Again, why "fast" lenses?

A tip for newcomers to SLR photography.

I often hear: “Why do I need so-called “fast” lenses – like the 50mm f/1.8 lens Michael keeps talking about? Surely my 18-55 lens also covers 50mm?”

Well yes it does. But:

  1. Less sharply. A “prime” (i.e. non-zoom) lens is sharper.
  2. A prime lens is also smaller and lighter.
  3. And especially: the prime lens has a lower minimum “F-number” – i.e. a larger aperture. The lower the “F”-number, the better. Your kit lens is f/3.5-5.6 (meaning zoomed out it can go as low as 3.5; zoomed in it can go only as low as 5.6. The 50mm f/1.8 can go as low as 1.8).

Why is this important?

So in today’s class I took two shots of a student in available room light. One at f/5.6, and that is what you would get with your standard “kit”-lens. It looks like this:

Two things happen:

  1. Because of the small aperture (high “F-number”), the camera has to keep the lens open for a long time. This means that unless I use a tripod and tell the subject not to move, in indoors light I will get camera shake (the shot needed 1/10th of a second). And sure , do.
  2. The lower the “F” number, the shallower the depth of field, i.e. the blurrier the background. The higher the F-number, the sharper the background.F/5.6 gives a background that is somewhat blury.

Now look what happens when I use an aperture of f/1.8 (for which you need a lens that can do that, like the 50mm f/1.8 lens):

Much better – a pretty dramatic difference on both counts!

So the best way to immediately get great portrait shots is to:

  1. Get yourself a 50mm lens. On most cameras this is simple; do note that on a Nikon D40/D60/D3000/D5000 you need to manually focus this lens (that is why I recommend Canon cameras at the entry level).
  2. Learn Aperture Priority mode (A/Av) and use a low “F-number”.
  3. Turn the camera sideways and get close!

Have fun.

(Wow, three numbered lists in one blog post!)

0 thoughts on “Again, why "fast" lenses?

  1. That is very generous of Santa: a superb lens, and at f/2.8 a fast zoom. This is THE standard “Pro General Purpose” lens, and on a rebel it is a great portrait lens. I have one too: it is the sharpest lens I own.

  2. Michael…I read your blog regularly. I have a newbie question — a lot of “fast lenses” are pretty pricy (excluding the 50mm 1.8 of course) – are you essentially paying for the ability of using the wide aperture or are they generally better a narrower apertures as well. I guess what I’m asking is in a situation that is well lit…will a fast lens provide notably better image quality?

  3. Quick answer: not significantly. Except as a side effect of the fact that fast lenses tend to be expensive (and often prime) lenses, and expensive and prime means sharper. And that IS an important side effect.

    I can write more later but am now about to get together with a client!

  4. hey Micheal!

    Great blog here…

    I got a Rebel XSi for Christmas and just got my first prime lens, 50mm 1.4f, a couple of days ago for my birthday… and I must say that this has been one of the best improvements I could have ever made towards my photography…

    The ability to shoot in low light with out flash and being able to blur the background and make such smooth bokeh almost make me feel like a professional when I take someone’s portrait!!! Love it!!!


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