Why aren't zoom lenses faster than 2.8?
- Thread starter JonB8305
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woollybear
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K3nt said:It is quite easy actually. If you want to know what size it would require just divide the focal length with the required f-stop and you'll see what size the front element diameter would be. So, for a 100-400mm f/1.2 the front element would be 33,3cm in diameter.
If you divide the focal length with the diameter of the front element you'll get the maximum f-stop for that lens.
33,3 cm...wow!!!
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Malte_P
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woollybear said:K3nt said:It is quite easy actually. If you want to know what size it would require just divide the focal length with the required f-stop and you'll see what size the front element diameter would be. So, for a 100-400mm f/1.2 the front element would be 33,3cm in diameter.
If you divide the focal length with the diameter of the front element you'll get the maximum f-stop for that lens.
33,3 cm...wow!!!
i don´t quite get the math.... 8)
can you show how f1.2 gives 33,33 cm on a 100-400mm lens?
it´s more like 83mm for the 100mm end.
by the way:
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Loswr
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Malte_P said:woollybear said:K3nt said:It is quite easy actually. If you want to know what size it would require just divide the focal length with the required f-stop and you'll see what size the front element diameter would be. So, for a 100-400mm f/1.2 the front element would be 33,3cm in diameter.
If you divide the focal length with the diameter of the front element you'll get the maximum f-stop for that lens.
33,3 cm...wow!!!
i don´t quite get the math.... 8)
can you show how f1.2 gives 33,33 cm on a 100-400mm lens?
it´s more like 83mm for the 100mm end.
A 100-400mm f/1.2 lens (which will never exist) would need to have a 333.3 mm (13") front element - a practical impossibility. Your comment about needing an 83mm front element for the 100mm end isn't relevant - the front element can't change size so the optics must be sized for the long end. That's why the 100-400mm lens is a variable aperture zoom with an f/5.6 long end - if it was f/4.5 throught the zoom range, it would need an 89mm front element.
Malte_P said:
While the above is true, it actually depends on the design of the lens (Roger Cicala has an article on lens designs that's worth a read). With a telephoto design, the 'apparent working diameter' or 'virtual aperture' of the lens actually sits right at or just behind the front element.
Put another way, the front element generally needs to be at least as large as (focal length ÷ f/number), and with many lens designs the front element is significantly larger than that.
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every time you add a stop, the size of the lens more than doubles - 70-200 F4 to 70-200 F2.8. imagine the size of 70-200 F2.0, it would weigh 5-6 pounds. And since L lenses cost more than $1000/lb, this monster would be close to $10,000 (USD). Still interested? I am not.
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Weight, technology, materials and price.
The trend today have bodies increasing ISO so the need for faster glass lessens.
The trend today have bodies increasing ISO so the need for faster glass lessens.
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dolina said:
The need is not always for "faster". Sometimes the need is for thinner DOF, which you can only get with a wider aperture.
The fact is, it IS more difficult to develop high quality lenses that perform well at apertures wider than f/2.8. The wider you go, the harder it is to control optical aberrations...and controlling them means more glass, glass with more precise characteristics, and often higher grade materials (high end optical glass, aspheric elements, fluorite or DO elements, etc.) Throw in "zoom", and those issues become even harder to control, as you have to control them such that they balance out at all of the key focal lengths along the zoom range.
I don't think there really is a trend towards lenses with slower apertures. I think f/3.5-5.6 and f/4.5-5.6 lenses, and even at times f/5.6-6.3 lenses, have been used in consumer-grade lenses for decades, by both brand names as well as off-brands. I don't think there are generally more of them now that we have cleaner higher ISO than there was before. I think they just become a little more relevant now that we have clean higher ISO...they are more useful in a broader range of use cases than they were before.
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To get an idea on the weight - I'll make a wild assumption and say we scale the optical formula. This doesn't really work in practice... whatever 
Say you go from f/2.8 to f/2. Achieving this means the aperture has to increase by 41.4%. Keeping the same slope on the glass means the glass means the thickness increases by 41.4% too. The result is that the weight of the glass must increase by 1.414^3 = 2.83x.
The structure that supports each lens has to be stiffer, and since it's going to be bigger, the mechanical bits get a lot heavier.
but that's not all....
Optical glass increases in price at roughly the fourth power of the aperture. This means the cost of glass has increased by 4x. Similarly, faster lenses demand tighter tolerances so the manufacturing costs increase. Say this quadruples too.
Suddenly a 70-200 weighs at least 10 lbs and material / manufacture costs $8000.
The price is very high, so the market size is now about 1/100 of the market size for the f/2.8 model. Amortise the development cost makes the lens even more expensive... say $10k.
If you think the numbers are wild, take a look at the Sigma 200-500 / 2.8. $26k and 35 lbs. Compare this with the 120-400/4.5-6. 10x as heavy and 26x more expensive.
Say you go from f/2.8 to f/2. Achieving this means the aperture has to increase by 41.4%. Keeping the same slope on the glass means the glass means the thickness increases by 41.4% too. The result is that the weight of the glass must increase by 1.414^3 = 2.83x.
The structure that supports each lens has to be stiffer, and since it's going to be bigger, the mechanical bits get a lot heavier.
but that's not all....
Optical glass increases in price at roughly the fourth power of the aperture. This means the cost of glass has increased by 4x. Similarly, faster lenses demand tighter tolerances so the manufacturing costs increase. Say this quadruples too.
Suddenly a 70-200 weighs at least 10 lbs and material / manufacture costs $8000.
The price is very high, so the market size is now about 1/100 of the market size for the f/2.8 model. Amortise the development cost makes the lens even more expensive... say $10k.
If you think the numbers are wild, take a look at the Sigma 200-500 / 2.8. $26k and 35 lbs. Compare this with the 120-400/4.5-6. 10x as heavy and 26x more expensive.
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Reading these last few posts reminds me of how surprised I am that my 135/2 isn't larger, especially compared with my 200/2.8
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