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02-16-2015, 11:49 AM   #1
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Question about apertures on zooms (fixed vs variable, why ever have fixed?)

so maybe i'm completely not getting it, but this is puzzling to me.

i understand that the aperture is the ratio of the focal length and the diameter of the "pupil"

so on a variable aperture zoom you may be able to have f4 at the wide end and only f5.6 at the long end. fair enough, that makes sense.


however, on a FIXED aperture zoom, why can you only achieve say f2.8 at the wide end when you can achieve it at the long end? why wouldn't it be an f2-2.8?

is it just a matter of difficult optics to manage this? because clearly if you have the pupil diameter to have f2.8 at say 135mm you'd need 48.2mm of pupil diameter, which would give you almost an f1 at 50mm.

so how come you don't see lower f stops at the shorter end?



Thanks

02-16-2015, 12:04 PM   #2
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you do, tokina has a 2.6 -2.8 28-70mm lens but aside from that, it is scarce. on a zoom you have to take into account... the zooming, meaning you have to have space for movement. And you have to correct the image distortion. This equals that whatever part that moves bust be smaller than the front element, and depending on the position relative to the front element it will allow different amounts of light, the closer it is to the front element the less light can be allowed through it. The same goes for all the little optical parts. you could design a zoom with 1.4 to 2.8 without any problem regarding light transmission but you would have severe barrel or pinch distortion on all focal lengths other than the one where the very few optical components work. A zoom often has 10 elements in different groups that move relative to eachother, this is the reason you get less light in one or another focal length.
02-16-2015, 12:22 PM   #3
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This is the second thread on this topic topic. The F2.8 being the Gold Standard is related but different so I won't merge the 2. That said, it is hard to get fast lenses 35mm and below. They start getting big. In a zoom, it complicates things even more. An 28 f2.8 lens maybe considered average in speed where as a 28-70/2.8 zoom is considered fast.
02-16-2015, 12:25 PM - 1 Like   #4
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QuoteOriginally posted by wibbly Quote
however, on a FIXED aperture zoom, why can you only achieve say f2.8 at the wide end when you can achieve it at the long end? why wouldn't it be an f2-2.8?
Because it's fixed, and it's 2.8 at the long end.

No need to overthink it :-)

02-16-2015, 12:47 PM   #5
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QuoteOriginally posted by SpecialK Quote
No need to overthink it :-)
Indeed...doing so will only lead to headaches.


Steve
02-16-2015, 12:53 PM - 1 Like   #6
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It could be faster at the wide end, as you infer. However, not making it faster gives the lens designer more latitude to optimize other parameters and not sacrifice too much for the design. Considering that most photographers consider that a fixed aperture means a better quality lens (whatever the aperture...) it's like a free lunch for the designer.
02-16-2015, 01:34 PM   #7
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Samsung did this with their premium 16-50mm lens for the NX mount. It's f/2-2.8 and impressively sharp, but the lens suffers from excessive chromatic aberration towards the wide end. You might want to look at the SLRGear review, which has some useful charts.

—DragonLord
02-16-2015, 01:36 PM   #8
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QuoteOriginally posted by stevebrot Quote
Indeed...doing so will only lead to headaches.


Steve
Yeah, I mean why not make it f2 at the long in f4 in the middle and f2.8 at the long end.

Hysterical

02-16-2015, 02:16 PM   #9
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QuoteOriginally posted by wibbly Quote
i understand that the aperture is the ratio of the focal length and the diameter of the "pupil"
It's my understanding that the f-stop is the ratio of focal length to the "entrance pupil" aka "apparent size of the aperture" aka "how big the hole looks when viewed from the front".

In a constant aperture zoom, the hole will look bigger at longer focal lengths. This happens to some extent in a consumer zoom as well, otherwise a 50-200mm f4-5.6 would also not work as the numbers suggest, so stare down the barrel of whatever zoom you have and see what happens.
02-16-2015, 02:27 PM   #10
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And that is the explanation I needed. Assuming its correct which I am


Thanks
02-16-2015, 03:03 PM   #11
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BrianR is right.
If @stevebrot is around, he will probably dissuade you from going too deep into dangerous territory... :-P
02-16-2015, 03:06 PM - 1 Like   #12
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QuoteOriginally posted by BrianR Quote
It's my understanding that the f-stop is the ratio of focal length to the "entrance pupil" aka "apparent size of the aperture" aka "how big the hole looks when viewed from the front".

In a constant aperture zoom, the hole will look bigger at longer focal lengths. This happens to some extent in a consumer zoom as well, otherwise a 50-200mm f4-5.6 would also not work as the numbers suggest, so stare down the barrel of whatever zoom you have and see what happens.
Now you've done it. My head really hurts now thanks to you

Now somebody is going to have to explain entrance pupil in more precise terms and somebody else will note that its location is not where you might expect...and then somebody else will butt in and assert that it is not the same as T-stop and ...


Steve
02-16-2015, 03:23 PM   #13
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QuoteOriginally posted by stevebrot Quote
it is not the same as T-stop and ...
And don't forget the equivalence on FF.
02-16-2015, 04:58 PM   #14
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Here is a decent Wikipedia article on Entrance Pupil:
Entrance pupil - Wikipedia, the free encyclopedia
02-16-2015, 07:24 PM   #15
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QuoteOriginally posted by SpecialK Quote
And don't forget the equivalence on FF.
Yes! That too!

Rabbit hole here we come!


Steve
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