Manual flash

With manual flash, you set the flash power yourself rather than leaving the TTL system to work it out automatically. You have to work out what the flash power setting or lens aperture need to be, but once you’ve done that you can shoot away, knowing that you’re going to get predictable and consistent results. It’s important to understand the difference between manual exposure and manual flash. You can set the flash to manual, but still use the auto exposure modes on your camera (it’s best to select Aperture Priority mode, because setting the camera aperture is a crucial part of manual flash control). You can use your camera in full manual mode too, of course, but make sure that you’re using a shutter speed no faster than the maximum flash-sync speed.

When to use manual flash

You’d use manual flash for the same reasons you might use manual exposure – so that you don’t have to keep second-guessing what the camera is going to do. For relatively static subjects, it can often be quicker to use manual flash than to try to arrive at the perfect flash strength using TTL flash, flash compensation and trial and error. Manual flash is particularly useful in the studio if you’re using your flash off-camera and to one side of your subject; you can arrive at the correct flash power fairly quickly via calculation and experiment, and you can swap between light or dark backgrounds, vary lighting angles and adjust reflector positions without having to check and counteract any automatic variation in flash strength. When you’ve set up controlled conditions, you don’t want your flashgun continually making its own mind up about the exposure.

The statue in the foreground had a very dark tone that needed careful exposure with the flash. In the end, it was quicker to set the flash to manual and set the aperture according to its guide number than it was to experiment with different TTL flash compensation settings.

Calculating exposure

Manual flash is like manual exposure; it’s up to you to do the calculations and get it all right, because if you don’t the shot will be a failure. As with automatic exposure, however, automatic flashguns make the whole process seem more complicated than it actually is. Some external flashguns allow you to vary the power output, but we’ll stick to the basics here. The further away your subject is, the wider the lens aperture needs to be to counter the fall-off in light. The aperture you need is related mathematically to the subject distance; it’s a very straightforward calculation, which can be made using the flashgun’s guide number, and which is explained in more detail over the page. Don’t forget to use a shutter speed that’s no faster than the camera’s sync speed.

Guide numbers and zooming

Every flashgun has a guide number, which is both a measure of its power output and is essential for working out the correct lens aperture for manual flash. Modern automated flashguns might hide this guide number away in the specs, and for built-in flashguns that don’t have a manual mode it’s really only useful for judging flash power against rival models. Modern flashguns, however, can zoom to match the angle of coverage of your lens, and this introduces an extra complication, as you need to know the guide number for the zoom setting you’re using; you should find this information in the flashgun’s manual.

How to choose the right f-stop

You can work out the lens aperture using the flashgun’s guide number as follows: aperture = guide number/distance; the guide number usually assumes an ISO of 100 and distance quoted in metres. Remember to use the guide number appropriate to the zoom setting; on the Canon 550 EX, with a zoom setting of 35mm, for example, the guide number is 36. For a subject five metres away, you’ll therefore need an aperture of 36/5, or f/5.0 (to the nearest aperture value setting). That’s at ISO 100; if you’re using ISO 400, the guide number is doubled (you need two ISO increments to double the guide number because of the inverse square law). The aperture then becomes f/10.

How to choose the right distance

You may want to work this out the other way round, and start off by assuming a fixed aperture and then working out the appropriate subject distance, in which case, the calculation is rearranged to become: distance = guide number/aperture. With our Canon 550 EX and an aperture of f/8, for example, our subject will need to be at a distance of 36/8, or 4.5 metres. Incidentally, there’s an important point to make here about flash zooming and guide numbers. When equating the zoom setting to your digital SLR, remember the focal multiplier and to use the lens’s equivalent focal length. 35mm on your flashgun will correspond to 22mm on your DSLR’s zoom.

Using a flash meter

There’s another way of working out the correct exposure for flash, and that’s to use a handheld flash meter. But why would you, when TTL metering is automatic and straightforward? Many professionals prefer to use handheld exposure meters rather than built-in camera meters, and it’s the same with flash. For studio work or carefully-crafted location shooting, it can be important to measure the exposure in different areas of the scene. A flash meter will also be able to measure incident light – the amount of light falling on the subject rather than that reflected from it – and with unusually bright, dark or contrasty subjects, this can be more useful than TTL measurements (reflected light only, remember) will ever be. You’ll need a flash meter sooner or later for studio flash work, particularly if you use more than one flash head. Otherwise, though, you may find you manage perfectly well without.

What are power ratios?

Sometimes your flashgun can be too powerful, and this is especially true with the larger external units; the closer you get to your subject, remember, the greater the intensity of the light, and the smaller lens aperture you need. If you’re shooting a close-up at a range of one metre, or you want fill-flash for a portrait shot at the same distance, a flash with a guide number of 36 will technically require an aperture of f/36! In practice, automatic flash systems will reduce the flash duration to compensate, but manually turning down the flash power is another option. The flash ‘ratio’ is also a factor when you’re using two or more automatic flash units at the same time in a studio set-up. You can set the relative power output of each flashgun to make sure that one is always the ‘key’ light, and the other always offers weaker ‘fill’ light. Using a flash in manual mode, you can reduce the power by halves, with some flashes going from full power to 1/2, 1/4, 1/8, 1/16 and lower. This has the added benefit of producing correspondingly faster flash durations – essential for high-speed flash photography.