# Equivalent focal length for MFT lenses

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

Detail Man wrote:

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 ...

.

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.

Of course, you, DM, and KB are correct.  Many of the others have an incorrect understanding of focal ratio, f-stop, (what we commonly refer to as the f/ ) when setting the APERTURE in a lens.

"In optics, the f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens's focal length to the diameter of the entrance pupil. It is a dimensionless number that is a quantitative measure of lens speed, and an important concept in photography"

The above is from the WIKIPEDIA ARTICLE ON F-NUMBER.

Unfortunately some photographers use the term "aperture" instead of "relative aperture" or f/stop when discussing these matters.   Many books on photography also screw up learners heads by misusing the terms.  Even the wiki entry referred to above has a stupidly misleading caption to the well-known diagram on the right hand side of the page.

WIKI CAPTION FOR ABOVE DIAGRAM:  "Diagram of decreasing apertures, that is, increasing f-numbers, in one-stop increments; each aperture has half the light gathering area of the previous one."

It would be clearer if the caption said "Diagram of decreasing apertures (or entrance pupils), corresponding to increasing f-numbers for a fixed focal length lens."  (I would leave the discussion of the geometric series of f-stops and SS to another paragraph.  Sufficient for the newbie camera operator to learn that f/ = F/A where F=Focal length in mm, and A= Aperture Diameter in mm.)

I have recently returned to photography and tried to read various texts, articles and forums relating new techniques to modern camera optics and electronics.   Huge difficulty with the amount of very confusing information out there.  So I am indebted to guys like you, KB, GB, Macx, Hogan, AO, crames Tan68, Anders, and others who are trying to help clean up the language used on these forums.

GOLLYWOP'S ARTICLE  is a good start to help new photographers get a clear understanding of Exposure Vs Brightening.  Unfortunately many folks still believe in the Exposure Triangle and the software houses will continue to mislabel their sliders.

Hopefully GB will write an article on equivalence to help clear up the huge misunderstandings on the subject.

But somehow there are some outstanding images out there by folks who are obviously disadvantaged by not understanding the fundamentals.  Imagine how much better they could be if they learned the basics.

t

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