Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
KenBalbari Regular Member • Posts: 276
Re: Equivalent focal length for MFT lenses

draleks wrote:

1) The lens on the FF system is of worse quality then the MFT lens, so that the softness (and possibly other optical imperfections) are vaguely twice as large as the MFT lens.

This is still not quite right, as I think you would want the lenses to be of equal quality.  The FF system won't have a chance using an older sensor with no resolution advantage unless it is also using good quality lenses.  Image sharpness doesn't generally change as you reduce the focal length.

Think of it as though the longer focal length lens is just spreading the same image out more to cover a larger area.  Spreading the light out more doesn't change the image sharpness or otherwise improve image quality.

Nonetheless, the FF system will offer a more shallow depth of field. If this holds, then a high quality 300 mm MFT lens on a high quality MFT body produces as good photos as an older FF system with a 600 mm lens, despite the latter being much more bulky and probably also more expensive.

Yes.  I've been arguing that for more complete equivalence, you might use the higher f-stop and ISO on FF, and so have no advantage in noise or dynamic range, either.  In which case the only advantage is if the sensor can produce a higher resolution, which a sensor from a couple of years ago can't.

However, to be fair, FF users aren't often shooting at f/13  and high ISO.  In many cases, they would either open the aperture or lower the ISO, as needed, whether to increase the shutter speed, or reduce depth of field, or lower the noise levels.  But if none of these is needed or desired, there is really no image quality advantage at all on a sensor with no more resolution.

But even on a current generation sensor, which can achieve more resolution, I think you may need to use a pretty good lens and get most other things right, maybe use a tripod for example, to really see the resolution difference in a real world photo, if you are comparing to an OM-D and a good lens.

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