Fast lenses, and High ISO

Started Jul 18, 2014 | Discussions thread
Great Bustard Forum Pro • Posts: 42,576
Re: Understanding ISO

PalmettoFellow wrote:

Great Bustard wrote:

Chikoo wrote:

Fast lenses, as they are called allow for more light to hit the sensor and in turn allow for fast(er) shutter speeds. The F number provides a relative measure of how much this ability is.

In this age of ever increasing ISO, are fast lenses needed anymore? The only ability I see the fast lenses provide was actually a disadvantage that happened to become a feature, and that is shallow DoF, allowing for separation of subject from the background.

That said, should they be called Fast Lenses or Shallow Lenses?

When people say things like "In this age of ever increasing ISO...", what they mean is "In this age of ever more efficient sensors". This is not a small, or pedantic point. The reason is that there are two primary sources of noise in a photo:

  • Photon Noise (noise from the light itself)
  • Read Noise (additional electronic noise from the sensor and supporting hardware)

So far as photon noise goes, modern sensors have a QE of right around 50%. What this means is that half of the light that makes it through the CFA (Color Filter Array) onto the pixel is recorded. Thus, for a Bayer CFA, there's only a stop more to go to 100% efficiency.

In terms of read noise, most modern sensors are at around 2-3 electrons per pixel at high ISO settings. The role played by the ISO setting is that higher ISOs result in lower read noise (up to a point) due to the analog amplification applied to the signal through the ADC -- Analog to Digital Converter. That means, for example, that ISO 3200 is less noisy than ISO 100 pushed 5 stops. However, most all sensors are ISOless by ISO 3200 (some are ISOless right from base ISO), so that ISO 25600, for example, has the exact same noise as ISO 3200 pushed 3 stops.

In other words, what we want is lower read noise, not "higher ISO". The thing is, though, a read noise of 2-3 electrons per pixel is already pretty low (considering that the pixels on FF DSLRs, for example, are able to produce signals of around 100,000 electrons), so why are high ISO photos noisy? The answer is simple: they get very little light, and less light means more noise.

In other words, it's not like sensors can keep on getting more and more efficient and low light photography will get less and less noisy. For sure, the read noise represents a kind of floor beyond which the noise in the photo quickly diminishes. But even with zero read noise, we are limited by the light itself.

So, do we still "need" fast lenses? Inasmuch as we want less noise for any given shutter speed, yes, we do. Of course, that lower noise comes with the concomitant effect of a more shallow DOF, which sucks for those that want a deeper DOF in low light for a given shutter speed.

No, but I have a pretty decent wand.  Wanna see a pic? 

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