Aperture Question.

[PJ]

Ok, All lens have a maxium Aperture right! (lowest number)

Why is this?
Could someone be so kind as to explain or give me a link that explains the mechanics of a lens and why it can only work up to a certain aperture?

I would just like to understand the techinical reasons why all lens can't have a large aperture.

thanks, and sorry if this seems like a noob question

pj

[Scottes]

Realize that Aperture is a ratio of lens length compared to the diameter of the opening of the iris or diaphragm that lets light through. So a lens with a aperture of 1 would have to have an iris diameter equal to it's length. Imagine for a second a lens that is 400mm in diameter and length!

In actuality the lens would be larger, since there are some mechanics that go around the blades that open and close. Take a look at the Canon 85mm f/1.2 (http://consumer.usa.canon.com/ir/controller?act=ModelTechSpecsAct&fcategoryid=152&m odelid=7446) - it's an 85mm lens which is 91.5mm is diameter. You need some room for the mechanics and for the blades themselves.

So generally the longer the lens the smaller the max aperture. It requires room. The Canon 600mm f/4 is a whopping 168mm (6.6") in diameter. So the front element is a BIG piece of very precise, highly machined glass - which is very time-consuming to make because it's so exacting. And that lens has 17 elements - each one is large and exacting. So it's very expensive. $7,300 I think. And it weighs 12 pounds.

So it *could* be possible to make lenses with larger apertures - but they'd be big and expensive.

[PJ]

Thanks Scottes for the Info. I was curious about it because I was watching videos about how Canon makes their lens' and a few questions arose.

thanks again

[timmyquest]

Thanks Scottes for the Info. I was curious about it because I was watching videos about how Canon makes their lens' and a few questions arose.

thanks again

Where is this video you speak of 8)

[PacAce]

Realize that Aperture is a ratio of lens length compared to the diameter of the opening of the iris or diaphragm that lets light through. So a lens with a aperture of 1 would have to have an iris diameter equal to it's length. Imagine for a second a lens that is 400mm in diameter and length!


So why is it that there aren't a lot of wide-angle lenses with bigger max. apertures? Seeing as how much smaller the focal length is, it should be a lot easier to accomodate a bigger diameter opening for these lenses without it getting too big or costing too much. :)

<Actually, I know the answer to this question but I just wanted to throw it out just for the sake of discussion. :mrgreen:>

[timmyquest]

Realize that Aperture is a ratio of lens length compared to the diameter of the opening of the iris or diaphragm that lets light through. So a lens with a aperture of 1 would have to have an iris diameter equal to it's length. Imagine for a second a lens that is 400mm in diameter and length!


So why is it that there aren't a lot of wide-angle lenses with bigger max. apertures? Seeing as how much smaller the focal length is, it should be a lot easier to accomodate a bigger diameter opening for these lenses without it getting too big or costing too much. :)

<Actually, I know the answer to this question but I just wanted to throw it out just for the sake of discussion. :mrgreen:>

Can i guess...

Something to do with defraction :oops: :oops:

:?
????

Or are they just holding out on us.

[PacAce]

Realize that Aperture is a ratio of lens length compared to the diameter of the opening of the iris or diaphragm that lets light through. So a lens with a aperture of 1 would have to have an iris diameter equal to it's length. Imagine for a second a lens that is 400mm in diameter and length!


So why is it that there aren't a lot of wide-angle lenses with bigger max. apertures? Seeing as how much smaller the focal length is, it should be a lot easier to accomodate a bigger diameter opening for these lenses without it getting too big or costing too much. :)

<Actually, I know the answer to this question but I just wanted to throw it out just for the sake of discussion. :mrgreen:>

Can i guess...

Something to do with defraction :oops: :oops:

:?
????

Or are they just holding out on us.

Actually, it has something to do with the fact that SLRs use a mirror that prevents a true wide angle lens from being utilized in such a body. Due to the short focal length, a true wide angle lens would have to be placed close to the "film" plane. But that won't do because of the mirror getting in the way. So, to get around this problem, lens makers for SLR bodies actually end up making a short telephoto lens and then mounting them in reverse! :shock: Go ahead, try looking through your telephoto lens the wrong way and you'll see what I mean.

This is called retrofocus. Unfortunately, the result of using a retrofocus lens is that it's going to be bigger than a true wide-angle lens and hence heavier and more expensive. And I hear that contrast suffers as well.

[PJ]

Where is this video you speak of 8)

actually it wasn't a video but a gallery of how L lens are constructed and the glass is made.

It's in the cannon museum
Construction of the 500mm f/4.5 USM L

http://www.canon.com/camera-museum/tech/l_plant/zairyou/za_1/zairyou1.html