Night photography and basic principles

24 April 2012


Photography at night

I recently was asked at a night time photo shoot what to set the camera for this situation.
I paused whilst I considered how to best answer the question to give the questioner the information they sought.
As I was doing so they continued “maybe it’s just that I need flash”
In the end I set the camera to av mode with a large aperture and high iso but I thought I would use this blog to explain the fundamentals.

To make a correct exposure you need a certain (correct) amount if light energy to strike the sensor( or film).
The light source can be either from an external ( usually sunlight) or from flash. At night although it is dark there is still external light around from streetlights, moonlight even stars etc.

There are 3 factors to consider
Shutter speed

Aperture this describes the size of the hole made by leaf blades, behind the lens that allows light to pass to the sensor behind when the shutter is depressed. A smaller number means a larger hole because the numbers relate to the inverse of the radius. So that f 2.8 is a large aperture and f 22 a small aperture.
Different apertures have different characteristics. A large aperture has a small depth of field, meaning that only a small range infront and behind of the focus point will be sharp. This is useful if you want to isolate a subject from it’s background eg a bird infront of foliage.Conversely a small aperture has a large range infront of and behind the focus point in focus and is useful in landscape photography. There are more subtleties to aperture but I won’t overcomplicate things.

Shutter speed denotes the time that the shutter is open. If you are hand holding the camera then this has to be fast enough to prevent camera shake. The rule of thumb on this is to take the inverse of the focal length of the lens as being the minimum speed required, so for a 200 mm lens 1/200sec is needed, for a 50mm lens 1/50 sec etc.

ISO this relates to the sensitivity of film (or now the electronic sensor) to light. The higher the ISO number the more sensitive the sensor to light and so less light is required to give a correct exposure. The trade off, however, is the higher the iso the greater the noise (for digital sensors) or grain (film).

So to tie it together the correct exposure requires a correct combination of all three of these variables. Many cameras will do this automatically and give a good result. I would recommend understanding how to do this manually as it gives 1) an understanding of typical settings for a situation which may pay off in the future 2) it allows more control which then allows you to convert your previualised image to a finished result.

Eg1 landscape in midday sun. Typical settings ISO100, f11,250th sec
The ISO gives good quality, f11 afford depth of field for say a 35mm focal length lens of 10feet to infinity and for a similar lens 1/250 sec will avoid camera shake

Eg 2 portrait in low light with 200mm lens
ISO 400, f 2. 8 1/1000 sec
The higher Iso is required to get a shutter speed that will safely avoid camera shake. The wide aperture gives a shallow depth of field. Overall this allows for an image where the subject is sharp and the background do out if focus that it is more colour than form. This isolates the subject and simplifies the image.

One final note the histogram.
This measures frequency invent y axis and wavelength of light on the x axis (0-256). Unless you wish to have pure black (0) or pure white (256) them all readings should lie between these values. It will look like a bell shaped curve ( or variation thereof ).

So what to set the camera at night?

Happy shooting!