[jfdavis58]

The guide number is a contrivance--it's made up!

Engineers, technicians and their ilk have electrical, mechanical and optical theories and models and equations and big fancy computers to actually calculate things like power to light flux vs distance, and the ideal guide number, but who of us Photographers has the time to think about that when the scene of a lifetime presents itself in our viewfinders? {Well, I do, but I got too much time on my hands and lots of letters from a fancy university after my name, and I get paid to do that thinking. But...)

The important points are these: light intensity 'falls off' (it gets dimmer as you move away from the source) as the square of the distance. F-stops, i.e. aperture values follow the same mathematical sequence. Flashes are fast so shutter speeds don't generally matter but aperture does. It's much easier to calculate a guide number with a tape measure , an aperture setting and a calibrated light meter than all those fancy theories and mathematics--so that's how it's done.

Because it's done this way, we need only the camera, flash, a tape measure and some simple math.

There are two math formulas to remember. The first will require simple multiplication and division. The second uses square roots so you may need a calculator. You will use the first equation a lot more than the second-that's good, right?!?!?!

Here is the first equation: Guide Number (GN) divided by distance equals aperture. You must have a GN in meters if the distance is in meters or a GN in feet if the distance is in feet. Aperture doesn't have any named units. Distance is indicated on the barrel of most lenses; usually in both feet and meters.

This distance is the camera to subject focus distance and the same as the flash to subject distance if and only if the flash is mounted on the camera. (Yeah, on camera flash is bad; carry a carpenters tape measure for off camera flash.)

There are two other forms of this equation: GN/f-stop=distance and f-stop*distance=GN. This is simple algebraic manipulation. Examples will follow.

Film/sensor speed affects guide number; along the same sequence as f-stops-and hard to remember. Most manufacturers give the GN in distance units at some ISO. Some, like Pentax, provide a daunting table of numbers with two speed ratings and a host of other settings----you really need only the manual settings chart! {Truthfully, there is a third equation to handle the zoom function. It's complicated, make a copy of the chart from the manual, listing all the GNs for zoom values. Laminate it. Keep it with your flash.}

You can use the second formula to get a GN at any speed from a GN at some speed. Here is the second formula: GN2=GN1*squareroot(Speed2/Speed1).

Let's do some examples.

For the 360 flash the manual GN table is on page 31. Somewhere in the manual for the camera it says that the 1.5 magnification factor for the sensor size difference with respect to 35mm film needs to be accounted for. Let's ignore it for the moment; we can use the LCD/histogram to make 'on-the-fly' adjustments and this stuff doesn't need great precision.

Mount every bodies favorite 50mm f/1.4 lens and the iso 100 guide numbers are:
full power 30
1/2 21
1/4 15
1/8 10.5
1/16 7.5
1/32 5.4

These are in meters; multiply them by 39.37/12 (3.28) to get GN in feet.

Put your subject 9.5 feet away--that's about 2 meters. By power setting here are the f-stops (aperture values):

full 30/2=about f/15
1/2 21/2=about f/11
1/4 15/2=about f/8
1/8 10.5/2=about f/5
1/16 7.5/2=about f/3
1/32 5.4/2=about f/2.5

I made my subject a friends face. F/8 is nice for faces so I need 1/4 power. Shoot, check your preview/histogram. Before you change power try moving a small step forward to make the subject brighter, backward to make it dimmer----it doesn't need to be precise!

Again, by power settings I'll change GNs@iso 100 to GNs@iso 400--you can look on page 31 to check the accuracy:

full power 30 30*sqrt(4/1)=60 {sqrt(4/1)=sqrt(4)=2}
1/2 21 21*2=41
1/4 15 15*2=30
1/8 10.5 10.5*2=21
1/16 7.5 7.5*2=15
1/32 5.4 5.4*2=10.8

Some considerations: GNs are optimistic-a polite way of saying inflated! Step forward 1-3 small steps in most cases-preview/histogram/bracket by stepping forward(/backward) in increments. As a flash ages the circuitry and the flashtup deliver less pop. In general all the other techno-babble in the manual will reveal that the extra features cause the flash to generate less output---smaller GNs! Still, the light physics don't change because of the extra functionality--when in doubt "GO MANUAL!"

PS: I could clean this up if somebody will tell me how to make nice neat columns in those tables above.

John

[Mr. H]

So, if you've stopped laughing, can you recommend either a print or web resource that clearly explains the basics? Or perhaps post a thread in the tips and techniques section, you do seem to have a way with words. It'd help a lot of confused people!

Julie[/QUOTE]

I agree!! Most of us have questions and could use some help with using our external flashes, so if jfdavis58 or anyone else can pass on some tips or websites that we could go to, that would be appreciated!

Like, how to set the flash or camera when using a M42 lens?

[foxglove]

Some considerations: GNs are optimistic-a polite way of saying inflated! Step forward 1-3 small steps in most cases-preview/histogram/bracket by stepping forward(/backward) in increments. As a flash ages the circuitry and the flashtup deliver less pop. In general all the other techno-babble in the manual will reveal that the extra features cause the flash to generate less output---smaller GNs! Still, the light physics don't change because of the extra functionality--when in doubt "GO MANUAL!"

Ahah! I knew it was yet another rule of thumb. I've been doing more or less what you describe, minus the math. So the math will give me a better starting point. And I like understanding how things work, so thanks for the explanation and examples. But I do wish the "manual" that came with my flash were more than a single sheet of paper! I feel like I must be missing something, but maybe I'm not. Time to haul out the flash, tape measure and calculator and have some nerdy fun!

Julie

[BarneyCG]

As I said in another thread I got a Metz 40mz-2 and a ttl shoe for it from Walters Photo-Video Online for all your photographic needs
I got one that was marked as fitting a Canon which came with a basic single pin adaptor for £25 and then the ttl adaptor was another £15 ... obviously this wont work on the K10/K100D but it is fine on the OP's DS.

It has LOADS of features that would make jfdavis58 laugh like strobe, motorized zoom, trailing curtain sync but it works really well in just basic ttl mode which is the most important bit and it does have the basic 3 things that jfdavis58 talks about an I for one am not frustrated any longer ... I was when all I had was the built in flash

[jfdavis58]

Whatever floats your boat Barney. Truth of the matter is that I do own several 'mega' flashes--including a trio of high end Pentax poppers. Hey, unless you got at least two 360s or two 540s or one of each you simply cannot do contrast control mode, or wireless mode. And those modes are incredibly useful!

As a whole, and with no disrespect toward any single member: as this group collectively matures and grows in it's knowledge of photography-especially Pentax based photography, most will want to go off camera with their lights. Sooo, before one can understand light ratios and such, a solid foundation in manual flash technique is essential.

In fact as the picture develops, we will all soon see that much of the portable flash gimmickry is absent in the studio flash-and that the gimmickry of those small flashes is just that---small.

Rear-curtain sync is neat, but out past a certain distance you just don't get action smears. Contrast control is cool, but the low power of portable flash limits it to table-top work and very small group portraits (1-3 heads). Similar with high-speed sync---it reaches about 15 feet into the typical basketball court (do you have sideline tickets?)

So that leaves ttl and p-ttl---useful? To continue my tease something then present an answer approach, you will need some cheap toys: a solid color tablecloth---a yard square of bright felt (red or green or blue, not black or white or yellow) and some small objects--I use wooden shape blocks found at the local teacher supply. With these Items and your on-camera ttl flash, I bet I can make many of you laugh! And scratch your head and question why you bought all the extras.

[BarneyCG]

Don't get me wrong I don't disagree with what you've said at all, though being a relative newbie I'm not really up to speed on the more "advanced" techniques you are talking about.

I just wanted better pictures than the built in flash could give and for the princely sum of £40 I found a flash that will do that ... It does a lot more but I doubt I'll ever use those features, however should I ever need them they are there. I'd not have spent money to get them ... I was looking for cheap with swivel/tilt and ttl.

I've done some reading of the strobist web site and one day I'm sure I'll end up going off camera but for what I need and want at the moment this will do me fine

[jfdavis58]

The camera must be set to manual for this type of lens. It also helps to have the digital preview set to viewfinder and the custom function for lens off A set to allow. At the end of this thread/this username is some other metering information: i.glisin

Personally I have several lightmeters and I work from them for settings. Some experimentation is needed to set a shutterspeed that allows 'dragging'- i.e. including some ambient light on the image. Otherwise max sync is 1/180.

You will also need to experiment with the zoom if the strobe has that function.