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I want to use the HSS guide in my 550ex flash manual to answer the following questions:

1. If I have my camera set at 1/1000, F5.6, ISO 400, and I'm using a 20D and 500mm F4 lens, and I am using full power manual flash with 550EX and better beamer what is the max distance of flash coverage I can expect?

2. What camera settings would I use to get a distance of 100 feet?

3. How did you use the info in the manual to answer the questions above. Please explain what equations you used and why.

Thanks

Jim Neiger wrote:I want to use the HSS guide in my 550ex flash manual to answer the following questions:

1. If I have my camera set at 1/1000, F5.6, ISO 400, and I'm using a 20D and 500mm F4 lens, and I am using full power manual flash with 550EX and better beamer what is the max distance of flash coverage I can expect?

2. What camera settings would I use to get a distance of 100 feet?

3. How did you use the info in the manual to answer the questions above. Please explain what equations you used and why.

Thanks

To start, the 550 ex ISO ratings are arrived at indoors where the Gn is arrived at by including relected light off of interior walls...for the "true" guide Number, divide the numbers in the manual by a factor of 1.4. You can confirm this with a flash meter.

The ratings are also based on ISO 100, so as you move up or down with your ISO setting, either divide or multiply by the 1.4 factor for each stop you increse/decrease the ISO.

The basic formula is GN= Fstop x Distance..so in your case

You Corrected guide number is 13.8 (from HSS chart @ 1000 sec) /1.4 (derate flash) x1.4 (adj for 1 stop to ISo 200) x 1.4 (adjust for 1 stop to ISo 400) or GN 19.3

Using the formula 19.3= 5.6 x distance or 3.5 Meters (11 ft) full flash

It's not much distance and what I was alluding to in an earlier post. Add 2 stops for a better beamer and you have 19.3 x 1.4 x 1.4 or 22 ft. Full flash

Use as fill at minus 1 stop, then it's 22ft x 1.4 factor again..or 31 ft.

I think you get the feel for the limitation....I would also test the actual output using a flash meter along with checking the Flash "alignment" when using the better beamer....my test on a brick wall showed the beamer "hot spot" low and to the left of the centerline it should have been on. Note the GN's in the manual are in meters....multiply those GN's by 3.2 if you prefer working with ft.

To get to 100 ft range ....you can't in practical terms unless you can find four more "stops" of light either through raising the ISO, or using more flashes.

The bottom line is the HSS mode severely cuts you range as the flash is pulsed during the exposure....you will also only get "one shot" before you have to let the flash recharge.

Hope I did the math right...if I didn't...someone will probably chime in.

Forgot to explain that the column you will be looking at for the GN will be the "105mm" column under the HSS listing. Using the 500mm lens the 550ex will default to the 105mm setting. Once "derated", tests I have done with a flash meter agree with the chart for outdoor use.

I had a bad experience with this yesterday Jim, with the Waxwings. When they were about 70 to 100 feet away, I was intially using Av mode at F8, not HSS on, Better beamer. The issue was severe white-sky backlighting so I was hoping flash would reach. Of course in Av mode, the bright day dictated fast shutter speeds but the flash changed it to 1/250 so everything got blown out. I switched on HSS on the flash and got ok ambient exposure but the birds were silhouettes. I tried + flash comp as high as possible but still silhouettes. Bottom line, distant birds against whitish skies do not work for me.

jnadler wrote:I had a bad experience with this yesterday Jim, with the Waxwings. When they were about 70 to 100 feet away, I was intially using Av mode at F8, not HSS on, Better beamer. The issue was severe white-sky backlighting so I was hoping flash would reach. Of course in Av mode, the bright day dictated fast shutter speeds but the flash changed it to 1/250 so everything got blown out. I switched on HSS on the flash and got ok ambient exposure but the birds were silhouettes. I tried + flash comp as high as possible but still silhouettes. Bottom line, distant birds against whitish skies do not work for me.

Jeff I think that is part of the problem... using Av mode...you will never know if the flash power was limited by the higher (+2 stops) of the"white" sky, or you just didn't have enough ooomph to reach at the distance you were shooting. I'm prety sure what Jim is talking about is camera in manual mode, HSS flash in manual mode....in other words, not using Ettl or TTL. In manual HSS you can rely on full flash power occuring at the HSS shutter speed you select. You guys have digital bodies...just go outside at night, set up a target at 100 ft, and keep moving it closer till you get a good exposure. This will certainly give you a pretty good idea of your maximum range. If you have a wall as a background so much the better as you can see light fall-off if the beamer is mis-aimed. (If you used a beamer) I also don't know with such a bright background if the Canon ETTL does it's "flash reduction" thing...just one more complication with flash in the auto mode. I would assume in digital bodies you have the option to turn that off...but I still think manual mode at least makes you aware of your max range which is probably the problem if your gettin silouettes.

As a last note...as you up the ISO to get more range, in sunny 16 situations you could bump into an "upper limit" with the shutter speeds going so high that the corresponding HSS GN values are so small the effective distance is reduced to being impractical.

Jeff,

Plus comp on the flash becomes irrelevant once you reach maximum capacity of the flash output.

Sometimes it is just best to expose for the subject and let the background fall where it may.

Chas

Steve,

Thanks for your very detailed answer. It was exactly what I was hoping for and what I was afraid of at the same time. I was having trouble figuring it out from the manual, and I haven't seen it explained this clearly here before. It gives me a good starting point for my experimentation in Homer next month. I hope to be able to use fill flash on the dreary days. If I walk the edge of minimum shutter speed, DOF, distance, and fill flash, it just might be doable. It won't be easy though.

Anyone have any advice that might help with this?

Jim-sometimes the theoretical just doesn't bear out in the reality. I would go out in the yard during low ambient light conditions , set up a tripod for camera, and one for meter, and run some actual tests at measured distances and camera settings. From that make a short list(Maybe taped on the lens shade) to keep handy in the field until you have the settings you most often use intuitively set in your mind. How can you go wrong?