FZ200 Diffraction Limit - Panasonic Tech Service

Started Aug 27, 2013 | Discussions thread
Detail Man
Detail Man Forum Pro • Posts: 16,688
FZ200 Maximum Desirable F-Number to achieve adequate "Sharpness"

Stephen Barrett wrote:

Bridge Cameras

As Detail Man has indicated, there are various factors that affect resolution. The two most important factors, however, are the diffraction limit of the lens and the resolution of the sensor. At low F-numbers, it is the pixel density of the sensor that limits resolution. Around f/3.5 to f/4.0 the lens resolution and the sensor resolution are becoming comparable. When you get to higher F-numbers, it is the lens diffraction that limits the resolution. So, perhaps the Panasonic tech people were trying to indicate that diffraction effects begin to dominate once the F-number gets above approximately 4. This is the same for all of the small-sensor cameras.


It is interesting that the resolution of large-sensor cameras (including DSLRs with full-frame sensors or APSC size) is dominated by the sensor until you get to very long focal lengths. (Focal length enters the problem because of the interplay of the lens and sensor resolutions.) Because of the low pixel density on a DSLR, it requires a very expensive long-focal length lens to match the overall resolution of a bridge camera. There is an article that I wrote on this topic with some simple equations in the appendix for calculating the approximate 9% MTF resolution, including the lens and sensor components and their combination:


The equations are for white light and are not as detailed as using Magnitude Transfer Functions (MTF), but they are much simpler and I have found them to match what I see in tests for telephoto, macro and "telemacro" situations.

Other Related Articles and Postings:

My article on Telemacro using the same equations:
My article on Macro Test Targets:
http://www.dpreview.com/articles/5039116594/my-search-for-an-inexpensive-macro-test-target More on Macro Test Targets (Challenge: How Small Can You See) :http://forums.dpreview.com/forums/post/50011528


Thank you for your interesting post. Will be having a look at your published articles with interest.

The spatial sampling frequency at which the (combination) of Photosite aperture/pitch combined with an optical low-pass ("AA") filter has a significant effect - where no "AA" filter is most "sensitive" to higher F-Numbers and Wavelengths, and stronger "AA" filters are less "sensitive".

Additionally since even the crudest de-mosaicing algorithms combine 2x2 Bayer-arrayed photosites, a "fudge factor" on the order of (at least) 2 seems reasonable to add to the model.

My limited understanding of more sophisticated and more commonly used de-mosaicing algorithms is that they interpolate photosite image-data wider than 2x2 (and up to the 4x4 photosite spatial periodicity of color-filtered Bayer-arrayed photosites).

As a result, my calculations (below) take into account the first zero magnitude spatial frequency of the (convolution of) Photosite aperture/pitch combined with the optical low-pass ("AA") filter, as well as adding a (de-mosaicing-related) "fudge factor" with a conservative value of 2.


I calculate the "critical" (minumum) Photosite Aperture dimension to be:

Pa = ( Pa / ( K * Pp ) ) * ( Fz ) * ( W * N )


Pa is Photosite Aperture;

K is the Bayer-arrayed and de-mosaiced "fudge factor";

Pp is Photosite Pitch;

Fz is the fraction of the spatial sampling frequency (which is equal to the reciprocal of Photosite Pitch) at which the first zero magnitude response occurs in the composite Optical ("AA") Filter combined (convolved in the spatial domain, multiplied in the spatial frequency domain) with the Photosite Aperture);

W is the Wavelength;

N is the F-Ratio of the lens-system.


Solving for the simple case of a 100% Fill Factor (Photosite Aperture equals Photosite Pitch), setting the value of K to conservative value equal to 2, and setting the value of Fz to 1/2 (the strongest possible "AA Filter" (resulting in a zero magnitude response at the Nyquist spatial frequency), the identity presented above simplifies to the following form:

Pa = ( W * N ) / 4

Re-arranging to solve for the maximum F-Ratio (N) as a function of Wavelength (W) and Photosite Aperture (Pa):

Nmax = ( 4 * Pa ) / W



For the DMC-FZ200, Pa ~ 1.5 Microns. For a worst-case Wavelength (W) of 700 nM, it appears that (in the base case), diffraction "extinction" is not an issue until F=8.571 (which exists, in fact, above the maximum F-Number adjustment value of F=8.0 for the FZ200).

The above case being for an optical low-pass ("AA") filter yielding a zero response at the Nyquist (1/2 of the spatial) frequency itself, a more likely situation is one where the optical low-pass ("AA") filter yields a zero response at (around) 2/3 of the spatial sampling frequency. In that case, the result of the above calculations being applied result in a maximum value equal to F=5.714.



It seems to be also important to take into account the FZ200 lens-system's specific Focal Length being considered - as net composite system spatial frequency (MTF) response characteristics are (also) going to depend on optical lens-system aberrations at various Focal Lengths.

DM ...

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