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01-28-2011, 02:23 PM   #1
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so what are the advantages of a steady F stop rating?

I see a lot of people who buy a lens an it is rated (example) 17-70 F4. what is the advantage of a steady f stop as apposed to a 17-70 2.8-4.5?
(this is just an example, any lens configurations could apply)

thanks

randy

01-28-2011, 02:28 PM   #2
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The only thing I can think of is flash exposure. It doesn't matter in p-TTL mode, but if you're using manual or auto flash mode, not knowing the exact aperture can be a problem.
01-28-2011, 02:30 PM   #3
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QuoteOriginally posted by SOldBear Quote
The only thing I can think of is flash exposure. It doesn't matter in p-TTL mode, but if you're using manual or auto flash mode, not knowing the exact aperture can be a problem.
To add to that, just a constant exposure in general so your settings aren't randomly changing on the fly based on zooming a lens in/out. Also, fixed aperture lenses tend to be faster than the variable aperture equivalents (at least F4, or F2.8) which allows for faster shooting in available light, along with shallower depth of field (at least with F2.8 lenses).
01-28-2011, 02:40 PM - 1 Like   #4
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Variable aperture zooms are smaller and lighter than fixed aperture zooms, which as others have noted, tend to (at least the good ones) hold the same maximum aperture through out the range.

While modern metering and TTL/P-TTL flash, have taken away most of the issues associated with variable aperture zooms, one point remains.

WHen you zoom in, you need higher shutter speeds for sharp images, and holding the same maximum aperture makes this easier to deal with. Reducing aperture will result in reducing shutter speed as you zoom, which is the exact opposite of what you want to achieve.

it is an ease of use thing more than anything else.

01-28-2011, 02:44 PM   #5
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QuoteOriginally posted by slipchuck Quote
I see a lot of people who buy a lens an it is rated (example) 17-70 F4. what is the advantage of a steady f stop as apposed to a 17-70 2.8-4.5?
(this is just an example, any lens configurations could apply)

thanks

randy
no real advantage really... it's just different version of the lens i guess. you get a bit faster at the longer end, which is important to some people.
01-28-2011, 03:08 PM   #6
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Constant aperture is very helpful - to me - because at any zoom length the lens will have the same minimum and maximum apertures. It's one less thing to think about when shooting and/or fight.
01-28-2011, 03:19 PM   #7
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QuoteOriginally posted by blackcloudbrew Quote
Constant aperture is very helpful - to me - because at any zoom length the lens will have the same minimum and maximum apertures.
Right. Quite useful if you like to shoot in full manual mode.
01-28-2011, 03:26 PM   #8
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Most, not all, constant aperture lenses have good to exceptional IQ, and they cost 2-4x as much as variable aperture lenses.

01-28-2011, 03:36 PM   #9
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the biggest advantage for me is that i know that no matter where i am in the zoom range what the aperture is.

that is, if i set it to f4 i can zoom in, zoom out, whatever, and it will stay f4.

otherwise, there is no real advantage. however as others have pointed out constant aperture lenses tend to be higher quality although this is just a generalization rather than a rule.
01-28-2011, 04:37 PM   #10
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Most of the time the variable max aperture lenses are the slower consumer lenses - but this may be dispelled with the rumoured DA* 25-350 (at least in being termed 'consumer').

Nevertheless it doesn't mean much in the scheme of things other than to be assured of the aperture being shot at, which in itself is no big deal unless exposure values are in the critical range, or at max aperture if the lens performs poorer than desired. Depth of field issues with variable apertures are less of an issue but strictly speaking this too can be a cause of disinterest in these variable aperture lenses.
01-28-2011, 05:22 PM   #11
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For A series lenses, it definitely matters, at least on the K100d I have used. These cameras don't recognize the variable maximum aperture, and treat the lens throughout its zoom range as having the largest aperture available at any focal length. As a result, as your true maximum aperture shrinks, you need to dial in exposure compensation or get underexposure. I am not sure if this was fixed on later models, but I think not.

Last edited by Nick Siebers; 01-28-2011 at 07:04 PM.
01-28-2011, 07:36 PM   #12
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QuoteOriginally posted by SpecialK Quote
Most, not all, constant aperture lenses have good to exceptional IQ, and they cost 2-4x as much as variable aperture lenses.
This answer wins the Coopie Doll. (Spelling on Coopie?)

They're simply better lenses, and the fast and constant aperture is simply a REFLECTION of that better quality.
01-28-2011, 09:39 PM   #13
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QuoteOriginally posted by Ira Quote
This answer wins the Coopie Doll. (Spelling on Coopie?)

They're simply better lenses, and the fast and constant aperture is simply a REFLECTION of that better quality.
It's Kewpie. But, I missed the point about the significance of the constant aperture itself - not about lenses with them.
01-30-2011, 04:37 PM   #14
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QuoteOriginally posted by SpecialK Quote
It's Kewpie. But, I missed the point about the significance of the constant aperture itself - not about lenses with them.
I'm missing THIS point!

01-30-2011, 06:47 PM   #15
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Variable aperture lenses tend to be consumer lenses. It's not the variable aperture that gives it a bad reputation, but the type of lenses you tend to find them in.
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