Help this noob understand Fast Lenses

[Helen B]

Darrel,

You have used the example of lenses of modest aperture, not an example where faster lenses become easier to make because they are faster. You're doing a bit of bait and switch and hoping that no-one realises, I think.

Best,
Helen

 

[Gaerek]

It does so for all Canon and Nikon super telephoto lenses. If you check the specs, the diameters of the lenses is generally between 10mm and 20mm larger than the required size of the objective. For example, the Nikon 300mm f/2.8 lens is 124mm in diameter. The front objective must have at least 107mm diameter of clear glass, leaving 17mm. So the housing holding that big objective to the rest of the lens is about 8mm thick.

Refractor telescopes work the same way. The clear opening of the front objective defines the max aperture, as there is no diaphragm in the system.

Good call. My knowledge of lens design theory is apparently somewhat limited. My original point, however, still stands. That is, that the size of the front element doesn't always dictate the max aperture. Although you can usually assume large front element = large max aperture.

 

[rusty9]

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!

it's like saying "why can't every car be a lambo?"
well not everyone can afford one

 

[cfusionpm]

Darrel,

You have used the example of lenses of modest aperture, not an example where faster lenses become easier to make because they are faster. You're doing a bit of bait and switch and hoping that no-one realises, I think.

This is the basis for most of his argument structures.

 

[DJH]

To answer the OPs question in my own method, without getting into a bunch of convoluted technical jargon: The kit lens is a very budget minded lens with sort of economy glass, body and bits and pieces. Typically when you are upgrading to a faster lens, its not because you are the run of the mill housewife taking snapshots. So not only do you get an upgrade in aperture, you get better glass ( better optics, better coatings etc. ) Typically you will get a better build quality as well. The fact of the matter is, they can make cheap lenses with with wide aperture ( look at the bower line, or even the canon 50mm f1.8 ) Fast ZOOM lenses are a different ball game because there is movement of lenses which means all of the properties of light hitting that glass are going to change when you zoom and need to stay as consistent as possible ( refraction/distortion). Thats why many zooms get soft at certain focal lengths. . The kit lens needs to be a versatile lens, thats why you get that and not a 50mm f1.8 offered typically.

So I guess in a sense, they could make a kit lens that is fast ( the 50mm f1.8 is the same price as the kit lens actually but no zoom ) but its not going to be the best quality ( just as the 50mm f1.8 isn't as nice as the 50mm f1.4 which isn't as nice as the 50mm f1.2 etc etc.

Thats my own take on it.

 

[Petraio Prime]

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.

Hah. You're completely mistaken, as usual.

 

[Petraio Prime]

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.

You have completely misunderstood the quote. Why don't you get out of threads about lenses. You know nothing about them; and you cannot comprehend anything you read.

 

[Petraio Prime]

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 it costs more to produce them.

 

[Robin Usagani]

Thank you everyone who actually try to explain why (no matter if it is right or slightly wrong). Telling me because it costs more to produce, got to pay more for good sh1t, aren't good enough and totally missing my the point of my question. I guess the dumbed down answer is, it is not as simple as opening the blades more, you will need bigger front elements hence the hefty price.

 

[Petraio Prime]

Thank you everyone who actually try to explain why (no matter if it is right or slightly wrong). Telling me because it costs more to produce, got to pay more for good sh1t, aren't good enough and totally missing my the point of my question. I guess the dumbed down answer is, it is not as simple as opening the blades more, you will need bigger front elements hence the hefty price.

What are you talking about? The blades open all the way on almost all lenses. The f-number represents the speed of the lens with the blades fully opened.