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04-02-2010, 04:22 AM   #1
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Zoom faster than 2.8

I was wondering why there seems to be no zoom lenses faster than 2.8, even below 50mm - or have I overlooked something?

50/1.4 lenses and in general zooms are available from decade and technology is very much matured. While I understand that zooms are relatively more complex to build, it is hard to believe that it is so complex that even small range zooms (say 16-45mm) cannot be produced as fast as f1.4 or even f1.8.

Any insight please.

04-02-2010, 04:40 AM   #2
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Designing a zoom wider than f2.8 is not realistic for two reasons:

1-have you looked tat the f2.8 zooms from Pentax? Can you imagine letting twice as much light pass through? Can you figure the size?

Such a lens would be ludicrously large. Finding filters would be a nightmare, the price would be outrageous (double the glass area, more or less double the amount of glass, increase the price a loooot)

2-control of aberrations. There's a reason why lenses are better in the center than in the corners, generally. Increasing the lens area would also increase the effect of whatever aberrations you are trying to control. vigneting would, by definition, be worse. Coma, also. Chromatic aberration and spherical aberration would be off the charts.

It's easy to optimize a lens design for ONE focal length, but when you increase the range of focal lengths, you start to face choices and compromises. That's why most zoom lenses are better somewhere in the middle of their range. Move away and they decrease in quality.
04-02-2010, 04:59 AM   #3
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What i dont entirely get is why there are slow zooms with varying apertures, and zooms with constant aperture, but not zooms with fast varying apertures. A 70-200mm with f2.8 would need a bigger size at 200mm for the f2.8 than at 70mm. Shouldnt it then be easy to have a faster aperture at the lower range?
04-02-2010, 05:23 AM   #4
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QuoteOriginally posted by LinXitoW Quote
What i dont entirely get is why there are slow zooms with varying apertures, and zooms with constant aperture, but not zooms with fast varying apertures. A 70-200mm with f2.8 would need a bigger size at 200mm for the f2.8 than at 70mm. Shouldnt it then be easy to have a faster aperture at the lower range?
The diameter of the aperture isn't directly related to its physical size, but rather the apparent size as viewed through the front of the lens. So the aperture diameter does not remain fixed throughout the range; the constant aperture zooms just ensure that it changes apparent size in a 1:1 ratio with the focal length.

04-02-2010, 09:05 AM   #5
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There are fast zooms with variable apertures... like the Vivitar Series 1 70-210mm F2.8-F4. I've had three of them, it's a nice lens (though I don't use it very much because zoom lenses that don't transmit focal length to the camera are kind of a pain to use stabilization with.) On a DSLR, it shows F2.8 through the range but I assume that at 210mm, you'll get the same exposure whether it's set to F2.8 or F4, but I haven't tested that.

Olympus has some F2.0 zooms for the 4/3rds system (14-35mm and 35-100mm) but they are very expensive. Presumably, the smaller sensor makes the speed possible.
04-02-2010, 11:34 AM   #6
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QuoteQuote:
The diameter of the aperture isn't directly related to its physical size, but rather the apparent size as viewed through the front of the lens.
Sorry, that's not true. The aperture, as reported in f-numbers, is a ratio of the physical aperture and the focal length.

QuoteQuote:
On a DSLR, it shows F2.8 through the range but I assume that at 210mm, you'll get the same exposure whether it's set to F2.8 or F4
Sorry, that's not true either. F4 is f4 whatever the focal length, because it's a ratio of aperture and focal length.

The Olympus system has a shorter distance between the lens and sensor, meaning it's easier to make larger apertures.

Most people, when discussing lenses, make the implicit assumptions that they can change a parameter and that everything else remains the same. Sadly that's not true, everything is closely interrelated. So using a wider aperture might change aberrations performances, sharpness across the whole image plane, vigneting, etc. There is also a huge difference between a lens that focuses externally, for instance, by moving the whole lens (think M lenses) and a lens that focuses internally, by changing the relative position of the lens elements. The latter will have much more varying performances as you focus or zoom.

Lens design is an art AND a science. It's really not as easy as it sounds. Which is sad
04-02-2010, 11:38 AM   #7
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QuoteOriginally posted by bdery Quote
Sorry, that's not true either. F4 is f4 whatever the focal length, because it's a ratio of aperture and focal length.
What I'm saying is that the lens maxes out at F4 at 210mm, so the fact that the camera thinks that it's still using F2.8 is probably not accurate.

I assume that the lens can only tell the camera the max and min aperture, and since no focal length is given, the camera always assumes that F2.8 is possible.
04-02-2010, 11:56 AM   #8
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constant aperture zoom lens design: Nikon SLR Lens Talk Forum: Digital Photography Review

04-02-2010, 02:04 PM   #9
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This exact topic has been discussed in depth last month:
https://www.pentaxforums.com/forums/pentax-beginners-corner-q/91480-why-there...w-f-2-8-a.html

Here you'll find all the reasons and their discussion.

Ben
04-02-2010, 03:07 PM   #10
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angenieux 28-70/2.6-2.8
04-03-2010, 01:06 PM   #11
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QuoteOriginally posted by bdery Quote
Sorry, that's not true. The aperture, as reported in f-numbers, is a ratio of the physical aperture and the focal length.
Sorry, pretty sure you're wrong.

Entrance pupil - Wikipedia, the free encyclopedia

QuoteQuote:
In an optical system, the entrance pupil is the optical image of the physical aperture stop as 'seen' through the front of the lens system.
QuoteQuote:
The f-number ("relative aperture"), N, is defined by N = f/EN, where f is the focal length and EN is the diameter of the entrance pupil.
And specifically for you:
QuoteQuote:
The common assumption in photography that the pupil diameter is equal to the aperture diameter is not correct for many types of camera lens, because of the magnifying effect of lens elements in front of the aperture.
04-03-2010, 01:24 PM   #12
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Olympus has a whole line-up of a really sharp set of m4/3rd f/2.0 zoom lenses but they are extremely expensive. The size of two of these lenses and the focal length crop transfers down to a typical 24-70 and 70-200 full frame range in m4/3rd. If there were similar for APS-C or even more, an FF sensor, the size and cost would be much larger so we're stuck with f/2.8.

Last edited by LeDave; 04-03-2010 at 01:31 PM.
04-03-2010, 02:48 PM   #13
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QuoteOriginally posted by Groucho Quote
What I'm saying is that the lens maxes out at F4 at 210mm, so the fact that the camera thinks that it's still using F2.8 is probably not accurate. I assume that the lens can only tell the camera the max and min aperture, and since no focal length is given, the camera always assumes that F2.8 is possible.
Funny thing about another lens: I've been doing some rough tests with (among others) a Tak-A 28-80/3.5-4.5 on my K20D in Av and other modes. Zooming in and out wide-open over the entire range, the cam-top LCD always reports the aperture as f/4.0. Curious, eh wot?
Four are the things I'd be better without:
Love, curiosity, freckles, and doubt.
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04-03-2010, 05:44 PM   #14
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QuoteOriginally posted by pingflood Quote
Sorry, pretty sure you're wrong.
I'm pretty sure he knows a lot more about that stuff than all of us together. It's his job.
04-03-2010, 05:55 PM   #15
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Then I'd like to know how I am misinterpreting all the info on the subject I find.

edit: maybe someone can go look at ISO 517:2008 and report their findings.

Last edited by pingflood; 04-03-2010 at 06:29 PM.
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