This is, in theory, the first of a series of contributions in which I will try and explain my mental model of how a camera works. Actual physics, chemistry or electronics is not going to be available here: while I’m reasonably good with the technical side of photography I’m not all that interested in the intricacies - I just want to be able to extract the effects I want from the tools I have. Other people can correct my idea of how the optics work, how exactly the silver grains in negatives get that way and so on. Not my problem. If I’m not clear, ask away.
A camera is, at it’s core, a simple thing: it’s a box that allows light to fall onto a piece of film or a digital sensor in a controlled way.
At the back of the box is the piece of film or the sensor (5), the details of which will obviously affect the eventual image produced - if you put black and white film in there you’re not going to get a colour picture out. At the front of the box is a hole (1) that allows light in. The distance from the film/sensor - the focal length (4) controls the angle-of-view of the image and the size of the hole controls how much light falls on the sensor. In the hole is a lens (2), which controls how the light is focused on the sensor, and a shutter (3), which controls how long light is allowed to fall on the sensor for. This gives us five basic controls: the nature of the sensor, the size of the hole, the distance of the hole from the sensor, the lens focusing and the shutter speed. Depending on your camera, some of these will be fixed, some will be chosen automatically and some can be controlled by you.
Film or sensor I don’t actually want to talk too much about the thing that we’re letting the light fall on: the film or sensor. For the moment we’ll just say that every sensor has an ISO rating, and the higher the rating, the more sensitive the sensor is and the less light it needs to form an image. Film with a rating of ISO 200 needs half as much light to form a good image as does film rated at ISO 400. There is a trade-off though: the quality of the image tends to suffer as the ISO speed increases. High ISO film tends to have much larger “grain”, while digital sensors tend to get “noisy” as the speed increases:
Low ISO = Slow = lots of light to make a picture = smooth High ISO = Fast = little light to make a picture = noisy/grainy
Low ISO = Slow = lots of light to make a picture = smooth
High ISO = Fast = little light to make a picture = noisy/grainy
Aperture - the size of the hole The hole that lets light in is covered by a magic device that allows us to set the size of the hole, it’s aperture. This is measured in f numbers, running from maybe f1.0 or so up to f64 or even higher. Ranges from f2.8 to f22 are not unusual to see on a lens. The larger the f number, the smaller the aperture - they’re fractions of the diameter of a notional circle, so that f2.8 is 1/2 the circle allowing light in, f4 is 1/4, f5.6 is 1/8 and so on.
Small f number = 1/small number = big hole = lots of light allowed in Big f number = 1/big number = small hole = little light allowed in Aperture also affects the depth of field of the image - more on that later.
Small f number = 1/small number = big hole = lots of light allowed in
Big f number = 1/big number = small hole = little light allowed in Aperture also affects the depth of field of the image - more on that later.
Focal length - the distance of the hole from the sensor The distance of the hole from the sensor, which is the focal length of a lens, controls the field-of-view of the image formed on the sensor by the light.
If the focal length is long, the angle of view will be small and the image will appear bigger on the sensor. If the focal length is short the field-of-view is large and the image appears small in the sensor. The details of this depends on the size of the sensor, so it’s normal to convert to a 35mm equivalent so that we can compare focal lengths on cameras with different size sensors. It would make more sense to talk about the field-of-view, but we don’t generally. Changing the focal length, either with a zoom lens - which allows you to effectively change the distance of the hole from the sensor - or by changing lens, does not have the same effect as moving closer or further from the subject. There are subtle differences in how the subject will be presented between the two.
The lens The lens, which we’re just going to consider magical for the moment, allows us to choose how far away light has to be coming from for it to be in sharp focus on the sensor. Focus 1m away and light coming from 1m will be in sharp focus. Focus at infinity and light coming far away will be in sharp focus. If we didn’t have a lens we wouldn’t get sharp pictures at all - all light would be more or less equally focused, depending on the size of the aperture. With a lens, the aperture affects how much of the picture is in sharp focus - the depth of field. Roughly, the larger the f-number and the shorter the lens the greater the range of distances around the focus distance that will be in sharp focus.
In the “drawing” above the camera is focused on the stick figure. With a small depth-of-field - typical of a long lens (85mm+) and/or a small f-number (say f1.8) only the figure would be in sharp focus, the rest of the picture would be blurred. With a large depth-of-field - typical of wide angle lens and/or large f-number (say f16) almost the entire picture would be in focus, with little blurring showing anywhere.
Controlling how long the light falls for - the shutter The length of time the shutter is open is the other part of what controls how much light falls on the sensor. It also controls how much freezing or blurring of movement can appear on the image. High shutter speeds allow in little light and freeze movement, while slow shutter speeds allow in lots of light but movement will blur. 1/500 of a second is fast. 1/30 of a second is slow.
Putting it together The ISO of the sensor controls how much light it needs to make a good image. The aperture and the shutter speed control how much light falls on the sensor. The focal length controls the field-of-view that the light will come from Taking a picture is a matter of balancing those controls.
In the shot I posted in photos as usual of the spiral shell on the sand I wanted shell and sand to be in full focus because the detail of the grains worked really well with the shell.
In this one I wanted the eye to be drawn to the holes in the stone - although I wonder if the DOF could have been a bit larger because the blurred side of the stone on the right hand side is a bit distracting. Ad astra per aspera
Film with a rating of ISO 200 needs half as much light to form a good image as does film rated at ISO 400.
Is there a mistake in this sentence, or does "higher ISO" means lower number, or am I not awake yet to comment on technical matters?
Great tutorial, BTW. Helps clarify my misformed notions that came from a physics rather than photography viewpoint. Un roi sans divertissement est un homme plein de misères
For a film camera, a bright day would use ISO 100. An overcast day would use ISO 400 which needs less light to capture a photo with.
With digital you can get very large ISO numbers for night shots and very low light but it does get very grainy eg at ISO 1600. Ad astra per aspera
Is it worth adding that, in old money, the ISO was sometimes referred to as the "speed" of a film?
In other words, for a given light level, you can take a picture with a higher shutter speed/shorter exposure if you're loaded with a "fast" (ISO 400) film than a "slow" one (ISO 100).
Where you really notice this is in photographing moving objects that you want to keep in focus. Like children.
