If you have at your disposal fleshmetr - then you can stop reading. Although no one knows when this knowledge can be useful. Suppose arrived on the set, you find that the batteries are exhausted or fleshmetre where banal, in a hurry, just forgot to put it in a carrying case.

Calculator to find today because it's easier than narrow-profile device as fleshmetr. Article is for those who do not have this handy device in its arsenal, but has a strong desire to learn how to calculate the correct aperture in studio photography, where the construction of the cut-off lighting configuration using multiple lights at different angles and distances to the subject.

There are many ways to find the correct exposure for a given cut-off lighting configuration. In this article I will reveal one of the following calculations aperture using simple formulas which once successfully used himself. To commemorate the start of the geometry curriculum. I recall some basic points:

Cosine √cosφ 20° = 0,97... 30° = 0,93... 45° = 0,84... 60° = 0,71... 75° = 0,51, etc.

The number in the box, for example: 26^{2} is 26x26 = 676. Remember? Excellent. For ease of reading material, I use images and symbols for clarity. Here's how the calculation of f.

it is a simple formula by which you can easily calculate the aperture forward direction illuminator location, angle less than 20° to the axis of the lens, in fact illuminator is on the axis of the lens.

For example, the flash power G38, object distance 3,5 m (38 ÷ 3,5 = 11). Standard Guide Number divided by the distance from the flash to the model, we obtain the value of f.

It is quite another thing when a few flares and angles to the axis of the lens. First we need to know G (guide number) of each outbreak.

G flash in your passport, usually tied to ISO 100. In our example, it would be for a single flash 26 and, for the other 33. First, consider the example of a single light source - G33, 45° to the axis of the lens, and the distance from the flash to the model 2,5m quality ISO 100, the figure is R1 (Reflektor).

formula for calculating the corrected aperture G∠

Learn G ∠ corrected by using the formula: G∠ = √cosφ.

Where, G33 multiply, such as √cosφ 0,84 (angle 45°) and get 27,72 G∠, rounded to the G28

As we can see from the example, when setting out flares at 45° GN of little drops. We learned G flash in the direction of light at an angle to the axis of the lens. However that is not the correct f, because we are going to send the light through the reflector, which, in turn, slightly reduces the intensity of the reflected light. This is reflected by the surface due to the absorption of some part of the world, the so-called "absorption coefficient."

This ratio for the various reflectors/individual reflectors, the percentages are on a reflective equipment, designated by the Latin letter "ρ". If no such data, found the aperture is increased by one or two divisions.

That is, if G∠ 28 sections, for example, 2,5m = 11,2f, then divide the number by 2 found = 5,6f. Found aperture 5.6 is well suited for portraits, but if you want to shoot in a big hole, this can increase the distance of up to 3 meters, and counted on the formula, the aperture ratio equal 1:4,6f.

Please note that the distance from the flash to the subject , is the distance from the flash to the reflecting surface of the reflector, see below.

This formula will help determine the best Go to several outbreaks.

Once we found the adjusted G∠ for each fixture (G33-45° and G26-20°), we can start the basic calculation of the total number of master Go.

- 28x28 = 784, - 25x25 = 625, 784+625 = 1409, take out the root, and we Go ≈ 37,5.

Further, as in the previous example: Go 37,5 ÷ 2,5m ÷ 2ρ = 7,5f.

R2 may be identical to the distance from the subject as the R1, but can be pushed upward from the object, thus, give the portrait a huge relief and volume. Playing the distance between the object and the angles of lighting, we can achieve the desired result. Electricians, who are abroad drawing a, do not take an active part in the construction of the main light, and therefore do not require the inclusion in the correction of Go.

If you change the camera's sensitivity iso, of course, so does the guide number of outbreaks.

You may need to change the sensitivity iso100 to another, depending on the shooting conditions, lighting and technical capabilities of the lens. For example, at 200, 320, 400, or any other.

- How to Find G flash in any other sensitivities that are different from the standard iso100?

- If you have the instruction to use a flash - you can find the corrected G on the table.

And if one is not - use my formula calculating corrected GK below.

For example, G = 28, with the flash mounted on the camera sensitivity iso100 (iso standart), but you need to take off at a value of, say, iso200.

To do this, divide 200 (iso new) 100 (iso standart) = 2. Take out the "2" in the root (on a calculator), and find the value of ≈ 1,41.

Now, Gf G28 is multiplied by 1,41 = G39, 48 - rounded and get GK = G39.

Thus, the Guide Number G28 (with standard iso100) will meet at G39 iso200, - simple as that.

I hope that in this article you will find for themselves the answers to questions if the information is not enough - in a personal email or a guest. Thank you for your attention.

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© Andrey Ko 2010***