down and dirty of fstops

[fmw]

As focal lenth increases, you need a larger and larger front element to gather the same amount of light. The f number is a mathematical relationship between the diameter of the front lens element and the focal length of the lens. It is done this way so that f5.6, as an example, would produce the same exposure on the same film at the same shutter speed on any lens. The resulting photograph could look very different with the two different lenses but the exposure should be about the same.

So, Lorrir, because all of your lenses have the same maximum aperture doesn't really change anything. You still need to (or should) use the other aperture settings and settings with the same f number would produce about the same exposure at a given shutter speed regardless of the maximum aperture of lenses involved. The fact that the f2.8 maximum aperture provides another option for exposure and depth of field over a lens with an f4 maximum aperture, doesn't change basic light gathering capability of f4 as an aperture setting. Shooting everything at maximum aperture not only operates your lenses where they have the worst image quality but also removes you from a whole world of depth of field and motion management. I would recommend you get busy learning what the other apertures will provide for you photographically.

There is always a price to pay for the faster lens. Not only are faster lenses more expensive to buy, but they are larger, heavier and more complex. They are more complex because the larger front element requires more correction of lens aberrations than a smaller one. With complexity comes a degradation in image quality - perhaps just a minor one but a degradation nevertheless. For most photographers, the additional fexibility of a wider range of apertures is worth dealing these issues, however.

 

[Don Simon]

Lorrir, I assume you're shooting on Aperture Priority or Manual?

If so and you do use f/2.8 all the time, do you not find that somewhat limiting, particularly the depth of field? Plus the fact that the lens is likely to perform significantly better stopped down a bit.

 

[ksmattfish]

If so and you do use f/2.8 all the time, do you not find that somewhat limiting, particularly the depth of field? Plus the fact that the lens is likely to perform significantly better stopped down a bit.

Those fancy L lenses should perform pretty well wide open. I recently ran my Sigma 18-50 f/2.8 through some tough tests at f/2.8, and was really surprised at the results. I had been afraid to use it all the way open, prefering to stick to f/4, or switch to a prime lens to go faster. I'm afraid no more. It's plenty sharp at f/2.8, as long as I'm focused correctly.

 

[fmw]

Those fancy L lenses should perform pretty well wide open. I recently ran my Sigma 18-50 f/2.8 through some tough tests at f/2.8, and was really surprised at the results. I had been afraid to use it all the way open, prefering to stick to f/4, or switch to a prime lens to go faster. I'm afraid no more. It's plenty sharp at f/2.8, as long as I'm focused correctly.

The issue isn't good or bad. The issue is better or worse. The optical issues of high quality lenses at maximum aperture are always at the corners of the frame. If you use lenses designed for 35mm on a smaller digital sensor, then the edges of the image circle won't even show in the images and aren't an issue. If you use lenses designed for the digital sensor, then, even if they perform well wide open, will performe better at the corners when stopped down a little.

 

[RacePhoto]

Don't forget to mention that the faction (ratio) in question is the focal length over the diameter of the aperture. So if you have a 100mm focal length, and your F stop is F4...the diameter of the aperture is 25mm.

Also, when we look at F-stops on a scale
f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45, f/64, f/90, f/128...
the F numbers don't increase in a linear fashion. In other words, F8 is not twice as much light as F16. They increase by a factor of 1.414 (square root of 2).

There's everything in one message. :thumbup:

And the 1.414 part (or 1.4 for simple math) explains why the standard f stops are what they are. 2 x 1.4 = 2.8 Isn't that magic?

There are probably long confusing articles that explain all of this, and I think you hit the nail on the head.

So to get this straight. Do I get more light with a higher F number or a lower one?

1/16 is a smaller piece of the pie than 1/4. Big numbers smaller pie = less light. (although the pie in this case is just a smaller circle)

I just wanted to add that there is a direct inverse relationship between f-stops and shutter speeds. Go one speed faster and you need to open on f-stop more. Go one speed slower and you need to close down one stop to get the same exposure.

Lets say you are shooting at f/8 125th of a second. A nice central point.

But you wanted to stop the action a little bit more. 1/250th of a second, at f 5.6 - Speed up 1 = Open up 1.

Same starting point, you are at f/8 1/125th and you want more depth of field? f/11 at 1/60th of a second. Size goes down 1 = Speed goes down 1.

Light exposure on the sensor or film, stays the same.

In this day of everything automatic, I wonder if it matters, but the idea of what's behind those numbers can be useful?