I’m not sure you can. The theory goes that because an EVF relays exactly what the sensor is capturing, it is by definition an accurate representation of the image you will record.
This is in stark contrast to the optical viewfinder of a DSLR, which shows you what your eyes eye, not what the sensor ‘sees’.
EVF evangelists will also point out that an EVF will be able so simulate exposure adjustments, white balance settings, image effects and styles and more.
All this is true. Well, largely true. But I think it’s leading us to trust EVFs altogether too much. In fact, I’d go as far as to say, trust your sensor, not your EVF.
Why? Two reasons:
1. An EVF is just a digital display, with all the weaknesses of digital displays
An electronic viewfinder is, after all, just a tiny digital display viewed through a magnifying eyepiece. The fact that it’s being fed directly from the sensor is no guarantee of its accuracy.
Digital displays vary a lot in quality, just like desktop displays. They don’t necessarily have a high brightness range or dynamic range, they don’t necessarily display colors accurately and even the sharpest can’t reproduce the detail captured by the sensor.
In fact, based on having used dozens of mirrorless cameras, I would say they are (obviously) excellent framing guides, pretty good at showing you how different effects and settings will look and actually quite bad at indicating the range of shadow and highlight detail the camera will actually capture.
Many’s the time I’ve thought the shadows look blocked in in the EVF, adjusted the exposure to bring them back and little and then found later that I didn’t need to. Just because the EVF is clipping the shadows, it doesn’t mean the sensor will. Indeed, there may be details in the shadows that you didn’t notice when composing the shot that are as plain as day in the captured image.
I’d go so far as to say that I can get a better idea of the dynamic range of a scene and the camera’s ability to capture it with the naked eye, or the optical viewfinder of a DSLR. At least it will show me the full range of tones in the scene.
Of course, if I use the camera’s histogram I can see what will be captured and I don’t have to trust the EVF. I’m just saying that as a visual guide EVFs are not quite all they’re cracked up to be… and there’s a second reason why.
2. If you’re shooting RAW you’re only being shown a JPEG preview
EVFs show a JPEG preview of a scene based on the current camera settings. They don’t show you the range of tones in the RAW file even if you’ve got the camera set to shoot RAW.
Photographers who shoot RAW will do so in order to get, amongst other things, extended dynamic range. Your EVF won’t show you that extended dynamic range – and nor will its histogram, because the histogram too is based on the camera’s JPEG preview.
With the long, slow decline of DSLRs, we can assume mirrorless cameras and electronic viewfinders are the future. So be it. But just because they are digital and fed straight from the sensor, don’t imaging for one minute that they are an accurate representation of the tonal range you will actually get.
It would be an interesting exercise to test the dynamic range of EVFs to find out just how potentially misleading they actually are.