Why fast lenses?

Beginners often ask me “why should I buy those lenses with the low F-numbers? Why not buy the standard lens with the 5.6 on the front? They say the lens with the 2.8 on the front is better but I see it takes the same pictures: it’s just more expensive, right?”

Not exactly. The low F-number means  the lens has a larger maximum aperture. This means two things: it lets in more light, meaning faster shutter speed; and the ability, should you choose this, to create blurrier backgrounds.

Here’s my hand at F/11. Recently in Scarborough, while the other instructor, Christine, was explaining the effects of the Aperture setting. My hand is as close as the lens will allow while still achieving focus. I am in Aperture mode (“Av”) and have set the camera to a setting of F/11:

f11

Now the same at f/5.6: a much blurrier background, see:

f56

And now, since I have an expensive lens, I can go even farther, to the extreme end of this lens, namely f/1.4. Meaning very wide open. Meaning very blurry background – and when you look carefully, even foreground:

f14

So that is why people buy these lenses with those low F-number – i.e. “fast”lenses. If you want to blur the background dramatically. Or if you want to have the resulting faster shutter speed (at the same ISO, f/1.4 gives a shutter speed four times faster than f/2.8, and 16 times faster than f/5.6).

18 thoughts on “Why fast lenses?

  1. Excellent no bs explanation of ‘expensive’ lenses. I always tell people who ask- get an ok body but buy the best lenses you can afford. That’s the actual investment.

  2. It seems like we’re talking about two things here and I don’t fully understand how they inter-relate.

    1. Lower F numbers mean that more light hits the sensor or film. Therefore you can take better exposed pictures in low light at a given ISO / shutter speed.

    2. Low F numbers also mean decreased DOF. That’s desirable in many situations, e.g. portraits but I don’t think I want it all the time. For example sports photographers surely need a higher DOF to get more than the scorer’s nose in sharp focus but shooting at high shutter speeds in available light.

    Are the two related by a law of physics or is it just our technology limitations? Why can’t I have a F1.4 lens that has a high DOF performance?

    To put it another way, are all F1.4 50mm (say) lenses created equal? Maybe the Canon has different characteristics from its Nikon competitor?

    And how about history? If I looked a an old Leica F1.4 lens would it show the same DOF performance as one I purchase today?

  3. Yes, lower F-numbers mean:

    1. More light can get in (yes this is simple physics)
    2. Shallower DOF (yes this is simple physics).

    Because:

    “The F-number is the diameter of the lens opening as a fraction of the lens focal length.”

    Bear with me. This means a 100mm lens with an f/4 aperture can be opened to a maximum opening diameter of 100/4=25mm. A 100mm f/2 lens would open to 100/2=50mm diameter when all open.

    This means, the lower the F-number, the bigger the opening. Said differently: the lower the F-number, the more glass. In fact, every stop faster (lower F-number) means you need to double the amount of glass.

    And yes, DOF is physics, and is the same across time as well as brands.

    Does that help?

    PS Trivia Quiz question: what is the reason for those numbers: 2.8, 4, 5.6, 8, 11, 16, 22, 32? Specifically, what is the ratio between successive numbers?

    • You’ll need to help me with a bit more detail or point me in the right direction for #2. As I dimly recall learning a wide open lens has a shallow depth of field because of imperfections in the lens design. Lenses work best when the light passes through the middle. That’s why for consumer grade lenses we are told to shoot at F8 for best DOF.

      With smaller apertures than F8 (bigger F-numbers) other lens imperfections start to apply.

      So you have not yet convinced me the DOF performance is a matter of physics, cannot change and is the same for all lenses of a given max aperture / focal length.

  4. Oh, one more thing: the f-number is the MAXIMUM aperture. You can ALWAYS set the lens differently. So I can set my f/1.4 lens to any aperture between 1.4 and 22. (But when I set it to f/22, only a little light gets in!)

  5. The engineer in me couldn’t resist answering your question “what is the reason for those numbers: 2.8, 4, 5.6, 8, 11, 16, 22, 32? Specifically, what is the ratio between successive numbers?” :).

    The answer is √2 or approx 1.414. This is derived from the fact that the area of the aperture opening is (pi*d^2)/4, so if the area of aperture one is twice that of aperture two then the diameter is √2 times larger.

  6. Spot on! The square root of two, for exactly that reason. The numbers are chosen so that each stop down gives you half the light. And that means reducing the diameter by the square root of two!

  7. Oh no, it’s not lens imperfections! Unless “being larger than a zero size point” is an imperfection! try this: hold a piece of cardboard in front of your eye – or even a small circle made with your index finger Close to your eye. The smaller you make that circle, the greater the dof (test that by looking at something close by while also seeing the background).

  8. Excellent. I’ll check. Oh and indeed with small apertures (say, f/11 and greater numbers) indeed diffraction effects come into play, making it harder again rk get super sharp images.

    Now I’m going to teach 25 people how to use a Canon Rebel , in three hours. 25!

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