Re: [Cameramakers] Spotmeter
Or, better yet, why not modify a pair of binoculars. Have one side function as a finder and the other as the meter. That way, you wouldn't have to worry about metering through a one way mirror that may affect the reading by polarizing the light. You could put a plane of glass at the location of a virtual image inside the binocular lens assembly (if there is one) with a circle on it to show the area being metered. This is also where you would want to put the display that gives the meter reading. Of course the meter would not be accurate at close ranges, but you can't expect a homebrew meter to work just as well as a commercially made one. John - Original Message - From: Frank Earl [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Thursday, December 19, 2002 7:53 PM Subject: [Cameramakers] Spotmeter Murray, I have a Cambron SP-1. I can't find any patent numbers on it, but I would make one suggestion. The Pentax, Cambron, Minolta, etc. are all shaped like guns. I start to get nervous using it sometimes, especially with the paranoia going around now. There have been stories in the press about the police shooting people who have things that look like guns but aren't. Make the shape like a small monocular - not like a gun. Message: 1 From: Uptown Gallery [EMAIL PROTECTED] To: [EMAIL PROTECTED] Date: Wed, 18 Dec 2002 21:19:02 -0500 Subject: [Cameramakers] Spotmeter? Reply-To: [EMAIL PROTECTED] Hello: Sorry for duplicating a message on the pinhole list too, but I have an idea for a homebrew spotmeter, but want to read up on 'real ones' first. If any of you own one, I'd appreciate if you'd take a peek on the case and see if there any US Patent numbers so I can start reading. Thanks Murray __ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
Re: [Cameramakers] Spotmeter
It would be very hard to make a homebrewed spot meter as nice as a Pentax, for example, but homebrewed meters in general can be VERY accurate. I made a small series of meters made from cheap handheld digital V-O-M meters which used a photoresistor . I had a chart to convert ohms to shutterspeeds at a couple different apertures and filmspeeds. Kind of clunky, but very accurate, and extremely sensitive. I thought about trying to make a spotmeter but never did. I was stuck on a way to freeze the display so that when I took my eye from the peeper, the display would still show what I was looking at. I now have a nice analog Pentax Spotmeter so I can fiddle with other things. Gene - Original Message - From: John Yeo [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, December 20, 2002 6:56 AM Subject: Re: [Cameramakers] Spotmeter Or, better yet, why not modify a pair of binoculars. Have one side function as a finder and the other as the meter. That way, you wouldn't have to worry about metering through a one way mirror that may affect the reading by polarizing the light. You could put a plane of glass at the location of a virtual image inside the binocular lens assembly (if there is one) with a circle on it to show the area being metered. This is also where you would want to put the display that gives the meter reading. Of course the meter would not be accurate at close ranges, but you can't expect a homebrew meter to work just as well as a commercially made one. John - Original Message - From: Frank Earl [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Thursday, December 19, 2002 7:53 PM Subject: [Cameramakers] Spotmeter Murray, I have a Cambron SP-1. I can't find any patent numbers on it, but I would make one suggestion. The Pentax, Cambron, Minolta, etc. are all shaped like guns. I start to get nervous using it sometimes, especially with the paranoia going around now. There have been stories in the press about the police shooting people who have things that look like guns but aren't. Make the shape like a small monocular - not like a gun. Message: 1 From: Uptown Gallery [EMAIL PROTECTED] To: [EMAIL PROTECTED] Date: Wed, 18 Dec 2002 21:19:02 -0500 Subject: [Cameramakers] Spotmeter? Reply-To: [EMAIL PROTECTED] Hello: Sorry for duplicating a message on the pinhole list too, but I have an idea for a homebrew spotmeter, but want to read up on 'real ones' first. If any of you own one, I'd appreciate if you'd take a peek on the case and see if there any US Patent numbers so I can start reading. Thanks Murray __ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
Re: [Cameramakers] Spotmeter
You are right; a home-made meter could be as good, and with little extra effort, better than a commercial model. It would not even be too hard if you do not insist on too many bells and whistles. I finally bought a commercial one and modified it to live without mercury batteries and to overcome some troubles with loss of sensitivity (wrong readings!) over the years so I gave up on actually making a meter but I see nothing difficult if you know a little optics and electronics. The problem mentioned below, freezing the reading is also not serious. There are commercial digital voltmeters which have a freeze button, but if you start at the IC level, there are IC DVM chips which include the freeze function (look at the old Intersil stuff or the newer but similar parts from MAXIM). My own plans included Display in the viewfinder, best done with small LED readouts but LCD has some valuable advantages (and backlighted LCD s do it all) (incidentally, you must keep light for / from the display out of the photodiode.) Freeze of reading Diode, not photoresistor sensor, for great linearity Good input amplifier for excellent sensitivity meaning taking pictures in terrible light. Today amplifiers measuring to a couple pA are easy to find, also in low power versions, and getting well below 1 pA is not hard if you know what you are doing (leakage currents are deadly!) Make the amplifier have a log output of the input current. Not only is this convenient for the photographer, it makes obtaining wide dynamic range almost a snap. You do have to include some temperature compensation for the logging transistor(s) but that is adequately described in the appropriate books. Simple optics in the measuring path to avoid unnecessary reflections, and carefully designed baffles for the same reason. I have read this is where some commercial meters loose out to other models. Do any image inversion in only the viewing path. Probably easiest with a penta prism combined with the beamsplitting mirror to form an erect image (but maybe large format folks are happier if the image is inverted!) An old penta prism from a dead camera would be fine, though maybe larger=heavier than ideal. You could consider including the bells and whistles like a microcomputer chip to give a diaphragm setting to match your shutter speed, and all the similar features which eliminate most work. Today this is relatively easy at rather low power consumption (low power means slow speed to some extent, but how much computing speed is needed to calculate a shutter speed/diaphragm pair.) Some modern microcomputer chips are quite cheap and easy to program if you have the correct software. Probably lithium battery, though any kind will suffice if you regulate voltage. I considered trying to guess which types will be available for the remaining duration of my life (somewhat less than another generation) and designing around that. And I wanted a type which is unlikely to leak and then probably in a leaktight housing in the sense that if there were ever a leak it would be unable to reach the optics and electronics. Sensitivity goal; photos by moonlight (perhaps to quarter moon or better) Anyway, all this looked fairly easy in the sense that risk of failure was low and very little development would be required beyond spending enough time to engineer a comfortable housing and a good method of being sure the circle in my viewfinder was very similar to the circle sensed by the diode (That looked the hardest!) Bob At 07:25 20.12.02 -0800, you wrote: It would be very hard to make a homebrewed spot meter as nice as a Pentax, for example, but homebrewed meters in general can be VERY accurate. I made a small series of meters made from cheap handheld digital V-O-M meters which used a photoresistor . I had a chart to convert ohms to shutterspeeds at a couple different apertures and filmspeeds. Kind of clunky, but very accurate, and extremely sensitive. I thought about trying to make a spotmeter but never did. I was stuck on a way to freeze the display so that when I took my eye from the peeper, the display would still show what I was looking at. I now have a nice analog Pentax Spotmeter so I can fiddle with other things. Gene - Original Message - From: John Yeo [EMAIL PROTECTED] To: [EMAIL PROTECTED] Sent: Friday, December 20, 2002 6:56 AM Subject: Re: [Cameramakers] Spotmeter Or, better yet, why not modify a pair of binoculars. Have one side function as a finder and the other as the meter. That way, you wouldn't have to worry about metering through a one way mirror that may affect the reading by polarizing the light. You could put a plane of glass at the location of a virtual image inside the binocular lens assembly (if there is one) with a circle on it to show the area being metered. This is also where you would want to put the display that gives the meter reading. Of course
[Cameramakers] Spotmeter
Murray, I have a Cambron SP-1. I can't find any patent numbers on it, but I would make one suggestion. The Pentax, Cambron, Minolta, etc. are all shaped like guns. I start to get nervous using it sometimes, especially with the paranoia going around now. There have been stories in the press about the police shooting people who have things that look like guns but aren't. Make the shape like a small monocular - not like a gun. Message: 1 From: Uptown Gallery [EMAIL PROTECTED] To: [EMAIL PROTECTED] Date: Wed, 18 Dec 2002 21:19:02 -0500 Subject: [Cameramakers] Spotmeter? Reply-To: [EMAIL PROTECTED] Hello: Sorry for duplicating a message on the pinhole list too, but I have an idea for a homebrew spotmeter, but want to read up on 'real ones' first. If any of you own one, I'd appreciate if you'd take a peek on the case and see if there any US Patent numbers so I can start reading. Thanks Murray __ Do you Yahoo!? Yahoo! Mail Plus - Powerful. Affordable. Sign up now. http://mailplus.yahoo.com ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
[Cameramakers] Spotmeter?
Hello: Sorry for duplicating a message on the pinhole list too, but I have an idea for a homebrew spotmeter, but want to read up on 'real ones' first. If any of you own one, I'd appreciate if you'd take a peek on the case and see if there any US Patent numbers so I can start reading. Thanks Murray --- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.391 / Virus Database: 222 - Release Date: 9/19/02 ___ Cameramakers mailing list [EMAIL PROTECTED] http://rmp.opusis.com/mailman/listinfo/cameramakers
