Aperture theory question

[bigfatbadger]

Ok, so thanks to Michael Langfords Basic Photography, I am getting more of an idea about all the theory of photography, which is a good thing. It's also makig me realise how little I actually know, which must also be a good thing.

Therefore, I'm putting a question to the collective:

I understand that F number= focal length / effective aperture. Hence f2 on a 50mm lens has an effective aperture of 25mm. This much I get (I hope that's right).

This explains why those 400mm f2 (or whatever) lenses have such massive fron elements, because the effective aperture size must be 200mm, whereas the effective aperture on a 50mm at f2 only needs to be 25mm.

So here's my question: Why do lenses like the 17-55 f2.8 have such massive front elements? Surely if the effective aperture is smaller then the front element doesn't need to be as large?

I know I don't need to know this, but I want to! Is it going to hurt my brain?

TIA!
Jon

 

[Tiberius]

There's no reason the aperture can't be larger than required. Plus it allows Nikon to stick to one standard filter size (77mm) for their Pro lenses.

 

[Unimaxium]

I don't think the size of the front element is necessarily related directly to the aperture size. When dealing with large-apertured telephotos, you're right that they do need a big front element to collect all the light, but I would imagine that lenses that go wide like yours (or like my 17-35 f2.8-4) have large glass in the front in order to be able to see the full angle that you get at 17mm. I'm not sure of this, and I could be very wrong, but that's my guess anyway.

 

[Torus34]

For short fl lenses the problem is to avoid vignetting. Thus the large front element.

There, now. That didn't hurt, did it? [concerned expression . . .]

 

[bigfatbadger]

Aha! Thank you!

@unimaximum - I wish I had one, I was just thinking about it!

 

[journeyman]

For short fl lenses the problem is to avoid vignetting. Thus the large front element.

There, now. That didn't hurt, did it? [concerned expression . . .]

You can have vignetting in 35mm cameras?

 

[Unimaxium]

Of course you can.

 

[Torus34]

Journeyman;

Just take a glim at some of the kooky lensshades for the extreme 35mm WA's and then reconsider your question!

 

[Luke]

i don't know how all of it works but basic theory you said is right.
thats why zoom lenses have f 32

 

[ksmattfish]

Besides vignetting, they may make the lens larger, with more coverage, so that they don't have to use the edges of the lens. Thereby using more of the "sweet spot", and not having to deal with edge/corner abberations.

Final appearance of the lens will be influenced by many factors, of which maximum aperture is just one. Making a zoom lens that is decent over all focal lengths and apertures is extremely complicated.

I have the Sigma 28mm f/1.8. It's huge. Bigger than my zooms. 77mm filter size, but it should only need an aperture of a little over 14mm right? I had no idea when I ordered it. It's more than twice the size of the Canon 50mm f/1.8. Why? The only thing I can tell is that it can focus very closely (about 4 cm).