During the testing and configuration of the Panasonic P2 cameras with the 1/3 inch mos sensors, the optical effect known as "Small Aperture Diffraction" reared its head. This effect has always been present in lenses, but with the small size of the sensors, it has become an important factor. The mos sensors in the camera have a 16x9 diagonal with the full 1,920 pixels horizontally. Vertically, well, you figure it out: ratio 16:9, 16 goes into 1920 um, er, suffice to say that the pixels in these sensors are REALLY SMALL. The interesting thing is that we have been accustomed to seeing sharper pictures at larger f numbers (smaller aperture). That was due to an increased depth of field, which helped to cover your sloppy focusing skills. The cure for the small aperture diffraction softness problem is to GO THE OTHER WAY... open up. The quick answer, if you want to avoid reading any further is YOUR PICTURES ARE GOING TO BE SOFT AT f-11 AND f-16. To sharpen up, rotate the filter wheel to one of the ND (neutral density filters) settings to get your lens open to 5.6 or 4. |
| Diffraction is a phenomenon that happens when light rays
pass through a small opening, and has always been present in optical
systems (lenses to you). Light rays at the edge of the aperture are
slightly bent. Below is a rendering of the light pattern through an
aperture. You can see this for yourself if you punch a small hole in a
card, and let sunlight pass through it onto a white piece of paper. The
edges of the circle are fuzzy.
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| On the left is a diagram with the rays passing
through a small opening. The rays at the edge of the opening are
bent. Those that pass right through remain straight. On the
right, the opening is smaller. The same thing happens, but note
that the ratio of straight rays to bent rays is smaller. The
bent rays are now a bigger percentage of the light.
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Now, There's your problem...
The above isn't really to scale. The top is your point of light through a wide open aperture (iris, f-stop) and the bottom is through a fairly small f-11 hole. The grids get smaller as the image sensor does. Up until now, we have been familiar with 2/3 and 1/2 inch sensors, where the spread due to diffraction has not been a noticeable problem. In the 1/3 inch sensor, the diffracted rays slop over onto several adjacent pixels, blurring the image. THE SOLUTION is to open the lens up more, by using an ND filter to reduce the amount of light. Our new Panasonic P2 cameras have three ND settings to this end. |
| The pictures below show the result of shooting the same
chart with the same focus at f-5.6 and at f-11. To get the f-11
exposure, the camera was set to 18db gain. So, what have we learned?
Avoid shooting (especially outdoors) at small apertures by using ND
filters.
Compare the resolution wedges at the 800 mark.
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