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Vivitar 283. Info on varying power.

 
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C Krebs
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PostPosted: Sat Dec 11, 2004 6:28 pm    Post subject: Vivitar 283. Info on varying power. Reply with quote



We've been discussing electronic flash set-up in the microscopy section. I've settled on using the venerable Vivitar 283 primarily because it allows easy and remote (via wire) setting of power levels. The lower power levels have the short flash durations that have great motion stopping ability. I tried TTL flash, but I found I wanted to make + or - adjustments fairly often, and the position of the flash made this cumbersome. With the feedback received from digital cameras (histograms) manual flash is not a problem.

The picture above shows the two contacts on the front of the unit that are used to change power levels. You only need to vary the resistance between these contacts. I have set mine up using potentiometers in two "ranges". You could also set it up using multipole switches with discrete resistor values. I took four Vivitar 283's (all were "used") units, and using a flash meter, determined the resistance values required to decrease the power in "stops". There was some difference from unit to unit, but these values are the average, and should be a very good guide. A flash meter (or studying the exposure information from a digital camera) allows determining values with great precision for a particular flash unit.

-.5 stop/ 150,000 ohm
-1 stop/ 78,400 ohm
-1.5 stop/ 57,000 ohm
-2 stop/ 35,290 ohm
-2.5stop/ 26,500 ohm
-3 stop/ 17,720 ohm
-3.5 stop/ 12,350 ohm
-4 stop/ 9,550 ohm
-4.5 stop/ 7,600 ohm
-5 stop/ 5,680 ohm
-5.5 stop/ 4,400 ohm
-6 stop/ 3,150 ohm
-6.5 stop/ 2,400 ohm
-7 stop/ 1,530 ohm
-7.5 stop/ 800 ohm
-8 * stop/ 155 ohm *

* all units went down to at least -7.5 stops. One went to -8 and two other were about -7.8


Additional info added 5/4/05....

The center contact is not used. The two contacts on the left side of the picture above (one at about "10 o'clock" the other at about "8 o'clock") should be connected ("dead short") to each other. There is a 200pF capacitor across the leads to the variable resistance.


The picture below shows how these contacts are "set up" inside a Vivitar module that plugs into the sensor socket shown above.



Also... to be on the "safe" side, if you will be connecting the flash directly to a digital camera, you should look into a device such as a Wein "safe-sync" that insures the trigger voltage is sufficiently low so as not to damage the camera electronics. Of course, the flash can always be safely "fired" using a flash slave unit.


Last edited by C Krebs on Wed May 04, 2005 9:04 am; edited 1 time in total
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Steve West



Joined: 10 Oct 2004
Posts: 545
Location: Tucson, AZ USA

PostPosted: Sat Dec 11, 2004 8:50 pm    Post subject: Reply with quote

Great stuff Charlie, and much appreciated! That's a very large range of resistances. My vivitar is here, but the wife has it locked up under the tree. Hopefully, I'll be up and running by the new year.
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Steve West



Joined: 10 Oct 2004
Posts: 545
Location: Tucson, AZ USA

PostPosted: Sun Dec 26, 2004 8:28 pm    Post subject: Reply with quote

Hi Charlie,

I'm messing around with the 283, and I have a question. In the manual, it says that full output is 1/1000th sec. If you can adjust 8 stops, that corresponds to a duration that is 256 times less, but the manual says that the shortest duration is 1/30,000th which is only 30 times faster. Do you have more control by varying the resistances you provide than you would normally get with the on-flash controls?

Thanks

Steve

PS-- I have made my 45-deg beamsplitter, and I found PVC fittings that almost fit the Diavert perfectly, so it's a very clean setup. I glued on a 1mm thick piece of slide. Now I'm trying to figure out how to mount the flash to microscope frame. With some bare slide tests, I can oversaturate the camera through a 73x phase objective and annulus with full output. This is just placing the flash up next to the beamsplitter. With just brightfield, full power oversaturates a 100x objective by a good portion.
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Charles Krebs



Joined: 13 Jul 2004
Posts: 1200
Location: Issaquah, WA USA

PostPosted: Mon Dec 27, 2004 1:02 am    Post subject: Reply with quote

Steve,

The output/flash duration relationship is not really "linear" like that. You can get from 7.5 to 8 stops of light output variation with proper resistance values. But that doesn't mean you are getting flash durations any where near 1/256000 second. The graph of a flash pulse shows a "rise", a "peak", and then a "decay". There are several ways duration is measured depending on the points on the graph you choose to select as the beginning and end of the pulse. The amount of total light output is the area under the discharge curve.

I have no way of measuring (or knowing) the actual flash duration when I've got it tweaked down 7 or 8 stops. I do know the ability to "freeze" motion is great. It's very rare that I can use -7 with a microscope (low magnification bright-field with a high na objective). But when you have it down to -3 or less you will be stopping the motion of just about anything you will encounter.

Unless you have the Vivitar Varipower accessory, there are no "on-flash" manual power modes. The Vivitar Varipower will dial down to -6 stops. But I have found that I can get at least 1 to 1.5 stops less light output when I make my own "control" device.
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Charles Krebs



Joined: 13 Jul 2004
Posts: 1200
Location: Issaquah, WA USA

PostPosted: Wed Dec 28, 2005 11:38 am    Post subject: Reply with quote

I have recently had a few people contact me about the "control" that I made for the Vivitar 283. While it is discussed above, the picture below may be helpful as well.

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puzzledpaul



Joined: 20 Dec 2005
Posts: 3
Location: uk

PostPosted: Sun Jan 01, 2006 11:07 am    Post subject: Reply with quote

I came across this and wondered if it might be of interest?

pp

http://www.hiviz.com/activities/guidebook/sensor.pdf
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