So confused about camera lenses and f-stops... HELP?

The opening known as the aperture or f-stop is formed by the movable blades of the diaphragm in the lens. The smaller the number, the larger the opening. The larger the number the smaller the opening.

You are correct that an f2.8 lens is "faster" than an f4 lens. Although I always suggest the faster lens when possible, in reality f2.8 is only 1 stop faster than f4. Unless you plan on a lot of low-light, non-flash photography the f4 lens should serve for your needs.

You are completely incorrect in thinking that a larger f-stop will produce "... better/sharper pictures ...". The opposite is true - optimum sharpness is usually somewhere between f5.6 to f11.

Now lets go back to f2.8 vs f4 and look at the real differences as related to shutter speed. ISO also plays a role since the exposure triangle consists of f-stop/ISO/shutter speed and all 3 are interdependent. One of the best ways to see the relationship is by using the old "Sunny 16 Rule" which states: "On a sunny day, set your f-stop to f16 and your shutter speed to 1/ISO." So lets set ISO 100 and go out on this sunny day.

ISO 100

f16 @ 1/100 sec. Sunny 16 Rule

f11 @ 1/200 sec. + 1 stop

f8 @ 1/400 sec. + 2 stops

f5.6 @ 1/800 sec. + 3 stops

f4 @ 1/1600 sec. + 4 stops

f2.8 @ 1/3200 sec. + 5 stops

f2 @ 1/6400 sec. + 6 stops

As you can easily see, the difference between f2.8 and f4 is a 1/3200 sec. shutter vs a 1/1600 sec. shutter speed. There are very few instances when you'll need a 1/3200 sec. shutter speed.

Now lets look at low-light, non-flash situations. This will be purely hypothetical but the ISO/f-stop/shutter speed relationship is valid, even if you never duplicate the examples used. Here we'll start at ISO 400 since its a good ISO for indoors.

ISO 400

f2.8 @ 1/125 sec.

f4 @ 1/60 sec. - 1 stop

ISO 800 + 1 stop

f2.8 @ 1/250 sec.

f4 @ 1/125 sec.

Plus 1 stop of ISO cancels the 1 stop loss from f2.8 to f4 and your camera should perform adequately at ISO 800. You can always use a program like Noise Ninja to clean up any annoying digital noise.

Now if you're really into low-light, non-flash photography lets look at a really fast lens - the 50mm f1.4 - compared to the f2.8 and f4 lenses.

ISO 400

f1.4 @ 1/500 sec.

f2 @ 1/250 sec. - 1 stop

f2.8 @ 1/125 sec. - 2 stops

f4 @ 1/60 sec. - 3 stops

ISO 800 + 1 stop

f1.4 @ 1/1000 sec.

f2 @ 1/500 sec.

f2.8 @ 1/250 sec.

f4 @ 1/125 sec.

ISO 1600 + 2 stops

f1.4 @ 1/2000 sec.

f2 @ 1/1000 sec.

f2.8 @ 1/500 sec.

f4 @ 1/250 sec.

ISO 3200 + 3 stops

f1.4 @ 1/4000 sec.

f2 @ 1/2000 sec.

f2.8 @ 1/1000 sec.

f4 @ 1/500 sec.

This chart really shows the advantage of a fast f1.4 lens compared to our slower lenses.

In my opinion, the 70-200mm f4L would be your best choice. I've used a 70-210mm f4 for a very long time with excellent results. Use the money you save to buy a 50mm f1.4 lens and a good tripod. Don't forget to buy a Haze/UV filter to protect the front element of your new lens and a circular polarizer for it.

IS (Image Stabilization) is available on either lens but I don't consider it necessary - as long as you keep your shutter speed at 1/200 sec. or faster. Long before IS was even a dream we hand-held 200mm, 300mm, 400mm lenses by using good camera-holding technique and keeping our shutter speed equivalent to or higher than the focal length. You can easily do the same. IS would only be desirable if you plan on shooting a lot of indoor sports - but only to correct camera movement, It has zero effect on subject movement.

So unless indoor sports photography will be your main use for your new lens, save your money with the f4L non-IS lens. If your main use of your new lens will be indoor sports, then go for the f2.8L IS lens.

The 70-200 f/4L IS is a good lens, and half the price of the 2.8L, which is one stop faster. Lens speed pertains to its maximum effective aperture [diameter], the smaller the f/number, the larger the opening - the larger the opening [aperture], the more light the lens can gather and expose to the image sensor.

The three exposure settings are: aperture, shutter speed, ISO. The aperture determines how much light is exposed to the image sensor. The shutter speed determines how long that light is exposed to the image sensor. And the ISO determines how sensitive the image sensor is to that light, higher ISO = more sensitive.

Larger maximum effective aperture lenses will allow you to make a shot in an area with less light, at a lower ISO (less sensitive). This makes for an image with less noise. Less noise means more sharpness, better detail.

Your 40D has good sensitivity. And is how you decide on the importance of speed when buying a lens. It's always a trade off, and in this case your trade off is 1 stop of sensitivity vs. cost. And in the case of canon's 70-200 telezooms, cost can be steep as I am sure you are aware. But canon does something that nikon [or others] do not. They offer this lens in 4 different flavors, each either f/2.8 or f/4, with or without IS - all based on cost. Because either way, canon's pro telezoom is about the best lens ever (and I'm a nikon/olympus guy, so you know they must be good).

So, if you've got $1900, get the 2.8 IS. You couldn't run out of situations that its use would be highly beneficial to you. But, f/2.8 does not mean "better". Because there is no better, compared to the f/4 version, only appropriate. With image stabilization, you can use f/4 as you would an f/2.8 without it, at up to 4 stops (so canon claims, anyway).

The f/4 IS is cheaper and lighter, and still as sharp, just not as fast - which is not a big deal. Were it me, I would get the f/4 IS and save money.

You're right that having a larger aperture lets in more light - this allows faster shutter speeds. So-called 'fast glass' (ie large apertures) is always more expensive than slower lenses (ie f4 or f5.6) or variable aperture lenses (the aperture changes according to the focal length of the zoom).

A larger aperture does not necessarily mean better/sharper pictures. In fact, wide open, lenses are often not performing at their best and may be a little 'soft'. The reason why a large aperture is more often sought after on long lenses is because these are often used for sport/wildlife, where a fast shutter speed is wanted to 'freeze' action, not always possible on a lens with a smaller aperture.

Larger apertures in smaller focal length lenses like a 24mm is not so much to obtain a fast shutter speed (though of course it will achieve that), but because the shorter lenses are more often used for portrait work where you may want the larger aperture for a more pleasing shallow depth of field.

Well the larger aperture allows you to use a faster shutter speed, this can be important to reduce camera shake , even with IS

I think better/sharper pictures is misleading.

When using a large aperture your depth of focus is reduced.

With the wider aperture you wil probably have a bigger filter thread, and any filters you want to use will be more expensive

Lots of factors, but Do you need the faster lens ??

Having a large aperture allows you to take shots with less light, create better bokehs (subject sharp, blurred backgrounds) take hand held shots at faster speeds etc... depending on the type of photography you are doing and how dedicated you are to photography in general will help you in lens purchases.

THe 70-200 f4 is a great lens for the average photographer... but being able to shoot through the entire zoom range at f2.8 is GREAT!!!