Considering that the mirrorless fever (and war among manufacturers) has been started in the past years, more and more users faces with the issue of flash photography with EVF (electronic viewfinder in a mirrorless camera).
Until you’ve used a DSLR, there were no problems: you saw what you saw trough the glass, regardless of harsh sunlight or a dark studio environment with 100% flash lighting. Although if you use an EVF it is not all the same. If you use wrong settings for your EVF, a studio session will be a nightmare.
EVFs are simulating your exposure, that means what you see in your viewfinder is what you get on your photo. Obviously this is only true in case of continuous light, but if you mix ambient and flash light, moreover if you use 100% flash lighting on your photoshoot, then this is not a good method, since the EVF were not able to simulate flash lighting.
You totally underexpose the ambient lighting in a studio, so your EVF will be dark by default.
I guess that even newbie mirrorless-owners know that the exposure simulation can be turned off on your camera. If you didn’t know this, then read the followings carefully. Turned off simulation will show you a balanced, midtone-weighted picture in your EVF every time, regardless of the exposure settings of your camera. In case of Sony cameras this is called Live View Display: Setting Effects ON/OFF. I suggest you to store this function in your Fn (Quick) menu or in My Menu tab because if you do flash photography frequently, you’ll need this feature a lot. BTW if you put a TTL flash or trigger in your camera-hotshoe, it will active the Setting Effects OFF automatically. It will turn off exposure simulation.
Exposure simulation can be turned off on every mirrorless camera, moreover the Canon EOS R has a half-solution as well. It has a Metering Timer function where you can let your camera ignore exposure simulation only for a couple of preset seconds for focusing and composing. This half-solution may be good when you are mixing ambient and flash light, but not in the studio.
In the studio you may still have issues with a turned off simulation, because
When you turn off the exposure simulation, your EVF will show Live View picture based on the settings of your camera’s metering mode.
If you have dark background in the studio and you use dramatic lighting like spot-lighting, side-lighting and such, then the default Matrix metering mode of your camera (hence your EVF view) will not be good. The EVF will brighten up your live view because of the total darkness in the viewfinder: your black background and shadowy parts of your subject will be dull 50% grey and the partly lit spots of your subject will be blown out in your viewfinder (provided that you are using modelling lights during your work). As a result you will not see the eg. face of your model because his/her face will be blown out in the Live view. See below:
The solution is obviously correct setting of the metering mode in your camera. It will not affect your final picture because you shoot manually in the studio with fixed lighting setup. It will only affect the view in your EVF. You have to change the default Matrix metering mode to something more special.
If we are talking about Sony (I do not know how it goes with other brands), there is a Highlight weighted metering mode. It can be enough for a partly lit portrait at a dark background. I mean you will see the reality in your EVF with this.
The other solution may be the spot metering mode in your camera, especially if the spot metering is not at the center of the frame (like most Canon and Nikon DSLRs can do), but linked to the focus point or to a detected face. In this case the camera (hence the EVF) will do the light metering on that point where your focus point is.
All information above are not evident things. Mirrorless is a different dimension that requires different thinking sometimes. These new cameras are much like computers than cameras, they are like switching from an old Nokia to a smartphone. This EVF issue wasn’t evident to me either despite my 10yrs photography and 1yr Sony mirrorless experience. Being an old Canon user, I even could not imagine that the spot metering can be linked to a focus point. At the Canon-universe this feature is the privilege of the 1D-series only.
This article is not a Sony advertisement. I just have a Sony camera and I can only talk about what I have. I’ve used and reviewed Fuji cameras earlier, but I do not remember how they managed this EVF-thing. As for Canon, Nikon and Panasonic mirrorless cameras, I’ve not tried them yet, so I cannot tell how their menu and such features work.