Photography - 1

 Although point-and-shoot photography, provided by my smartphones, have served me well in the past, occasionally, I need to have more control and my study of biology has brought that forward, especially here in the desert where the intense light adds an extra factor and need for adjusting exposure.

So I decided to take Professor Marc Levoy's online digital photography course (Digital Photography). And I'm learning quite a lot.

Exposure, the amount of light reaching the photographic element affecting the brightness of the image, is a subtle balance of shutter speed, lens aperture, and ISO which is a measure of the sensors sensitivity to the light.

The trick is that each control involves a different trade off.

For instance, the longer the shutter remains open, the more motion you will catch and the more the subject will blur, which is an effect that you might want occasionally, but not usually.

These two scenes show what progressive shutter speeds will do to brightness.

If you expand the photos above, you can see that the dog is more blurred in the brighter (top) photo.

You can really see the trade off of motion blur and brightness in the photographs of the ceiling fan below  Only the shutter speeds were changed between photos.

Aperture width controls brightness but it also controls depth of field, the area a distance away from the lens where subjects are sharp. Only my Photon camera allows the virtual aperture to be changed and the differences aren't great, so that might be why the other apps don't provide the option.

Top right f/1 clockwise by steps to f/16. Expand each photo and you can see a difference in the sharpness of leaf veins 

ISO is the last element of exposure. It was originally a measure developed by the International Organization for Standards to describe how "fast" a photographic film was, that is, how sensitive it was to light. Since sensitivity was related to the graininess of the photographic emulsion used, it also was associated with the graininess of the image produced. That translated well into digital photography 

For a CCD, (photographic sensor) each pixel can be amplified to increase its sensitivity to light. That amplification also introduces noise. Enlarge the following photographs and note the "graininess" of the images.

An ISO of 100 is the image as it is produced by the sensor without any amplification. ProShot will adjust ISO from 100 to 19200 and those values are shown in steps above. The top left image has an ISO of 100 and the sequence gets brighter to the lower right. But enlarge the lower right image to see how grainy it looks, so there's a trade off. And you might want that graininess. I sorta like it 

Autofocus (automatic mode) adjusts everything for you and usually does a good job except for some special effects. You can even adjust depth of field in automatic mode on some cameras (like ProShot and Photon) by tapping on the part of a scene you want to have the focus.

Bracketing is a useful tool for planning a picture. It gives you a number of successive shots of the same scene with progressive exposures. The following shows three exposures differing by 2.2 stops.

So, before I go on to my second photography blog, let me tell you what Professor Levoy says about choosing a camera 

If you have the money, get an SLR (or DSLR) or MIL, not a point-and-shoot. 

Don't worry about megapixels, most cameras have too many.

Don't worry about brand or body 

Worry about lenses. Variable zooms are nice but consider the quality.

Avoid large focal ranges because they tend to be "soft".

Maximum aperture (lower f-stops) is good 

Some accessories you might want: 

50 mm f/1.8 "prime" (fixed focal length) lens for low light work

A good, light tripod isn't too expensive, and you might want a table tripod 

An external flash can be used to advantage. You can control where the light goes....and the shadows 

Video capability.

Now, that said:

Bryan Peterson's Understanding Photography Fieldguide is subtitled How to shoot great photographs with any camera, and for fieldwork I take it as gospel. For professional work, spend the money. 

What about that sky color?

There are a couple of hints as to why the sky is blue that are easy to obtain.

First, look straight up through a polarized sunglasses lens. Rotate the lens and see what happens. You'll see it lighten and darken. Second, note that the sky is not always blue. There's a definite sequence of colors as the day progresses: reds and oranges, yellows, blues, then the reverse.

What polarizes light? Reflection! When light bounces off something, it comes off organized. That's what causes glare and glare is why there are polarized sunglasses to start with.

Light is sorta complicated and maybe you don't want the excruciating details, but, at base, it's a wave (like ocean waves) made up of electric and magnetic fields. The fields move at right angles to each other. The waves can be oriented in any direction and they are when they leave the sun.

But when light hits a surface, whether it's a molecule or a lake, it's absorbed by the atoms in the surface. Some of the atoms send out light when they calm down, sometimes in a different color but often unchanged. Different materials are better at this than others. That's why metals reflect better than glass.

But the gas molecules in the atmosphere absorb and re-emit sunlight and the light they re-emit is polarized. Although molecules tend to be neutrally charged overall, most of them have different charges in different places. Remember that like charges repel so molecules next to each other tend to drive each other's electrons away from each other. So molecules tend to organize themselves. I say "tend to" because it's not a strong phenomenon and not all the molecules in air are lined up like a high school marching band. But light coming off these molecules also tends to be lined up. It's called Rayleigh scattering.

Here's a shot of the sky through a polarizing filter.

After turning the filter ninety degrees, it looks like this.

Notice the gradation of light toward the sun. There is another kind of scattering in the atmosphere. Mie scattering is from larger particles: dust and other aerosols like water droplets. A big difference between Mie and Rayleigh scattering is that Rayleigh scattering is affected by light wavelengths. The shorter wavelengths (violet, indigo, blue) are scattered more than the longer wavelengths (red, orange, yellow). Mie scattering disperses all the colors about the same. 

Space is black and the sun is almost white. As sunlight comes into our atmosphere, ultraviolet (eaten up largely by ozone in the upper atmosphere), violet, and indigo goes first. What reaches us is blue. As a spot on the Earth rotates away from the sun, light has to travel through more atmosphere to reach it. Green drops out, but you can't see much green since it's still mixed with a lot of blue. Yellow drops out, and then you get the oranges and reds of the sunset.

Keep in mind that the same sun creating beautiful sunsets over the Rockies is creating a blue California sky at the same time.

The sky closer to the sun looks whiter due to Mie scattering. So that's why the sky looks blue...and white and orange and red…

So, using color filters.

Filters have a coding system that looks rather arcane. I mentioned it in the last blog. The filter I used was a CTO ¼ 6500 to 4500 K ½ f/stop gel correction filter.

CTO stands for "color temperature orange". There are also CTB (color temperature blue) and "plus green" or "minus green" filters. CTB filters are specifically there to make tungsten light look like sunlight. They "cool down" yellowish light. CTB does the opposite. I wanted to take blue out of my picture so I needed a warmer filter. ¼ is just the strength of the color change. ¼ is a light colored filter.

The K number is color temperature. All bodies above absolute zero have molecules that vibrate - that's what heat is - vibrating molecules. And if molecules are vibrating, so are the charges within them. Moving charges are what causes magnetic fields. Moving magnetic fields create electric fields and when magnetic and electric fields start moving together, you get electromagnetic fields - light.

Relatively cool bodies like a summer sidewalk (see my blog for Friday, August 30, 2019, "Trail temperature vs. air temperature") might seem hot, but they're not hot enough to glow. But they are giving off light in the invisible, infrared range.  Infrared light is how heat is radiated because when infrared light hits matter, the matter heats up. 

As objects get hotter, they begin to glow a dull red, then orange, then yellow, then blue, and then they go white. The K number in filters stands for "Kelvin" as in "Lord Kelvin" after whom the Kelvin temperature scale, preferred by scientists, is named. The K number is the color that most closely resembles the color given off by a hot body around that temperature. A hot body from 6500 K to 4500 K will give off a yellow color. Larger numbers denote bluer colors, smaller numbers mean redder colors.

The f/stop is how much light the filter lets through. On a camera, the f-stop is the ratio of the lens' focal length to the diameter of the aperture. That might sound complicated but remember that the camera's focal length is related to its light gathering power and the larger the aperture, the more light can get in, so the smaller the f/stop number, the larger the aperture, and the more light is passed through.

An f/stop of ½ does not mean that half the light gets through. It's actually a very light filter that lets a lot of light through. An f/stop of, say, 8 would be much darker.

Correction or compensation filters tend to be pretty light. There are also color filters that are intended to really alter the color of photographs for special effects. They cut out a lot more light in specific colors. There are also neutral filters that just cut light in all wavelengths, polarizing filters, and special effects filters like diffraction gratings that give you rainbows.

Some photographic filters are made of glass or thick plastic. Gels are thin plastic films. I like them because they're inexpensive but they do the job and the thicker filters are harder to mount on my cell phone. I just slip the gel between my camera lens and the phone case and I'm ready to go.

The sky is what you have to look through to do astronomy so, if you're interested in sky watching, it behooves you to understand the weather.