Aperture Formula...

[sublime]

I took several pictures of the same subject with different apertures and exposure times and noticed that I often get similar pictures by inverting the values of both.

For example, a picture shot at 1/120 and f.6 might look the same as 1/1500 and f.2.4. Those numbers aren't the actual numbers of similar looking pictures, but the premise behind the difference in the numbers are the same. A picture shot with lower aperture and faster exposure time looks similar to one shot with higher aperture and slower exposure time.

...but I'm working with an untrained eye, and I doubt that what I see is what the pros see. So what is would be the difference between these two pictures that look so similar? What should I see that is different?

[backpackerx]

The difference comes in depth of field and "speed"

For example, a low aperature value means that the aperature, the hole the light passes through to the film or sensor, is bigger. This gives you a shallow depth of field that is in focus. That is why you see portraits and other shots where the foreground is in focus and the background is blurred. A high aperature value or small opening gives you a big depth of field and the foreground, middle, and background will all be in focus. Usually anything above 8.0 to 11.0 will ensure that everything is in focus but will cause you to have to shoot slow shutter speeds to compensate for the lack of light comming through the lens. Just remember that with aperatures, the lower the number the bigger the aperature hole is.

The speed part of the shutter is important for movement capture or reducing the effect of hand camera shake. You should'nt shoot anything under 1/60 without a flash if you're not using a tripod because that is the slowest shutter speed most people can shoot and get a clear picture. A fast shutter speed allows you to capture fast moving objects like sports shots or race cars without getting the blur effect. A slow shutter speed can allow for nice shots like shooting a stream where you want the moving water to be blurred.

You can get very artistic just by varying these two settings. You are right, the aperature and shutter are proportional and if you halve one you need to double the other value.

[backpackerx]

To see how this works do two things:

Take a picture of someone who is about 6 feet away from you outside with the biggest aperature (smallest aperature value) and then shoot the picture the same way but with the smallest aperature. Make sure there is a distant background to see this the most effective way.

Next, find a moving object like cars in front of your house or similar and shoot one at 1/30 shutter speed and one at 1/500 or 1/1000.

You'll see the difference immediately.

[Lee Yuan Sheng]

Whoa, let's start with the basics.

Aperture controls depth of field, and the basic definition of it is

"The region in front and behind the point of focus in the photo in which it is deemed to be acceptably sharp."

Right. I'll illustrate with a very simple ascii drawing

Say from the camera (C), you focus on a point (F)

(C) -----------F-----------

Now, in reality, only that particular point in which you focused on, F, is truly in focus. Anything behind and in front of that point isn't. But thanks to depth of field, you can have a range of points in the photo that looks sharp.

So if you had a shallow depth of field,

(C) ----------fFff----------

Where F is the point in focus, and f the point where it looks acceptably sharp.

Large depth of field

(C) ---------ffFfffff------

You get a larger range where the photo is acceptably sharp.

The other point to note is the phrase "acceptably sharp" What might be acceptably sharp on the web might not do so well when printed out on a 8x10.

Also, DOF doesn't just affect what looks sharp; conversely it'll affect the out of focus areas; if it's shallow you can get backgrounds that meld into each other.

[Lee Yuan Sheng]

The aperture controls the amount of light entering the camera by a mechanical diagphram. The shutter does it by opening the itself to expose the imager for an amount of time. One controls by amount of light entering at a point in time, the other controls the amount of time light is allowed to enter.

Now for the relationship between aperture and shutter speeds.

When we measure light in photography, we use a term called a "stop". Everytime we go by a single stop of light, the amount of light is halved or doubled.

For shutter speeds, a halving or doubling of the shutter speed is a stop of light. From 1/500 to 1/250 is increasing it by a stop. From 2s to 1s is halving it by a stop.

For aperture, understand the value giving is a fraction. f/2 means that the aperture size is 1/2 the focal length. So if your focal length f is 50mm, f/2 means the aperture is of the size 25mm.

Since aperture controls the amount of light entering via it's size, that means we're talking about the area of which it's open. So instead of doubling, we increment aperture values in terms of the square root of 2.

Normally it's rounded down to the first decimal point, so one stop down from f/1 is f/1.4. One stop down from f/4 it is f/5.6.

So in terms of exposure, there is no difference between 1/125 f/4 and 1/30 f/2.

Oh, and one more thing. For handholdable shutter speeds, use general rule of thumb; don't let the shutter speed fall below 1/f. On a 50mm, don't let the speed drop below 1/50.

[sublime]

It took me a couple of days to understand, but I think I understand it now. The exposure will be the same, but the focus will be totally different...or at least I think that's what you're trying to say.

But I don't understand why I can't let shutter speed fall under 1/f. Is it because my hands will shake? If I get a tripod can I shoot under 1/f?

[Lee Yuan Sheng]

Shucks, I thought I simplified a bit. Guys, was it too difficult to understand?

Focus doesn't change. The amount of the area in which the photo looks sharp changes. Focus is always at a single point. If you focused at the same point, but used f/2 and f/11 for the two photos, the amount of sharp areas in front of and behind the point you focused on will be different

Yes, handshake is a problem, and as you've said, getting a good tripod will let you shoot below 1/f shutter speeds.

[backpackerx]

First, I'm not sure what he meant by 1/f for shutter. But yes, the general concept is that at slow speeds camerashake from your hands affects the picture. With a tripod you can shoot at any speed. This is why many higher end cameras have a bulb setting which means the shutter stays open for as long as you want. Those are the city pictures at night you see where all the traffic is blurred in red and yellow lines.

I think you generally understand aperature now and I know what you mean by focus but it's more depth of field. It affects the part of the shot that is in focus in a round about way. If your aperature is small ie. large value then everything is in focus. If the aperature is large ie. small value, then the object that is focused on will be in focus but part of the foreground and background will be blurred because there is a shallow depth of field.

[sublime]

Focus doesn't change. The amount of the area in which the photo looks sharp changes.

Sorry, that's what I understood, and meant to say. I accidently used "focused" as a synonym for sharp. My mistake.

[backpackerx]

To see how this works do two things:

Take a picture of someone who is about 6 feet away from you outside with the biggest aperature (smallest aperature value) and then shoot the picture the same way but with the smallest aperature. Make sure there is a distant background to see this the most effective way.

Next, find a moving object like cars in front of your house or similar and shoot one at 1/30 shutter speed and one at 1/500 or 1/1000.

You'll see the difference immediately.

You should really try this if you haven't so you get a firm grasp on the real world applications of how these 2 values affect your shots.