Monday, 24 February 2014

The new Triangle of Exposure

Only a few years have passed since we were shooting film and now everybody is making images with their Smartphones. So where has Photography gone?

In a nutshell taking a photograph since the 1850s involves exposing a sensitive surface to light for a short instant through a hole of variable size.

This is still what we do: through the size of the aperture we control the quantity of light, but also the Depth of Field: we determine how many things are in focus along the perspective. The smaller the hole the more objects in focus.

By controlling the time of exposure, the Shutter Speed, we freeze (or not) the action we are recording.

The third element is the Sensitivity of film, that in Digital has been replaced by that of the Sensor. 

The sensor is made by wells which are filled by light. If you increase the ISO of the camera it becomes more sensitive. How? In the same way any a radio, a Hi Fi, works: by electronic amplification: Gain.

Increasing the ISO however will translate in Noise. the image will become more dirty, just like your sound when you increase the volume: you will hear background noise. In the camera it will show with little dots and colour smearing.

None of this will be perceived by you if you use a smartphone, but being a photographer is precisely wanting to control personally the values of the Triangle of Exposure. 

(Courtesy Cambridge in Colour)

In a camera there is another control in addition to those above: EV, Exposure Value correction: with a wheel (or button) you can control the brightness of the image i.e. the proportion of the image which is in light compared to that which is in the shadow.

In film times this acted like a small correction to the aperture of the camera, but in digital it works by a small  electronic 'gain'. Exactly like in the ISOscale, +1 EV stop correction is exactly the same of the difference between 100 and 200 ISO.

Since the camera companies adopt ISO scales that are not always the same, many have cried foul, and protested that there should be only one dimension: Brightness - and no sensitivity scale anymore.

There is an interesting discussion at DP review on the concept, and the subtle semantic differences between Exposure, Brightness and Brightening here at DPR:

Although I am aware of the problem it doesn't influence me one bit. When the light goes down I increase the ISO (or I let the camera do it) like everybody - in order to keep elbow room in my choice of camera Aperture and Shutterspeed.

What I am aware however is that by doing so I increase  Noise. So perhaps the Triangle of Exposure could be revised in Aperture - Shutterspeed - Noise.

One should be aware that a digital camera, differently from film, converts photons to electrons at the cost of noise. So you rate a camera for its Signal to Noise, its S/N. 

The more a camera has S/N the more it will be tone sensitive. The wider the tonal scale i.e. the Dynamic Range of shades between absolute white and absolute black.

See here for the different kinds of noise, in relation to gain:

So shall we abolish ISO scales? Both companies and gearheads would never had it. They are the conventional way to rate a camera sensitivity, and thus to promote sales. People want to shoot as late at night as they can, presumably because they have their fun after work.

I prefer to shoot at Dawn or Sunset, with daylight anyway. I don't go over ISO 800 - I was going to say. But in fact I leave my AutoISO climb up to 2000, so little is the noise in my EM-5.  I relish in the operational speed. But how would I even express this without an ISO scale?

It is important to know that the camera processor knows only about brightness in Digital. The ISO scales are just a conventional way to express how the camera is able to offset  the decline of light, by electronic gain.

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