Flash comes in two flavours.
The first is TTL which means “through the lens”, referring to metering that is done by the camera and flash as one unit. This sounds great, but has its issues and the units themselves are more complicated, brand specific and like for like, more expensive.
TTL sounds ideal, but like camera metering it can be tricked and some times even be at odds with them. It fires a series of pre-flashes off the subject surface and decides, based on the average 18% grey rule, what the correct exposure is. Just like camera exposure, high reflectance, or a mostly dark or light subject can force the camera or flash to make poor decisions.
This is still automation of the highest level and sometimes it is the best option. It is ideal if you are in a hurry, unsure what to set, have a fluid situation to deal with, only get one chance, or want to experiment. It does however lack consistency.
This inconsistency is where it fails most often. If you need to shoot, then adjust and shoot again, you may as well use manual, which then avoids having to do it again.
TTL worked well here as I was juggling a modifier held by my journalist, with time pressure and felt the flash would not be tricked by anything in the scene. Reflective surfaces are the tricky bit, especially very light or dark ones. Shooting across like this is pretty safe and the subject was not moving, or in strong light or a long way off.
Manual, as the name suggests is a throw back to the original days of flash photography, but in the modern world it is oh so much easier.
Instead of having to set all your camera settings around fixed light, you have levels usually ranging from 1/1 ratio to 1/128th in increments of half or a third. This is a lot of fine control, probably more than is actually needed.
Instead of having to fire off Polaroid test shots, even if that was possible, or even pre-test, then develop film and note down the needed exposures, digital gives us instant feed back. This leads to empowered intuition, which comes from experimenting and experience, i.e. getting to know your gear.
The other main advantage of manual is power drain.
The easiest way to kill flash batteries is using TTL and/or high speed sync flash, because to flash has to file multiple times per activation to do its job. HSS, needs to fire multiple pulses*, TTL does also for metering. I do not know the technical details here other than to say my usually well behaved YN560’s and Godox 685/860 units only ever obviously warm up in these modes or Multi flash mode**. There is often a more obvious “popping” sound from the flash fire also.
On the other hand, a single set of batteries in a single YN560 did for me, over 1200 fires using 1/8th to 1/32nd power last Saturday and still had full bars at the end.
Butterfly light from a shoot through brolly above at 1/64th and a silver reflector below. I would guess that probably 5-6000 flash fires would be possible with these settings.
My road to manual flash has come from necessity and a little foolish bravery at first. It is now a well settled process, so here are my take aways.
If I use the same base camera settings, which are f2.8, ISO 400, shutter speed to suit ambient light, then I can rely on my units working usually in the range of 1/64th to 1/8th with bounce, modified or even direct flash. This is a little fluid, but the more I do it, the more I get it right or nearly right the first time.
As an example, my standard setup is one upper right convex (shoot through-main) 42” brolly and one lower left concave (reflected-fill) 42” brolly at about 2m to the subject with the above camera settings, which is almost guaranteed to be 1/16th first go. If not perfect, it falls well within processable parameters. If I do change, it is now only in full steps as any more precise is pointless. Basically after a single test shot, I am in the ball-park next frame.
This image was shot with a slightly high shutter speed (for dance floor moves), so the background has dropped out. In this case it was fine, but normally I would go with more ambient at a slower shutter speed. There was still a nice feel to the background lights.
Another example is I shot the school dinner the other day with the same camera settings, shifting the flash power between 1/32th and 1/8th and the aperture from f2 to 2.8 all night by feel. If a shot was a little under or over, I adjusted until the same happened again, but did not lose a single frame to irretrievable exposure error, except the very first fired at 1/1 because I did not check.
Ok, the basics.
Set your camera to manual exposure (same, but different to the flash).
Set your aperture to the widest workable setting which for M43 is f1.8 to 2.8, FF might be f2.8 to 5.6. This is dependent on the needed subject depth (f2.8 on M43 and a semi wide lens guarantees a multi row group) and distance to subject as the ISO and aperture will effect the flash power needed.
For ISO, usually 400 is fine, but do not be afraid to use higher settings as flash cleans up exposures nicely. I use 1600 often with M43 expecting “A” grade results even on older cameras.
These to are now the determining factors for your flash power setting. They have almost nothing to do with ambient light, just flash power.
If you are using bounce flash, these settings will likely be good for most closer distances (6-20ft), but if you are using direct flash, which is linear by nature, meaning it will drop of dramatically over distance (the inverse square law), you may need to adjust the aperture, ISO or flash power to compensate, or switch to TTL, which is more useful here.
Set the flash setting to any setting (1/16th is my starting point), and fire it off as a test. This is the bit where experience and intuition come into play. After a while you will set a power value that you will likely not have to adjust. It comes with experience and there is flexibility.
Now look to your shutter speed. The shutter speed has a special role here. It controls the ambient light recording, which is to say it determines how much light you will record without flash and this has two very important effects.
The first effect is controlling the naturalness of the background. If you want a black background, set a high shutter speed, if you want a more harmonious one, use a lower setting, but be careful of subject movement.
Subject movement was “staged” so not too hard to capture, allowing for plenty of ambient light in the frame. The effect is ideally invisible flash enhancing the image naturally.
If there is enough light recorded to capture subject movement without flash, but it is at a slow enough shutter speed to blur that movement, you will get a “ghost” image overlayed on your “frozen” flash subject.
Flash when used, effectively becomes your shutter speed and it is fast, at lower power settings super fast, like 1/4000th to 1/20,000th or even faster on smaller units. Ambient shutter speeds of 1/60th to 1/90th are not. If they share the same image, you will see the effect of both like a badly registered 3D image.
This was a keeper, but many others like it were not. Notice the slight fringe around the girls face and dress edge? The background subjects, lacking as much flash are mostly just blurred. This is the blurred ambient exposure sharing the same space as the perfectly frozen flash element. The picture is effectively a double exposure, one part caught perfectly by the flash at probably 1/6000th of a second or so, one part blurred at about 1/90th.
This bit is up to you and may need some caressing on a case by case basis. The desire to add ambient light for balance is strong, but a random movement can look odd or ruin a frame. You can use HSS with manual, but again flash and battery power both suffer.
*
So to sum up, my working method for roving candids would be something like this;
I set ISO 400 at f2.8 and a flash with a “white foamy thing” mod bounced off the ceiling would likely at about 1/16 to 1/32 power on a YN560 flash. If flagging the front of the flash relying only on bounce, the higher setting is usually chosen, for forward fire, the lower, but ceiling height/reflectance and other factors may come into play. If I am struggling with a high and/or darker ceilings or distant wall I will push the ISO up or open the aperture before raising flash power.
I set the shutter as needed to bring out ambient light, slower is fine for posing groups, but for dance floor images I will usually set 1/250th to (a) freeze motion better and (b) allow the dance floor lighting to do the work. If this fails I will resort to HSS, staying at the lower end around 1/500 to help the flash out.
If you are using warm background lights and flash together sometimes the flash is colder and bluer, making the subject look a little at odds with the background. A CTO correction Gel can be used on the flash, or in my case I use the EM10 mk2 and a Panasonic lens and a soft white foamy thing, which seem to balance things to slightly warmer. Also as flash units age, they tend to warm up, so avoid using brand new ones. For static shots, creamy white brollies have the same effect.
If dealing with differing distances such as red carpet arrivals shot in the open with no bounce available, which makes the flash output very linear, I will use the aperture to adjust output (front dial), simply because it is easier to do quickly. For the red carpet on Saturday I switched between f1.7 (other end where interviews were held) and f2.8 (close end posed shots), which effectively changed flash power by 1-2 stops. There were a few that were a bit off, but most were fixed easily enough in post. TTL would have likely been easier her, but it also may have had the odd miss (reflective surfaces, black background and movement) and likely killed a set of batteries.
*High speed flash or HSS is used when you need a faster shutter speed than the cameras maximum flash sync speed (usually 1/180-250th). It fires a burst of flashes to guarantee coverage of the sensor, allowing you to use very wide apertures or shoot in strong light or both. It severely reduces flash range and drains batteries, so another way is to use an ND or neutral density filter to reduce the light getting through without straining your flash.
**Multi is where the flash fires multiple times in a second during a single exposure. A technique I am keen to master, it is a flash/battery killer, the instructions of most units even warning against repeated use without a rest.