Wednesday, November 28, 2012

Apertures Explained

As I re-read my recent post about a photo I had taken, I realized that I might have left some people hanging with regard to camera apertures.  I touched on what an aperture is, but didn't really tie it back to the 50mm lens I wrote about.  Nor did I properly explain just what apertures have to do with photography other than making parts of the image blurry and others in focus.  This, then, is sort of a follow-up to my "Mockingbirds, Photoshop, and Miss Maudie" post but it's really just designed to be a tutorial on one of the basic functions of a camera. If you have not read that entry yet, I encourage you to do so as it introduces some concepts that will be covered here like depth-of-field.

Let's start at the beginning: cameras takes pictures by capturing light on an electronic sensor.  Prior to digital cameras, this sensor was a piece of film but nowadays it's a microchip that essentially performs the same function.
Kids today will have no idea what this stuff is.
The light that hits the sensor is controlled by the lens of the camera, which is the big cylinder-like object that sticks out from the front.  All cameras have a lens, even small ones like on an iPhone.  The lens is what lets light come in and hit the sensor which is inside the camera body.
Every camera camera lens also has a mechanism that controls how much light is allowed to enter through the front and pass to the shutter.  The size of this opening is called an aperture, and it is one of the three key pieces that, when put together, control much of how photography works (the other two being shutter speed and ISO, but we'll get to those another time).  It's a lot like how our own eyes work, actually.  Have you ever woke up in the morning, turned the lights on too soon, and had to squint or close your eyes quickly because things were so bright?  That's because when it's dark, our pupils open wide in order to let in more light.  And in bright light, our pupils shrink because there is so much light they simply don't need to be open in order to let enough in.  For example, in the image below the pupil on the left is dilated and will let in a lot of light. The pupil on the right is contracted and not much light will be let in.  At night or in dark conditions, our pupils open big to let in every bit of available light, but in the daytime our pupils contract because there is so much light around us that they don't need to let it all in for us to see properly.
Image courtesy of Aurora Health Care
Camera lenses work in the exact same way: when the aperture is wide open, a lot of light is able to enter.  And when the aperture is small, not much light will enter. But what does all this have to do with taking pictures?  Good question.  For what I hope is a decent answer, here's a video I made that addresses this:


When my wife and I were looking at DSLR cameras, I was surprised to find out that most of the standard lenses they came with could barely zoom in and out.  A common 18-55mm "kit" lens doesn't really zoom in that much, and this confused me.  Our Panasonic ZS7 had a monstrous zoom lens, so why wouldn't a fancy DSLR have the same thing?  Because even though our ZS7, and most pocket cameras nowadays, have lenses that can zoom in and out, they make a critical compromise in order to do so: they can't let in very much light.  And not only that, but the image sensor (or film) inside most pocket cameras is much, much smaller than those found in DSLR cameras.

When a lens goes from wide-angle (you can see lots of stuff) to telephoto (you zoom in), the physical elements of the lens's construction behave in such a way that the maximum aperture (or opening) of the lens almost always shrinks.  It's just not physically possible to keep a really big aperture when zoomed in, unless you spend thousands of dollars on a mega-fancy lens.  And in bright light situations, like a nature hike or backyard picnic or outdoor sporting event, this is fine because there is so much available light that the camera does not need a very big aperture in order to get enough light to take a nice photo.  You can even zoom in on things, like a woodland critter or a single athlete, and take a decent picture because the camera has so much light to work with.  But when taking pictures indoors or at night, it is simply not possible for the apertures on many small cameras to be big enough to let enough light in.  This is why most cameras have a built-in flash: they have to create their own light in order to make up for their small lenses, which are incapable of letting a large amount of light through the lens to reach the image sensor.
A 110 camera with a stack of single-use flash bulbs.
I actually had one of these when I was a kid :)
Image courtesy of Wikipedia
Another way to compensate for a small aperture is to leave the shutter open longer, which gives the image sensor a longer period of time to collect the available light coming in. This is fine if the subject of the photograph remains still, but in most situations either the subject is moving or the camera is moving. Using a tripod can eliminate the latter problem, but if you are taking pictures of, say, your daughter's evening soccer game it's somewhat impractical to ask the team to freeze motionless in the middle of a play while you snap a picture. But like I said earlier, in bright daylight this sort of thing is not a problem.  A camera with a small aperture doesn't need to have the shutter stay open very long in order to let in enough light for a picture to be captured, which is why a pocket camera with a long zoom lens is just fine in many circumstances. On the flip side, leaving the shutter open for a longer period of time can produce very pleasing photographs given the right conditions.

Ever wonder how photographers get shots like this? They shoot in low
light with a tripod and leave the shutter open for a long time :)
Image courtesy of user TuffTuffTuffTuff on Reddit
Let's back up for a minute here, and take a look at the 50mm prime lens now that we have discussed various aspects of a lens aperture.  This lens, and others like it, doesn't zoom in and out, but it does have a big maximum aperture of f/1.8 (for an explanation of how apertures are measured, check out the excellent Wikipedia entry on F-numbers or the DPReview glossary).  This means that it captures a lot of light--enough such that a flash is rarely necessary, even indoors or in lower-light conditions.  It is also capable of a very shallow depth-of-field, which I discussed in my Mockingbird entry as well.  These features make it an incredibly versatile lens, provided you are not trying to capture a wide angle of view. Let's compare this to the kit lens from earlier.  While the kit lens does zoom in and out, it has some significant drawbacks that must be considered.  When it is zoomed out all the way to 18mm, its maximum aperture is f/3.5.  When the lens is zoomed in all the way, its maximum aperture is f/5.6. (For an explanation of what it means to say 18mm, 50mm, etc., check out the excellent Wikipedia entry on focal lengths or the DPReview Glossary.)  This means a couple of things:

• In order to make the lens let in as much light as possible (f/3.5), the lens has to be at its widest-angle setting of 18mm.  Indoors or in low-light settings, a flash might not be necessary.
• When zoomed in all the way to 55mm, the lens is not capable of letting in much light (f/5.6). Indoors or in low-light settings, a flash is almost always necessary.

These limitations are not necessarily a bad thing, but they are important to know when shooting.  Also, a kit lens is relatively cheap, which makes it a good choice for photographers who want a lens that, while not perfect, is decently suited for a variety of settings.  For the sake of comparison, let's look at the Nikon 27-70mm f/2.8 lens.
*drool*
Image courtesy of Nikon USA
This lens, while heavier and significantly more expensive than a kit lens, has the significant advantage of maintaining a maximum aperture of f/2.8 across its entire zoom range of 24-70mm.  This means that indoors or in a low-light setting, a flash is probably not necessary and the lens is capable of a very shallow depth-of-field.  These properties make this a far more versatile lens, and ideal for an incredible variety of settings...provided you are willing to fork over almost $2000 for it.

Let's bring this back to the realm of pocket cameras, like my Panasonic ZS7, which I said had a "monstrous zoom lens."  Keep in mind that in order to get a telephoto zoom lens on a little camera with a little image sensor, the lens aperture has to be (thanks to the laws of physics) super duper tiny.  Compare that to the Sigma 200-500mm lens which really is a monster, and has an incredible zoom while maintaining a freakishly large aperture of f/2.8 for the entire range:
Kind of impractical to carry around to your daughter's soccer game, eh?

Before I wrap this up, I want to touch on one critical question that many people still have: how do you set the aperture on your lens?  On many pocket cameras, you can't.  Most of the time this is controlled by the camera's internal software, though many cameras have scene-specific modes such as "Sports," "Fireworks," "Beach," etc. that contain aperture-and-shutter-specific presets for a few given scenarios.  Some pocket cameras do have manual control options that allow you to set the aperture, shutter, and ISO, but they are often hidden in various menus or control dials.  If you have a pocket camera it's worth checking out, though, and some even have dials that say something like "M A S P" or "M Av Tv P"

M = Manual. You have full control over the aperture and shutter values.
A/Av = Aperture mode.  You control the aperture of your lens, and the camera determines the appropriate shutter speed.
S/Tv = Shutter mode. You control the shutter speed, and the camera determines the appropriate aperture setting.
P = Program auto. The camera determines what it thinks are the best aperture and shutter values.

To adjust the aperture, switch to A or Av mode (you can also use the M mode, but that's a tad more complicated) and you will then be able to set the aperture of your lens using a dial or series of buttons on your camera.  Keep in mind that a smaller number means a bigger aperture, or more light coming in to the lens.  A bigger number means a smaller aperture, or less light coming in to the lens.

So that's the basics, folks! There's a lot I didn't cover here, and a lot that I didn't really explore in depth, so if you have questions just leave me a comment below and I will be happy to help as best as I can :)

2 comments:

Julie said...

Again, very informative post! Of course all this does is make me wish I had a better camera... :/

Simon Ringsmuth said...

I know what you mean, Julie! But your flower and nature pics are beautiful, so whatever camera you do have must be doing the trick :)