What is Aperture? Tutorial

[kilasting]

I found this and wanted to share to everyone that is fairly new to photography and want to begin advance stages like me hehe

http://www.photoxels.com/tutorial_aperture.html

Hope this helps some people.

Enjoy! ;)

[FLiPMaRC]

:cool: Posted by a friend ...


Shutter speeds make sense. We can wrap our heads around them. F stops are just weird. It's a form of math we aren't used to. Rick explained a little about the focal length ratio in another thread, but I'll expand on it and perhaps it will make sense why 'fast' lenses cost so much more.

Every F stop is an abbreviated number. F4 really means 'focal length divided by 4'. If I am using a 100mm lens, or a zoom lens at 100mm, the opening in my aperture at f4 is 25mm across. That's a circle about an inch in diameter. The area of that circle is pi*r squared. Therefore, the open area of your aperture is .79 square inches.

Ok, so if I want to let in half the light, I just need to close the opening of my aperture by half, right? So, f/8? That would make my aperture diameter half of 25mm- 12.5mm or half an inch. It's also 1/8th my 100mm focal length. Simple. Now, the area of that circle of light at f/8 is 3.14*(.25*.25). So that comes out to... .20 square inches? What? That doesn't make sense. That's a quarter, not a half?? It should be .40 ish, right? NO! This is where those weird aperture numbers come into play. Circles don't play by halves. To halve the area of a circle, you need to reduce it in DIAMETER by .7 times. To double the area, you need to increase the diameter by 1.4 times. Therefore, starting with F/1, we get this order - F1, F1.4, F2.0, F2.8, F4.0, F5.6, F8, F11, F16, F22 and so on. Your camera probably has half stops in between as well.

The cool thing about all these numbers is that F4 on a 300mm lens lets in exactly the same amount of light as F4 on a 16mm lens with the same shutter speed. The actual aperture opening on the 16mm lens is MUCH smaller, but its looking at a much bigger cone of light. The 300mm lens aperture is much bigger, but the cone is a tiny slice of the size of the 16mm's.

Now, if you realize this is a ratio to focal length, you can think about how big a fast telephoto lens has to be. For a 400mm lens to have a 2.8 aperture, the aperture has to be 143mm across! That's 5.6 INCHES. Lens engineers can do a lot with geometry and optical design, but they can't overcome physics. This is why long, fast lenses are HUGE, and very expensive. Lots of glass, lots of engineering..

Now, a point and shoot camera like the S3 IS is always showing the 'equivalent' 35mm focal length- 35-432mm, even though it is actually 6mm-72mm. Much less glass, less resolving capability, and small abberations can be seen easier. However, it allows for quick lenses and obvious portability, since at 72mm, the f3.5 maximum aperture is only 21mm across, or less than one inch.

Hope this helps!!