Some Flash Basics

[yelvertoft]

I found a very useful article on the Pentax User forum giving information on some basics of flash. It was written originally (not surprisingly) with lots of Pentax specific advice. With the permission of the original poster, I’ve edited it to be non-brand specific as there’s some useful information in here that I’m sure would be of use to others. The original article can be found here:
http://www.pentaxuser.co.uk/forum/viewtopic.php?t=4950

Reproduced with thanks to MattMatic of the pentaxuser.co.uk forum who originally wrote this article.

I can’t guarantee the results will be perfect for all users of all brands and models, but hopefully it will give somebody some pointers and get them further along the learning curve.

Points (in order of importance):
* Flash is limited in power
Pre-flash TTL (i-TTL in Nikon speak, or E-TTL in Canon speak) will sort out the exposure but only within the limits of the flashgun. Let's use a car analogy. Most of us have cars with limited power, so we may see a situation where we'd like to accelerate from 0 to 60 in 2 seconds, but even when we push really really hard even to the floor the car won't go any faster.
The flash can be like that - it's limited and has a limited range. Even if P-TTL says "I want loooooads of light for this shot" the flash may very well not have the power and the shot will be dreadfully underexposed. (Ever see people in a stadium trying to use flash photography? How much effect do you think their flashguns will have?? Absolutely zero.)

* Flash distance is dependent on ISO rating and aperture
This is where physics comes in and the "inverse square law", ie double the distance and you quarter your light (because it's falling in 2 dimensions not just one).
For photography this means you need to balance the ISO and Aperture (note: NOT the shutter speed. More on that later) so you get the correct distance.
For those with flashguns that have a distance scale readout on their LCD panels, try the following:

- Set the camera to ISO 100 (fixed)
- Set the camera to Av mode and rack it up to f/22. Your distance will be 0.7-1.5m. Anything further away than 1.5m will be dark!
- Now set the Av to f/5.6 and the distance changes to 0.7m to 6m.
- Set the camera to ISO1600 (fixed) and at f/5.6 the distance will change to 3m-22m

(your specific flashgun may give different distance figures depending on its output, but I’m trying to illustrate the principle here).

So, you should be able to see that with that setting of ISO1600 and f/5.6, if you have someone standing 2m away they will be drastically overexposed! With a minimum distance of 3m, the flash can't turn off quick enough! (A bit like asking a Bugatti Veyron to nudge forward 2cm - you probably won't be able to stop in time)

* Bounce flash messes up the distance
This is where those guns with an LCD distance scale stop giving direct advice! The flash distance is no longer the camera-to-subject distance, but camera-to-ceiling-to-subject distance.
Depending on where you are this distance could be enormous. If you're outside then it's infinite.

* Balancing the light
This is where some skill comes in (and a tiny bit of knowledge). When you're trying to take realistic images using flash you really want to balance the ambient lighting with the flash.
For example, let's suppose the ambient reading (ie without flash metering) is ISO200, 1/30s, f/5.6. If you set your flash to use f/11 then you're going to "punch" your way through the image with the flashgun. The flash lighting will be two stops (f/5.6 to f/8 to f/11) brighter than the surroundings and you'll get the horrible "cardboard cutout" look.

Actually, if you set your camera to auto ISO in Av mode you'll notice that the ISO changes according to the ambient lighting even when the flash is powered on. Your camera is making some pretty reasonable guesses at balancing the light.

* Metering aims for grey
Don't forget that the metering aims for mid grey. If you're taking shots of mostly black or mostly white scenes then you'll almost certainly need exposure compensation (on the camera, not the flash) to "inform" the metering system what you're taking.

* Shutter speed is (almost) unimportant
The duration of the flash is blindingly quick, something like 1/10000s. As a consequence, the shutter speed is not that important - especially when you want to "freeze" action. But this is where you need to learn about balancing the light...
If you want to freeze action and isolate the subject from the surroundings, then you want to make sure the flash is at least one or two stops more powerful than the ambient light.
If you want to make the shot look natural then you need to at least keep the flash and ambient lighting within about -1 to 0 stops.

* Leading/Trailing curtain synch
If the shutter speed is very slow (or the subject is moving) such that a non-flash exposure would produce a streak or motion blur then you'll probably want to put the flash in "trailing curtain" mode if your camera and flash unit both support this mode. This mode operates the flash just before it closes the shutter curtain rather than just after it’s opened the curtain, as it is when the more usual leading curtain synch is used. The flash will produce its punch at what will appear to be the "head" of the object so the object looks like it's moving in the right direction (exposure is blur-then-flash). Using normal, i.e. leading curtain synch, mode makes the subject look as though it's going in the wrong direction (i.e. flash-then-blur). Think about the time ordering of things...

* Use fully manual for controlled situations
For studio type shots where the conditions don't change - use fully manual.
A suggested starting point…….

- Set the camera to Manual, 1/60s, f/8
- Set the flash to Manual, 1/4 power
- Take a test shot
- If it's too bright do one or more of the following:

> Pick a small aperture (ie large f-number)
> Move the flash further from the subject
> Reduce the flash power
> Reduce the ISO setting on the camera (smaller number)

- If it's too dark, do one or more of the following:

> Pick a larger aperture (ie smaller f-number)
> Move the flash nearer to the subject
> Increase the flash power
> Increase the ISO setting on the camera (bigger number)

- Once you've got a good flash exposure you can balance with the ambient lighting:

> Lower the shutter speed to allow more of the ambient light in. (e.g. 1/15s)
> Raise the shutter speed to darken the background (e.g. 1/180s)

With a bit of practice you can get the right aperture and power combination within a couple of test shots - and having digital preview is extremely helpful indeed!

* Some other suggestions to try
- For snaps at family gatherings, try Auto-ISO and Av for general portraiture, the camera will generally get it right every time.

- If you want to override the flash "effect" you can set the exposure compensation on the flashgun. Something like -0.5 or -1.0 will allow more ambient light in.

- If you are shooting outdoors and want fill in flash, if your flashgun supports it, set the flash to High Speed Synch. Note that the distance is typically halved when in this mode (though if the shutter speed goes below 1/180s the flash will go back to "normal" mode).

- If you want attractive catchlights in the eyes (especially for close portraiture say about 2m away) then if your flash has a small pull out translucent catch light panel, extend this panel and point the flash straight up (90 degrees). The small amount of reflected light from the catch light panel will produce a subtle fill in with a lovely sparkling catch light.

- If you use a Stofen Omnibounce then you'll halve your effective flash distance. But the flashgun won't know - so you'll have to make mental adjustments when you look at the flash distance scale.

- The same rules apply to using Wireless mode. But remember, the flash distance is the flash-to-subject distance, and not the camera-to-subject distance!

- For controlled situations you can use an "non intuitive" setting. Try the following.

• Turn off the flash
• set the camera to "M"
• meter the scene using spot metering to try and expose near the highlights – check the shutter speed is under 1/180s and adjust ISO if necessary
• turn on the flash in Pre Flash-TTL mode (E-TTL, i-TTL, P-TTL)
• take the shot

The flash fills in the shadows without changing the exposure. This technique can be successfully used for taking shots in dark rooms that have light streaming in and got shots that have everything properly exposed. As an addendum to this "non intuitive" mode, you can also:

• Set the camera flash mode to "Slow sync"
• Switch to Manual on the camera
• Meter as before and use Pre Flash-TTL mode on the gun

By enabling one of the slow-sync settings the camera will continue to meter the ambient light even when the flash is powered.

Hope this helps someone,

Duncan (but really MattMatic).

[Birdsnapper]

Many thanks, Duncan and MattMatic. I've kept away from flash photography as it has seemed so daunting, but might give it a go (when I can afford a decent flash).