Equivalent focal length for MFT lenses

Started Apr 12, 2013 | Discussions thread
OP draleks Forum Member • Posts: 76
Re: Equivalent focal length for MFT lenses

Detail Man wrote:

draleks wrote:

Detail Man wrote:

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.

You've scaled the Focal Length by the ratio of the Crop Factors (for FOV), but haven't scaled the F-Number by the ratio of the Crop Factors (for DOF). "Twice as soft" is a vague phrase. The spatial frequency response of subject-matter across the image-frame is different than Depth of Field.

I don't think this is how it works.

That is exactly how it works to achieve equal DOF and Total Light delivered to any two formats.

If you maintain equivalent total light delivered, then you will have to have four times higher ISO sensitivity on the FF than on on MFT. The FF looses at least some of its advantage.

An MFT f/6.7 will illuminate the sensor with the same amount of light per unit area as an FF f/6.7. The difference is of course that the FF area is four times as large, so an FF sensor should be able to take equally "good" picture with four times less light per unit area. So my idea is that:


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.

and apparently also,

2) The sensor on the FF system is of worse quality (for example older), and has the same ISO capability as the MFT sensor,

then the IQ of those two system should be a very close match.

Nonetheless, the FF system will offer a more shallow depth of field.

That sounds like a lot of "ifs and buts", there. Yes, the FF DOF will be shallower (if the MFT F-Number is the same). My point was that if the MFT F-Number equals one-half of the FF F-Number (divided by the ratio of the Crop Factors), then the DOF will be the same (because the diameter of the Entrance Pupil, and thus the Total Light delivered, will be the same).

Depending on the shot DOF may or may not be an issue. The total light delivered is of course always important.

Then, on equal ISO values, those two lenses will be an exact match. Right?

Shutter Speed is what you want equal. Exposure/Total Light are not functions of ISO Sensitivity.

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