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

Started 3 months ago | Questions thread
Re: Actually...

Great Bustard wrote:

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.

No? Are you saying lens aperture affect DOF differently depending on the sensor size?

Yes, there are other factors that affect the DOF and each factor factor affect the DOF the same regardless of the sensor size and the sensor size itself affects the DOF.

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.

It's not arbitrary at all - 0.030 mm or smaller is considered within DOF limit for the FF sensor. CoC size of 0.015mm is required for 4/3rd sensor because it is enlarged by 2x relative to FF image.

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).

CoC diameter is affected 4x for the 2x FL change. CoC diameter is affected 2x for 2x aperture change. Sensor size does not affect the CoC diameter. However, the viewed CoC is enlarged by the crop factor.

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) - bold added
• 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.

Ya, so? if you keep the SS and f/stop the same, the exposure will be same for all the systems, also.

Yes - it called math. DOF is proportional to the aperture diameter and proportional to the sensor area, which, together, compensates for the inverse square of FL.

• same aperture diameter (aperture diameter = focal length / f-number)

The above needs clarification as many folks believe the above means the DOF is same if the aperture diameter is the same. It works when the sensor size is also scaled. On the same sensor, change of 2x FL requires 4x change in f/stop to keep the same DOF.

Try it with DOF calculator.

Obviously, the above has nothing to do with diffraction.

Actually, it has everything to do with diffraction because the same principles hold.

No. Diffraction is a wave interference phenomena and is present even with the perfectly focused point light source. DOF blurs because the lens focuses only at the focal plane. They are unrelated optical property.

You know that, of course, but you have tendency to equate the physical properties if both have same mathematical relationship

The below perfectly illustrates what I mean by your tendency to equate.

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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