How can a lens have a 'speed'?

This week's Photography Fundamentals column answers a question that came to me as an anguished plea in my Twitter feed from a relative newcomer to photography. In short: How can a lens have a speed? Whilst he knew that speed equated to aperture and that a 'fast' lens was one with a large aperture, the terminology felt far from intuitive. How could an opening behind a lens be described in terms of 'fast' or 'slow'?

I'm guessing that if he were perplexed by the use of the the word 'speed' in conjunction with 'lens', there are a few other people out there who find the concept that lenses can be 'fast' puzzling, too. Or illogical. Or something that's accepted terminology. Or something that made sense many years ago but has since become obsolete. This might help.

It's not actually as illogical as it sounds to refer to lens speed. It's more a case of joining the dots.

We know that large apertures are referred to as 'fast'. A lens with a large maximum aperture, let's say ƒ/1.8 for the sake of an example, is described as a 'fast' lens. We know that the larger the aperture, the more light is able to reach the sensor (or film, if you're old school). We know that by the virtue of the exposure triangle, the more light that is able to hit the sensor, the shorter the time the sensor needs to be exposed to capture the image and the faster the shutter speed we can use. The shorter exposure time is a direct result of the larger aperture. The larger aperture allowed for a faster image capture, hence a large aperture is a fast aperture.

If you were wondering, yes, it's for this reason that higher ISOs are referred to as fast ISOs: increased sensitivity allows for faster capture.

A lens can have a 'speed' then, because it refers to how fast it can allow you to capture your images, which I hope isn't nearly as illogical as it first seemed.

Rule of thirds << Photography Fundamentals >> Time-lapse

How exposure works

To understand exposure in photographical terms, EV is probably the single most important number you will have to understand, to understand the theory behind the art of photography. This goes from your tiniest, least significant compact camera, to your cock-on-the-table style medium format camera with a digital back.


Let us imagine a value called TCE. This TCE (The Correct Exposure) does not exist, because you might for a variety of reasons want a different exposure than the TCE. But for the sake of argument, let’s assume TCE exists, and this is what you will want when you take a certain picture.

To get a correct exposure, you will want to have EXACTLY the right amount of light to capture your image. Not too much, and not too little.

So, what is it that might affect how much light comes to the film or imaging chip?

  • Shutter speed – Imagine a mug with a lid containing a mysterious source of light, and the room you stand in is covered in darkness. Shutter speed would be how long you open the lid.
  • Aperture – Same cup, same concept, but this time, how far you open the lid (if you open it a little – small aperture, i.e. high aperture numbers (for example f/22). If you open it all the way – large aperture – i.e. low aperture numbers (for example f/2.8)
  • These are the two basic ones. The last factor that comes into play is your film speed, or the light sensitivity of your surroundings while holding the cup if you will.

That’s all there is to it – these three factors combined allow you to manipulate the light in all kinds of ways (big depth of field through small apertures, freezing motion through fast shutter times, etc).

So, to get TCE, you will want to combine these three factors into JUST the correct way. Now, if you replace TCE with TCEV (The Correct Exposure Value), you understand what I have been on about.

EV is a number describing an exposure – any exposure – regardless of its “correctness”.


The definition of EV=0 is an exposure of 1 second at f/1 using ISO 100 film, or any equivalent thereof (2 seconds f/1.4, 4 seconds f/2.0 etc)

The technical definition of EV is 2EV = LS/C.

EV = the exposure value – explained above
L = field (or zone) luminance –
C = Exposure Constant – This is a constant that depends on what unit you are using to express the luminance (L)If you use candelas/ft2, it is 1.3. If you are using candelas/m2*, it is 12.5*. If you use apostilb, it is 3,98.
S = film speed following the ISO standard

*) some of you might know cd/m2 as lux or lumens/m2,

This also means that 2ev = A2/T

A = the f-stop number of the aperture
T = shutter time in seconds

Combining these two; EV = log2(A2/T) = log2(LS/C) – which is the only formula you are likely to need, if you want to understand the basics of mathematics behind photography.

So what is the EV number used for?

Ah. Well, the EV number is used internally in cameras – an EV number of 10, for example, would refer to all the combinations of shutter times and apertures that would give a given exposure using ISO 100 film. This is useful, because a camera only has to add one thing to this equation; A light measurement. A camera with a lookup table or an algorithm to calculate the correct EV is all set for using all the different combinations that are able to give you the exposure you want.

But why would I care, if the camera handles everything?

Because the camera doesn’t always get things right. You may also want to use alternative exposures for artistic reasons.

Most cameras have an EV compensation wheel/dial, allowing you to choose how much you want to over/underexpose an image. This is usually measured in +/- 2EV, 1/3 steps. This means that you can over- or underexpose an image by two whole EV steps (which, incidentally, would mean the same as two full f-stops either way), in steps of 1/3 EV.

I hope that made things a little clearer – if not, leave a comment, and I’ll see what I can do!

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