The Exposure Triangle is the bedrock of photography.
All images, no matter what they are taken with have these three elements in balance to get the desired exposure.
Light passes through a hole (the Aperture), for a period of time (the Shutter Speed) into a photo sensitive surface (film or sensor) which has an ISO (light sensitivity rating) to create a captured image.
Adjusting any or all of these three values is how we control the amount of light needed to expose the image properly, but also, to adjust depth of field we can use the aperture, to capture motion blur we can set the shutter speed and to control the image quality vs light sensitivity we use the ISO setting (although these are all up to other factors also).
Aperture
Shutter Speed ISO
Simple when you know it, applicable to most situations with a little adaption of thinking or terminology.
When you have determined the correct exposure, whether it is “technically perfect” or to your own creative tastes, changing any of these values requires changing at least one other to compensate.
If you want to open up the Aperture to make your depth of field shallower, you will also be letting in more light (through a bigger lens opening), so you need to reduce light by either dropping the ISO value lower (making the sensor less light sensitive) or increase the shutter speed (less exposure time) or a combination of the two.
Mastering these and their inter-relationship is the core of photography.
*
Using flash is a rare exception, but when you get it, it is as easy to apply, possibly even easier.
The above triangle is still just as applicable for ambient exposure when using flash which is to say, to capture the amount of environmental light you want before the flash capture, but for the flash exposure itself, this is the new triangle;
Aperture
Flash Power ISO
Shutter speed has been replaced by flash power for a couple of reasons.
Shutter speed has an effect on flash in that it is often restricted to a maximum “flash sync” shutter speed otherwise the flash fire may not cover the whole sensor or film plane when it fires. High speed or “FP” flash can fix this but requires substantially more power and some flash and camera combinations cannot use it.
The second reason is, flash is substantially faster than camera shutter speed, anything up the 1/100,000th of a second. The camera shutter is like a snail compared to a bullet, so flash works effectively independently (except for the sync speed thing above). Aperture and ISO still matter, because they are not speed dependent.
Changing the shutter speed will have effectively no effect on flash exposure, only background exposure levels.
Balance is key.
This is an ambient only exposure, using the room lights and no flash. The shutter speed is just fast enough to avoid subject blur at the distance shot with a wide angle lens.
Once you have chosen a shutter speed, aperture and ISO that gives you enough environmental light, but also fast enough shutter to freeze ambient motion if needed*, then you set your flash power to balance it’s own exposure, effectively like a second shutter speed for a second exposure requirement.
This is the same space with ambient light capturing the background and flash capturing the foreground. In this case the flash was fired upwards with most coming off the ceiling and some bounced forward off a white card to help even out the coverage. Notice that the background is warmer looking than the more daylight balanced flash lit area.
In this image, the background is deliberately under exposed because there is nothing of great interest except the background pin lights (the table to the right is only just visible).
A catch here can be adding in enough ambient light to see the important elements as wanted as the flash exposes the main subject, but not at too slow a shutter speed to capture it’s motion, which will result in ghosting of the subject (blurred motion around a sharp flash capture).
If your main subjects are in shadow, the shutter speed is not overly important as the flash “speed” is plenty to arrest movement. It is when you mix both flash and strong ambient light so they are fighting for the same subject that problems can arise.
This image avoids too much dance movement blur by capturing the foreground shadowed subjects. These would otherwise have little exposure so would have been black silhouettes. The dancers under the lights in front of these girls would be at risk of blur, because they are better lit naturally and the shutter speed was only 1/100th, not fast enough to freeze motion. The laser on the right looks a little blurred at this shutter speed.
For me, switching from TTL to manual gave me a stronger understanding and also more control over this space. TTL, effectively auto flash, has it’s uses, but manual is more reliable, more efficient and unlike the film era when you had to do mildly complicated math every exposure, it is as easy as turning a dial and getting a feel for the space.
I know I can walk into most rooms with a M43 camera set to ISO 800, 1/100th and f2.8 (depending on ambient light levels) with the flash set at 1/8th power* and I will be roughly right to go. Flash power may shift from 1/1 to 1/16th depending on other elements, but 1/8th seems a good average.
If using a fixed light on a stand (bounced brolly), it is almost always 1/16th power**, ISO 800, f2.8, 1/100th at my normal working distance (about 3mtrs).
If your camera settings are consistent, you only have to learn your flash output by eye and guestimation can be plenty close enough, especially if the ceiling is an even height and distances similar. I find that thanks to the clean nature of flash exposure, being about 1 stop under or 2 over does little harm, so shoot first, adjust, shoot again if needed.
Flash is a great tool and needs to be mastered if you want to be able to handle most situations at a professional level. It can seem daunting, but the reality is, it only takes an awareness of a few new variables, often easily controlled ones and with some experimentation you are quickly in a much better space.
I now find flash a safe place to play, not a mysterious beats with teeth.
*This is done by either capturing the motion if the shutter is fast enough, or cutting the light down so only the flash captures anything.
**Godox 860 or 685 or **YN430 IV. I use the Godox for walk around with TTL as a safety net, the all manual YN’s for static setups, but either can do either.