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Let's Photograph Comet 46P Wirtanen
Paul Schlyter:
If the numerator is always 20, what does this 20 mean? Is 20/20 in some way different from, say, 25/25 or.30/30? 20/20, or metric 6/6 = 1. It's explained at https://en.wikipedia.org/wiki/Visual_acuity#Measurement. 20/25 for a person like me with macular degeneration is considered to be very good. Again, that's my left eye. I don't have a number for my right eye, but subjectively, it is not very useful at the moment. We'll "see" what the new eyeglasses bring in a couple of weeks. -- I agree with almost everything that you have said and almost everything that you will say in your entire life. usenet *at* davidillig dawt cawm |
#32
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Let's Photograph Comet 46P Wirtanen
On Mon, 24 Dec 2018 09:26:32 -0500, Davoud wrote:
20/20, or metric 6/6 = 1. It's explained at https://en.wikipedia.org/wiki/Visual_acuity#Measurement. Ok, so it measures the visual acuity at a distance of 20 feet. But wouldn't a measure of (practically) infinite distance be more interesting to skywatchers? No stars are only 20 feet away. |
#33
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Let's Photograph Comet 46P Wirtanen
On Mon, 24 Dec 2018 22:33:39 +0100, Paul Schlyter
wrote: On Mon, 24 Dec 2018 09:26:32 -0500, Davoud wrote: 20/20, or metric 6/6 = 1. It's explained at https://en.wikipedia.org/wiki/Visual_acuity#Measurement. Ok, so it measures the visual acuity at a distance of 20 feet. But wouldn't a measure of (practically) infinite distance be more interesting to skywatchers? No stars are only 20 feet away. The distance is largely irrelevant. It's measuring resolution... the ability to detect close high contrast features as separate from one another. The distance of 20 feet is simply a standard so that the test chart is always the same, and it's a distance that is practical in a typical testing situation. Optically, 20 feet is pretty much the same as infinity for the human eye in terms of accommodation. Modern refraction techniques project patterns on the back of the eye and directly assess accommodation, focus, and astigmatism. But this is usually translated to the 20:X notation for simplicity. |
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Let's Photograph Comet 46P Wirtanen
On Sun, 23 Dec 2018 18:44:35 -0600, hleopold
wrote: Fast moving indeed. As I mentioned earlier, few things move fast in the sky, other than meteors which move very fast. But this was surprising to me even knowing that it is fast. Thanks for that. How far from the Earth was it at the time you took those shots? At the imaging time of 17 Dec 2018 UT 10:00, the distance was 0.078 AU, 11.7 million km, 30.3 lunar distances. That was just about its closest approach to Earth. |
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Let's Photograph Comet 46P Wirtanen
On Dec 23, 2018, palsing wrote
(in ): On Sunday, December 23, 2018 at 4:38:10 PM UTC-8, hleopold wrote: “I have an inferiority complex, but it's not a very good one.“ - Vintagewheels 'When my kleptomania gets bad, I take something for it" - Ken Dodd Love it, and stolen. I have grabbed some good stuff from around the web for a couple of decades, lost a bunch about a year and a half ago with the only hard drive crash I have ever had, on a nearly 20 year old drive. I had 3 other drives on my computer, but I had the bad habit of not moving to a newer drive for running my newsgroup reader AND my OS, so I lost 20 years of mail as well. Now that really hurt. -- Harry F. Leopold The Prints of Darkness (remove gene to email) “It would be a horrible embarrassment to be a a flesh-and-blood human and still be unable to pass a Turing test.“-ErikF |
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Let's Photograph Comet 46P Wirtanen
On Dec 24, 2018, Paul Schlyter wrote
(in t): On Sun, 23 Dec 2018 18:38:19 -0600, wrote: On Dec 23, 2018, Paul Schlyter wrote (in et): On Sat, 22 Dec 2018 20:09:17 -0500, wrote: My corrected vision in my left eye is 20/25, i.e., very good. That's the same as 0.8 right? And full visual acuity would then be 25/25? Or 100/100 etc if you prefer that. Or just 1.0 20/20 is, as I understand it, perfect vision. Mine is 20/400 in the right eye uncorrected, corrected to 20/15. left is somewhat worse, but corrected to 20/15 also. Or it used to be, these days I am probably more 20/20 or slightly worse. Plus they have to correct for the bad astigmatism I have in both eyes. If the numerator is always 20, what does this 20 mean? Is 20/20 in some way different from, say, 25/25 or.30/30? As I understand it, you should be able to see at 20 feet as the average person would see at 20 feet. (An average person with good eyesight of course.) If you are shown to have a 20/30 in your right eye it means that you see at 20 feet is like that “average” person would see at 30. In other words, your eyesight is not that great. My right eye, without glasses means that at 20 feet I see about as well as the “average” person would see at 400 feet. In other words, pretty poorly, than you add in the majorly bad astigmatism. Fortunately my eyesight is pretty much correctable. to slightly better than average. I can, or could, see as well at 20 feet as that “average” person can see at 15 feet. Of course using glasses may correct your eyesight to “perfect” but there are drawbacks, especially as you get older and need bifocals, which I have used for about 12 years, my latest pair of glasses are line-less bifocals. They are pretty good, but you have to actually use head movement to get the right area to look through. Parallel lines are no longer so parallel and they change as you move your head side to side or up and down. Plus things swell up/shrink down as they move across the lens. (At least now I can actually see the speedometer while driving, unlike when I used the old type of bifocals, with those I had to move my head to actually read it, now I can keep my eyes on the road as still read the speed.) Oh yeah, and you can have either near-sightedness or far-sightedness. Mine at age 12 was “perfect,” at age 12 and a half went near-sighted and kept going that way. These days I am getting slightly better as my eyes age and they attempt to become somewhat far-sighted, which in my case means that I become very slightly less near-sighted. And then of course there are lots of other problems you can have with your eyes, one I have to keep track of is cataracts, I have them in both eyes, but very mild so far and have been that way for at least a dozen years. -- Harry F. Leopold aa #2076 AA/Vet #4 The Prints of Darkness (remove gene to email) Greater love hath no man than this, that he lay down his self-respect for his online enemies. - Teresita |
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Let's Photograph Comet 46P Wirtanen
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#38
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Let's Photograph Comet 46P Wirtanen
On Dec 24, 2018, Paul Schlyter wrote
(in et): On Mon, 24 Dec 2018 09:26:32 -0500, wrote: 20/20, or metric 6/6 = 1. It's explained at https://en.wikipedia.org/wiki/Visual_acuity#Measurement. Ok, so it measures the visual acuity at a distance of 20 feet. But wouldn't a measure of (practically) infinite distance be more interesting to skywatchers? No stars are only 20 feet away. It is always nice to be able to actually FIND your telescope so you can look at the sky. ;-0 -- Harry F. Leopold The Prints of Darkness (remove gene to email) “Damn. Someone's been ****ing in my genepool again.“-Kermit |
#39
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Let's Photograph Comet 46P Wirtanen
On Dec 24, 2018, Paul Schlyter wrote
(in et): On Sun, 23 Dec 2018 18:30:49 -0600, wrote: Way back in the late 70s early 80s I did, for a while, try photography with my scopes, but otherwise I have been strictly a visual observer. I cant believe the great stuff that we have these days. I am not really a fan of digital photography in many ways, I loved shooting film, and that was how I did astro photos back then. You can still shoot film if that's what you enjoy to do. The camera stores near me hardly carry any film anymore, except for 4x5 or 8x10. And I have gotten out of the habits I used to have. I love my smart phone for its ability to grab shots that pop up unexpectedly. I also have a very nice Leica digital camera (about 17 years old now. But I can never find the charger for it half the time. My first camera was picked up in Hong Kong when my ship stopped there in 1972. I picked up a fair amount of film, and the battery died two days after we pulled out. This limited me to either .125 of a second, or Bulb. No light meter either. I picked up a couple of extra batteries and a light-meter as soon as we hit Subic Bay a few weeks later. That light meter was the second best buy I ever made. What I really loved about my favorite film cameras was that they were grab and shoot. I picked up in San Francisco’s Wolfes Cameras a couple of old Leicas, one a red-dial 3F (I think) and a black-dial somewhat earlier model. One was just the body, the other had the usual collapsable 50mm. I ended up putting that lens on an old Canon B-2 I picked up as a much cheaper alternative and found a couple of lovely Nicon screw-mount lens, one a 85mm and one a 38mm. One was F 1.2 and I think the other was F 1.1 (Thank you for holding those for me until I got back to shore, Ciddy). We are talking about some big, heavy glass here. The 85mm was literally the size and shape of a US hand grenade and just about as heavy. But it took beautiful photos. A big, heavy, polished chrome hand grenade with a huge glass center. I kept the Leica collapsable 50mm on the Canon B-2 as my “I am only carrying one camera” "And no, I am not a camera geek” camera. “See, I can practically put it in my pocket." Running around on a ship at sea you don’t want to carry your favorite that might get damaged slamming into a round-down or soaked in seawater. (Or strapped to the front of a bike or motorcycle, which I did a lot riding through the hills of California.) I find few cameras these days that do what I like to do, night-photography, or as I used to call it, Available Darkness. This was nothing like those wimps who did available light. ;-) That is where The Prints of Darkness in my sig comes from, partly. I have been a printer for decades, mostly as a night worker, I am a night person to begin with, I love astronomy. And I printed up a lot of available darkness photos I took. Some even turned out pretty good. Just about 18 years ago I found myself seriously thinking to buy a brand new Voitlander (spelling?) A near copy of my old 3F. I had to step back and really think about that and when I did I sadly passed on it. Lovely, but even then I could see what was happening to film and I just could not justify it. So a few years later I bought the digital Leica, if I had only waited a few more years... -- Harry F. Leopold The Prints of Darkness (remove gene to email) “(B)iological evolution is a team sport.“-Louis Friend |
#40
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Let's Photograph Comet 46P Wirtanen
On Dec 24, 2018, Chris L Peterson wrote
(in ): On Sun, 23 Dec 2018 18:44:35 -0600, wrote: Fast moving indeed. As I mentioned earlier, few things move fast in the sky, other than meteors which move very fast. But this was surprising to me even knowing that it is fast. Thanks for that. How far from the Earth was it at the time you took those shots? At the imaging time of 17 Dec 2018 UT 10:00, the distance was 0.078 AU, 11.7 million km, 30.3 lunar distances. That was just about its closest approach to Earth. That was about what I thought from what I had previously read. Yeah, that was moving pretty good. -- Harry F. Leopold The Prints of Darkness (remove gene to email) "We used to be afraid of comets. Now it's their turn." The Register |
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