Basic Flash Instruction

[Ces1um]

I have a basic Pentax Flash. I also have a camera that syncs at all flash speeds. There is no electronic communication between the two to tell each other what is going on and what settings are being used. On my flash there is a table giving basic iso, distance and aperture information - but there is no information about shutter speed. I was wondering if these tables assume a standard shutter speed?
The scenario goes like this- I know my distance to my subject, iso of my film. I look on the table to pick the corresponding aperture to be used. Now if the flash is to be believed the shutter speed is irrelevant in the calculation, which seems wrong to me. I understand flash is usually used in the dark so the film is only being exposed by whatever the flash puts out, but what about situations where there is ambient light and you're just trying to add more light to the scene? There must be some kind of standard shutter speed they assume you're going to use. That way you can safely alter aperture/shutter speeds so that you can get the effect you're after. I may want to do a portrait at F4.5 but the table tells me to use f8. If there was a standard shutter speed I could just halve that time to compensate. I see a lot of slr's sync at 1/60 second. Is this the "standard" speed these tables assume you're using? Thanks for your help.

 

[AgX]

I was wondering if these tables assume a standard shutter speed?

These tables expect you to know that:

-) with leaf shutters you can practically set any shutter speed.
-) with focal plane shutters you can set all shutter speeds that do not crop the image. (With 35mm cameras you are mostly safe with a 60th or longer.

Synchronisation (if switchable) should be "X".

Within this margin you can use the shutter speed to control the ratio ambient/ flash light, as only the ambient light will be effected by the shutter speed.



With bulb flashes the situation is more tricky, as the lighting time of these typically is quite long and there also is some delay. Here there are more complex tables, that take into account the shutter speed and type of synchronisation and the related effective output of the flash.

 

[BrianShaw]

If your flash calculator indicates f/11 and you want to shoot more open you can move the flash farther away. Remember the inverse-square law from high school science?

With flash the aperture is adjusted relative to flash output and distance; shutter speed is adjusted to alter the influence of ambient light.

BTW, flash exposure is not estimated for complete darkness but for some degree of ambient light. Using a strobe in complete darkness leads to incorrect exposure.

A flash meter, even a cheap one, is a good thing to have. It really helps understand flash exposure.

 

[AgX]

BTW, flash exposure is not estimated for complete darkness but for some degree of ambient light. Using a strobe in complete darkness leads to incorrect exposure.

Flash effective output (Guide Number) is based on darkness, but a within a room with white walls and ceiling.

 

[BrianShaw]

Flash effective output (Guide Number) is based on darkness, but a within a room with white walls and ceiling.

Yes, more correctly expressed. Thank you.

 

[AgX]

Synchronisation (if switchable) should be "X".

"Hot" shoes always are X-synchronized, but when there is a PC-socket, there may be choices, either by switch or by sockets.


Some (compact) cameras in addition automatically set a certain shutter speed (typically 30th) when you use the hot-shoe, whatever flash you use.

 

[Ces1um]

My flash is on-camera and is quite basic. I don't have the option to move it further away. It has a hot shoe. So given that my camera syncs at all speeds, my flash has two "auto" modes that are selected based on distance and that the camera and flash do not talk with one another. This is not for studio work if that clarifies things as well. So what I'm getting from here is basically as long as I'm using the correct f stop, film iso and distance based off the chart, then I can set whatever shutter speed my little heart wants. To me this makes very little sense though. I find it hard to believe that my shutter speed doesn't affect the photo.

 

[Mr Bill]

As a note, the typical mechanical shutter is not fast enough to significantly reduce the light from an electronic flash. So if the table says to use f/8, then the flash is expected to supply all of the necessary light at f/8, regardless of the shutter speed.

The shutter speed comes into play when there is ambient light. Say, for example, that your exposure meter says to expose at 1/100 second at f/8, using ambient light. If you shoot this way, you'll get more or less correct exposure.

Now, if you add the flash, which also says f/8, then you will get two units of "correct exposure," meaning that you are overexposed by one f-stop. This is the worst overexposure situation you'll get into, and if you want to, you can think, oh, at f/8 I'm one stop over, so I'll just close down one stop to f/11. But with negative film this is not such a big deal, and a lot of people would probably just let it be overexposed.

Aside from the situation where both flash and ambient give equal exposures, you would normally expose for the one that dominates. For example, say that you want to use a fill flash, but have it be one stop under. Continuing the same conditions as before, you'd say, ok flash exposure is normal at f/8; to make it one stop weaker, the camera goes to f/11. Now for the ambient part, you still want this to produce correct exposure. Earlier you metered the ambient at f/8, but since you now plan to shoot the camera at f/11, you can see that you need to lengthen shutter speed to 1/50 second. So you shoot 1/50 at f/11. There is still some extra exposure, but less significant than before; it's roughly a half stop over; I personally would not bother adjusting for that in normal shooting. But you could close down the aperture a half stop if you want. If you use a weaker fill light, then the overexposure effect is even less.

Sometimes you might want the flash to predominate the exposure, it works basically the same way. Just keep the ambient exposure contribution less than that of the flash, and you will always be less than a stop over.

If you want to be finicky, go ahead and make the exposure corrections; probably best to write them down and tape it on the side of the flash. But if you're ever shooting something under pressure, remember that this is when one tends to make dumb mistakes, so make the adjustments at your own peril.

Last note; if you have an incident flash meter, this will properly combine both the flash and ambient . You set whatever shutter speed you want to use into the meter, take a reading, and it will give you the correct camera aperture.

Ps, Some studio flash units have a longer flash duration, and fast in-lens shutters can actually cut into the flash exposure. Probably test exposures would be the best way to determine this. But if you keep shutter speeds slower than 1/500, you'll more than likely never have an issue.

Too much info? I dunno. Hopefully it helps.

 

[Sirius Glass]

So what I'm getting from here is basically as long as I'm using the correct f stop, film iso and distance based off the chart, then I can set whatever shutter speed my little heart wants. To me this makes very little sense though. I find it hard to believe that my shutter speed doesn't affect the photo.

Basically the flash duration is so short and all the shutter speeds are so relatively long, the flash is over long before the shutter can close.

 

[Mr Bill]

To me this makes very little sense though. I find it hard to believe that my shutter speed doesn't affect the photo.

In very rough terms, the flash is mostly over in the order of about one thousandth of a second. If you are using in in an "auto" mode it will "shut off" the flash part way through, with the flash duration being perhaps 5 or 10 times shorter.

If you have a really fast leaf shutter, that could expose at say, 1/2,000 second, this is probably fast enough to cut off part of a full-power flash. You can't do this with a focal plane shutter, though; when they are set at faster than the "sync speed," this means they are no longer fully open at one time; essentially a "slit" is moving across the film. The flash would only illuminate the open part of the slit.

 

[Ces1um]

In very rough terms, the flash is mostly over in the order of about one thousandth of a second. If you are using in in an "auto" mode it will "shut off" the flash part way through, with the flash duration being perhaps 5 or 10 times shorter.

If you have a really fast leaf shutter, that could expose at say, 1/2,000 second, this is probably fast enough to cut off part of a full-power flash. You can't do this with a focal plane shutter, though; when they are set at faster than the "sync speed," this means they are no longer fully open at one time; essentially a "slit" is moving across the film. The flash would only illuminate the open part of the slit.

Thank you! That had to be the clearest answer to my question. I get it now! @Sirius Glass Yours was great too- I just saw Mr Bill's answer first.

 

[MattKing]

Fill flash can be really fun to experiment with.
Fill flash can also be frustrating to experiment with.
The important thing to remember, and Mr. Bill describes it well, is that when you have flash and lots of ambient light, the two different sources of illumination add to each other for those parts of the subject exposed to light from both of them.
I tend to use fill flash most when subjects are backlit. The fill brings the illumination up on the parts of the subject that would otherwise be cloaked in shadow.
When you meter the light, be sure that you are metering carefully. If you are trying to use the flash to adjust the shadow illumination, make sure that you are metering the ambient light in the areas that are shadowed.

 

[AgX]

I find it hard to believe that my shutter speed doesn't affect the photo.

As said above, your shutter speed has effect on the ambient (continuous) light.
The flash burst of nearly all electronic flashes though runs out of itself before a leaf shutter could cut it.

 

[M Carter]

I can't tell you how many corporate shoots I've done where someone says "oh my god, should I turn off the lights???" And I'm like "it will take a hell of a lot more light to beat these strobes!"

That's one of the great things about working with flash - it's usually very independent of the ambient light, giving you a lot of control. The biggest issues is shooting color, and learning to "see" the difference in ambient light when it's not perfect daylight. These days that's tough, when so many flos and LEDs look like perfect daylight but are usually (in commercial spaces, if they're fairly recent fixtures) an orange-green color.

All that flash-guide-number-math gets crazy when you're bouncing or using diffusion though. Bought a flash meter years ago and I can't remember a whit of that stuff.

 

[Soeren]

Flash effective output (Guide Number) is based on darkness, but a within a room with white walls and ceiling.

Nope doesnt matter. You may get softer shadows, some fill etc dependent on bounce surfaces but output is fixed.

 

[AgX]

The german standard on establishing guidenumbers is very detailed, but just on this matter is either vague or even lacks requirement. So with intent a manufacturer may add reflected light to the metering by design of his metering set up. But I agree, at least with a realistic set up, such addition would be small.

 

[BrianShaw]

The flash in discussion is a two-range automatic “exposure” system. Who knows exactly what their formula is. My experience with that kind of flash mode is very different from manual mode with GN computation or use of a flash meter in complete darkness and non reflective surroundings where the auto mode generally underexposed almost 2 stops. Specifically, that was using Vivitar 283/285 in underground mines.

But a lot of this discussion is a bit of a diversion, albeit an interesting diversion, from the question of shutter speed.

 

[mjork]

The german standard on establishing guidenumbers is very detailed, but just on this matter is either vague or even lacks requirement. So with intent a manufacturer may add reflected light to the metering by design of his metering set up. But I agree, at least with a realistic set up, such addition would be small.

I have no idea about GN standards for electronic flash. But I have looked at a lot of flashbulb documentation and that always clearly says the GN given assumes a light-colored, medium sized room. When used outside or in large/dark rooms with little reflection, the GN has to be adjusted by one full stop(!)
And I have verified this with a flash meter: the GN given on flashbulb packages is indeed one stop too optimistic because it assumes flash bounce by the walls/ceiling.

 

[AgX]

Flash bulbs are a bad example as they lack the reflector, thus in practice just by using different flashes (luminaires) the outcome would vary.

In realistic surrounding the wall/ceiling reflection should only be of significance if hit directly by the flashlight. With bulb flashes this would be more probable than with tubes in a reflector housing.

But this whole thing indeed would be a nice enterprise for checking.

German standard asks for establishing the guide number by metering in direction of the main axis of the flash. In conservative interpretation this means using a snoot on the meter. Thus lateral reflection would be cancelled out. Mjork likely used a spherical head on his meter.

 

[mjork]

The flashbulb GN actually does assume a reflector and that's also stated in the fine print: for example the GN printed on a package of #5 flashbulbs would assume a 5" polished reflector.
I did my measurements outside in total darkness with no reflective surface nearby. I had the flashbulb in the right type of reflector pointed directly at the flash meter. Both were raised up on tripods at a distance of 9' from each other. The GN I computed from the measurement was exactly one stop below the manufacturer's given GN.
That result was perfectly consistent with the manufacturer instruction to open up one stop if there is no wall/ceiling to provide bounce light.

Would be easy enough (actually easier) to do the same test with an electronic flash.

 

[mjork]

My conclusion was that the GN for flashbulbs were marketing numbers and for most practical use cases one has to subtract a stop from that number.
I can imagine that electronic flash manufacturers are also tempted to provide the most optimistic number they can...