mm

Because I hope we know how to measure diameter. And I'll understand that you're talking about actual aperture diameter = 6.7 mm.

And when you write me that A = 6.7 or f/6.7, - I'll read it as a result of dividing from wiki snip

Note sure what all these references to the term "snip" are about. Use of the term "aperture" to refer to the F-Ratio (which is the ratio of Focal Length divided by Aperture) represents an  incoherent mode of communication. Choose it if you will, but don't expect others to understand you.

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