Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
Anders W Forum Pro • Posts: 21,468
Re: Equivalent focal length for MFT lenses

draleks wrote:

There is a lot of talk about comparing lenses for MFT with APS or full format lenses. So I want to check if the following comparison is correct. If I have an MFT 300 mm f/6.7 lens, and a full format 600mm f/6.7 lens that is of a worse manufacturing quality and just happens to be twice as soft as the MFT lens. Then, on equal ISO values, those two lenses will be an exact match. Right?

I am not sure exactly what you are getting at here. But as far as resolution is concerned, an MFT lens mounted on an MFT camera has to have twice the resolution per mm on the sensor as an FF lens on an FF camera in order to yield an equally sharp image when the two images are displayed at the same size. This is because the image from the smaller MFT sensor has to be magnified twice as much to reach the same display size as the image from the FF sensor. In this regard, the MFT lens faces a tougher challenge than the FF lens. On the other hand, the FF lens faces the difficulty of having to cover an image circle with twice the diameter of the MFT lens. This evens out the score.

As to exposure and ISO, the two camera-lens combos (300 on MFT versus 600 on FF) will behave the same way. However, at any given exposure setting, e.g., f/6.7 and 1/1000 s, the FF combo will accumulate four times as much light due to the sensor area being four times larger. At the same time it will have more shallow depth of field at the same subject distance (and thus the same magnification). In order for the two combos to gather the same amount of light and have the same depth of field, the FF lens would have to be stopped down two stops further than the MFT lens, e.g., f/6.7 on MFT versus f/13 on FF.

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