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12-08-2008, 01:00 PM   #1
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2 myths i would like clearing up quickly please...

1. "longer exposures produce more noise": I always thought thihs was true but since doing 7 minutes exposures at the weekend and getting clear results i think i was wrong. maybe because i was just underexposing in many low light situations?

2. "constant aperture zooms" are in fact limited at the wide end to smaller apertures than they are capable of? my 16-50 appears to have a less than circular aperture at the wide end, so i assume that if the aperture was left open it would be an f/2.0-2.8 lens? is this the same with all pro lenses? Not that i would want to use it that much open, but it seems like an interesting idea to me.

12-08-2008, 01:12 PM   #2
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QuoteOriginally posted by IsaacEastgate Quote
2. "constant aperture zooms" are in fact limited at the wide end to smaller apertures than they are capable of? my 16-50 appears to have a less than circular aperture at the wide end, so i assume that if the aperture was left open it would be an f/2.0-2.8 lens? is this the same with all pro lenses? Not that i would want to use it that much open, but it seems like an interesting idea to me.
That's right. When zooming to greater focal length the viewing angle is reduced, therefore less light comes in because the lens has less angle to receive ligth from. Therefore to maintain constant aperture it has to be limited to match the long end's.

Don't think of it as losing something though. Most likely if you were to "open" it up you'd find the resulting image unacceptable due to softness, vignetting or other aberrations.
12-08-2008, 06:15 PM   #3
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Your first one is not a myth. The longer the sensor 'runs' the more noise it generates. This may or may not be visible in your results. If it is cool out there will be less noise, also if you use a low ISO you may not see any appreciable noise.
12-08-2008, 07:16 PM   #4
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Turn off dark frame noise subtraction and see how you like (1)
It's not a myth ;-)

12-08-2008, 07:24 PM   #5
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i had always wondered about the fixed aperature lenses. That doesn't make sense to me. Fixed 2.8 lenses at the widest angle should be able to go to like 2.0 or lower technically right? why don't they at least give you the option if there is no reason not to, besides having a possible soft image?
12-08-2008, 07:25 PM   #6
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QuoteQuote:
Turn off dark frame noise subtraction and see how you like (1)
It's not a myth ;-)
OP does not specify camera - may have K20D in which case it is always on and doing its job.

Question 2.
No, it is not a 2.0 lens. Otherwise, manuufacturers would advertize it as a 2.0 lens. And charge you more.
12-09-2008, 10:03 AM   #7
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QuoteOriginally posted by bobloadmire Quote
i had always wondered about the fixed aperature lenses. That doesn't make sense to me. Fixed 2.8 lenses at the widest angle should be able to go to like 2.0 or lower technically right? why don't they at least give you the option if there is no reason not to, besides having a possible soft image?
No. The way fixed aperture lenses work is that the aperture opening physically changes size as you zoom in and out. They are more correctly called constant aperture.

Take an example lens that is 50-100mm f/2. At 50mm f/2, the aperture opening is 25mm. At 100mm f/2, the aperture opening is 50mm.

Conversely, for variable aperture lenses (50-100mm f/2-4), the physical aperture size doesn't change as you zoom in and out. Consider the same hypothetical 50-100mm lens.

At 50mm f/2, the aperture opening is 25mm. When you zoom out to 100mm, the aperture opening is still 25mm, which gives you 100mm f/4.

The constant aperture lenses are more complicated to build because they have to match the aperture opening to the focal length.
12-09-2008, 10:44 AM   #8
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QuoteOriginally posted by arbutusq Quote
Your first one is not a myth. The longer the sensor 'runs' the more noise it generates. This may or may not be visible in your results. If it is cool out there will be less noise, also if you use a low ISO you may not see any appreciable noise.
Yep, the 7 minute exposures were on a very very cold night...

Thanks for the explanation on the constant aperture lens ^^, i completely understand now. But still, technically they could allow the aperture to open up at the wide end leaving me with a f/2.0-2.8 lens

12-09-2008, 12:28 PM   #9
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QuoteOriginally posted by IsaacEastgate Quote
1. "longer exposures produce more noise": I always thought thihs was true but since doing 7 minutes exposures at the weekend and getting clear results i think i was wrong. maybe because i was just underexposing in many low light situations?
the idea is that your sensor starts to heat up from long exposures, hurting your light recording and introducing more noise.

maybe it was just chilly outside.
12-09-2008, 02:57 PM   #10
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QuoteOriginally posted by alohadave Quote
No. The way fixed aperture lenses work is that the aperture opening physically changes size as you zoom in and out. They are more correctly called constant aperture.

Take an example lens that is 50-100mm f/2. At 50mm f/2, the aperture opening is 25mm. At 100mm f/2, the aperture opening is 50mm.

Conversely, for variable aperture lenses (50-100mm f/2-4), the physical aperture size doesn't change as you zoom in and out. Consider the same hypothetical 50-100mm lens.

At 50mm f/2, the aperture opening is 25mm. When you zoom out to 100mm, the aperture opening is still 25mm, which gives you 100mm f/4.

The constant aperture lenses are more complicated to build because they have to match the aperture opening to the focal length.

oh that makes sense, thanks man. so why not leave the option to have the opening aperature opening at 50mm even when you are at 50mm zoom? that would be like F/1?
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