The Hole Thing

Take away a camera’s mount, viewfinder, electronics, optical system (including lens), and case, and what’s left? It’s not “nothing;” it’s a hole. Holes treat light very differently from the way nothing treats light, and the image business is very much involved with light. One of the key effects of holes on light is diffraction.

Imagine a small road, a two-lane highway. Imagine that there’s a lot of traffic on it, but it’s moving nicely, as fast as anyone would like to go. Now imagine that the highway suddenly expands from two lanes to four (or six or eight). What happens? In my experience, the cars from the two-lane highway, even though they are moving as fast as their drivers would like, will spread out into the newly available space.

Light does something similar. It is bent by edges. The phenomenon is called diffraction.  Sean T. McHugh’s CambridgeInColour photography site offers an excellent interactive tutorial on the subject here: http://www.cambridgeincolour.com/tutorials/diffraction-photography.htm Many of the illustrations in this post are taken from that site (with permission).

As can be seen from these images, a big hole bends light less than does a small hole. Think of a 12-lane highway expanding into a 14-lane; cars won’t spread out as much as when a two-lane highway expands into a 14-lane.

Now consider the diagram on the right above. Each arrow, each ray of light, may be considered to be consist of waves. If the center arrow hits the wall at the right at the peak of a wave, it’ll make a bright dot. One of the bent rays might hit the wall at the same instant at the trough of a wave, resulting in a dark ring (a ring because the hole is two dimensional). The resulting diffraction pattern is called an Airy disk. As two Airy disks overlap, the dark part of one might be co-located with the bright part of another.

As a result, the dark gets brighter and the bright gets darker, a loss of contrast, and contrast is essential for sharpness.  If they overlap enough, they no longer look like two disks but one.  At that point, when individual pixels can’t be distinguished, resolution is lost. More »

Tags: aberrations, cameras, contrast, diffraction, hdtv, optics, resolution, sharpness

Angry About Contrast

If you are looking at the above picture on a nominally sized screen at a nominal viewing distance, you probably see an angry man on the left.  What’s an “angry man”?  Me, when I think about technical descriptions of HDTV.

Think about it.  Maybe you hear HDTV described as being 1080i or 720p.  Maybe it’s 1920 x 1080 or 1280 x 720.   Maybe it’s 2 megapixels or 1.  An engineer who remembers such things as analog bandwidths might refer to 30 MHz or 37 MHz.  Someone concerned with lenses might talk about 100 line-pairs per millimeter.  Someone describing visual acuity, screen sizes, and viewing distances might offer 30 cycles per degree.

Someday, I’ll probably get around to explaining how all of those are related and how many of them are pretty much the same thing.  But, when it comes to the sharpness perceived by viewers, they’re all pretty bogus because they’re all missing something of vital importance.

Of course, that isn’t the only silly spec.  Look at “sensitivity,” or, one of my all-time favorites, “minimum sensitivity.”  I just went to a web site of someone called an “expert” and found a sensitivity figure of 1 lux. More »

Tags: angular frequency, aperture, contrast, gain, hdtv, heynacher, mtf, schade, sensitivity, sharpness, spatial frequency