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Phenomneek posted:Aren't these magnets created by cooling a superconductor with liquid helium, making it impossible to actually turn them off without venting the helium? You can shut them off by turning off the power but doing that makes the field degrade slowly over time, most of the MRI magnets are also variable strength, sometimes down to a couple of dozen milligauss. Since the cores are usually held inside a glass vacuum bottle, you don't really want to vent the helium until the modality is down for repair. That's one of the routine maintenance tasks, depressurizing the bottle, recovering the helium, and flushing the system. Then you repack, revac, and you're super conducting again. Ok, fun stories. At the training center where I attended classes for this stuff, there were several rooms given over to MRI abuse. The idea was that nurses and rad techs beat the hell out of the machines, so it was good to see machines that weren't perfect. After work though, those things became like amusement park rides. The right kind of dart board can be hung in the center of a 1.5T bore or stronger. You can throw darts at that thing and not even be close but you'll still hit the board. You can throw darts at about a 90 degree angle, and as long as the tip of the dart intersects the field, it will get sucked right in to the center. When we got the 7T magnet in, the first thing the Modality Director did was spray paint some lines on the outside of the building that showed the edges of the room inside. Since the 7T magnets are tiny and on wheels (usually used for small extremities like wrists, ankles, etc.) we could truck it back and forth. So, stick a trash can to the wall, and watch it slide between the yellow lines.. by mAAaAAaaAAgic! I told you we stuck a van to a wall, well once we figured out that little trick, we stuck the magnet on a lift truck (funny story there, we couldn't get it unstuck till we PM'd the magnet) and got the van a couple of inches up off the tires. Lesse... did you know, several 3T magnets back to back turn in to a pretty effective railgun if you throw metal marbles through the bore?
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Jun 3, 2010 03:33
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HPL posted:It's actually kind of mesmerizing when they squash. It's sort of like when you pour pancake batter onto the griddle. Thanks for ruining boobs for me for a while.
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Jun 3, 2010 03:50
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Pompous Rhombus posted:Thanks for ruining boobs for me for a while. Yeah, but pancakes are now awesome.
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Jun 3, 2010 04:07
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spog posted:Yeah, but pancakes are Fixed that for you.
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Jun 3, 2010 04:34
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squidflakes posted:You can shut them off by turning off the power but doing that makes the field degrade slowly over time, most of the MRI magnets are also variable strength, sometimes down to a couple of dozen milligauss. Since the cores are usually held inside a glass vacuum bottle, you don't really want to vent the helium until the modality is down for repair. That's one of the routine maintenance tasks, depressurizing the bottle, recovering the helium, and flushing the system. Then you repack, revac, and you're super conducting again. Why didn't you record any of this??? -- How accurate is the software calibration that Mac OS X includes? I understand it's going to be off a bit since you're judging with your own eyes, but should it be a pretty close ballpark? Waiting to get a replacement Huey Pro at work, so hoping this will get me close in the meantime. scottch fucked around with this message at 15:05 on Jun 3, 2010 |
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Jun 3, 2010 12:28
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scottch posted:Why didn't you record any of this??? Did you read the part where it rips out piercings and fillings? I can't imagine that you would be able to get a video camera in very close, let alone without serious image distortion.
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Jun 3, 2010 21:13
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Ok two questions. If you focus on something with a telephoto lense, and then zoom in or out without physically moving, do you need to refocus? Does DOF change in relation to distance? Like, if I focused in on something at f1.8 at a foot away the DOF would be razor thin, but if I focus on something 20ft away the DOF expands, right? Sorry if these are super-noobish questions, but they've been bothering me haha.
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Jun 3, 2010 23:00
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Covert Ops Wizard posted:Ok two questions. 1) Your terminology is a little confused. A "telephoto" lens is actually just a specific optical design that lots of different kinds of lens use. Most people use it to mean a "long focal length lens", though. A lens that can "zoom" in or out, ie. change its magnification, could either be a "variable-focal-length lens" or a "zoom lens". The difference is that a true zoom lens doesn't need to be refocused, while a VFL lens does. Nearly all lenses with variable focal length today are zooms. So to answer that question: 99% of the time no, you don't need to refocus. Weird older lenses you might have to, though, and you could always bump the lens slightly out of focus while zooming. 2) Yes, the depth of field is a function of the subject distance, and works just the way you've described. You can think of it as always being a ratio of the subject distance -- so if you have a depth of field of 1 foot at 20 feet away you might have a DoF of 10 feet when focused at 200 feet. Doesn't quite work that way but it's near enough to get the idea.
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Jun 3, 2010 23:17
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A parfocal lens retains its focus when zooming in or out. Almost all lenses today are parfocal. Edit: Actually, I take that last part back. There are still varifocal (non parfocal) lenses made. TheAngryDrunk fucked around with this message at 23:36 on Jun 3, 2010 |
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Jun 3, 2010 23:27
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orange lime posted:Did you read the part where it rips out piercings and fillings? I can't imagine that you would be able to get a video camera in very close, let alone without serious image distortion. That's a very good point. I just want to see a magnet that powerful doing ridiculous things. I suspect I am not alone.
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Jun 4, 2010 01:48
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orange lime posted:Did you read the part where it rips out piercings and fillings? I can't imagine that you would be able to get a video camera in very close, let alone without serious image distortion. As part of my original safety orientation, yes. This is the reason modality techs aren't allowed to have piercings or impacted metal wounds i.e. shrapnel or gunshots. This is also how I found out I still had metal fragments in my leg from the time I was shot. This is also why I was put on PACS (which is the computer system that controls all of this stuff) and PET, which is so awesome as to be called wicked awesome. As for distorting video, this really would only be possible if the camera were nearly in the bore, and even then it would be because of the magnetic field effecting the read/write heads on the media or bit positions in the solid state devices. Unless the field were spinning, then it might grab the focus ring and screw with it. I don't think we can make medical magnets so strong they bend light.
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Jun 4, 2010 07:22
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squidflakes posted:Lesse... did you know, several 3T magnets back to back turn in to a pretty effective railgun if you throw metal marbles through the bore? Perpetuum mobile? I don't see why the marbles would gain net energy here.
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Jun 4, 2010 07:31
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orange lime posted:1) Your terminology is a little confused. A "telephoto" lens is actually just a specific optical design that lots of different kinds of lens use. Most people use it to mean a "long focal length lens", though. A lens that can "zoom" in or out, ie. change its magnification, could either be a "variable-focal-length lens" or a "zoom lens". The difference is that a true zoom lens doesn't need to be refocused, while a VFL lens does. Nearly all lenses with variable focal length today are zooms. Alright, thanks!
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Jun 4, 2010 08:58
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scottch posted:That's a very good point. I just want to see a magnet that powerful doing ridiculous things. I suspect I am not alone. There's this cool thing called YouTube on the Interwebs... http://www.youtube.com/watch?v=_lBxYtkh4ts http://www.youtube.com/watch?v=4uzJPpC4Wuk http://www.youtube.com/watch?v=_pkRqbxDTcw Back to the parfocal/zoom vs VFL design. How does one know which they are buying?
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Jun 4, 2010 13:42
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KennyG posted:There's this cool thing called YouTube on the Interwebs... http://www.youtube.com/watch?v=fxC-AEC0ROk
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Jun 4, 2010 15:32
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none of those are nearly as cool as what squidflakes was describing
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Jun 4, 2010 15:45
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KennyG posted:There's this cool thing called YouTube on the Interwebs... If it was made in the last 20 years for an SLR, it's a zoom. Another good indication is if the lens says "zoom" on it. It's next to impossible to get a plain old vari-focal lens any more unless you're looking at niche applications like CCTV lenses.
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Jun 5, 2010 09:53
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I somehow got specks of dust not on the sensor, but on the mirror, the part that flips up. Whats the best way to get rid of this? Its really noticeable when looking through the viewfinder and its annoying the piss out of me.
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Jun 5, 2010 16:59
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Saint Celestine posted:I somehow got specks of dust not on the sensor, but on the mirror, the part that flips up. Whats the best way to get rid of this? Its really noticeable when looking through the viewfinder and its annoying the piss out of me. You should own one of these: http://www.the-digital-picture.com/Reviews/Giottos-Rocket-Air-Blower-Review.aspx
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Jun 5, 2010 17:07
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What if I have a spot that won't seem to come off? I thought it was a hair, but no matter how much I use the blower I can't get it off. I'm having to clone it out of all of my photos. I'm afraid to touch the sensor!
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Jun 8, 2010 18:59
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Zapf Dingbat posted:What if I have a spot that won't seem to come off? I thought it was a hair, but no matter how much I use the blower I can't get it off. I'm having to clone it out of all of my photos. I'm afraid to touch the sensor! Then your choices are 1) send it to the manufacturer for cleaning ($50-$150, plus 3 weeks without the camera, plus cameras often come back with new dust spots) 2) maybe get a better deal on cleaning at a *good* local camera shop (but they are likely to just use an air blower and charge you $30) 2) get over the fear of touching the inside of your camera and do it yourself. You're not actually touching the sensor. The surface that you clean off is a piece of hard optical glass with some anti-reflective coatings on it; the sensor is mounted below that, with an air gap in between.
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Jun 8, 2010 21:46
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Well, I guess cleaning the sensor myself doesn't sound so bad. What should I use? It's crazy, the only camera shop in town does exactly what you said: $30 for an air pump.
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Jun 9, 2010 02:00
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Zapf Dingbat posted:Well, I guess cleaning the sensor myself doesn't sound so bad. What should I use? Giottos Rocket Blower. Most camera stores should carry them. Also handy for de-dusting negatives and film holders, if you shoot film.
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Jun 9, 2010 02:01
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Pompous Rhombus posted:Giottos Rocket Blower. Most camera stores should carry them. Also handy for de-dusting negatives and film holders, if you shoot film. Yes, but what if I have to touch it? What should I use then?
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Jun 9, 2010 02:13
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http://www.photosol.com/eclipseproduct.htm i use this fluid + their swabs, but I also use a pec-pad to replace the swab heads (which they don't recommend)
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Jun 9, 2010 02:55
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I bought a cheap 3 euro nylon artist brush, washed it 10x in dishwasher soap, dried it with compressed air, tested it by swabbing a filter 50 or so times and when it didn't leave any residue, just brushed off my 5D sensor with that. Worked like a charm, and 1/30 the price of an arctic butterfly.
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Jun 9, 2010 11:47
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poopinmymouth posted:I bought a cheap 3 euro nylon artist brush, washed it 10x in dishwasher soap, dried it with compressed air, tested it by swabbing a filter 50 or so times and when it didn't leave any residue, just brushed off my 5D sensor with that. Worked like a charm, and 1/30 the price of an arctic butterfly. There are also sensor brushes that aren't anything more than this. They just come sealed in a pack. The only time you really need liquid is for more than hair, or if the hair has left oil. If you have oil on the glass covering the sensor, then you need a cleaning solution.
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Jun 9, 2010 13:17
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poopinmymouth posted:I bought a cheap 3 euro nylon artist brush, washed it 10x in dishwasher soap, dried it with compressed air, tested it by swabbing a filter 50 or so times and when it didn't leave any residue, just brushed off my 5D sensor with that. Worked like a charm, and 1/30 the price of an arctic butterfly. This is what I wanted to hear. I'm such a cheap bastard.
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Jun 9, 2010 14:53
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Zapf Dingbat posted:This is what I wanted to hear. I'm such a cheap bastard. Just be absolutely sure it leaves nothing on the filter. Clean the filter beforehand so itīs perfectly smooth when looking at reflective light. If the brush leaves anything on it, residue, streaks, wetness, you did it wrong. Wash again the way painters clean their brushes (put soap in palm, and rotate the brush around in a spiral, working the soap up into the head) and be sure to rinse it incredibly thoroughly. Then do not let it touch anything at all, no tissues, towels, or otherwise, and use a compressed air like for a shop, not a can.
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Jun 9, 2010 15:44
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Zapf Dingbat posted:This is what I wanted to hear. I'm such a cheap bastard. http://www.amazon.com/Lenspen-LENSK1-Sensorclear-Dslr-Cleaner/dp/B000I30HOS/ref=sr_1_1?ie=UTF8&s=electronics&qid=1276102669&sr=8-1 Its like $10 and it works perfectly. Cleans lenses too. Batcat! Batcat! fucked around with this message at 18:02 on Jun 9, 2010 |
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Jun 9, 2010 17:59
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Im having trouble getting flashes to work on either of my cameras. Ive tried a National PE-280C, a Hanimex TS855 and a Hanimex TZ325 on both a Pentax K1000 and a Fujica STX-1. The only one that will actually flash is the national and then it takes about 3 minutes to charge. All of the flashes power up as you can hear them, but as for getting them to flash I haven't got a clue. I strongly suspect I'm just doing something stupidly wrong but I thought it might be worth asking here if anyone knows what my problem is?
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Jun 9, 2010 18:01
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poopinmymouth posted:and use a compressed air like for a shop, not a can. I thought it was the opposite? Because air from a compressor has a tiny bit of oil in it?
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Jun 9, 2010 18:10
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Zapf Dingbat posted:This is what I wanted to hear. I'm such a cheap bastard. The cheapasses way out is to never stop down past 2.8.
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Jun 9, 2010 18:38
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Could anybody recommend an .rss feed I could tap into to keep up with dSLR tech news (lenses, bodies, technologies, etc.)?
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Jun 11, 2010 11:19
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Stregone posted:I thought it was the opposite? Because air from a compressor has a tiny bit of oil in it? Among other things like dirt and water.
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Jun 11, 2010 15:40
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Obama 2012 posted:Could anybody recommend an .rss feed I could tap into to keep up with dSLR tech news (lenses, bodies, technologies, etc.)? dpreview has an RSS. http://www.dpreview.com/feeds/
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Jun 11, 2010 18:49
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Compressed air in a can is usually 134a, the stuff used in air conditioners. The downside to them is unless the aerosol can uses one of those fancy valves that allow you to invert the can while spraying, you run the risk of spraying out some liquid 134a along with the gas when you depress the actuator. This is bad because the liquid evaporates extremely rapidly, which can then cause condensation to form. This normally happens with full cans, or cans that are not held perfectly level while spraying, so you would be better off using a half-empty can.
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Jun 12, 2010 05:28
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I don't know if this thread is the place to ask, but i couldn't figure out a better place. My camera is hosed up and i'm curious about a few things. I took my Canon 5d to the wedding of my girlfriend's brother, thinking i could finally get some experience with weddings. I was in the middle of a shot, when i heard a loud snap out of loving nowhere. It appears something came disconnected. I have no idea what i'm really talking about, but it's a small, square piece of glass with an equal section of black rubber-like material that comes out with it. The strange thing is, it still took a photo after this, though not a normal one. I've been afraid to even touch it since then. It's probably close to 3 years old, and i've taken relatively good care of it. I've never had any problems apart from a time where i hosed up a lens. So, despite how poorly i've described it, does anyone know what i'm talking about and can tell me what is wrong with my camera? What kind of repair cost am i looking at, and where should i take it? I'm not in a big area, so there aren't many camera shops around.
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Jun 14, 2010 16:57
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You can't find where it came from?
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Jun 14, 2010 17:00
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Nausea posted:I don't know if this thread is the place to ask, but i couldn't figure out a better place. My camera is hosed up and i'm curious about a few things. Did the mirror fall out?
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Jun 14, 2010 17:03
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