Understanding focal length for M43


New to Mu-43
Aug 21, 2014
I know this is a dumb question but I can't find the answer anywhere. I'm trying to figure out the functional focal length of my lens. I am using the M.Zuiko ED 12-50mm f3.5-6.3 EZ. When zooming in or out with the lens, the "mm" of the focal length is reported in my view finder ranging from 12mm to 50mm (obviously). Now the question I have is because this is a M43, the 12mm-50mm range is actually equivalent to 24-100mm. SO, does this mean when my view finder indicates the lens is shooting at a focal length of 17mm, that this is roughly equivalent to a 35mm focal length?

Basically, I'm trying to decide between a the Zuiko 17mm and the Zuiko 25mm prime lens and am trying to get a feel for each focal length using my stock 12-50mm lens.

Thanks for any help you can offer!


Instagram: @MRSallee
May 3, 2013
Yes, your understanding is correct.

Your camera will always report the real focal length of the lens. If you want to compare that number to full-frame terms, you will always need to double that number.


Mu-43 All-Pro
Aug 23, 2012
Denver, Co
Real Name
Sean Rastsmith
Do you shoot/have you shot with a 35mm camera currently/in the past? If so, then comparing field of view may make sense (remember, 35mm is a 3x2 aspect ratio, m4/3 is 4x3). If this is your first interchangeable lens camera, then just learn what ACTUAL focal length gives you the framing you want. The 17mm and the 25mm will both compare to the kit lens at 17mm and 25mm. All too often, people try and start talking equivalence, when it doesn't help them understand anything any better. It is just that everyone else is doing the same, and they think they need to.


Mu-43 Top Veteran
Jun 26, 2010
Key West FL
... SO, does this mean when my view finder indicates the lens is shooting at a focal length of 17mm, that this is roughly equivalent to a 35mm focal length? ...
Technically, your implied statement is false by error of omission.

It should read "... equivalent to a 35mm focal length on a full frame 35mm camera". Without the reference to another format the statement is inaccurate and very, very misleading. There is nothing magic or holy about the old 35mm full frame format. It has become the default reference format but not being clear about that fact is what creates all of the massive confusion.

Back before the modern digital revolution and the internet there was no problem like this. You never encountered this type of confusion in the old photo magazines, either primary articles or "help" columns despict the wide range of film formats (e.g. 35mm FF, 35mm SF, 110, 6x4.5, 6x6, 6x7, 6x9, 3-1/4x4-1/4, 4x5, 5x7, 8x10, ...). People dealt with the field of view equivalencies without any mystical belief that 35mmFF was the "one true format".

Focal length is focal length independent of senor/film format. The only equivalencies are in field of view, not focal length. The real correct wording would be "the field of view of a 17mm lens on m43 format is roughly equivalent to the field of view of a 35mm lens on 35mmFF". I say "roughly" because the comparison in this case is to the diagonal of both formats. If you compare either the horizontal or vertical view the equivalent pair of focal lengths are somewhat different do to the fact that the two formats have different aspect ratios (different shapes).


Mu-43 Hall of Famer
Mar 13, 2014
Central Ohio, USA
Real Name
The numbers you see on the lens and in the view finder are the lens focal length.

The actual physical view you see in the viewfinder or in the finished image is the field of view of the lens/sensor combination. M43 field of view can be calculated as the reported focal length X 2
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