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05-02-2009, 03:36 AM   #1
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A few random questions:

1. Was looking at my Sigma 70-300mm zoom off the camera. I noticed when I manually set the aperture at some given setting, say f/8, that the aperture size did not change when I zoomed from 70-300mm and back. Or, at least, it didn't appear to looking through the lens from the front or back.

Question:
How does the lens maintain a constant light level, say f/8, throughout it's zoom range without also compensating by changing the aperture opening? Perhaps it does this when connected to the camera body?

2. A sensor question.
Lot's of discussion about the problem of high density sensors like that on the K20D compared to the old 6mp Sony sensor. Noise for instance.

Question:
Does using the 14.6mp sensor of the K20D at 6mp give you quality results, apart from resolution, comparable to or better/worse than the Sony 6mp sensor?

Thanks,
Wildman


Last edited by wildman; 05-02-2009 at 03:41 AM.
05-02-2009, 09:13 AM   #2
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1. Optical trickery, it works, don't try an understand it.

2. Sensor quality isn't so much dependent on the number of pixels more the technology emploied in the sensor. For example the 11mp full frame EOS 1DS has significantly worse noise performance the the EOS 5D 12mp full frame. Newer technology allows for beter noise control. But to answer your question with the K20D vs the 6mp sony sensor, at full resolution i have found that my K20D give far better image quality as far as noise is concerned then my *ist DS was ever capable of.
05-02-2009, 09:53 AM   #3
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1. It would likely take a lesson in physics to explain it well. Watch someone come along, though, and sum it up in one sentence. I can't.

2. The size of each pixel (on the sensor) doesn't change when switching resolutions. Therefore, the noise floor remains (essentially) constant. What you gain is a bit of sensitivity by summing the signal from adjacent pixels. I believe that cutting the resolution in half & summing would give you roughly 1 more stop of dynamic range (someone correct me if I'm wrong).
05-02-2009, 02:33 PM   #4
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1. I'll try. You're seeing the aperture through the changing optical geometry, ie as the aperture changes, so does the lens' light-bending, so it looks the same, automagically compensating. Except on the DA 18-250, where it looks the same from the back but nor from the front. Oy.

2. When my sensors are noisy, I stick in earplugs and pay no attention.

But I was going to ask, does shifting to a lower resolution - setting my K20D to 5mpx, for example - affect the diffraction limit (DL)? (Yeah, that again.) And are there any other (side)effects of down-rezzing? And is there a tech reference somewhere? But then I note that setting resolution ONLY affects JPG output - with RAWs, the final resolution can be selected in development (like PhotoLab3). That answers the DL question - DL is a function of sensor size/density, and that hasn't changed, only the way the pixel site data is put together. Does JPG down-rezzing really increase sensitivity? Is there a reference somewhere?

Since Sony was mentioned, I'll bring up a curious point. Down-rezzing the K20D does not change the crop factor. The full area of the sensor is still being used. With my old 5mpx Sony V1 and P10 PNS cams, however, the crop factor DOES change, with the SmartZoom feature, basically an in-camera crop. Down-rez from 5mpx (2592x1944) to VGA (480x640) and the V1's 4x becomes 16x, the P10's 3x becomes 12x. The effect is less extreme at the 4.5, 3.1, and 1.2mpx resolutions, of course, and it's just a trick, some pre-processing to save on PP work. Hey, it's in-camera, so it's Pure! Somewhat less of the sensor is being used. This SHOULD affect the Diffraction Limit somewhat. Always trade-offs, of course. TANSTAAFL.

05-02-2009, 02:36 PM   #5
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To answer question 1, it is just due to the fact that a longer lens will transmit less light for a given aperture than a shorter one. If you look through a constant f: zoom, you will see a change in the opening while you are zooming. It is this change in opening that necessitates larger glass for a constant f: zoom lens.
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