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Old Sep 11, 2003, 4:13 PM   #11
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Very interesting. I have a 10D (closer to a 35mm than a consumer camera) so I learned it in relation to that. On the other hand, I have never heard anyone say that that logic doesn't apply to consumer grade digicams as well.

Could you please explain why this is dependend on the sensor size and not the quality and amount of glass elements. This makes no sense to me. I thought this had to do with the physical portion of the lens being used (not that I've read or been told that, it just seemed logical.. and therefor I could easily be wrong.)

If true, though, I completely agree that it would be a great thing for digiscoping!

Eric
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Old Sep 11, 2003, 8:03 PM   #12
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"Could you please explain why this is dependend on the sensor size and not the quality and amount of glass elements. This makes no sense to me. I thought this had to do with the physical portion of the lens being used (not that I've read or been told that, it just seemed logical.. and therefor I could easily be wrong.)"
.................................................. ..

The physical portion of the lens being used is only half the story. On a 35mm and to a slightly less extent on a DSLR, the stopping down of the lens reduces optical abberation by using less of the outer areas of the optical elements that bend the light through greater angles. But there is a point of diminishing returns since with smaller apertures diffraction becomes more of a factor. This results in a "sweet spot" where you minimize optical flaws from the lens but don't suffer from diffraction problems.

A digicam with a small sensor could surely benefit optically by stopping down the iris as well. But unfortunately, diffraction becomes a problem for the small sensors at larger f-numbers. Why? Well consider that they have almost as many pixels packed onto a sensor that is much smaller than your DSLR's sensor. So they resolve more detail per square millimeter. Because they are resolving more detail, diffraction effects become apparent sooner.

So with a digicam, the observed image quality tends to be somewhat constant (at least that is what I observe on my Coolpixes) through the limited aperture range (f2.8-f10). Anything gained by stopping down tends to be offset by increased diffraction.

Read the "Pixel size, sensor size, pixel count, and image quality" box at:
http://www.normankoren.com/Tutorials/MTF7.html
for more info.
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Old Sep 11, 2003, 11:01 PM   #13
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Thanks for the info. Very interesting indeed. That first paragraph I knew, but I hadn't though about how the smaller lenses (and sensors) effected things in the consumer cameras.

I will have to read that link.

Eric
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Old Sep 12, 2003, 3:12 AM   #14
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Thanks wturber. I have had a great offer off a "friend of a friend" to go digiscoping this weekend. I dont think the guy is an experienced digiscoper but it gives me the chance to see what it's all about first hand.... A few people have now mentioned that a hide (or blinds ) have limited use when digiscoping.... What sort of ranges are we looking at for taking close, sharp bird shots ( with your equipment )?

I am familiar with fieldcraft and am aware that some bird species are elusive and extremely "switched on" to say the least, so the ranges that we are talking about must be considerable.

Regards

Simon
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Old Sep 12, 2003, 3:44 PM   #15
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Bigwigg asked,"What sort of ranges are we looking at for taking close, sharp bird shots ( with your equipment )? "
...............................
My experience is that atmosperic disturbances can start to become significant factors as soon as 75 feet. Hence my rule of closer is better. Of course it depends a lot on the quality of the light and especially the size of the bird.

I don't use anything more than a 2500mm equivalent on my rig. I know that Lin, the list moderator, seems happy to go out to as much as 6000mm equivalence. I've yet to see a full resolution image that is sharp at that level of magnification, but if one exists, I'm pretty sure that it would be taken at close range. I think 3000mm is a realistic limit for most 3-4MP cameras.

I calculated that the distance to the men in this shot was 240 meters - basing the calculation on the assumption that the man in the beige turban closest to the camera was 5'10" (average) height. You can see that the image is soft - though the faces are recognizeable.

The same rig produced this image which looks nice and sharp.
http://www.jayandwanda.com/digiscope...allard0928.jpg
As I recall, the subject to camera distance here was closer to 50 feet.

Of course, web-sized images can be deceiving. Resizing an image also makes shortcomings in the image smaller. If you have the time, look at some of the full resolution versions of these images in this folder:
http://www.jayandwanda.com/digiscope/rubinar/

All of the shots in that folder were taken with a Rubinar 1000mm f/10 lens hooked up as a telescope using a homemade eyepiece.
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Old Sep 15, 2003, 3:25 AM   #16
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Thanks wturber..... Nice images...

I didn't manage to go out over the weekend, but have been sworn that we will be photographing Red Kites this Sunday..I wont hold my breath though.

Cheers

Simon
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Old Sep 17, 2003, 7:12 AM   #17
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Just a quick observation on hides (no pun intended). Wooden floors are the kiss of death in terms of vibration....
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Old Sep 17, 2003, 7:16 AM   #18
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LOL! I would go for "natural" hides anyway....
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