FZ200 Diffraction Limit - Panasonic Tech Service

Started Aug 27, 2013 | Discussions thread
Detail Man
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Diffraction Pattern Dimensions are a function of Lens-System F-Ratio
In reply to Ron Tolmie, Aug 30, 2013

Ron Tolmie wrote:

Jerry:

What the equation states is that if you want to achieve a given angular resolution then there is a corresponding aperture dimension that will produce that resolution so long as some other factor (like optical aberrations) does not override the consideration.

Looking at it the other way around, if the aperture diameter is 3mm and the lens is intended to be used for imaging a "normal" field of view (equivalent to what you get with a 50mm lens on a full frame camera) then the image will be sharp, and it doesn't matter what the focal length of the lens is. You might have a different opinion on what constitutes a "sharp" image, but in that case the dimension might be a little bigger or a little smaller than 3mm, but the point is that there is a particular diameter that will satisfy your objective.

You have not responded to the following inquires addressed to you (posted on this thread):

http://www.dpreview.com/forums/post/52070220

http://www.dpreview.com/forums/post/52064518

Depth of Field is inversely proportional to the diameter of the entrance-pupil (virtual aperture as measured from looking into the outer lens-system element) - which is the "aperture diameter" which is used in the derivation of F-Ratio ("F-Number").

Note that that the "virtual aperture" is not the same physical size as the actual mechanical aperture opening wothin the lens-system, and it's measuarble physical diameter changes in the case of a variable Focal Length lens-system.

A pin-hole camera can achieve a very high numerical value of Depth of Field - but always at the price of (also) very significant loss in the spatial frequency (MTF) response magnitude due to diffraction effects. Deep DOF and low lens diffraction effects (upon MTF response) are mutually exclusive.

The intensity of the Fraunhofer diffraction pattern of a circular aperture (the Airy pattern) is given by the squared modulus of the Fourier transform of the circular aperture:

where:

is the maximum intensity of the pattern at the Airy disc center,

is the Bessel function of the first kind of order one,

is the wavenumber,

is the radius of the aperture, and

is the angle of observation, i.e. the angle between the axis of the circular aperture and the line between aperture center and observation point.

, where q is the radial distance from the optics axis in the observation (or focal) plane and

(d=2a is the aperture diameter, R is the observation distance) is the f-number of the system.

http://en.wikipedia.org/wiki/Airy_disk#Mathematical_details

.

Cameras like the FZ200 and ZS20 have pushed the choice of sensor size right down to the point where diffraction is a critically important design consideration (although certainly not the only one!). They work well, especially if you apply some simple post processing. I printed up a batch of 11x14" ZS20 prints this afternoon and they were sharp enough to satisfy me. However, I did not print any of the images that had used the longest telephoto settings because they were not sharp enough for my tastes.

I would argue that these cameras are operating right at the limit of what is practical. Examining the impact of diffraction is one of the most basic considerations that determine if they perform satisfactorily - or do we need to revert to using much larger sensors like M4/3 or full frame?

What the equation implies is that we really only need to make modest changes in the aperture size (and hence the camera size) to get away from the diffraction limitation. A really big sensor may offer other advantages, such as wider ISO settings and a wider choice of apertures, but I am happy to give up those advantages if it means that I can put a wide-zoom camera in my pocket.

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