Saturday, November 20, 2010

A Practical Guide to Working with Guide Numbers

Recently I've had a number of inquiries about the articles I wrote about Guide Numbers (in feet and in meters).
The embarrassing thing was that each time I had to re-read my own articles and work through the whole thing just to remind myself how it works.  More embarrassingly was that I couldn't figure it out straight away and had to take quite a bit of time to work it all out again.

So after listening to my readers (and getting frustrated with myself) I decided that I would write this article to make it easier how to use guide numbers - and the truth be told so that I don't have to work so hard next time I am asked how it works!

I think that looking at the calculations by themselves is what makes it confusing. It's better to put yourself into the situation and ask yourself what would you be doing in a real situation? So let me try to explain using this approach.

Situation 1
A person has asked you to do a portrait of them sitting on a chair. You have a camera (Canon 5D MKII) and a flash (Canon 580EX II) with a remote trigger and a light stand (but you forgot to bring your umbrella and soft box!).

The person doesn't have much time, who does nowadays? so you need to quickly set things up and fire away. You already know a few things.

1) You would like to have enough depth of focus to have the whole head in focus, so will use your 24-105mm lens at somewhere between 50-90mm depending on how much of their body they want to include. To get the right depth of focus you will use roughly F/7.1.

2) The other thing you know is that, as you do not have an umbrella or soft box, to get softer lighting you will use a fairly wide flash zoom, maybe 24mm or 28mm (you may also like to try a 35mm flash zoom).

3) Because it's indoors and relatively close you know that you'll probably use a 1/8 or 1/16th power. What you need to know now is how far your lights need to be away from the subject.

So now you know that:
a) You're using F/7.1 on your lens.
b) You will try first with a 24mm zoom on your flash head
c) You will use between 1/8th and 1/16th power

So now you do some calculations based on the calculation (D=GN/F : Distance (Flash to Subject in feet) equals the Guide Number divided by the F-Stop)

You look up the guide number (G/N) for 24mm flash coverage (zoom) and 1/8 power which you find is 32.5. To follow the formula you need to divide this G/N by your lens aperture 7.1 which gives you 4.5 feet. If you were to use 1/16th power the GN is 23 and for the same 7.1 lens aperture would give you the distance of 3.2 feet.

Situation 2
You have a small room where you want to photograph a bunch of items you want to sell on e-Bay. The room is small so you are limited to where you can place your lights. After placing the light in the room on the side of the table you measure the distance from the flash head to the place where you will position the items and find that the distance is 2.6 feet. Now you can calculate what settings you need for your flash and/or the F-stop setting for your camera.

For this you will use the calculations:
a) F*D=G/N : F-Stop by Distance (Flash to Subject in feet) equals the Guide Number.
b) F=GN/D : F-Stop equals the Guide Number divided by Distance (Flash to Subject in feet).

You're going to be using around an 80mm lens and you would like a fair depth of focus for the whole product. As your camera is quite close to the items being photographed you will choose an aperture of F/11 for good clarity.

So now you know that:
a) You're using F/11 on your lens
b) Your distance from flash to subject is 2.6 feet

So to get the guide number you multiply the F-Stop (11) with the Distance (2.6) which gives you a guide number of 28.6. Now you look up on the table in the white numbers for the closest number to 28.6. The closest you can find with be the flash coverage of 70mm with the flash output at 1/32 power (G/N=28.9).

Alternately you could think about moving the lights closer to 2 feet which would give you a G/N of 22 (2 * 11) which would give you some options of 35mm @ 1/32 or 24mm @ 1/16.

Say you weren't concerned about the F-Stop number so much but you knew that you wanted to have fairly hard shadows so wanted to have a narrow flash coverage say 105mm and you decided you would use 1/8th power. Now you would have to calculate what F-Stop you would need to set on your lens.

So now you know that:
a) Your distance from flash to subject is 2.6 feet
b) Your G/N number is 67.3 (105 mm with 1/8th power)

Your camera F-Stop would therefore be 67.3 divided by 2.6, which would give you an F/Stop of F/25 (or F/22 which is the closest one).

You may then decide that you don't want to use such a high f-stop, but you still want to use 105mm flash zoom. Your only option is to use a lower flash output, maybe 1/32. The G/N number for those settings is now 33.8. So now your F-stop would be 33.8 (G/N) divided by 2.6 (Distance) which would be 13 (or F/11).

Obviously these things are averages and you will need to make minor adjustments to get the exposure right. Remember that these settings are good for finding a good starting point, which you can then adjustment from.

The last table I created helps you to change the guide number based on if you use a light modifier or you change the ISO settings.

If you were to add a shoot through umbrella you will need to multiply the G/N by 0.5 before you use it in the calculations.

In the previous example above the G/N for 105mm flash coverage at 1/8 power with a shoot through umbrella becomes 67.3 multiplied by 0.5 which equals 33.65.

So now your F-stop would be 33.65 (G/N) divided by 2.6 (Distance) which would be 12.9 (or F/11).

Well that wraps up this post.  I hope that the examples above help to make more sense of how to use guide numbers.

Please feel free to leave comments or questions below or email them to me.