So the sensor size Tom was asking about originally would relate to film speed, interesting.

Not exactly. Sensor size does not translate into sensitivity. Rather on most DSLR camers the sensor is smaller than a 35MM frame of film, so a 50mm lens behaves more like a 35mm lens. I believe my memory on that is correct. On the higher end cameras like the Canon 1D and 5D the sensor is the same size as the 35mm film.

So the sensor size Tom was asking about originally would relate to film speed, interesting.

The size doesn't matter, you can just set the sensor to whatever you want. One shot can be ISO 100 equivalent and the next can be ISO 1600 if you want.

The size of the sensor will change the effective focal length of the lens. A bigger sensor will also generally improve image quality.

Or, increasing the f-stop (e.g., from f/4 to f/16) gives you greater depth of field.

You see, this is exactly where I get confused. What exactly is an f-stop measuring? It's a ratio of what to what? Why can't they just measure the diameter of the hole?

I used to love the manual-ness of my Pentax K1000, I would manipulate shutter speed and aperture to widen or narrow the DOF.

That's certainly still possible in the SLR world. It's kinda the point.

Where's my 100 dammit! Not to mention Kodachrome 64. Sniff.

Because you can barely take pictures anywhere with those speeds! Or with any zoom lenses...

No idea where they came up with the system, but I believe each increase in f/stop (the size of the lens opening) halves the amount of light getting to the film.

What exactly is an f-stop measuring? It's a ratio of what to what?

[link]

f/stops are a bit more confusing because the numbers appear so arbitrary. This is the standard sequence of f/stops from f/1.4 to f/22. Although it doesn't seem intuitive at first, in this sequence the f/1.4 setting lets in the most light while the f/22 setting lets in the least. Also, each of these f/stops has precisely the same halving/doubling relationship as the shutter speed sequence.

1.4 2.0 2.8 4 5.6 8 11 16 22

On the face of it, going from f/4 to f/5.6 doesn't sound like halving the amount of light. What's more, 5.6 is a larger number and sounds like it ought to be more light, not less. Neither does f/4 to f/2.8 sound like doubling the amount of light. In fact, each of the numbers in this sequence is a halving/doubling of the amount of light from its immediate neighbours, just like the shutter speed settings are. Not only that, but it makes sense, as I shall show below.

The reason that both the halving and doubling and the smaller numbers mean more light things make sense is that the f/stop is a ratio. The ratio is between the diameter of the aperture in the lens and the focal length of the lens. The focal length is generally measured in millimeters, so we'll stick with those as our unit of measure. On a 50mm lens, f/2 is saying that the diameter of the aperture is 25mm. The ratio is this 50/25 = 2. A good question might be, what is the area of that aperture? Well, the aperture is usually a set of five to fifteen blades which form a roughly circular hole, so we'll use the formula for the area of a circle, which as you all remember from fifth grade math is π * radius2. For π I'll use 3.14159265. On our 50mm lens, the aperture at f/2 has a diameter of 25mm which is a radius of 12.5mm. The area of the aperture is thus π X 12.52, or 3.14159265 X 156.25, or 490.9 square millimetres.

This fact by itself isn't all that useful. It is useful in relation to the adjacent f/stops. What is the area of the aperture at f/2.8? Well, because the f/stop is a ratio of the focal length to diameter, our 50mm lens at f/2.8 would have a diameter of 50/2.8 = 17.86mm. The area of the circle thus formed would be π X (17.86/2)2, or 250.5 square mm. That's about 250 sq. mm at f/2.8 and 500 at f/2, a double/half relationship. Aha! So that's it! The area of the hole doubles and halves, it's just represented by a ratio on the lens! No wonder it's so darn confusing.

Because you can barely take pictures anywhere with those speeds! Or with any zoom lenses...

I've rarely had problems with 100, but I do mostly landscape and nature photography, so I can manipulate the f-stop and shutter speed accordingly. 200 is easier sure, but 100 and below are so much nicer in enlargements. I never use 400.

The simple version, is the f-stop is lens focal length divided by aperture diameter.

The simple version, is the f-stop is lens focal length divided by aperture diameter.

Even that makes my brain hurt.

I do a fair amount of portraiture, candids and action, so speed is of the essence--when shooting film I'd happily go up to 1600 in black and white. Even sometimes for architecture too.