FROM: ted andresen SUBJECT: Effect of air currents on image clarity & resolution? DATE: Sat, 25 Sep 1999 21:18:15 -0400 ORGANIZATION: Suncoast Free-Net NEWSGROUPS: sci.optics,rec.photo.technique.nature,rec.photo.equipment.35mm I've been experimenting to determine the impact of thermals or air currents on image clarity and resolution. To see the effect I used a laser pointer to determine how much movement a light beam would experience as it traveled over the distance that I shoot. Normally, this is about 250' (75 m) with 2500 mm. When I did the measurement I found that the laser beam moved about 0.2" (5 mm) when the light beam traveled a distance of 240' (73 m). That worked out to an angular variation of 69 microrads or 14 arcsecs. That kills any hope of getting an image with the 5 arcseconds of resolution that I would like. It seems to me that the main reasons for poor clarity and resolution when shooting with high focal lengths (greater than 500 mm) is camera shake due to subsystem shock (mirror flip up and shutter curtain stops) and thermal currents. Has anyone else come to the same conclusion? Ted Andresen St. Petersburg, FL 33703-1721, USA Floating habitat homepage: http://members.aol.com/Tjacmc/ FROM: zanekurz@sansnetcom.com (Zane) SUBJECT: Re: Effect of air currents on image clarity & resolution? DATE: Sun, 26 Sep 1999 02:23:08 GMT ORGANIZATION: Netcom NEWSGROUPS: sci.optics,rec.photo.technique.nature,rec.photo.equipment.35mm ted andresen wrote: >I've been experimenting to determine the impact of thermals or air >currents on image clarity and resolution. To see the effect I used a >laser pointer to determine how much movement a light beam would experience >as it traveled over the distance that I shoot. Normally, this is about >250' (75 m) with 2500 mm. > > >When I did the measurement I found that the laser beam moved about 0.2" (5 >mm) when the light beam traveled a distance of 240' (73 m). That worked >out to an angular variation of 69 microrads or 14 arcsecs. That kills any >hope of getting an image with the 5 arcseconds of resolution that I would >like. I've made resolution measurements over horizontal paths under a variety of air turbulence conditions and never seen it that bad. I think you may have some type of experimental error there, or some unusually bad conditions. Here's a good way to test the atmosphere (and your lens capabilities) and separate it from the camera movement: Put a resolution chart at your 75 m place. Illuminate it with a slaved flash which is close to the chart. Synch your camera to a local flash and use it to trigger the remote (at the chart) flash. The flash should freeze most, if not all, of the camera movement and vibration. Use a fresnel lens in front of the local flash if you have to. Use very high resolution film and a shutter speed that will make the ambient light exposure negligible. A slow ISO rating with a strong flash will help suppress the ambient light. (You can even go faster than your camera synch speed if you put the chart in the right part of the frame.) Bracket the exposure and focus. Shooting several f/no's may be interesting. Examine the negatives (or slides) with a magnifier or microscope to determine the resolution on the film. >It seems to me that the main reasons for poor clarity and resolution when >shooting with high focal lengths (greater than 500 mm) is camera shake >due to subsystem shock (mirror flip up and shutter curtain stops) and >thermal currents. > >Has anyone else come to the same conclusion? That's been my experience also. Even with mirror lockup, the camera will move some when the shutter curtain starts to move, and usually will continue to move for the whole exposure time. Anything you can do to stabilize it will help. If the ground is cool (e.g. during cloud cover), the atmosphere will sometimes give you pretty good resolution over only a 75 m range. Good luck Zane FROM: philfflash@my-deja.com SUBJECT: Re: Effect of air currents on image clarity & resolution? DATE: Sun, 26 Sep 1999 03:23:08 GMT ORGANIZATION: Deja.com - Before you buy. NEWSGROUPS: sci.optics,rec.photo.technique.nature,rec.photo.equipment.35mm This is why astronomical photogs prefer to shoot in the cold months. BTW an astronomical photog is one who shoots the sky through a telescope NOT one who weighs 700 pounds. -- An iota of wisdom from Ol' Phil Accept NO substitutes The BEST is none too good! FROM: "W Scott Elliot" SUBJECT: Re: Effect of air currents on image clarity & resolution? DATE: Sat, 25 Sep 1999 21:34:03 -0700 ORGANIZATION: via Internet Direct - http://www.mydirect.com/ NEWSGROUPS: sci.optics,rec.photo.technique.nature,rec.photo.equipment.35mm philfflash@my-deja.com wrote in message <7sk3il$sqa$1@nnrp1.deja.com>... >BTW an astronomical photog is one who shoots the sky through a >telescope NOT one who weighs 700 pounds. Would that be a gastronomical photog? FROM: Acer Victoria SUBJECT: Re: Effect of air currents on image clarity & resolution? DATE: Sat, 25 Sep 1999 22:44:51 -0700 ORGANIZATION: University of California, Riverside NEWSGROUPS: sci.optics,rec.photo.technique.nature,rec.photo.equipment.35mm On Sat, 25 Sep 1999, W Scott Elliot wrote: :philfflash@my-deja.com wrote in message <7sk3il$sqa$1@nnrp1.deja.com>... :>BTW an astronomical photog is one who shoots the sky through a :>telescope NOT one who weighs 700 pounds. : :Would that be a gastronomical photog? Wouldn't that help stabilise (inertia, LOTS of it) the rig? /Acer "steppenwolf" Victoria -- at spes non fracta FROM: w.j.markerink@a1.nl (Willem-Jan Markerink) SUBJECT: Re: Effect of air currents on image clarity & resolution? DATE: Sun, 26 Sep 99 08:00:09 GMT ORGANIZATION: A1 Internet news-server NEWSGROUPS: sci.optics,rec.photo.technique.nature,rec.photo.equipment.35mm In article <37ed7d04.441049@nntp.ix.netcom.com>, zanekurz@sansnetcom.com (Zane) wrote: >ted andresen wrote: > >>I've been experimenting to determine the impact of thermals or air >>currents on image clarity and resolution. To see the effect I used a >>laser pointer to determine how much movement a light beam would experience >>as it traveled over the distance that I shoot. Normally, this is about >>250' (75 m) with 2500 mm. >> >> >>When I did the measurement I found that the laser beam moved about 0.2" (5 >>mm) when the light beam traveled a distance of 240' (73 m). That worked >>out to an angular variation of 69 microrads or 14 arcsecs. That kills any >>hope of getting an image with the 5 arcseconds of resolution that I would >>like. > >I've made resolution measurements over horizontal paths under a variety of >air turbulence conditions and never seen it that bad. I think you may have >some type of experimental error there, or some unusually bad conditions. > >Here's a good way to test the atmosphere (and your lens capabilities) and >separate it from the camera movement: Put a resolution chart at your 75 m >place. Illuminate it with a slaved flash which is close to the chart. >Synch your camera to a local flash and use it to trigger the remote (at the >chart) flash. The flash should freeze most, if not all, of the camera >movement and vibration. Use a fresnel lens in front of the local flash if >you have to. Use very high resolution film and a shutter speed that will >make the ambient light exposure negligible. A slow ISO rating with a >strong flash will help suppress the ambient light. (You can even go faster >than your camera synch speed if you put the chart in the right part of the >frame.) Bracket the exposure and focus. Shooting several f/no's may be >interesting. Examine the negatives (or slides) with a magnifier or >microscope to determine the resolution on the film. For practical reasons you might want to do this at night. Simply walk over to the target, and fire the flash manually (or have an assistant fire it at command). This will avoid the practical problem of flash syncronisation at such distances (don't think more than 10m is very reliable, definately not in daylight). And it will also rule out any shake of camera+lens. Don't think you have won with mirror lockup....the shutter itself is the next vibrational source down the line (Bob Atkins from rec.photo.* cq photonet has done some interesting testing....the shutter could clearly be identified on the scope). > >>It seems to me that the main reasons for poor clarity and resolution when >>shooting with high focal lengths (greater than 500 mm) is camera shake >>due to subsystem shock (mirror flip up and shutter curtain stops) and >>thermal currents. >> >>Has anyone else come to the same conclusion? > >That's been my experience also. Even with mirror lockup, the camera will >move some when the shutter curtain starts to move, and usually will >continue to move for the whole exposure time. Anything you can do to >stabilize it will help. But nothing will be as perfect as bulb-mode and a flash at night....:-)) >If the ground is cool (e.g. during cloud cover), >the atmosphere will sometimes give you pretty good resolution over only a >75 m range. -- Bye, Willem-Jan Markerink The desire to understand is sometimes far less intelligent than the inability to understand [note: 'a-one' & 'en-el'!] FROM: zanekurz@sansnetcom.com (Zane) SUBJECT: Re: Effect of air currents on image clarity & resolution? DATE: Sun, 26 Sep 1999 16:36:08 GMT ORGANIZATION: Netcom NEWSGROUPS: sci.optics,rec.photo.technique.nature,rec.photo.equipment.35mm w.j.markerink@a1.nl (Willem-Jan Markerink) wrote: (snip) >For practical reasons you might want to do this at night. >Simply walk over to the target, and fire the flash manually (or have an >assistant fire it at command). >This will avoid the practical problem of flash syncronisation at such >distances (don't think more than 10m is very reliable, definately not in >daylight). >And it will also rule out any shake of camera+lens. Yes, night is the easiest, but if you're interested in testing what daytime turbulence does you have to do it then. I've done this at distances over a mile---it's surprisingly not too hard. A collecting lens jerry-rigged in front of the remote flash synch and at the local flash will give good synch at really large distances. No different than in a studio. You can mount the remote flash on a tripod and only have to go to the remote site to change the flash output or distance for testing different f/no's. (The turbulence can affect different apertures differently.) Slow film or ND filters can be used to make the flash swamp the ambient light. BTW, this is a good way to get sharp pictures of wildlife at a large distance if you know where they're going to be to within the coverage of the flash. The only problem for testing is that it effectively "freezes" the air turbulence, too, so you don't get the effect of slow-moving cells drifting across the FOV. You can infer some of this by looking at distortion of straight lines. >Don't think you have won with mirror lockup....the shutter itself is the >next vibrational source down the line (Bob Atkins from rec.photo.* cq >photonet has done some interesting testing....the shutter could clearly be >identified on the scope). Agreed. The shutter puts a torque into the camera/lens/tripod assembly, besides any plain vibration. A camera body with light moving shutter parts is what you want. You can often see blur in the direction of the shutter travel considerably higher than in the orthogonal direction. I think I mentioned this once before, but there is one thing people sometimes overlook. They will use a very heavy duty tripod and head, but with a rubber or cork mounting surface. This lets the camera/lens "twist" at the mounting surface. Replacing this with a metal-to-metal contact with as large a contact area as possible can make a big difference. Adding weight to the lens/camera ends to increase the moment of inertia around the mount point obviously helps, too, as does bracing the tripod legs to be resistant to twist. Zane FROM: w.j.markerink@a1.nl (Willem-Jan Markerink) SUBJECT: Re: Effect of air currents on image clarity & resolution? DATE: Sun, 26 Sep 99 23:46:17 GMT ORGANIZATION: A1 Internet news-server NEWSGROUPS: sci.optics,rec.photo.technique.nature,rec.photo.equipment.35mm In article <37ee44b5.6575683@nntp.ix.netcom.com>, zanekurz@sansnetcom.com (Zane) wrote: >I think I mentioned this once before, but there is one thing people >sometimes overlook. They will use a very heavy duty tripod and head, but >with a rubber or cork mounting surface. This lets the camera/lens "twist" >at the mounting surface. Replacing this with a metal-to-metal contact with >as large a contact area as possible can make a big difference. Yes, and the better incarnation of this concept are RRS-plates (ReallyRightStuff), customized/adapted to the exact form of several popular (pro) cameras, and compatible with the Arca-Swiss quick release system. Pure metal, fitting the camera like a glove. >Adding >weight to the lens/camera ends to increase the moment of inertia around the >mount point obviously helps, too, as does bracing the tripod legs to be >resistant to twist. Bracing/supporting the camera+lens on two different points is even better. Some tripod manufacturers sell such an additional camera-brace, to attach to one of the tripod legs....works only with a 'self-supporting' lens of course, and a long one too. -- Bye, Willem-Jan Markerink The desire to understand is sometimes far less intelligent than the inability to understand [note: 'a-one' & 'en-el'!]