What is the closest the SF-20 will work @ ASA 400?

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What is the closest the SF-20 will work correctly @ ASA 400 and an aperture of 1.4 in TTL and/or auto-mode? Anything around 1 or 2 meters? May-be by "cheating", aka underexposing? I'm considering to buy one if it can serve me as a real weak fill-in for available light photography with my M6s, TTL and non.

-- Lutz Konermann (lutz@konermann.net), April 09, 2001

Answers

According to the table in the manual, with ISO 400 @ f/1.4 and full power, the distance called for is 28m. So, at the 1/8 power setting (the lowest selectable manual power setting) it would be 3.5m. Whether in TTL or Auto modes the SF20 can be made to quench beyond 1/8 power, I can't answer. For flash at that subject distance I'd surely use some kind of softening diffuser, and that cuts the light some. Also, if you're always going to be working at 1-2m with ISO 400 film and shooting at f/1.4, you've got enough constants that with the aid of a borrowed flashmeter you should be able to find a little flash for next to nothing that should do the trick even if you need to put a layer of handkerchief over the tube.

-- Jay (infinitydt@aol.com), April 09, 2001.

Lutz:

I scoured my SF20 owner's manual and could not find much information for you... Here is what I can tell you: In the auto mode, you can only select f2.8, 5.6 or 11. In TTL, obviously you use whatever the lens is set at. According to the specs, the flash duration ranges from 1/400th sec. at max power to 1/30,000 sec at min power -- I don't know if the light output is linear, but if it is, you can do some math and see you get about a 7 1/3 stop range. The wide angle diffuser (included) decreases effective light output by 1 stop. Of course you could sandwich ND gel material between the flash and diffuser, and reduce the output further to whatever level you needed.

FYI, I did a test-fire at 1/8 power in manual mode and another in auto mode at a few feet -- There was significantly less light output and flash "pop" in the auto mode, but I cannot quantify the amount for you.

Hope this helps,

-- Jack Flesher (jbflesher@msn.com), April 09, 2001.


Also, in response to Jay's comment... Light from a flash (or any other source) falls off at the inverse square to the distance, so the range of 28M does not reduce to 3.5M at 1/8 power, but falls to 10M. Stated another way, doubling the power output at 10M will get you 14M; doubling it again will get you 20M; and once again 28M.

Cheers,

-- Jack Flesher (jbflesher@msn.com), April 09, 2001.


Gulp! I guess 35 years of using a flashmeter adds up to my memory falling off in inverse square proportion also. Thanks for the correction.

-- Jay (infinitydt@aol.com), April 09, 2001.

Jay & Jack Thanx for your maths. Still, I guess I'll have to do some tests to see whether the SF-20 can be of any greater help to solve my equation than what I put together myself, as a low-cost but non-TTL solution. ( http://www.konermann.net/flash.html ) Yours, Lutz

-- Lutz Konermann (lutz@konermann.net), April 10, 2001.


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