# Tag Info

15

The guide number represents the light output of the flash alone, with no ambient light factored in. Unless you are using slow sync flash, the ambient light is just assumed to have no meaningful impact. And, when you do want it to be a factor, the simple isolated number is much easier to actually use to figure out your light ratios. Why doesn't the ambient ...

10

"Magic" automatic flashes, whether TTL or using a built-in sensor, are relatively recent. Before that, a handy system was developed for getting correct flash exposure manually. This is the guide number system, which is used for calculating the right mix of lens aperture, subject distance, and flash power. The guide number itself is given in terms of ...

5

However, 25 is NOT 2/3 of the way between 16 and 32. You just have to remember that the scale is exponential, not linear. 4 2/3 stops allows 24.66 more light, and 24.66 = 25.28132..., or roughly 25. It's actually 9/16 (0.5625). Meaning that 25x less light is actually 4.5625 stops less bright, rather than 4.66 stops. Again, you're mixing linear and ...

5

You are correct, light is additive, so if the flash provides the same amount of illumination to the subject as the sun, and if no flash spills onto the background, then the background will be one stop darker than the subject. To underexpose the background by 2 stops would require a flash with three times the power, guide number 70. It's worth noting that ...

4

I don't understand what it means by 196.85' (60 m) ISO100 at 200 mm position You're looking at a unit where the flash head zooms to change the area that's illuminated. The head moves toward the front to light a wider area, and toward the back to light a narrow area. Changing the area that's illuminated means that the intensity of light per unit area changes,...

4

I guess there would be some some small amount due to absorption in the atmosphere. But in theory there's not much complicated to it, and I don't see why it wouldn't still be the same. The formula works because light follows the inverse square law — and that works for studio lights in a small space just the same as it does for stars. (I mean, the miasmas of ...

4

Imagining theoretically, twice the speedlight zoom mm would compute half size field dimensions with 1/4 the area coverage. Same flash power concentrated into 1/4 the area theoretically would be 4x brightness, which would be 2 EV increase at twice the zoom mm. However, that simply does not happen (not even close). The speedlight zoom is very simple, the ...

3

A guide number (GN) is the power of the flash to illuminate a subject correctly at ISO 100 at a specific distance using a specific f-number. Normally this distance is 10 ft for easier calculations, so the GN is GN = 10 x f/number The thing here is that we need to know if you have a correct exposure so the first approach would be to use an incident light ...

2

The original post is asking for a quantitative relationship, not a series of examples. There is nothing wrong about the basic guide number relationships used in the above answers, but the quantitative relationship between Guide Number and ISO is the square root of (ISO/100). In keeping with the use of a series of examples: the square root of (200/100) is ...

2

Well both calculation here are somewhat correct but the right formula should be GN = (Aperture*Distance)/ISO factor Distance = GN/(Aperture/ISO factor) Aperture = GN/(Distance/ISO factor) So if we're looking for the GN based on Aperture=f8, Distance=10m then our formula would be GN=(8*10)/1 ISO 100 =80 GN=(8*10)/1.4 ISO 200 =57 GN=(8*10)/2 ISO 400 = 40 ...

2

The formula you found with ISO included is wrong. The correct formula is.... Guide Number= Distance x Aperture. or Distance = Guide Number / Aperture or Aperture = Guide Number / Distance As you would know, Guide Number is given for ISO 100 always. We will deal with other ISOs a little later. As a photographer, my first worry would be how far my flash ...

2

Ignore shutter speed. As long as it's at or under the sync speed, it will be fine, and changing it won't affect the flash exposure. Assume ISO 100 for a start. Guide number of 32 divided by f/stop gives you the distance to your subject. f/8: 32 ÷ 8 = 4 feet f/4: 32 ÷ 4 = 8 feet f/2: 32 ÷ 2 = 16 feet If you bump up ISO, you can extend the range. ...

2

Guide Number is about the Inverse Square Law, which describes the way the direct flash intensity falls off with distance. At 2x distance, the flash is reduced to 1/4 intensity. At 3x distance, reduced to 1/9 intensity. At 4x distance, to 1/16 intensity. Like your flashlight or car headlights also fall off with distance. So distance does overwhelmingly ...

1

The Godox V860II and Canon 600EX both have the same 20-200mm flash head zoom range, and both have the same 60m guide number at 200mm, so I think you would be safe to use the Canon 600EX specs for a rough estimate.

1

I have 3 of these flashes and have not experienced this problem. Only thing I can suggest is maybe downloading and installing the YN flash firmware update. Maybe try to "reset" the flash and start over from the beginning.

1

According to Wikipedia's Guide Number entry: The aperture given by the guide number is only correct for certain locations, generally indoors where a reasonable amount of "stray" flash light will be reflected from surrounding walls onto the subject. The effective GN is slightly lower outdoors as any light not falling directly on the subject from ...

1

The flash will still throw light as far as it says regardless of whether it is inside or outside etc, the effectiveness of that light will vary greatly depending on the ambient light. Using a flash at its maximum range while in very bright ambient light will probably be pretty ineffective. You would likely need to shoot with the flash off-camera and close ...

1

With you camera set to ISO 100, take pictures of a subject 10 feet away, adjusting the f/stop until the exposure is good. Then multiply 10 (feet) by that f/stop. If f/5.6, then the guide number is roughly 56. If it's f/4, then the guide number is 40.

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