Bayronto photos

[bdombrowski]

Okie:

Guide numbers are not that hard to use. You can even make a table on a slate so you have zero math to do in your head while diving.

First, you need to know the underwater guide number for your strobe. I know Ikelite has these on their web site for all their strobes. Note that this number assumes a specific ISO film speed, usually ISO 100. You can adjust by a stop for different film speeds, but we'll just keep it simple for now.


You use the guide number to compute what the proper strobe to subject distance needs to be for a proper exposure as a function of aperture setting. This is the formula....

stobe to subject distance in feet = Guide Number / f-stop-number

Here an example...

I'm shooting with my shutter fixed at 1/125 and my stobe's guide number for 100 ISO film is 28. I'm shooting a balance light composition, meaning I want blue water in the background and a foreground subject illuminated with the strobe. So I aim the meter at the blue water and adjust the aperture to zero the meter (assuming I have an SLR with a reflective light meter built-in.) Let say this happens at f/8. So my strobe to subject distance needs to be 28 divided by 8, which is 3.5 feet.

Easy thing to do is put a table on a slate with your pre-computed strobe-to-subject distances for all f-stops for a given guide number.

Couple of details to mention....

1) Remember the strobe-to-subject distance is not the same thing as lens to subject distance.

2) Water magnifys and thus distorts your normal perception of distance. Aim the strobe a bit behind your subject and use the distance from your shoulder to your finger tip as an estimate of 3 ft when extended straight.

3) You may need to drop to the next farther distance (reduce light by 1-stop) when the subject is highly reflective to prevent over exposing it. Barracudas are a prime example.

4) Know that multiple power settings on the strobe can be used to reduce the light power in increments of 1 f-stop. Let's say in the above example, 3.5 ft is too far for the composition I want. If I reduce the strobe power from full to 1/2, the strobe to subject distance becomes 28/11 or 2.5 feet.

5) If you are using dual stobes you only need to be concerned with the guide number of the more powerful strobe, the one with the higher guide number. Stobes are not additive. They only increase coverage.

Hope this helps. Have fun in Bonaire and share your pics when you get back.

Regards,

Brian

 

[Dee]

Brian....would you post this in the Tip and Techniques thread to the top of the forum? This information needs to be kept handy for all to find.

 

[okiediver1]

Thanks Brian! That was simple!

My next question is....is this what this sticker is all about that I got with my ds-125?

And, do the "M's" stand for maximum?

 

[ReyeR]

My next question is....is this what this sticker is all about that I got with my ds-125?

Yes, that's the strobe distance to f-stop exposure guide for the DS-125.
If you apply the GN = D*F formula that Brian explained you will get those numbers.
For e.g. the stated UW GN of the DS-125 @ ISO 100 is 32(ft) or 10(m). If you select an f-stop of 8 then applying the formula D = GN/F we get 4ft or 1.25m. On ther chart, if you follow the ISO 100 column down to f-stop 8 then follow that row all the way across you'll see that the recommended strobe to subject distance at f8 @ ISO 100 is 4ft or 1.2m. It should be noted that these numbers presume a 'full dump' of the strobe.

And, do the "M's" stand for maximum?

The 'M' stands for manual ie manual flash mode. The bar next to the M is the recommended 'distance range' you should use while shooting in TTL flash mode. ie if you are using f8 and TTL exposure then the subject to strobe distance range should be between 1.5ft and 4ft to get a good exposure. The M indicates the recommended 'distance' the subject should be from the strobe to get the correct exposure when shooting in manual flash mode. ie if you are using f8 @ ISO 100 and a 'full dump' in manual flash mode then the strobe to subject distance should be 4ft.
HTH

 

[ssra30]

Thanks all for the info. This is very helpful. I have a couple of further questions though.
According to the table, this is for SLR film, it did mention that for ditital camera, you would not need as much light, so would you go up one f-stop ie smaller aparture for equavalent distance?
Since using a diffuser will bring the f-stop down by one (ie requiring more light), will the digital camera effect rougly cancel out the diffuser effect as far as f-stop is concerned. I am just trying to find a simple way to memorize a few useful numbers here.
Lastly, I am a bit embarassed to ask this but oh well, I should learn it sometimes. What is actually one f-stop?
Moving 1 f-stop, is it equal to moving from 4 to 5.6 to 8 as in the table or 5.6 to 6.0 to 6.7 as in the camera aparture's setting or is it just straight 5 to 6 to 7.
Thanks for your help.

 

[ReyeR]

According to the table, this is for SLR film, it did mention that for ditital camera, you would not need as much light, so would you go up one f-stop ie smaller aparture for equavalent distance?
Since using a diffuser will bring the f-stop down by one (ie requiring more light), will the digital camera effect rougly cancel out the diffuser effect as far as f-stop is concerned. I am just trying to find a simple way to memorize a few useful numbers here.

I wouldn't know the answer to that - perhaps someone more qualified might. I would suggest using it as a rough guide to get you started. You'll find after a while you won't need to refer to it at all. Being able to view your result instantly will accelerate your judgement and learning curve.

What is actually one f-stop?

The C5050 has broken the 1 f-stop to 1/3 increments. The traditional 1 f-stop range is f2.0, f2.8, f4, f5.6, f8, f11, f16, f22, and f32. The C5050 range goes like this: 1.8(W), 2.0 , 2.3, 2.6(T), 2.8 , 3.2, 3.6, 4.0 , 4.5, 5.0, 5.6 , 6.3, 7.0, 8.0 .

 

[ssra30]

ReyeR, thanks for the info, I will try to play with some of these number and my strobe setup and see what happens

 

[bdombrowski]

The full f-stops are 2.8,4,5.6,8,11,16,22. If you have small increments you are using fractional f-stops.

Closing down one full f-stop (ie from 2.8 to 4) cuts the light in half. Opening up one full stop (ie 4 to 2.8) doubles the light.

The numbers themselves are actually the distance between the film plane and the center of the lens divided by the diameter of the aperture. Not of much use, but interesting if you ever wondered where these crazy values come from.

Now regarding the sticker table printed on the DS-125 strobe. If your stobe's manual tells you to compensate for a digital camera by closing down one stop from what the table recommends for manual shooting, you could instead use a diffuser that decreases the light by one f-stop and get the same net effect. Note that this only works in manual mode, not TTL.

There are a couple of otherways to compensate w/o using the diffuser. One is to just use the the column with the next fastest film speed. So if you camera is set for ISO 100, use the ISO 200 column when finding your strobe to subject distance for a given f-stop.

Another way is to recompute the stobe's guide number. The DS-125's guide number for 100 ISO film is 32 at full power. The guide number for ISO 200 would be 44. So you could use a guide number of 44 when shooting manual with the digital camera. That's a lot of power, and you may find it's too much power to get close to many subjects with balanced lighting. That's why the fractional power settings on the DS-125 are such a great feature to have.

Brian