[justtakingpics]

Yes, I did search but I am confused. Looking at the 540 flash:

68' (21 m) at 20mm
105' (32 m) at 24mm
115' (35 m) at 28mm
128' (39 m) at 35mm
148' (45 m) at 50mm
164' (50 m) at 70mm
177' (54 m) at 85mm

on BH has this for guide numbers.

It doesn't have the fstop or the iso. I understand that it is typically ISO 100 and don't know the fstop.

What I don't understand other than a measurement of power, is what doest this mean? What does 177' (54m) at 85 mean? Even the smaller number 68. What would I shoot 68 feet away from me? How does this help?

I know it does something at 177 feet, but I don't know what it's telling me and I promise I've read and just don't get it. Maybe you can help. Thanks.

[COULDBE2]

Try here for info.
ShortCourses-Flash

Remember Goggle is your friend.
Mike

[justtakingpics]

Try here for info.
ShortCourses-Flash

Remember Goggle is your friend.
Mike

No disrespect but I promised above that I did search. I actually read that already. But when I applied what I learned to the catalog I did not understand.

[COULDBE2]

OK, then try this one.
Flash Photography - Understanding Guide Numbers
Mike

[SOldBear]

OK, let me try.

Let's look at one line:

148' (45 m) at 50mm

For now, just ignore "50mm." We have "148' (45 m)".

OK, 148 feet is the same distance as 45 meters. You can use either unit in the calculation. Since you are in the US, I assume you're more familiar with ft. Let's use that unit. The line now becomes:

148'

This says: the guide number (GN) of the flash is 148 at ISO 100, with distance expressed in feet.

If you want to take a photo of an object 15 ft from the flash, to have proper exposure, the aperture of the lens should be:

148'/15' = 9.87 (or f/10 in round number).

If the object is 22 ft from the flash, set the aperture at:

148'/22' = 6.7

The guide number increases by 1.4X (one stop more powerful) if you double the ISO. So at ISO 200, the GN becomes:

148' X 1.4 = 207'

at ISO 400:

148' X 2 (or 148' X 1.4 X 1.4) = 296'

OK, now you know how to calculate the aperture from GN and ISO setting.

------------------------

Let's look at the whole table:

68' (21 m) at 20mm
105' (32 m) at 24mm
115' (35 m) at 28mm
128' (39 m) at 35mm
148' (45 m) at 50mm
164' (50 m) at 70mm
177' (54 m) at 85mm

The last numbers on each line (20mm, 24mm, ....) are the settings of the flash's zoom head (the table uses 35mm equivalent focal length numbers). When you change the zoom setting, say from 24mm to 70mm, the light beam gets narrower (from the FOV of a 24mm lens to the FOV of a 70mm lens). But since the light is concentrated to a smaller area, it can project farther, and thus appears brighter. The GN increases.

In other words, if you set the flash head at 20mm, use 68' as the GN. At 24mm, use 105'. At 28mm, use 115'..... The calculation for proper aperture setting at each zoom setting is as in part 1 of this post.

Many flashes don't have zoom head.

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Sometimes, to create a photo with vignetting, the photographer can purposely set the flash head at a more tele setting than the lens. For example, if you use a 50mm lens, and set the flash head at 85mm, the center of the photo will have proper exposure, but the surrounding will be dark.

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Note that the table uses 35mm equivalent FOV. For the Pentax 540, you can change the zoom display to APS-C format FOV, or you can mentally convert the focal length equivalents (e.g. 24mm in 35mm format is equivalent to 16mm in APS-C format)

[justtakingpics]

thank you very much. I understand.

[selar]

With flash auto zoom, I wish my AF360AFGX would take the APS-C crop factor into account.