Dave-Platt%LADC@CISL-SERVICE-MULTICS.ARPA (Dave Platt) (01/18/86)
Well, here's some information I've gleaned from a number of photo and astronomy magazines on this general subject... I'm working from memory, so the figures etc. will be pretty inexact. For a shot of comet-and-kids, you should probably figure on using your 50mm lens; a 70-210 zoom (the Canon f/4?) is too slow for its focal length... you'd have to use it with a motor-drive to get even marginal results (the best choice might well be something like an f/2 100mm lens... if you've got money to burn). I assume that you're going to want to do something like a tripod-mounted exposure, without any sort of sky-following platform in use. This will tend to limit your useful exposure (with a 50 mm lens) to 30 seconds or less... any more than this, and your photo will begin to show "star trails", and the image of the comet will be blurred due to the Earth's rotation. For this reason, you'll probably want to shoot the comet with your lens "wide open", to minimize the necessary exposure time... this will cost you a bit of detail, and will make it more important to set your focus properly (and will also complicate getting kids-and-comet into a single exposure). I'd suggest using an ASA 1600 slide film... slide film will work better in situations of moderate underexposure, where print film handles overexposure better. You can buy good varieties off-the-shelf at decent photo dealers (remember that you want a daylight, not a tungsten film). You might want to experiment with a roll or two of "hypered" (hypersensitized) slide film... it's a slower (ISO 200-400) film that has been treated with a gas to increase its sensitivity and decrease its reciprocity failure... check ads in Sky&Telescope or Astronomy magazines. If you don't insists on having color, then a fast black-and-white negative film might be your best bet... you'll sacrifice the hues visible to the eye, in exchange for finer grain and higher speed (and, with some films, substantially lower contrast... which could result in much more detail being visible on the prints). Another idea... one of the new "chromogenic" black&white negative films... they're based on color-film technology (standard color C-41 negative development process at any one-hour photo shop), have a very wide ISO range and excellent exposure latitude, and very fine grain. Necessary equipment: camera body, 50mm lens, reasonably solid tripod, shutter-release cable (preferably one with a release lock that will hold the shutter open until you release it manually... beats standing there with your finger on the button for 30 seconds), flash unit, black electrical tape, fast film. To shoot the comet: mount the camera firmly on a tripod (in a location shielded from the wind, if possible, and from any electrical lighting). Set the lens to the wide-open setting (f/1.8 or f/1.4, depending on which Canon lens you have), and set the exposure to "B". Frame the comet in the viewfinder; then, cover the viewfinder opening with a small piece of black electrical tape (to keep out stray light during the long exposure). Use the shutter release cable to open the shutter, count 16 seconds, and then release the shutter. Wind to the next frame, and repeat the process adding 2-3 seconds to the exposure. Continue this process until you've reached 30 second exposure; then, go back down the sequence to 16. This process will use up most of a 36-exposure roll of film, and will result in a "bracketing" that stands a good chance of catching much of the comet detail properly. Now... for getting your kids into the picture. You've suggested doing a double exposure, and that's probably a good idea. Because you're going to have to shoot the comet with your lens pretty much wide open, there's going to be a very narrow depth-of-field... and with the lens focused on infinity (for the comet), you won't be able to get a decent (unblurred) image of your kids unless they're very far away from the camera (and probably outside of the range of your flash). For decent results, you'll have to reset the lens between exposures... stop it down to f/8 or so, and refocus on your children. Set the camera and flash for a normal flash exposure... and maybe plan to underexpose a bit, just to be sure that the result of your double-exposure doesn't oversaturate your film at any point. You'll have to pose your kids in a position that will not put them, or any other solid object, in the same portion of the frame that the comet filled. The trick in doing a double exposure, of course, is recocking the shutter without advancing the film. I'm not very familiar with the AE-1 (I have an A-1 myself), and I don't know if it has a double-exposure switch (the A-1 does). If it does, read your owner's manual for instructions. If it doesn't, you can "fake it", as follows: before taking your first exposure, turn the film-rewind knob [in the normal direction] until you can feel that the film is under slight tension, and that there's no slack left. Take your first exposure. Reach down under the camera, and depress the little button on the bottom that you normally push at end-of-reel; this disengages the film-winding gears. Hold the film-rewind knob in its current position with one or two fingers, and use the film-advance lever to recock the shutter; since the gears are disengaged, the film should not move. You should find that the gear-disengage button pops back up when the shutter has been fully cocked. Now you can take your second exposure. When you use the film-advance lever after the second exposure, the film should advance normally. This may sound as if you're going to be do a lot of switching back and forth between exposures. Yup... you're going to be busy. There is an alternate solution, with its own set of tricks... run the film through the camera twice. To do this, you'd mark the film's position carefully when you first put it in the camera (maybe place a small pencil mark at the edge of the film, lined up with the end of one of the film guides). Then, wind the film up to the first frame, and do your first set of photographs. You might, for example, want to do a complete roll of flash photos of your kids standing in the appropriate positions. When you reach the end of the roll, rewind the film... but ONLY until you have backed up past exposure 1 and feel the end of the film come out of the take-up spool... do NOT wind the end of the film back into the cassette! Now, open the back of the camera, and thread the film back into the take-up spindle... and line up the mark(s) that you made during the first loading with the film guides (or whatever reference points that you used). Close the back, and wind the film to the first frame. With a bit of luck, the film is now positioned very closely to where it was during your first set of exposures... so you could now run through the entire roll, taking photos of the comet, and have the comet images "overlay" the first exposures and (we hope) fall right into the areas of the frames that you reserved for the comet. [The risk with this technique, of course, is that if you don't get the film lined up properly, or if it slips, then the two sets of exposures will not be lined up properly, and the results will be quite confusing]. This whole process is going to be very much a "trial and error" proposition... if you're lucky, you'll get two or three good slides out of an entire 36-exposure roll (I, and just about every amateur or professional photobug I've ever spoken to, find that three-in-a-roll is about average even when shooting in daytime under good conditions). I STRONGLY recommend that you perform several nights' worth of experiments BEFORE you go over to Hawaii... try different types of film, different exposures, different camera techniques, and *KEEP*NOTES*!! This will cut your risk of yngvi'ing up an unrepeatable opportunity with a simple miscalculation or oversight... it'll be cheap insurance. If you want to go whole-hog, you might consider setting up some kind of star-tracking mount or platform. This would let you take a longer exposure, with the lens stopped down to f/4 or so; you'll get more detail without any "star tracks", and you could also use your 70-210 zoom (at about 100mm) [or your 50mm lens and 2x extender] to get a "closer" shot of the comet. Possible mountings include: a "piggyback" mounting on a standard equatorial telescope base with motor drive; a "barn door" hinged mounting, or a special rotating platform (the inventor's name escapes me at the moment... I think it starts with "P".). Check the last six months' issues of Astronomy and Sky&Telescope... I've seen at least two construction articles for the barn-door and rotating- platform devices... and the barn-door mount can be made to operate manually, rather than with a motor... relatively cheap & easy, by the look of it.
ems@amdahl.UUCP (ems) (01/20/86)
In article <8601180216.AA23509@s1-b.arpa>, Dave-Platt%LADC@CISL-SERVICE-MULTICS.ARPA (Dave Platt) writes: > Well, here's some information I've gleaned from a number of photo and > astronomy magazines on this general subject... I'm working from memory, > so the figures etc. will be pretty inexact. ... > I'd suggest using an ASA 1600 slide film... slide film will work better A few years ago at school, some of us were doing a project involving comet photography. The standard film used was high speed Ektachrome pushed to about 6400. Does anyone still do this? The extra speed of a high ASA/ISO film should be a good trade off of grain vs. blur from no motorized mount. ... > To shoot the comet: mount the camera firmly on a tripod (in a location > shielded from the wind, if possible, and from any electrical lighting). > Set the lens to the wide-open setting (f/1.8 or f/1.4, depending on > which Canon lens you have), and set the exposure to "B". You might also look up a friend with a f/1.2 lens. I have one. It does an OK job. Not the sharpest thing in the world, but blur and grain will be bigger factors in this scenario... > comet in the viewfinder; then, cover the viewfinder opening with a > small piece of black electrical tape (to keep out stray light during > the long exposure). Use the shutter release cable to open the shutter, Or buy a viewfinder cover. My cannon has grooves on each side of the viewfinder to accept one. They are cheap and nice to have. -- E. Michael Smith ...!{hplabs,ihnp4,amd,nsc}!amdahl!ems This is the obligatory disclaimer of everything.