# Micro four thirds - does the crop factor apply to MFT lenses as well?

Started 6 months ago | Questions thread
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 I appreciate the kind words! (nt) In reply to Aleo Veuliah, 6 months ago

Aleo Veuliah wrote:

Great Bustard wrote:

patrickestarian wrote:

To calculate the focal length and the aperture of a non-mft lens on mft sensors we use a crop factor of x2.

Neither the focal length nor the aperture of the lens change just because it's mounted in front of a different size sensor. However, the effect of both the focal length and aperture change in the same proportion as the ratio of the sensor diagonals.

Now I wonder how the aperture number on the "original" mft lenses are calculated?

The f-ratio (relative aperture) is the quotient of the focal length and the entrance pupil (virtual aperture) diameter. For example, a 50mm lens that has an aperture diameter of 25mm will have a relative aperture of 50mm / 25mm = 2. We write it as "f/2", where "f" stands for focal length: 50mm / 2 = 25mm.

Here's a little more in depth info on the point:

http://forums.dpreview.com/forums/post/40416852

are they before considering the crop factor or after that? For example, when an original mft lens is set on 2.8 is it really 2.8? or 5.6?

It's f/2.8, but the effect, in terms of the visual properties of the final photo, is the same as f/5.6 on FF (for a given shutter speed and sensor efficiency).

because those lenses are meant to be used for MFT cameras only and you would think that there is no point of printing 2.8 on the lens when it always means nothing but 5.6.

Lenses are marked for what they are. A 50mm f/2 lens is a 50mm f/2 lens. It is not a 100mm f/2 lens, nor is it a 100mm f/4 lens. However, a 50mm f/2 lens on mFT is equivalent to a 100mm f/4 lens on FF:

http://josephjamesphotography.com/equivalence/#introduction

50mm f/2 on 4/3 (mFT) is equivalent to 62.5mm f/2.5 on 1.6x (Canon APS-C), 67mm f/2.7 on 1.5x (DX), and 100mm f/4 on FF, where "equivalent to" means:

• The photos have the same AOV (diagonal angle of view) and aperture (entrance pupil) diameter: 50mm / 2 = 62.5mm / 2.5 = 67mm / 2.7 = 100mm / 4 = 25mm.

• The photos have the same DOF (as well as diffraction softening) when they have same perspective (subject-camera distance), AOV, aperture diameter, and display size.

• The photos have the same motion blur and the same total amount of light falls on the sensor when the aperture diameter and shutter speed are the same (usually attained by upping the ISO as required).

• The photos have the same same noise when the same total amount of light falls on the sensor if the sensors are equally efficient (less noise if the sensor is more efficient, more noise if the sensor is less efficient).

• Other elements of IQ, such as resolution, bokeh, flare resistance, etc., as well as elements of operation, such as AF speed/accuracy, size, weight, etc., are not covered in this use of the term "equivalent".

Hope this clears the whole matter up for you!

--
Simplicity is the ultimate sophistication.
God always take the simplest way.
Imagination is more important than knowledge.
God is the tangential point between zero and infinity.
Aleo Photo Site

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