# what is the diffraction limit of the Leica 25mm f / 1.4?

Started 3 months ago | Questions thread
Actually...

mostlyboringphotog wrote:

El Jeffe wrote:

Possessing a 4th grade math level I can not understand or answer your question, but generally, by what I read, f/5.6 seems to be the sweet spot for m4/3 prime lenses.

I am curious why you ask this question. Are you looking for a good landscape lens?

Consider m4/3 has an advantage over larger sensors for increased depth of field. So f/5.6 might for instance perform similar to f/11 on a full frame in that regard.

DOF effect due to the aperture is the same regardless of the sensor size.

No, 'cause DOF depends on more than just the aperture.

f/2 on 4/3rd or FF will have the exactly the same CoC size on the sensor.

CoC is an arbitrary value -- it's not a physical thing, like focal length or aperture.

However, for the same FOV, 4/3rd sensor uses 1/2 the FL of FF sensor, this has the effect of reducing the size of CoC by 1/4, which increases the DOF by 4x.

For photos displayed at the same size and viewed in the same manner, the CoC scales with the linear dimensions of the sensor, and thus the DOF as well (so 1/2, not 1/4; 2, not 4).

Since the 4/3rd image is enlarged by 2x (relative to FF - ignoring the aspect ratio), the DOF increase is now only 2x, which can be equalized by increasing the f/stop by 2 stops for the 4/3rd sensor.

It's much simpler to understand that for the:

• same perspective
• same framing
• same aperture diameter (aperture diameter = focal length / f-number)
• same display size
• same viewing conditions

the DOF will be the same for all systems. With all those things being equal, you'd think there was a term for it.

Obviously, the above has nothing to do with diffraction.

Actually, it has everything to do with diffraction because the same principles hold. For the same f-number, the Airy Disk (the spread of a point source) will have the same diameter, but span a smaller portion of a larger sensor than a smaller sensor, thus resulting in less diffraction softening in the final photo.

On the other hand, for the same DOF (twice the f-number on FF than on mFT), the Airy Disk spans the same proportion of the sensor, and thus the effect of diffraction softening is the same for all systems at the same DOF.

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