Why is the focal length doubled?

Discussion in 'Open Discussion' started by sebs_color, Mar 18, 2014.

  1. sebs_color

    sebs_color Mu-43 Regular

    Jan 5, 2014
    Virginia Beach, Virginia
    Real Name:
    I'm trying to look all over google, but why is the focal length doubled for m43 cameras? For example, when they say a 17mm is a 35mm equivalent, or the a 50mm is 100mm equivalent..
    Is it because it lacks a mirror?

    Thanks for the help!
  2. T N Args

    T N Args Agent Photocateur

    Dec 3, 2013
    Adelaide, Australia
    Real Name:
    call me Arg
    The focal length isn't doubled. A 25mm FL lens is a 25mm FL lens, no matter what camera it is on.

    But, with a smaller sensor, a smaller image is picked up. Less width and height fits on the sensor from the view that the lens is looking at.

    The µ4/3 sensor is half the diagonal of a 35mm sensor (22mm vs 44mm). So, only half the angle of view of the scene reaches the sensor. It looks like a lens of double the focal length was being used on the 35mm camera, to capture the same scene angle of view. So we say a 25mm lens on a µ4/3 camera is equivalent to a 50mm lens on a 35mm camera.

    This equivalence only applies to the angle of view. It does not apply, for instance, to the depth of focus.
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  3. esnift

    esnift Mu-43 Veteran

    Mar 17, 2013
    Boston, MA
    Real Name:
    Don't know if it'll help you to visualize it, but here is what T N Args said in picture format (for a comparison of full frame and all other sensor sizes) [​IMG]
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  4. sebs_color

    sebs_color Mu-43 Regular

    Jan 5, 2014
    Virginia Beach, Virginia
    Real Name:
    Thank you!!
  5. jonlong

    jonlong Mu-43 Regular

    Oct 25, 2013
    Not meaning to be argumentative, but rather satisfy curiosity... Isn't the angle of view a product of focal length and projected image size? It's not quite like a crop sensor camera which is actually only capturing the center crop of an image through a full frame lens. On a m43, the lens is full frame for the sensor. So it's not capturing the center crop like on a crop sensor body.

    Sent from my iPhone using Tapatalk
  6. madogvelkor

    madogvelkor Mu-43 Top Veteran

    Feb 22, 2013
    It is because it is smaller than an old 35mm film negative, which because it was the most common size of film is now considered "full frame". So as a point of reference, different size sensors are said to have a crop factor to let you know what the equivalent lens would be with a full frame sensor or 35mm film. I think it is becoming a bit dated, once upon a time most photographers were coming from 35mm film so it helped them to mentally adjust for digital cameras. But for people starting with digital cameras it is less helpful since most people aren't using full frame.
  7. brettmaxwell

    brettmaxwell Mu-43 Veteran

    Dec 8, 2012
    Correct, "full frame" has become a colloquialism for a 24x36mm sensor. Technically any camera format using the intended image circle of the lens is "full frame", including everything from an iPhone to an 8x10 viewcamera, including m4/3 in the middle. Ironically, "crop sensor" DSLRs are technically full frame when used with lenses designed for that format, like Nikon DX and Canon EF-S lenses.
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  8. Petrochemist

    Petrochemist Mu-43 Top Veteran

    Mar 21, 2013
    N Essex, UK
    Real Name:

    In which case the 8X10 isn't full frame as it needs considerably more image coverage than the 'sensor' to allow for movements (Shifts & tilts), In most cases the large format image will be a crop of around 50% or more of the image circle.

    Although many of todays users may not be comming from 35mm they are coming from a world of multiple crop formats where each has been referenced to 35mm. APSC includes both 1.5x crop & 1.6x crop so could hardly take over as a reference, and 4/3 or compact sensors are no better.

    It has far more meaning than the 'x32 speed' that used to be quoted for CD-Roms. I suspect few computer computer users ever had a single speed CD-rom.
  9. drd1135

    drd1135 Zen Snapshooter

    Mar 17, 2011
    Southwest Virginia
    Real Name:
    We should probably just use the angle for field of view instead of FL. Of course, since very few actually know the angles, this is not the path of least resistance.
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  10. speedandstyle

    speedandstyle Mu-43 Hall of Famer

    Lenses are designed to produce an image circle LARGER than the image size{sensor or film}. This is because the edges of the image circle are highly distorted. The exception is a circular fisheye. So technically speaking even a FF lens has it's image circle "cropped" by a FF sensor.

    This picture shows how much of the image circle the sensor actually uses, although it varies by lens and sensor design.

    If you use a lens designed for a larger sensor on a smaller sensor you use less of the image circle which changes the angle of view but also allows for a sharper image since the center of the lens is always the sharpest. Likewise if you use a lens designed for smaller sensor on a larger one the opposite happens. This can be used for artistic affect as when using a c-mount lens on m4/3.
  11. pellicle

    pellicle Mu-43 All-Pro

    Feb 10, 2010
    Southport, OzTrailEYa
    Real Name:
    Because when you look at the captured image a 50mm on a 4/3 sensor is about equal to the image captured by a 100mm on 35mm (which is where these things are compared against).

    For instance:

    Shows the differences and also discusses the depth of field differences (said to approximate 2 stops)

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  12. DigitalD

    DigitalD Mu-43 Top Veteran

    Jan 10, 2014
    Real Name:

    I have thought about this as well but, to be accurate a 25mm lens is a 25mm lens, and they way it behaves is like a 25mm lens cropped 2x. This is important to understand if you want or need to be technically accurate in your photography for a lot of reasons. For instance, take DOF. Though a 25mm lens shoots an image on to your sensor like a 50mm the DOF is still that of a 25mm lens and focus distances and calculations will have to be made based on 25mm not 50mm. So labeling it any other way would make it confusing.
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  13. pdk42

    pdk42 One of the "Eh?" team

    Jan 11, 2013
    Leamington Spa, UK
    The only meaning of "full frame" that matters nowadays is to define a 24x36 sensor. All this effective focal length nonsense :)wink:) is simply a way to compare the angle of view for lenses on different sensor sizes to that on a 24x36 sensor.

    For a u43 sensor(17.3mm x 13mm), a 25mm lens has the same diagonal field of view as a 50mm lens has on a 24mm x 36mm sensor. It's only doubled because the ratio of the diagonal between rectangles of 17.3x13 and 24x36 is about 2.

    If the world chose some other sensor size as the reference rather than 24x36, then we'd have another set of "multipliers" (or dividers if the reference was smaller than u43).
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  14. gryphon1911

    gryphon1911 Mu-43 All-Pro

    Mar 13, 2014
    Central Ohio, USA
    Real Name:
    From the Nikon USA website

    So in the simplest terms, it can be described as the distance from the aperture of the lens to the film/sensor plane.

    Optically, what makes a 50 on a FF35 versus the exact same lens on an APS-C different is the amount of coverage that the sensor gets from the lens. That is why a 50mm on FF appears to have the ability to have a narrower DOF at f/1.8 than it does on an APS-C. In actuality, the DOF is the same, it is just that the APS-C misses out on it because it only grabs the center most portion of the lens image. This is why the field of view is way more important to consider.

    It is also important to know that image compression is the same regardless of sensor.
    a 50mm on an APS-C does not magically get the compression of a 75mm lens.