Lenses and Optics

Things You Should Know About Your Lenses, But May Not

Published February 6, 2011

Floating Elements Don’t Float

I get asked about once a week what a floating element does (and a bit less frequently if that’s what’s rattling inside when a lens is shaken). Most lenses have poor performance at their closest focusing distance. Center sharpness may be good, but aberrations and corner softness increase when you’re shooting closeups. Floating elements (Nikon calls it close range correction) are lens elements outside of the primary focus group that change position when the lens is focused on a close object, correcting aberrations and improving close up performance. If you’ll be shooting things 20 feet away a floating element is probably not an advantage, but if you like close up work a lens with a floating element will work much better for you. Almost every macro lens has them, but many other lenses do too.

A wide angle lens with 2 focusing groups, giving a floating element.

And a telephoto lens with a single focusing group.

Fixed Apertures Aren’t Always Fixed

At least they aren’t with macro lenses. Focusing on very close objects reduces the effective aperture of the lens, so even if the lens is supposed to be f/2.8, it may not be when you focus very closely. The actual promise of fixed aperture is “throughout the zoom range you can get f/2.8 — unless you focus really closely”. If you shoot Canon you might not notice it because the Canon cameras will still report f2.8 (or whatever the aperture is supposed to be) but automatically adjust ISO or shutter speed, or whatever variable they need to in order to maintain proper exposure. Nikon and other brands will actually report that your f2.8 lens has a maximum aperture of f/3.5 or f/4 when you focus closely.

Variable Apertures Vary More Than You Might Expect

I bet you know your f/3.5 to f/5.6 lens goes from f/3.5 to f/5.6. The manufacturers will tell you that in a heartbeat. But try to find out where it changes and you may be interested in the results. Here’s an example: Lets look at similar lenses with slightly different apertures, the Sigma 50-500 OS f/4.5 – 5.6 and the Sigma 50-500 f/4 – 5.6. OK, so the old one has half a stop wider aperture at 50mm. But look at the point in the zoom range where the lens hits its smallest aperture, f6.3:

  • 50-500 OS f6.3>220mm
  • 50-500 f6.3>420mm

Don’t get me wrong, I’d rather have the excellent OS (optical stabilization) than the half stop of light at 400mm most of the time. But some cameras have decreased autofocus accuracy when the aperture decreases beyond f5.6, so it’s certainly possible that the old lens would focus more accurately on your camera at 400mm.

It’s not a big deal, but it is interesting. For example, if you compare 70-300 zooms, Nikon’s is at f5.6 by 130mm while Canon’s stays under f5.6 until 200mm. The Olympus’ 70-300 gets all the way to 260mm before it hits f5.6. And one generalization: if your lens is f3.5 to whatever, it will probably be f3.5 only at the very widest end of the zoom. Move it 2mm out from that and the aperture will change.

Author: Roger Cicala

I’m Roger and I am the founder of Lensrentals.com. Hailed as one of the optic nerds here, I enjoy shooting collimated light through 30X microscope objectives in my spare time. When I do take real pictures I like using something different: a Medium format, or Pentax K1, or a Sony RX1R.

Posted in Lenses and Optics
  • Cfreak

    I know a lot about cine lenses and less about still lenses. It was a very informative read: confirming things I do know, confirming things I suspected and informing me still more. Using still lenses to shot HD video I realize how spoilt I was in the movie business and why the true cine lenses COST SO MUCH 🙂 -Thank you.

  • Mini

    This article is also available in German.

    Thanks for mentioning after i have already read THE WHOLE ARTICLE in English!
    Anyway there are some interesting facts in here.

  • Jacek Z.

    One thing that seems closely connected, but might be missing on page 4 is “Aperture and Shutter Values Aren’t Totally Accurate”. For example, switching a 50 mm prime from f/4 and 1/500s to f/5.6 and 1/250s on the same object the amount of light coming to the sensor – at least theoretically – should be equal. But often IT ISN’T.

    So… either something is not too accurate with the shutter speed, or the lens diaphragm mechanism doesn’t work perfectly, letting more or less light than declared. Let it be just 1/3 EV. Small difference, but a difference, especially when this “pleasing circular aperture” gets a bit oval at some values.

  • David

    That bit about the magical wizard’s tube made me laugh. Oddly, my TC is the only lens I regret buying. Never seem to use the thing; never willing to sully the performance of the base lens.

  • Walter

    Ugh, the rattling IS.

    It turns out that Olympus sensor-shift IS systems calibrate/reset themselves every time you turn the camera off, which makes a rattling noise. Easily the most common newbie question on the 4/3 forums over at DPReview is “I got this E-whatever and it’s great, but it makes this horrid rattling noise every time I turn it off, is it broken?”

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