Thursday, 4 September 2008

Now for the Science: The Very Basics - shutter speed/aperture/exposure/depth of field

Go back to basics: shutter speed, aperture and ISO

The Very Basics
The way a camera takes a picture is through the lens - you press the shutter release, the shutter curtain is raised and the sensor is exposed to the light and then shortly after the shutter curtain drops and the sensor is hidden. An exposure has now been recorded and we have a photograph (of sorts)

The one important thing to note is that it is the light that is recorded. All photographs are possible because the light reaches the sensor.

Before we discuss the mechanical changes that affect the image created, it is first important to introduce terminology for how light is measured. Light is measured in stops of light, where a change of one stop is a doubling of the amount of light; the means for introducing and reducing the amount of light available for an image are detailed below.

Shutter Speed
The length that the sensor is allowed to record an image is controlled by the shutter speed and this is expressed as fractions of a second with a pair of scales:
32 .. 4, 2, 1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/125, 1/250 ...
25 .. 6, 3, 1/1.5, 1/3, 1/6, 1/12, 1/25, 1/50, 1/100, 1/200 ...

Each value in its set is a stop away from its adjacent value.

There are also other values such as 1/2.5, 1/1.3 etc but these are partial stops.

Notice that a full stop is (roughly) double the previous stride, and in fact think of 1/250 and 1/200 as values to derive other stops.

The amount of light that is admitted is doubled between each stop up; ie 1/125 admits 2x the light that 1/250 would admit.

There are also a setting speed called bulb which means that the shutter is open when the shutter release is pressed and closed when the shutter release is pressed again.

In a sentence:
the shutter speed affects the amount of ambient light that reaches the sensor as it controls how long the sensor is exposed

Click through here to see a larger image.

This image shows the effect of shutter speed on ambient light. We have 2 continuous light sources in the picture, one is (a torch) on the figure in the background and one is (a lamp) to the camera right. The camera's light meter tells us that (at our given aperture of f/4) the scene should be exposed at 1/8th of a second. Notice how the images (to the right) in the composite get lighter and whiter as the shutter is opened for longer - this extra light is an overexposure. Consequently, notice how the images to the left get darker.

So with shutter speed we can control how long the sensor is exposed to the light, but the lens can be controlled to allow a different amount of light through an opening at the back of the lens - this opening can be widen or narrowed; this is expressed as a f-stop:
f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32 ..
Each value is a stop away from its adjacent value.

There are also other values such as f/5.0, f/7.1 etc but these are partial stops.

Notice that a full f-stop is (roughly) double the previous stride, and in fact the base f-stop values are: f/1.4 and f/2. Every other f-stop is a double of these values.

The opening that admits light that is doubled between each stop up (widening); ie f/5.6 admits 2x the light f/8 would admit during the same timeframe.

Each lens, at a given zoomed focal length, will have a maximum widest aperture - the professional grade lenses will have a constant aperture throughout the zoom (ie f/2.8) whilst consumer grade lenses will have variable max aperture throughout the zoom. Prime (fixed focal length) lens generally come as f/2, f/2.8 or f/4 max aperture but can be faster (ie Nikkor 85mm f/1.4 or the obsolete Nikkor Noct 58mm f/1.2)

The aperture also controls the depth of field (DOF) - this is how shallow a focus we have into the frame. A larger (wider open) aperture (ie f/2.8) means a more shallow DOF whilst a smaller (closed down) aperture (ie f/16) means a deeper DOF.

As said before, a lens allows light through to the sensor through the size of the opening in the lens - but how much light allowed is controllable - an f-stop represents that size (as well as the DOF). Notice the way the f-stop is represented; f/2.8 for instance. think of this as a fraction of how much the lens is open. f/2.8 is more open than f/8.

In a sentence:
the aperture affects the manner in which the light (ambient and flash/strobe) reaches the sensor whilst the shutter is open as it controls the size of the opening, in the lens, which admits light.

Click through here to see a larger image.

This image shows how DOF works. The background figure in the f/4 shot appears to be out of focus. This is true and this is a way to view DOF - the focus does NOT extend far enough into the scene at the given aperture to allow the background figure to be in the field of focus. As the aperture closes down/gets narrower (especially at f/22) however, we are able to see the figure as the region of focus is extended into the frame, and this time far enough to bring the background figure into focus. The low-res composite here doesn't really show the difference between f/8 and f/22.

Importantly, notice as we close down the aperture we have compensated by opening up the shutter speed to maintain the same exposure.

f/4 @ 1/8th.. closing down to f/5.6 to f/8 (2 stops) results in the shutter opening to 1/4th and finally to 1/2th.

Similarly from f/8 @ 1/2th to f/22 (3 stops, f/11 to f/16 to f/22) we open the shutter to 1 sec to 2 sec and finally to 4 sec for the same exposure.

A note on stepping down: Although stepping down to smaller apertures will theoretically increase the DOF, in practice diffraction will affect the sharpness of the over image. For the topic of sharpness the topic of circles of confusion need to be considered.

So how do shutter speed and aperture relate
Well, they both offer us a way to control the amount of light that gets to the sensor. And now we get onto the concept of exposure: underexposed and overexposed. Each adjustment to either the shutter speed or aperture is a change of one stop.

For the natural light photographer, given a scene, generally, you start at a given aperture or shutter speed and you change the other until the camera's meter tells you that the scene is correctly exposed as per photographer's requirements.

If there is too much light, then the image is overexposed (too bright/washed out/white).

If there is too little light, then the image is underexposed (too dark/black).

Consider that we have our scene correctly exposed, metered at f/5.6 @ 1/60. What happens if we go to f/8 for greater DOF?

The scene is now underexposed by one stop - f/8 narrows the opening for the light to the sensor but we already know that we need a lens opening of f/5.6 for the desired exposure at the given shutter speed of 1/60.

Ok, but what if we adjust the shutter speed to compensate for the fact that there's less light reaching the sensor? So we open up the shutter one stop to 1/30.

We're now at f/8 @ 1/30. The meter now tells us the scene is at the same exposure as previous.

Therefore, for the given scene we have seen that: f/5.6 @ 1/60 gives exactly the same exposure as f/8 @ 1/30.

Now think about this.. and this is important. The amount of light that reaches the sensor is consistent in both cases but the manner in which it reaches the sensor has changed, and along with it, the DOF.

In the Bryan Peterson's book Understanding Exposure this control and relationship is paraphrased as such:
Think of 2 buckets, one empty sitting on the floor and the other full of sand. We place a funnel with a narrow opening over the bucket on the floor. We then start pour the sand through funnel. To transfer all the sand takes a couple of seconds (say) and we are left with sand in a nice tall, concentrated pile.

Now take the same scenario again, but this time the funnel has a larger opening. We start pour the sand through funnel. To transfer all the sand takes a shorter amount of time but we are left with sand in a flatter, more scattered pile.

Now, think of funnel as aperture and the bucket on the floor as the sensor.

The funnel with wider opening scatters sand and produces a less neat and less precise pile of sand - this is what happens with a shallow DOF (the background is scattered and not precise).

The funnel with narrower opening scatters sand less and produces a neater, more precise pile of sand - this is what happens with a deeper DOF (the background is less scattered resulting in a clearer background)

We see with a narrower funnel it takes longer for the amount of sand to get through - this time is equivalent to shutter speed. Therefore, the same amount of light to get through given a narrower aperture (bigger f number such as f/16 as apposed to f/2.8) requires a longer shutter speed.

Furthermore, there is also ISO (or what was also called ASA in film days). ISO is an another way to control the exposure and represents the sensitivity of the senor to light. The ISO steppings are: 100,200,400,800,1600.. with intermediate values between. Each step (ISO 100 to ISO 200) is a difference of one stop, where the higher the ISO the more sensitive the sensor is to light.

Therefore, an exposure at 1/30 @ f/8 ISO 200 can also be made with the same DOF at 1/60 @ f/8 ISO 400 - a typical use of ISO is to reduce the shutter speed, particularly for handheld only shots in low light to try to minimise camera shake. The down side of stepping through to the higher ISO levels is that there is an introduction of digital noise on the final image.

Another use of ISO is to increase flash efficiency. Given a flash exposure at 1/250 @ f/2.8 ISO 200, if we needed to increase DOF (to f/4 say), we could go and increase the flash output by one stop OR we could bump the ISO to ISO 400.

No comments:

Post a Comment