Help this noob understand Fast Lenses

Ok.. Lets use 18-55mm kit lens for an example. Why couldnt you just use the same optic and put bigger aperture when they produce it? Aperture is just an opening made out of blades. Why cant they just make the opening bigger? I just really want to undestand why we have to spend premium price to get fast lenses. Thanks!

 

because good stuff is expensive

 

In really really simple terms the reason is that the maximum aperture (the maximum diameter of the entrance pupil) is set by the lens elements, not by the iris blades. You want a faster lens, you have to make the elements larger. That means a more sophisticated lens design, especially if you want to avoid aberrations. We can go into much more detail if you wish.

As an aside, the maximum theoretically possible aperture is f/0.5. Very very few photographic lenses have been made that are faster than f/0.9, and f/0.7 is about the practical limit. The 1975 Kubrick movie Barry Lyndon was famously shot with an f/0.7 lens.

Best,
Helen

 

So why when I shopped for 50mm prime the Sigma was much larger in diameter but the aperture is the same as the canon one?

 

because sigma makes things big?

Sorry, I'm just being an @$$

 

It's not just the diameter of the outer element. If that were the case, the Canon 50mm f/1.2 would actually be about an f/0.7 (50mm/72mm). The maximum aperture, in mm, is essentially the size of the smallest opening throughout the lens. This is a bit of an oversimplification, of course, but it's close to what we're getting at. So, for the lens I mentioned above, the maximum opening size would be about 42mm, not the 72mm which is about the size of the front element.

I don't know exactly why the Canon is smaller than the Sigma, but my guess would be that it comes down to engineering. Canon has a process, or technology that allows them to make a more compact lens than Sigma, and still have a wide enough opening for that particular aperture. I hope that makes sense. It made sense in my head when I wrote it, lol.

 

The F number is not just the size of the aperture. F refers to the ratio of the length of the lens to the diameter of the opening in the blades. The physics behind understanding the F stop explains why a 600mm lens and a 15mm lens both let in the same amount of light to the sensor at F4 even though the diameter of the opening on the 600mm lens is going to be much larger.

 

Err, good call. I wasn't thinking about it when I posted that. My main point was that the front element doesn't determine physical aperture size. It's the smallest opening inside the lens. You could have (theoretically) a 100mm front element, and the biggest opening in your lens is only 10mm. Though I can't think of a practical application of this, it is possible. The front element needs to be at least as big as the largest opening in the camera (though, you could say this about ANY of the elements in the lens). However, rarely (if ever) does the front element actually reflect the largest physical aperture.

 

That's not quite true. It is the ratio of the focal length of the lens to the diameter of the image of the opening in the iris blades when seen from the front of the lens (this image is known as the entrance pupil), not the physical diameter of the blades.

The front element needs to be at least as big as the entrance pupil - which may be larger than the physical opening in the iris. Many telephoto lenses have a front element that is about the same diameter as the entrance pupil.

Retrofocus designs ("wide angles") tend to have a front element that is much larger than the entrance pupil - the entrance pupil needs to be seen from all angles of view that the lens is intended to cover.

The entrance pupil also happens to be the correct point of rotation of the camera for a stitched panorama.

Best,
Helen

Edit
Here's a diagram of the 35 mm for the Contax 645 that shows the front element as being about 8 times the diameter of the entrance pupil. Note that the exit pupil is about the same diameter as the last element. It is a retrofocus design, ie an inverted telephoto.



V1 = Front vertex
V2 = Rear vertex
N1 = First Principal Point or Node
N2 = Second Principal Point or Node (which is one focal length in front of the image plane when focused on infinity)
The iris can be seen faintly, just to the left of the exit pupil.

 

Larger-diameter elements are easier to grind,polish,and to fully correct for optical aberrations than are smaller elements. Lens designers have to strike a balance between lens size, optical performance, and final price. It is possible to design a very compact 50mm f/1.4 lens, or a significantly larger one, like Sigma did with its 50/1.4. Optical designers have to work within the constraints of the engineering and marketing departments; many people would love an 18-55mm f/2.8, but it could not be priced at $100, like the current pokey f/3.5~5.6 variable maximum aperture kit lenses. TOday's 18-55 and 55-200mm kit zooms are designed for lightness and low price with "reasonably good" optical performance. If you want better performance or better specifications, well then you have to pay a lot more money to buy those things,and in the bargain you will get a bigger, heavier lens and a lighter wallet.

 

I think that you should be working for Leica or Zeiss. They would love to be able to make large aspheric elements more easily than they make small ones.

(Did you get that the wrong way round?)

Best,
Helen

 

Not to get nit-picky, but when I say "Length of Lens" I mean focal length and I did say the diameter of the opening, not the "diameter of the blades".