Friday, June 6, 2008

Trying to be Photographer Part8

Aperture, ISO and Shutter Speed - The Good Kind of Threesome

PS: I have to admit that I have stolen this from blog site fearing that this information might go sometime

. I would like to thanks Choln for wonderful tutorial.

Canon 24-70mmL, f/2.8, ISO 1600, 0.125s

I thought I would take a little time and whip up an article about some photography basics. Please, please note the use of the word “basics” in that last sentence. If you’re been shooting for any appreciable length of time then you’re probably not going to get too much out of this. But given that we’re smack in the middle of the present giving time of year, it’s likely that in a few days there’s going to be a glut of new DSLR owners out there. What I’m aiming for is the article I wish I’d found when I first got my camera, to help explain the relationship between three critically important parameters: the ISO speed, aperture (or f-stop number), and shutter speed.

To lead with, the easiest way I think to approach these three things is to understand physically what they mean inside your camera. So, here we go.

  • ISO speed: A measurement of how quickly your camera’s sensor absorbs light.

  • Shutter speed: A measurement of how long your camera’s shutter is open for when you take a shot.

  • Aperture: A measurement of how wide the shutter on your camera’s lens opens up when you take a shot.

For completeness I should note that ISO speed is not specific to digital cameras with sensors. On film cameras, a particular roll of film will have a specific ISO speed that it is rated for. Fortunately for DSLR users like us, we can change the ISO speed without having to change film. All three of these parameters affect how light gets to your camera’s sensor to create an image, so let’s start looking at them in more detail.

ISO Speed

The ISO speed changes how quickly the sensor absorbs light. Lower numbers mean less quickly and higher numbers mean more quickly. Now, the thing to keep in mind is that at high ISO speeds, you will introduce some graininess into your image. Therefore, the basic rule of thumb is that you want to shoot at the lowest ISO speed that you can get away with given the environment you are in. For example, if you are outdoors in the middle of a sunny day, you can probably shoot at a very low ISO speed like 100 or 200. Conversely, if you are shooting indoors or at night (or both) you will probably need to shoot at a high ISO speed in order to properly expose your image.

Shutter Speed

This one is probably the easiest of the three to get your head around. It’s exactly what it sounds like: the length of time that the shutter is open and light can get in through the lens to your sensor. Shutter speeds on Canon cameras are displayed as inverse numbers against one second. So, if your camera’s shutter speed is set to 125, that means that the shutter will be open for 1/125th of a second. Therefore, higher numbers mean that the shutter is open for less time. The effect this has is on the sharpness of the image. If the shutter is open for a very short amount of time, then even if the thing you are shooting is moving you will be able to get a sharp image of it, because it won’t have time to move around before the shutter closes again. The downside is that if the shutter is only open for a instant, then not a lot of light can get in to the sensor.

Aperture (or F-Number)

The third and final member of our roster here is the aperture. This is a measurement of how wide the shutter opens up when you take the shot. In terms of the F-Number, it can be a little bit confusing at first. You’ll see lenses listed with specs that read like “35mm f/2″ which doesn’t mean too much at first, so let’s take a look at that. The “35mm” part is the focal length of the lens, in this case fixed at 35 millimeters. Now, the “f/2″ part means that the ratio of the focal length to the maximum width that the shutter can open up to is 2. Confused? Let’s use some real numbers. This is saying that on this 35mm focal length lens, the maximum diameter that the shutter can open up to is 17.5mm. So, the smaller the F-number is, the wider the lens can open up relative to its focal length.

Understanding the effect this has on your image is a little more complex than with the others because two different effects come into play. The first is the depth of field in your shot. I’ve personally always thought that the term “depth of field” is a bit, well, wrong for what it’s used to describe. In my opinion, a more accurate description would be “plane of focus” so I’m going to run with that. Here we go. Imagine you are taking a picture, and that there is a magical plane of glass somewhere in front of you that is parallel to the front of your lens. “Magical” because it can pass through other objects, like the ones you’re taking pictures of. Everything within the plane of glass will be in focus, and things that are in front of or behind the plane of glass will be blurred. Got that? Good. Now here’s the neat trick: the wider your lens opens up when you take the shot (low F-number), the thinner that plane of glass becomes. Conversely, if your lens opens up only a teeny amount (high F-number), then the plane of glass becomes very very thick. So, a very high F-number means that probably everything in your shot will be in focus. A very low F-number means that only a thin plane will be in focus in your shot.

Still not quite making sense? It’s a little hard to visualize at first. Fortunately, Ryan’s taken a great shot at a wide aperture that should help make things clearer.

Canon 100mm macro, f/4, ISO 400, 0.001s

As you can see, with a wide aperture, the caterpillar is in focus but the areas both in front of and behind it are a bit blurred out. This is often used to make objects in the foreground pop into sharp relief by blurring out the background, which is a standard portrait shooting technique shown here.

Canon 50mmL, f/2, ISO 1000, 0.013s

So the first of the two effects is related to which parts of your shot are in focus and which aren’t. The second again relates to light. The wider your lens opens up, the more light gets in to the sensor. So shooting with a wide open lens can help you in low-light situations. On the other hand, if you have tons of light, then you can shoot with much smaller apertures if you so choose.

Putting it Together

Now you should have a good understanding of what these three numbers are actually doing on your camera. Let’s talk a little bit about how the three are interrelated. All three of them share a common trait, namely that they affect how light gets into your camera to produce an image. This is one of the reasons that you’ll read over and over that lighting is the most important thing in photography. Let’s look at a few scenarios, starting with a situation in which you are doing social shooting in very low light. In this case, you will typically want to set your ISO speed very high at something like 1600, and you’ll want to open your lens up as wide as it will go (low F-number.) Then, you will set the shutter speed as fast as you can get away with while still getting a reasonable exposure. A good rule of thumb for the shutter speed is it’s the inverse of the focal length of your lens. Meaning, if you’re shooting with a 50mm lens, try to have the shutter speed 1/50th of a second or faster. Practically, I get away with a slightly slower shutter speed than this rule dictates all the time, but it’s a good thing to keep in mind.

Canon 50mmL, f/1.2, ISO 1600, 0.025s

You definitely see the grain here due to the high ISO setting of 1600. Also, you’ll note that while the subject’s left eye is right in focus, her hair over her right shoulder is a bit blurred. This is due to the very wide aperture of f/1.2. The shutter speed was 1/40th of a second, and fortunately she wasn’t moving very much so I was able to get a sharp shot. This was despite the fact that there were only a few tea light candles to illuminate this frame.

Let’s look at a more extreme kind of shot next. Low light, but where you want a low ISO to avoid the grain, and a reasonably small aperture so things are in focus. If that’s the situation you are in, then in order to get enough light into the camera to get the image, you have to have a very, very slow shutter speed.

Canon 17-85mm, f/8, ISO 100, 25s

In the shot above, taken by Ryan, the ISO speed is very low at 100, and the aperture is at f/8. In order to get the exposure correct, the shutter was open for 25 seconds. Obviously, in order to do this effectively, you need to be using a tripod or some sort of stabilizing device to keep the camera still so that the image comes out sharp.

Let’s look at one more that’s not in such an extreme low-light situation.

Canon 24-70mmL, f/2.8, ISO 100, 0.001s

This photo was taken in the late afternoon when there was a reasonable amount of ambient light out. The ISO speed is set very low at 100 so the image isn’t grainy. I have the aperture open quite wide at f/2.8, so Demian’s head is in focus but the background is nicely blurred out. The shutter speed was 1/1000th of a second in this shot, so I didn’t have to worry about movement from Demian or the camera affecting the sharpness of the shot.

Do I really have to worry about all of this?

At this point hopefully you have a better understanding of why these three magic numbers are important for your photography. Nonetheless, you may also be there thinking “Seriously? Do I have to worry about all of this stuff all the time?” The answer in this day and age is “no, you can make the camera do at least some of the work for you.”

If you’re just getting into your DSLR, you’ve probably noticed that it has a variety of different shooting modes. When I first started learning about all of this, my more experienced friends advised me to use Aperture Priority mode when taking pictures. I recommend that you do the same. In Aperture Priority mode on my Canon rig, you select an ISO speed and the aperture to use, and the camera will pick out the shutter speed for you. This is convenient because you don’t have to constantly readjust the shutter speed as you’re trying to get your shots. There’s also a Shutter Priority mode, which I’ve barely ever used, and it does more or less the opposite. In that mode, you pick the ISO speed and the shutter speed, and the camera will select the aperture for you. I know that on some cameras like the new Nikon D300 there are modes that will also select ISO speeds for you, but I don’t have a rig that has this functionality personally so I don’t know too much about how to use it.

At any rate, shooting in Aperture Priority mode is a great way to start out since it lowers the number of things you need to concern yourself with. And you can always see what the camera is recommending for you in the viewfinder, or by looking at the EXIF data later on when you’ve processed the image. I’ve personally been shooting in fully manual mode a great deal as of late, but that’s mostly because I keep finding myself in very dark situations and I’m generally insistent on getting a sufficiently fast shutter speed so that the shots are clear. That said, for normal daytime shooting, myself and many of my friends use Aperture Priority mode a great deal of the time.

In Conclusion

I hope this serves to clarify some of the jargon that you may be absorbing. There’s definitely a learning curve with photography, but once you get the basics down it’s an incredibly fun endeavor. If anybody has questions please ask them via comments and I’ll do my best to answer them. Happy Holidays!

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