Help this noob understand Fast Lenses

[Robin Usagani]

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!

 

[JeffieLove]

because good stuff is expensive

 

[Helen B]

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

 

[Robin Usagani]

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?

 

[JeffieLove]

because sigma makes things big?

Sorry, I'm just being an @$$

 

[Gaerek]

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?

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.

 

[oldmacman]

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.

 

[Gaerek]

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).
...
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.

72mm is the filter size...not the diameter of the front element. The front element is much smaller, as you can see in this image from digisniper.com...

I'm sure it's bigger than 42mm, but it's no where near 72mm, or even 60mm. I'd be surprised if it was more than 50mm in diameter. Maybe someone who has one can measure.

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.

 

[Helen B]

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.

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.

 

[Derrel]

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.

 

[Helen B]

Larger-diameter elements are easier to grind,polish,and to fully correct for optical aberrations than are smaller elements.

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

 

[oldmacman]

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.

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.

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".

 

[Helen B]

...I did say the diameter of the opening, not the "diameter of the blades".

I think that you may have missed my point. F-number is not calculated from the diameter of the opening in the blades, but from the diameter of the image of the opening when seen from the front of the lens. They can be very different.

Best,
Helen

 

[Derrel]

Larger-diameter elements are easier to grind,polish,and to fully correct for optical aberrations than are smaller elements.

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

No smart-aleck, I have it the right way around. You're the one who has it wrong, and who introduced aspherical elements inappropriately into a discussion where they are not relevant.--I did not mention aspherical elements, and besides, TODAY most aspherical elements are not ground, but molded. Speaking of working for Zeiss--here's my point, as seen from the POV of the Zeiss designers,as a matter of fact...

Larger-diameter elements are easier to correct fully because a larger element can be ground with a less-precise grind, and yet still be good enough to be considered well-corrected enough for use in a lens; a smaller lens element, which as I said is 1) ground and 2) polished, demands the utmost in precision to achieve the same degree of correction. Grinding smaller, miniaturized elements is much more costly and demanding a process than working with larger elements. If you doubt me, then please consult noted lens expert Erwin Puts and his article describing the process that the Zeiss company went through when designing its line of lenses for the Contarex camera line, some years ago.

Zeiss decided that they would allow each lens design to be as large as it needed to be, rather than to miniaturize the lens designs. This resulted in some very large lens designs, much larger than comparable lenses designed by Ernst Leitz or Nippon Kogaku (aka Nikon) BUT the Zeiss lenses were easier to correct fully because they went with larger elements, which have a more-gentle radius than smaller elements of comparable effect.

Here's an reference from one of the world's most-knowledgeable lens experts: "The Zeiss lenses for the Contarex system were the result of this second-generation design approach. I have stated it repeatedly, but the size of the lens, is one of the most important parameters for the optical quality. The Contarex lenses are proof of this statement. Every lens was optimized in performance without regard for the physical size and the designer allowed the lens to grow to its natural proportions. The result was a lens line of impressive performance for that time. There is some mystique around the Contarex lenses: they are sometimes described as the best lenses ever made with an optical quality never surpassed. This is not true. Some designs, like the 4/35 and the 4/135, are indeed close to perfection, but given the modest apertures, that is not a big challenge. The mounting and centring of the lens elements is indeed not yet surpassed."

ZM lenses


So,no Helen, I don't have it the wrong way around--you do. I did not mention aspherical elements, but yes, even when an aspherical element is hand-ground by a skilled technician, as was the case with the 58mm f/1.2 Noct~Nikkor, working on a LARGER surface demands a lower degree of absolute precision and a lower degree of accuracy, to achieve the same result,compared with working on a miniaturized lens. Sorry to disappoint you with the facts. Regardless, today, most aspherical elements are moulded, which has dramatically cut costs, but the fact still remains that larger is easier to work with--just as "regular surgery" like removing a gall bladder is much,much easier than micro-surgery, such as re-connecting severed blood vessels or re-connecting sliced nerves.

 

[Robin Usagani]

OK.. I am now more confused than how I was LOL JK. I am going to google it again and read more material. I went to 7 years of engineering school! It is more difficult for me to understand than I expected.