Photography 1A –Capturing Light
Photography is the process of creating images by capturing light. The modern digital camera captures images by focusing light on an electronic sensor which transfers the image to a digital memory device.
The camera consists of a lens, aperture, shutter, sensor, microcomputer/controller, memory device, and view finder. This is true whether it is a $50 point and shoot or $5,000 Nikon. Well the 5K Nikon may not come with a lens, but before taking pictures the user must supply one.
So, what is the difference? They both can take specular photos. The devil is in the detail, literally. The more expensive cameras can be used in a much wider range of light, from very dark to very bright, and can make exposures over minuets to hours or in 1/1000 of a second. The lenses may cover a 180 degree view or the head of an eagle ¼ mile away. They offer resolution great enough to make prints several feet on a side.
From sunrise to sunset you may never need any of these capabilities.
What is the most import part of a camera? Ansel Adams once said that the most important part of a camera is the 12 inches just behind it. The camera is simply a tool used to make photographs. The more skilled the photographer the higher is the chance of getting specular images. The more photos you take, the higher the chance of getting a specular image. Combine the two and you will get specular images!
OK, enough BS. Let’s get down to the fundamentals. They are not difficult to understand. Let’s start with the “capturing of light” part. The parts of the camera that play important roles are: The Lens, Aperture, Shutter, and Sensor.
I will explain how each works, and then how they work in combination with each other, and finally the grand finally, how you use these tools in the camera.
The lens focuses the light on the sensor. A wide angle lens will cover a large area while a telephoto will cover a small area. It is common today for a single lens to be adjusted from wide angle to telephoto. The parameter that defines the size of image is “Focal Length”. It is usually expressed in “mm” or millimeters. A wide angel lens may be 20mm and a telephoto lens 200mm. The “normal” lens may be between 40 to 75mm. This part will become important when we get to composition where we will find the difference to be much more than just coverage.
The second important parameter of a lens is how much light it can gather up and send to the sensor. The physics is simple: The larger the diameter of the lens, the more light it can capture. This parameter is usually referred to as the “f” or speed of the lens. Mathematically it is also simple: the f number is the focal length of the lens divided by the diameter of the lens. I will let you use your calculator to convert from mm to inches if you need to. Today pretty much everything to do with lenses is metric. So an example: You have a lens marked as 50mm f2.0. What is the diameter of the lens?
If f number=Focal Length/Diameter, then it follows that Diameter=Focal Length/f number or 50/2=25mm. This explains the big honking lens you see photographers on the sideline of the football field. The need a telephoto (200 to 500mm) to get “close” to the action and they need to capture a lot of light. Therefore the diameter has to be big.
Now the only time you would ever need to know this is if you were on the TV game show Jeopardy and was asked. The reason I got into it however is to help you understand that as the f number gets smaller, the hole light passes through gets larger and the amount of light gathered up increases. So a lens at f/2.0 will gather more light than the same lens at f/16. That is what you need to know.
The f number of a lens defines the maximum amount of light it can collect. What if you need less light on a bright day?
The aperture is usually a set of blades in the lens that form a hole that the light passes through. When opened all the way up the hole is the size of the lens. However it can be closed down to a relatively small opening. How does this affect the “f” number of the lens? The answer is that it reduces the effective diameter of the lens. Wide open, the lens determines the f number. Close it down to a 10mm hole on the example 50mm lens and you would get f 5.0. Again, the larger the f number the smaller the diameter of the hole and the less light that is collected.
If you have an old school lens and see the f numbers marked you will see that they are not 1, 2, 3, .. but rather an odd list going something like 1.4, 2.0, 2.8, 5.6, 8, 11, 16. Why is this? This series of numbers is called “Full f stops” and when you go from one to the next the amount of light passing thru the lens is doubled going down and halved going up. That is to say that when you change the f number from f/5.6 to f/8 you cut the light passing through the lens by exactly ½. Although these numbers are normally not marked on lenses today, they still appear on the LCD display of your digital camera. As digital cameras have more resolution in setting the f stop you may see numbers between these displayed, possibly half or quarter stops.
Now why do you need to know that? Stay with me and it will become clear. What we do know now is that we can lighten up a photo by going to a lower f number. In later sections we will learn that these f stops do more than just adjust the amount of light getting through the lens. There are other ways to lighten or darken a photo. These involve the shutter and sensor.
The shutter determines how long the light is sent to the sensor. It is expressed in seconds or more normal fractions of a second. The usual values include 1/25, 1/50, 1/100, etc. Be careful when you are looking at the small LCD display on your camera. To save space the above may be displayed as 25, 50, 100, etc. This can be confused with say 5 seconds which is displayed as 5 Sec or 5” or some way to be different than 5, which would be 1/5 of a second.
Shutter speed selection depends on a number of items. First it needs to be fast enough to reduce camera shake. Second if the subject is moving (i.e. sports) it needs to be fast enough to “stop” the motion. If using a telephoto lens you need to use a higher shutter speed as a small amount of movement of the camera will cause a lot of movement of the subject. Also the same is true if the subject is very close to the camera. I find that 1/60 or faster is usually OK for normal hand held photographs. Like shooting a gun, practice holding it steady and controlling your breathing will allow you to use slower shutter speeds. Note above that I said “reduce” camera shake, not eliminate it. There is always some camera shake and this reduces the sharpness of your photographs. When photographing landscapes I always use a tripod to reduce camera shake. Again I said “reduce”. Even on a tripod there is some shake caused by moving parts of the camera, wind, pushing the shutter lease button, etc. Good technique will reduce shake whether on a tripod or hand held. Money spent on high quality lens and high resolution cameras (high mega pixels) is all lost if the camera is not steady during the exposure. Subjects such as sports demand very high shutter speeds. On the other end, when shooting water, I like to go to very slow speeds in the range of a few seconds. This gives a smooth “milky” look.
With the Shutter, the longer it is open the more light reaches the sensor. If it is open twice as long, twice as much light gets through.
So now we will play a little game to re inforce what we covered about the Aperture. Let’s assume that if we set the f number to f/5.6 and the shutter speed to 100 (1/100) to get a perfect exposure, then we changed the f number to f/8, what change to the shutter speed would be required to get the correct exposure? Answer: going from f/5.6 to f/8 reduces the light in half. So the shutter speed would have to change from 100 to 50 (1/50) to let twice the amount of light in to end up with the exact amount of light getting to the sensor.
Now the plot thickens.
In the old days (that is to say the Film Days) the film was the sensor and the sensitivity was manufactured into the film and the only way to change the sensitivity was to change to a different film type. The sensitivity rating of a film was represented by a number called ISO. In the early days in the US it was call ASA, but as photography changed to metric it became known as ISO. ISO numbers were like 50, 100, 200. As the number got larger, the film was more sensitive to light. If the number doubled, it was exactly twice as sensitive. To get the same exposure on ISO 50 film would require twice the light as ISO 100 film. Guess what is coming next!
The Sensor Sensitivity
When film was replace by electronic sensors some very bright person decided to call the sensitivity ISO exactly as in film and even more brilliant, an ISO 100 sensor was exactly the same sensitivity as ISO 100 film! The difference however is that just like the volume control on an amplifier can be turned up so can the sensitivity of an electronic sensor. This means that one can select a different sensitivity for each photograph in the digital world. I know that my purpose is to take the mystery out of photography and about now I am guessing that you think I have done just the opposite. Trust me, we are moving toward an understand of the settings on your camera. We will get to why you might select one number over another, but first I need to give you a little understanding of what each control does.
Let’s play the same game as we did after exploring the shutter’s operation. This time we start with a correctly exposed photo with ISO set at 100, Shutter Speed set to 250 (1/250), and the Aperture set to f/8. Now if I changed the Shutter speed to 500 and the Aperture to f/16, what change would be necessary to the ISO to keep the exposure correct? Answer: The change from f/8 to f/16 cuts the light in half. The change from 250 to 500 also cuts the light in half. So I need to increase the total exposure 4X to stay properly exposed, thus the ISO must go from 100 to 400. Got it?
About now you are thinking of turning your camera back to “P”. But we have learned that the aperture can increase or decrease the light getting to the sensor. The Shutter speed can increase or decrease the light getting to the sensor. And the sensor sensitivity (ISO) can increase or decrease the light captured. Together, they form a three legged stool that defines how much light is captured. One can increase the light with one and then decrease it with either of the other two to end up with the same exposure. Now why did I not simply say that in the beginning?
More about Aperture, Shutter Speed, and ISO settings
If these settings only changed the amount of light being captured there would be no reason to understand them and the relationship between them. But each of these settings affects other properties of how an image will look after capture. Understanding this will provide you the tools to capture the same image with different combinations and although each will be exposed properly each have a very different appearance.
We are now about to leave the realm of snapping a picture to the realm of making a photograph.
I am now going to take you back through the different parts of the camera, this time discussing parameters other than exposure.
The Lens and Aperture
The lens is the most important part of a camera. All of the light that ends up being captured by the sensor goes through the lens and flaws in it can create distortion and flaws in the final image. We will talk of this more when we get to how to select your equipment. For now we will focus on the things that should happen because of different lens parameters.
As you know, the lens focuses the image on the sensor. It also may expand or compress depth.
When you look at an image you would like to see everything in sharp focus. Or you may want to see some of the image in sharp focus and some it is not in sharp focus to direct the viewer to the subject of the photograph. A new parameter used to create a photograph is “Depth of Focus”. When you focus on a subject you will see that some of the image in front of and behind the subject is in focus. This is “Depth of Focus”. There is not another knob you turn to change the depth of focus. It is a physical property of a lens and the primary parameters that affect it is the focal length of the lens and the aperture setting. As the focal length of the lens gets shorter (i.e. wide angle lenses) the depth of focus gets larger. As the aperture f number gets larger, the depth of focus also gets larger. This is true for any lens. As an example, let’s imagine a landscape with a flowering plant close to the camera and mountains in the distance. If you simply put your camera on “P” and shot away chances are that the plant would be in focus and the specular mountains in the background would be fuzzy or the other way around. If we really wanted to have all in focus we might zoom out to a wide angle setting of the lens and if that was not enough we could then stop down the aperture to f/11 or some larger number. Of course to get the exposure correct we will adjust either the shutter speed or ISO to get enough light to the sensor. Ah, starting to see how these are interconnected? In this example you need to force a large aperture f stop to get the desired depth of field and using the other light controls to accommodate the aperture. In effect, you are shooting with Aperture Priority, which by no coincidence is likely one of the modes on your camera. Often called A or Av (aperture value). In this mode the camera is adjusting either the shutter speed or ISO. As a photographer you must understand this and be aware that if the shutter speed needs to go very slow other problems may pop up, like camera shake causing the entire image to be blurred not by focus but by camera motion.
Sensor Sensitivity (ISO)
In the example above we set the aperture to f11 and then possibly the shutter speed had to go very low which would be OK if the camera is on a tripod, but a problem if it was hand held. How about raising the ISO to a point that we can shoot at f/11 and at a shutter speed fast enough to hand hold? This might solve the problem. However depending on how high you needed to go you may be creating another problem. Increasing the sensitivity of the sensor is like cranking up the volume on an amplifier. The output goes up but when there is no or little signal the speakers hiss. The equivalent problem with the sensor is that it also gets “noisy”, not in the audio sense, but the image starts to have a lot of specks called grain or noise. Some of this is OK and good software can really do a good job removing it but you will find a point that if you go passed it the image is so bad that it becomes useless. This point depends on your camera, and yes this is one of the things that generally improve with the more expensive cameras.
The normal useable range of ISO is in the range of 100 to maybe 800 or 1000. However on the higher end cameras it can be much higher. And low end cameras it may be lower. I frequently go over 1000 shooting indoors without a flash and get acceptable results. Having said that I normally use the lowest normal value available, which on my Nikon is 100. At this setting I get the highest quality of overall image. When I shoot the night sky I may use ISO 3200 or even 6400. But generally at these levels I am doing a lot in software to remove the noise caused by the sensor. When I am shooting water, as mentioned in the shutter speed section above, I use very slow shutter speeds. To keep the photo from being over exposed, I use a small f stop and go to the lowest ISO. Often this is not enough and I have to resort to using a neutral density filter to ever reduce the light more.
The usable range of ISO is getting better with each new generation of sensors.
Before moving on I want to point out the sensor noise is not only a function of how high you push the ISO, but also how high the temperature is. This can be a problem when shooting very long exposures like 30 minutes or several hours. In these situations power is turned on the sensor and it naturally will heat up. I am not talking about it burning up but it will become much noisier. For this reason making very long exposures with a digital camera (a common practice with film cameras) is generally not a great idea. The solution here would be to take many short exposures (10 to 20 seconds) and then put them together with software. This subject will be left for a course in astrophotography.
To this point I have covered some of the basic physics of photography and not the art of photography. The best composed subject is still not a good photograph until the physics of the camera is adjusted.