Low Light Performance – It’s all about the lens!

workshops-275 Low Light Performance - It's all about the lens!

This post follows on from my previous post about sensors and was inspired by one of the questions asked following that post.

While sensor size does have some effect on low light performance, the biggest single factor is really the lens. It isn’t really bigger sensor that has revolutionised low light performance. It’s actually the lenses that we can use that has chnge our ability to shoot in low light. When we used to use 1/2″ or 2/3″ 3 chip cameras for most high end video production the most common lenses were the wide range zoom lenses. These were typically f1.8 lenses, reasonably fast lenses.

But the sensors were really small, so the pixels on those sensors were also relatively small, so having a fast lens was important.

Now we have larger sensors, super 35mm sensors are now common place. These larger sensors often have larger pixels than the old 1/2″ or 2/3″ sensors, even though we are now cramming more pixels onto the sensors. Bigger pixels do help increase sensitivity, but really the biggest change has been the types of lenses we use.

Let me explain:

The laws of physics play a large part in all of this.
We start off with the light in our scene which passes through a lens.

If we take a zoom lens of a certain physical size, with a fixed size front element and as a result fixed light gathering ability, for example a typical 2/3″ ENG zoom. You have a certain amount of light coming in to the lens.
When the size of the image projected by the rear of the lens is small it will be relatively bright and as a result you get an effective large aperture.

Increase the size of the sensor and you have to increase the size of the projected image. So if we were to modify the rear elements of this same lens to create a larger projected image (increase the image circle) so that it covers a super 35mm sensor what light we have. is spread out “thinner” and as a result the projected image is dimmer. So the effective aperture of the same lens becomes smaller and because the image is larger the focus more critical and as a result the DoF narrower.

But if we keep the sensor resolution the same, a bigger sensor will have bigger pixels that can capture more light and this makes up for dimmer image coming from the lens.

So where a small sensor camera (1/2″, 2/3″) will typically have a f1.8 zoom lens when you scale up to a s35mm sensor by altering the projected image from the lens, the same lens becomes the equivalent of around f5.6. But because for like for like resolution the pixels size is much bigger, the large sensor will be 2 to 3 stops more sensitive, so the low light performance is almost exactly the same, the DoF remains the same and the field of view remains the same (the sensor is larger, so DoF decreases, but the aperture becomes smaller so DoF increases again back to where we started). Basically it’s all governed by how much light the lens can capture and pass through to the sensor.

It’s actually the use of prime lenses that are much more efficient at capturing light has revolutionised low light shooting as the simplicity of a prime compared to a zoom makes fast lenses for large sensors affordable. When we moved to sensors that are much closer to the size of sensors used on stills cameras the range and choice of affordable lenses we could use increased dramatically. We were no longer restricted to expensive zooms designed specifically for video cameras.

Going the other way. If you were to take one of todays fast primes like a common and normally quite affordable 50mm f1.4 and build an optical adapter of the “speedbooster” type so you could use it on a 2/3″ sensor you would end up with a lens the equivalent of a f0.5 10mm lens that would turn that 2/3″ camera into a great low light system with performance similar to that of a s35mm camera with a 50mm f1.4.

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