Extending this topic on Aperture a little more....
Big aperture lens... eyes open wider
Small aperture lens... eyes are narrower
So big aperture (like f2.8) allows more light I can understand.... but why depth of field is so thin? Doesn't this mean that in low light condition, it is easy to get focus on like a person but much harder to get whole picture focused when at f2.8?
Just a simple experiment....
1) open your eyes wide. (imagine f2.8)
2) look straight
3) notice that the sides of what you see is not as clear as what you are actually looking at?
4) open your eyes smaller... possibly half of the initial.... (mimicking f4)
5) notice that what u see now is clearer?
PS: on a more practical example, do you notice why people tend to squint their eyes when they can't see whats in front of them?
Having a smaller hole means that the light is coming through from a more specific and targeted source(for example f4 to f2.8). The bigger the hole, light strays from a bigger source which is why its blurred on the sides...
Therefore, imagine a typical setting of a landscape shot,
Say: F11, although the hole is smaller (imagine squinting the eyes), you can compensate it by a longer durations of squinting (long exposure or shutter speed) and therefore getting a well exposed picture.
While putting the above directly to your question:
but why depth of field is so thin?
f 2.8 = more light gets through. the thing you are looking at is clear but the sides get blurred off....
Doesn't this mean that in low light condition, it is easy to get focus on like a person but much harder to get whole picture focused when at f2.8?
Yes, it is harder to get the subject in focus...
try this at home: set your af points to automatic, take a shot at a subject under normal lit conditions with everything else equal except one using aperture f5.6 and another f2.8. notice that when you half press or af, lesser af points are lighted up.
Erm my explainations may be confusing. but the above is an agaration of how things work. Should there be any error or mistakes, other bros feel free to correct....
Last edited by Snapperholic; 21st January 2009 at 02:06 PM.
Canon: 5DMKIII & 40D|17-40L|70-200f4ISL|100Macro|Sigma30|580EXII| Olympus: EP3|25mm f1.4
I think that the image quality difference is due to the fact that its technically harder to make a lens that delivers good performance when opened up wide. As such, the quality of materials and research that goes into a f2.8 lens vs a f4 lens gives benefits of higher image quality all round.
That said, some lenses are optimised for certain apertures.
Thanks for all the replies. Looks like getting f2.8 lens is a better choice then. Cos... when I want, I can just set aperture to f4 anytime.
If there a site where it tells us the aperture sweet spot for each lens?
Thanks.. I tried clicking the other lenses and there is the info.
Very technical. Will try to understand it.