A (very) Basic Lens ~ Aperture ~ Shutter Speed ~ ISO Primer

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Introduction:
We all (well, almost all) know how a camera works - you press the shutter, which opens a little hole in the lens, which then lets light in, which strikes a sensor in the back of the camera (or film, if you prefer) and makes the pretty picture of your grandchild, favorite pet, flower, bug, Aunt Matilda, or the valley of the Yosemite, if you're into that kinda thing.

However, and behind the scenes, there is a fairly complex relationship between the parts of the above equation that goes a long way towards what that final picture actually looks like. Modern DSLRs on "Auto" take care of everything for you, more or less, but conditions often fool the camera into a setting that is not actually the best for your scene. Often, "Auto" is not the best choice.

Let's take a look how the three basic parts of the process work together, and how you can adjust them to learn how things work, and to take better pictures. After all, you ARE smarter than your camera, aren't you?

1. The LENS and APERTURE

Note : there is no terminating "E" in the word LENS, singular. The use of the spelling LENSE indicates unsophistication, ignorance, devil-worship, dog-kicking, cat-beating, mouth-breathing, knuckle-dragging, a history of close family intermarriage and a host of other very bad traits. So now that you've been warned, you'll NEVER do it again, right?

A lens is simply a device that focuses and gathers light rays and concentrates them to strike a smaller area in the camera. It does this through a combination of glass elements and something called an APERTURE. The aperture is similar to the pupil in your eye, in that it opens or closes depending on the amount of light coming through. Your eye does it automatically, your lens does not.

Here are the salient points:

When the aperture is very wide open, a lot of light gets through to strike film or the sensor.

When the aperture is closed down, a much smaller amount of light gets through.

WIDE OPEN and CLOSED DOWN are actual terms used by photog's to describe these instances.

A lens that can open very wide, compared to other lenses, is called a 'fast' lens.

The amount that the lens is open, compared to something called the focal length (which we'll get to) is noted by the 'f-stop', and is usually written or typed "f/xx", where "XX" is the ratio. You don't really need to be concerned with the actual numbers, per se - "1.2" doesn't really mean anything, after all, it's only the RESULT of the "1.2" or "4.5" or "8" that we're concerned about.
Let me repeat that:

DO NOT GET ALL HUNG UP ON THE FACT THAT ONE F-STOP IS TWICE AS WIDE AS ANOTHER AND MEANS THE FOCAL LENGTH IS 1/2 AS FAR AWAY AND ALL THAT DOPEY MATH. YOU DON'T NEED IT.

Yeah, it's cool to know and you can impress your geeky photog friends with a statement like, 'my Super Goofinall lens aperture is 4.5mm wide at an f/stop of 16, so I knew that the distance to my focal plane was 87 millimeters, and the picture of my dog came out in perfect focus." But all you really care about is that last bit. Is your dog in focus?

2. Focal Length, Prime and Zoom lenses

The focal length of a lens determines what kind of a view it presents to the sensor. Different lenses have different focal lengths for different purposes. A lens with a focal length of 15 millimeters, or "15mm" gives your camera a very wide view of the surroundings. A lens with a focal length of 400mm gives a magnified view, and is used to photograph far-off objects. A lens around 35-50mm approximates the field of view of our own eyes, and is often used as a baseline when comparing other lenses.

A lens with just one field of view setting, e.g. "35mm" or "100mm" is called a PRIME lens.

A lens with an adjustable field of view, e.g. "18-135mm" or "24-70mm" is called a ZOOM lens.

Prime lenses usually supply better image quality than zoom lenses because the glass elements can be manufactured to focus light at just one setting, and can therefore be more precise and accurate. However, many zoom lenses are of very high quality as well. They just cost more.

3. How the LENS and the APERTURE work together

Simply put, the wider the aperture is open, the more light gets through. What this means to YOU is that in any given scene, the wider the aperture you're able to use, the faster your shutter speed can be. This means that you can stop action, for example, in a sports environment, or a hummingbird flying by, or your dog chasing a Frisbee. It also means that you can take a picture in less light - say, candlelight around your kid's birthday cake.

So wide aperture ----> allows fast shutter ----> freezes action,
as well as
wide aperture ----> more light ----> indoor pictures possible without flash

As noted above, an aperture number like "f/1.2" is considered pretty wide open. An aperture of "f/4.5" is not, relatively.

If you want to shoot action indoors, such as gymnasium activity like basketball or volleyball, you will need a 'fast' lens - these areas typically do not have very good bright light, nor do they allow flash photography.

On most cameras that have an adjustment, you set your dial to the "AV" choice, for setting the "Aperture Value".

There is one other characteristic that comes into play with the lens and aperture settings - DEPTH OF FIELD.

Depth of field is the idea that, for any given aperture opening, a certain part of the scene in front of you will be in focus. Picture a picket fence, receding into the distance. At a wide aperture opening, just a very small part of that fence will be in focus. At a narrow opening, a much longer stretch will be in focus. Don't worry about the physics of the optics and all that involved in this, just remember that the WIDER your aperture, the NARROWER your depth of field.

This is why most landscape photos are best taken with a very narrow aperture - it's not uncommon to see a sweeping vista across miles and miles to taken with an aperture of f/16 or f/22 - virtually pinholes in your lens' aperture.

Portraits, on the other hand, are often taken with fairly wide open aperture, because you don't need a lot of depth to take a picture of a person's face. HOWEVER, you need to be careful, because at wide open apertures, it doesn't take much to get out of focus - you can take a picture of Aunt Matilda where her nose is perfectly focused, and her ears are fuzzy. Generally, if you focus on the EYES of a person, you'll get most of their head in as well.

Also, with portraits, a blurred background (e.g., out of focus) helps to concentrate the viewer's vision on the person in the picture. This is usually a good idea.

So our next set of relationships is

Wide Aperture ----> Narrow depth of field ----> nice, blurry background (called "bokeh", a Japanese term for 'blurry background')
and
Narrow Aperture -----> Wide Depth of field -----> Close and far all in focus, used in nature and landscape photos.

4. Shutter speed - quick, blink!

The shutter speed is pretty simple - it's how fast the camera blinks. The faster the blink, the better you're able to stop action, if that's what you want. The slower the blink, the more light hits the sensor or film. Shutter speeds are measured in tenths or hundredths or even thousandths of a second, e.g. "1/10th sec" or "1/500" or something to that effect. Most cameras also allow you to set a longer shutters speed by hand, from 1 second to 'as long as you hold the shutter down'.

The main thing to remember about shutter speeds is that FAST freezes action, and s.......l.......o.......w may cause blurriness due to camera shake if you don't use a tripod.

Modern cameras or lenses with "Vibration Reduction" or "Image Stabilization" can help reduce camera shake by overcoming your natural shakiness. It's important to note that these things reduce the effect of CAMERA movement, not SUBJECT MOVEMENT. If you've got a fidgety 2-year old in front of you, VR or IS will not keep him from being blurry or fuzzy.

The old rule was that your shutter speed had to be at least as fast as the focal length number, e.g. if you were zoomed to 200mm, your shutter speed should be at least 1/200th of a second to eliminate camera shake, or if you were at 50mm, you could shoot at 1/50th of a second. VR and IS have changed this a little bit, but it's still a good rule to follow when you can.
You need a fast shutter speed to stop action - sports in particular. For landscapes, bug close ups, portraits, or most everyday kind of shots with static subjects, you don't need a real fast shutter speed - getting things in FOCUS and eliminating camera shake are much more important in those situations.

5. ISO Sensitivity

One last little part that enters in to the equation: your camera's ISO setting. The ISO rating determines how sensitive your sensor is to light in the first place. In the days of film, it was called the ASA rating. (ISO = "International Standards Organization", ASA = "American Standards Association". Big deal.) In the old days, an entire roll of film was one ASA rating - but thanks to the glory of digital, we can change our ISO setting from picture to picture with hardly a thought. Ratings range from a standard, daylight or sunny environment of 100 or 200, to the 'you can take a picture of a raisin in a dark room in the middle of the night' number like 12000

The higher the ISO rating, the more sensitive your sensor is to a given amount of light. However, the down side is that the higher the ISO, the more 'grain' or 'noise' you might see in your picture.

Setting your ISO rating higher is one other way to take a picture in low light without having to use a flash, and you can also use a faster shutter speed as well.

ISO only affects how sensitive your camera is to light. It does not have any effect on, nor is it affected by, aperture, focal length, or focusing.

6. Putting it all together

One more time, with feeling on the whole aperture/shutter/light/depth of field thing:

Low f-Number - WIDE Aperture - Usable in LOW LIGHT because a lot of light gets through - Fast Shutter speed possible - Narrow Depth of Field - good for portraits and low-light action.

High f-Number - NARROW aperture - Needs GOOD LIGHT - slower shutter speed common - wide depth of field - good for static subjects, landscapes, nature shots.

Obviously, if you keep it in the middle, somewhere between f/8 and f/16, that will be good for a wide range of subjects and situations.

Ok, so now we know how the aperture setting affects and influences both shutter speed and depth of field (focus), and how ISO affects light sensitivity, and how shutter speed affects motion, movement, and potentially, focus.

As I said at the beginning, when your camera is on AUTO, it does all of that for you, based on what you're pointing the camera at. However, it can't read your mind. It doesn't know that you're trying to capture really speedy football action by your grandson, all it knows is that it's kinda dim outside, there's not much light, so set the aperture about mid way open and use a long shutter speed, and maybe even bump up the ISO. Oooops. Instant blurry kid, because of the narrow depth of field, long shutter, and high ISO. The camera only 'sees' the dim lighting.

However, if you can manage to remember most of what we talked about here, and you can think to yourself, 'Ok, low light here, but I need to capture action. I need a fast shutter speed. What's the best way to get a crisp, in focus picture? I need a wide aperture, maybe about f/2.8 or 3.5, and a shutter speed around 1/500th or so, so I'd better move the ISO to 400.' You're applying scene-specific needs to override what the camera thinks it should do. And in this case, as in many, there's ONE specific characteristic that is the linch pin of your settings - the need for a fast shutter to stop action.

In other cases, getting good focus is most important - most flowers, for example, don't need a fast shutter speed. They're not going anywhere. And, usually the light's pretty good. Few people shoot flowers in the dark. In this case, you think to yourself, "I want a good crisp focus, and a blurry background to draw attention to my flower. Shutter speed isn't too important, and there's plenty of light. So set the aperture to f/3.5, the ISO to 100, and let the camera pick the shutter speed."

When you're on the rim of the Grand Canyon, it's the other way around - "Ooooohh.. pretty colors. I want it all to be in focus. Set it to F/22. I'm on a tripod, so I don't care about shutter speed. Enough light for my purpose, so leave the ISO at 100. Click!"

So take your camera out of "AUTO" and put some of your knowledge to work. Have Fun!

12/2/2011
JMH