Aperture (max and min)

[rockstar]

I was reading my manual and I came across the Manual Settings section. It says "Aperture can be set to values between the minimum and maximum values for the lens."

I'm using a 18-70mm 1:3.5-4.5G lens, if you could explain it using those numbers to me.

How come on my camera body, aperature can go from 3.5 to 22 (on my D50). What does that have to do with my lens' aperture range?

 

[newrmdmike]

on the lens where it says 3.5 t0 4.5 that means that at the widest end the largest the aperature can get is 3.5 and at 70 4.5 is the largest.
lenses only show the largest possible aperature on them.
the reason it can go from 3.5 to f22 is because at 17mm thats how large and small it can be. at 70mm i bet it goes down to f/29.

 

[rockstar]

How do we know how high the f-numbers go? Is there a system for determining that for each lens?

 

[newrmdmike]

on manual lenses you can see it on the aperature ring, however, on yours its as far as you can go on your camera body (the d50).

to determine this you should also zoom in as far as possible, that may make a difference in the highest num.

so f/22 at 17mm and f/29 at 70mm

 

[newrmdmike]

or 18mm, sorry

 

[fmw]

The maximum aperture is determined primarily by the size of the front lens element. Lenses of similar focal length with wider maximums have larger front elements that can gather more light. The minimum aperture is determined by how small you make the diaphragm when you close it down. There is no theoretical limit to minimum aperture but there are practical limits. In 35mm or digital SLR's you will rarely see a minimum aperture of smaller than f32 simply because smaller isn't normally necessary given the focal lengths of the lenses. In larger formats it is common to see smaller apertures.

The reason your lens has a larger f number at the long end of the zoom range is that it is easier to design and cheaper to manufacture lenses in this manner. Since the f number is a ratio between focal length and aperture, longer lenses need larger front elements to produce the same amount of light on the film plane than shorter lenses. As you zoom to a longer focal length, this ratio causes the lens to be slower at maximum aperture. Hope that didn't confuse too much.

 

[rockstar]

So if my 18-70mm, 1:3.5-4.5 is cheaper to manufacture, are there lenses out there that don't have an aperture range but just one maximum? Like 18-70mm with only 1:3.5?

 

[fmw]

So if my 18-70mm, 1:3.5-4.5 is cheaper to manufacture, are there lenses out there that don't have an aperture range but just one maximum? Like 18-70mm with only 1:3.5?

Sure. All of my zoom lenses have the same maximum aperture at all focal lengths. They aren't unusual but they are more expensive.

 

[stigma]

I was just going to ask this! Thanks!