Why It Is Bad To Underexpose In Low Light

I shot this on auto-ISO, which means that I let the camera determine the ISO while I shot at the shutter speed and aperture at which I wanted to shoot.

The ISO was 4,000 because the light was very low in the late afternoon into evening. Still, something went wrong because the RAW file is underexposed by almost two stops. Here is the default, uncorrected setting:

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When the light is low, the camera is very unforgiving if the photo is underexposed. I am not sure what caused it to underexpose, maybe the settings hit the buffers and it couldn’t expose enough for the limits I had placed on shutter speed.

What I mean by unforgiving is that when I open up the exposure in Photoshop, there is a lot of grain in the photo. You can see it in the corrected version at the top, and more so in this crop.

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So What Is A ‘Stop’?

A ‘stop’ is a halving or doubling (depending on which way you are going) of the light entering the camera compared to the value of the adjacent ‘f-stop’

The standard ‘f-stop’ settings run something like f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22

And if you photograph, you may well have a lens with a maximum aperture of f1.8, which is a half-stop position.

So, for example, f2.8 lets in twice of much light as shooting with the same shutter speed and ISO at f4.

That’s why professional photographers use lenses that have wide maximum apertures such as f 2.8 or even f2.0 or f1.4. That gives them more freedom to use a lower ISO and/or a faster shutter speed.

The ‘problem’ with lenses with fast apertures, especially long focal length lenses is that there is more glass of greater diameter in them – and glass is heavy. Here for comparison are the weights of a couple of 300mm lenses. Of course, the f2.8 lens has more light-gathering power, but at the cost of size and weight.

AF-S NIKKOR 300mm f4E PF ED VR 755g (26 ounces)

AF-S NIKKOR 300mm f2.8 G ED VR II 2,900g (102 ounces)