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Old May 28, 2007, 5:24 PM   #1
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Hi everybody,

I wanted to know the Difference between F-stop for a Slave Flash which can be modified Versus Power ratio of the Flash Which can be Adjusted. As i understood it by reading different articles is that they both give the capability of using the Flash at the Faster speed but simply use a Different formula to mention the different power setting and in general the F-stop Power Reduction method seems to be a more favourable one as one can sync it with the Camera F-stop without having to calculate to get proper Exposure as with the Power ratio setting.

I also wanted to know which one is the favourable Slave Flash for FujiFilm S6000/S65000Fd Camera that i have.

1- Bower SF328AZ

2- Vivitar DF200

3- Phoneix D91- BZS

4- Sigma EF-430 ST

( Not Digital But Someone said that it works Fine with S6000fd, I have my Doubts Due to the Preflash and that its nota Slave Flash and would need a Slave trigger also)



Thanks In Advance

Muhammad Bilal Malik




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Old May 30, 2007, 2:19 PM   #2
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I'm not familar with those slaves (although I have seen a number of posts here mentioning them). So, some are pretty popular.

Most flashes are rated by GN (Guide Number) at ISO 100. Some will give GN in feet, some will give GN in meters. Some will state it both ways in their specs.

To determine the maximum range, you divide the GN by the Aperture you're shooting at (assuming ISO 100 is the ISO speed).

For example, if you're using a flash with a GN of 100 feet at ISO 100, and your lens is using f/2.8, your range would be approximately 36 feet (100 / 2.8 = 35.7)

Your Fuji's lens is brighter on the wide end of the zoom range. It starts out with a widest available aperture of f/2.8. But, as you zoom in more, less light gets through. So, at it's maximum telephoto zoom position, your widest available aperture becomes f/4.9

So, that same flash example with a GN of 100 feet at ISO 100 would only give you a maximum range of a bit over 20 feet if you zoomed in all the way (100 / 4.9 = 20.4). Note that some flash models assume a certain focal length is being used (so take the GNs with a grain of salt). They also assume a "straight on" flash. Bouncing will require you to take the ceiling height and characteristics into consideration. Use of diffusers of any kind will also impact flash range.

Note that each time you double the ISO speed, flash range will increase by approximately 1.4x

As for a slave that doesn't recognize a preflash, I'd avoid them (they'll fire on the preflash versus the main flash unless you're using a digital aware slave trigger with them). You can buy a digital aware slave trigger (for example, Wein makes some).


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Old May 30, 2007, 4:33 PM   #3
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Dear JimC,

Thanks for your Kind response. I have been reading about Flashes for a Couple of days on different sites and forums.

Some flashes, like the Vivitar DF200, list various power ratings that you can switch between (full, 1/2, 1/4, etc.) while others, like the Phoenix D91-BZS, list f-stop ratings to gauge intensity. Does this mean that the Phoenix flash will intelligently meter and expose the picture as it sees fit? .....As i Understood reading the Different articles there are Basically 3 Charactistics of a Flash

1- Color temperature (5500K-5600K)

2- Guide No (GN) @ ISO 100 ( Power of Flash) mentioned in Ft or Meter

3- Speed Of Flash (1/1000s - 1/20,000)

Adjusting the f-stops is an automated way of attempting correct flash exposure according to your lens aperture.
It's effectiveness is a little hit and miss as it doesn't take into account subject distance or luminance - but it works well if you're aware of these limitations.

Now with the Power Ratio change I know that the Speed of the Flash Differs....For Example if at Full power the Speed is 1/1000s then at 1/4 Power the Speed will be 1/4000s. This Flash speed adjustment ability is great for High Speed Action Stopping like the Wings of a Flying Bird.

Now the Confusion is that when i Differ the F-stop on the Flash am i changing the Speed of the Flash or Not? And if So how can i Calculate by how much is it being changed. This Confusion has also being increased as the Vivitar DF200 and Phoneix D91-BZS are Cosmetically Identical other than the Brand Name and Technically have the Same Specs Excepth of POwer Ratio Setting on Vivitar Df200 and F-stop Setting on Phoneix D91-BZS.

Thanks in Advance

Muhammad Bilal Malik


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Old May 30, 2007, 5:14 PM   #4
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Again, I'm not familiar with those flashes. But, the settings you seem to be referring to are manual power settings (the full, 1/2, 1/4, etc.).

As for the speed you're referring to, that's the way a flash controls it's output (flash burst length). It's common for a flash to use burst lengths between 1/1000 and 1/20000 second (longer burst for more range, shorter burst for closer subjects).

The way a modern non-dedicated strobe is best used is via an Auto Setting. Many have multiple auto ranges where you set the aperture and ISO speed on both the camera and flash to match. Then, the flash uses a built in sensor to meaure reflected light during the exposure, terminating it's own output when it seens enough reflected light for the aperture range selected.

Modern non-dedicated flashes like the Sunpak 383 Super or Vivitar 285HV use this technique, so that you don't have to resort to manual power settings and computing the distance to subject yourself.

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Old May 30, 2007, 5:18 PM   #5
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The flashes that come with an f-stop range have a light sensor built into the flash (also called auto thyristor flashes). You choose an f-stop and ISO combination, set both the flash and camera to that setting, and take your picture. The flash starts to emit light, and when the sensor reads enough reflected light that the flash thinks a correct exposure has been made, it will turn off the light. The advantage is that it is an automatic operation, you don't have to play with trial and error to adjust settings. The disadvantage is that you don't have control and you may think that the flash is overexposing or underexposing your subjects. You can work around this some by adjusting your camera ISO or aperture to a different setting than the flash setting to compensate. Another disadvantage is repeatability, changes in the subject might result in a change in the flash output i.e. you take a picture of someone in light coloured clothes and then two more people in dark clothes join and you take another picture, the two pictures may be exposed differently.

With a flash with a variable power setting, you set your exposure either using a flash meter, or by manual calculations of guide numbers and distances, or by trial and error through adjusting flash power, distance from flash to subject, camera ISO and camera aperture. The advantage is repeatability, the exposure will be the same every time, which is great if you are doing assembly line type portraits, i.e. pictures of students at a graduation. The disadvantage is the amount of time it will take to get set up. You may have a hard time grabbing a shot of your child doing something cute while you spend time fumbling with the flash.

Some digital cameras allow you to fire the flash in a manual mode without using preflash. Most cameras do not. I don't know what your camera can do. To be safe, make sure you get a flash that is a "digital" slave.

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Old May 30, 2007, 5:57 PM   #6
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Dear JimC and Edward,

Thanks alot for the Explaination. By the way i have got Minolta Dimage Z1 and a FujiFilm S6000Fd.

I will buy the Slave Flash and then post the results for all to see.

Thank you both of you for your Time.

Allah Hafiz

Muhammad Bilal Malik






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