# Question about variable aperture



## Hobby Shooter (Jan 16, 2013)

Not afraid to ask stupid questions as it's the best way to learn and normally people on this forum will give informed answers. 

My question is around variable aperture in lenses. Why do lenses with variable as opposed to fixed aperture cost less? Is it a mechanicel explanation to it? I can see that more advanced optics will cost more with more glass going into the lens, but don't understand the dynamics behind aperture. 

I'm in the IT business and have been for many years, hardware related features sometimes costs more to develop and produce and somteimes it is simply down to charging more for better functionality - higher margins from customers prepared to pay premium for higher performance.

I would be happy to understand more about this.

thanks in advance
J


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## paul13walnut5 (Jan 16, 2013)

I think you've hit the nail on the head.

Fixed aperture, especially fast fixed aperture zooms involve a lot of R&D to get right, and won't sell so many as simpler cheaper designs, so it's partly economics of scale.

The construction quality is almost always higher, more metal, metal mounts, often sealing, sometimes tripod rings etc.

They also tend to have more complex focus motor types (ring type manual over-ride USMs vs short throw micro-motors) and can often have more complex aperture designs with more blades etc.

I think there is an element (boom boom) of folk being willing to pay more for the best, and so they can charge more, but there is obvious physical differences in the bulk and weight of the lenses, they are quite different beasts.

There are some great budget lenses out there, like the 18-55 IS, the 55-250 IS (if you can live with the loss of light as you zoom) and the 40mm f2.8 etc, and the third party manufacturers do reasonably priced f2.8 zoom alternatives if you can't quite stretch to the canon versions.

It's not a stupid question at all, I hope I've helped, a bit at least.


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## neuroanatomist (Jan 16, 2013)

A variable aperture zoom lens costs less fixed aperture zoom lens because it means the glass elements can be smaller. Minimally, a lens elements must be sized to fill the iris diaphragm with light. The diameter of the iris diaphragm is (focal length / f-number), e.g. a 200mm f/2.8 lens has an iris diaphragm of 200 ÷ 2.8 = 71.4mm...and thus, a 70-200mm f/2.8 zoom must also have an iris diaphragm diameter of 71.4mm at the long end, although a smaller diameter is needed at the 70mm end of the zoom.

An easy example to see this is the 100-400mm f/4.5-5.6L IS zoom - at the long end, the diameter of the iris diaphragm (and thus, the glass elements needed to fill it with light) is 71.4mm (same as 200/2.8 - twice the focal length, 2-stop narrower aperture). If that lens were to be a 100-400mm f/4.5 lens, the diameter would need to be 88.9mm to support 400mm f/4.5. That's closer to a 400/4 or 200/2 in element sizes than the current lens, so figure probably ~$5K for such a lens. Not exactly a consumer-friendly price like the current 100-400...


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## IWLP (Jan 16, 2013)

The first part of this Tech Tips post from Chuck Westfall explains a little bit of the mechanics of a constant-aperture zoom lens. I found it an interesting read when it was first posted in 2009.


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## K-amps (Jan 16, 2013)

neuroanatomist said:


> A variable aperture zoom lens costs less fixed aperture zoom lens because it means the glass elements can be smaller. Minimally, a lens elements must be sized to fill the iris diaphragm with light. The diameter of the iris diaphragm is (focal length / f-number), e.g. a 200mm f/2.8 lens has an iris diaphragm of 200 ÷ 2.8 = 71.4mm...and thus, a 70-200mm f/2.8 zoom must also have an iris diaphragm diameter of 71.4mm at the long end, although a smaller diameter is needed at the 70mm end of the zoom.
> 
> An easy example to see this is the 100-400mm f/4.5-5.6L IS zoom - at the long end, the diameter of the iris diaphragm (and thus, the glass elements needed to fill it with light) is 71.4mm (same as 200/2.8 - twice the focal length, 2-stop narrower aperture). If that lens were to be a 100-400mm f/4.5 lens, the diameter would need to be 88.9mm to support 400mm f/4.5. That's closer to a 400/4 or 200/2 in element sizes than the current lens, so figure probably ~$5K for such a lens. Not exactly a consumer-friendly price like the current 100-400...



Thanks Neuro: Does it mean that the 70-200 mk.ii could possibly do 70mm @ f/0.98 ? (200/2.8 = 71.4, and 70/ 71.4 = 0.98) Since the lens element is large enough? So the lens potentially could be a Variable 70-200 f/0.98 -2.8L ? If they so chose to build it with similar glass elements?

Sort of reminds me of how Audio amplifier manufacturers cannot feasibly build a perfect voltage amplifer that doubles wattage when the impedance is halved as theoretically it should. so lets say a 100watt amp into 8 ohms should do 200 watts into 4 ohms. So what they end up doing is over building/ under speccing their amps . So in this example they will build a 130 watt into 8 ohm amplifier and this gives about 200 watts into 4 ohms, and they call it a 100 watter...


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## neuroanatomist (Jan 16, 2013)

K-amps said:


> Thanks Neuro: Does it mean that the 70-200 mk.ii could possibly do 70mm @ f/0.98 ? (200/2.8 = 71.4, and 70/ 71.4 = 0.98) Since the lens element is large enough? So the lens potentially could be a Variable 70-200 f/0.98 -2.8L ? If they so chose to build it with similar glass elements?



Chuck Westfall answers that in the link above (the answer is 'no', BTW).


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## unfocused (Jan 16, 2013)

K-amps said:



> neuroanatomist said:
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> > K-amps said:
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Agreed, I read Westfall's response and I don't think he ever answers the question. He "explains" how constant aperture lenses work – sort of; but he doesn't answer the question posed to him, which seems to be: why not a variable aperture lens with 2.8 at the long end and f1 at the short end.


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## K-amps (Jan 16, 2013)

unfocused said:


> K-amps said:
> 
> 
> > neuroanatomist said:
> ...



I deleted my post since I was not sure if i would get hammered for questioning Chuck... I remain unsure though. It seems they are playing with the definition of the aperture/ F-Stop conventions. In this case with the aperture being behind what he called the tele-converter, does not play with the classical definition of the F-Stop as it applies to irises and the aperture being up front...


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## neuroanatomist (Jan 16, 2013)

Basically, the virtual aperture for a telephoto lens design is right at (or very slightly behind) the front element. The _real_ aperture is smaller, but that doesn't matter in terms of element size - it's the virtual aperture that has to be filled with light (for other lens designs, e.g. retrofocus lenses like UWAs, the front elements are apparently much larger than they 'need' to be - it's a different design). What Chuck is saying is the 70-200/2.8 has a real, physical aperture that approximates 70/2.8 (e.g. ~25mm), and that the elements in front of that physical aperture 'magnify' it to form the virtual aperture, which gets larger as you zoom to longer focal lengths. 

You get a sense of that from the block diagramL






When looking at one of these, though, it's not just the position of the iris diaphragm or the sizes of the elements relative to the approximated size of the iris diaphragm - you also need to consider the elements themselves, whether they're converging or diverging lenses. Roger Cicala wrote a series on the basic lens designs a while back, on his lensrentals blog (and IIRC, CR reposted here with permission) - worth a read.


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## Mt Spokane Photography (Jan 16, 2013)

Hobby Shooter said:


> Not afraid to ask stupid questions as it's the best way to learn and normally people on this forum will give informed answers.
> 
> My question is around variable aperture in lenses. Why do lenses with variable as opposed to fixed aperture cost less? Is it a mechanicel explanation to it? I can see that more advanced optics will cost more with more glass going into the lens, but don't understand the dynamics behind aperture.
> 
> ...


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## Hobby Shooter (Jan 17, 2013)

Wow, thanks guys. Makes it clearer, will take some more time to read through more thoroughly. But I understand the basic dynamics better now. I didn't make out first that it had to do also with the elements.


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