How does a fixed focal length lens work? - The TrekEarth Forums


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Old 09-17-2007, 05:34 AM
nithin1 nithin1 is offline
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Default How does a fixed focal length lens work?

I read that even fixed focal length lens have auto-focus. I just wonder how does it work? Doesnt the lens moving in and out would mean that its not the same focal length?

Or do I have to walk near or away from the subject to get the focus right?

I see these two models- Canon 50mm 1.4 and 1.8 . The latter comes in one third of the price as the former. Is the 1.4 really worth the price? I mainly want to have a good shallow dof. Also, at 1.4, wouldnt there be a problem of overexposure for outdoors. I can get max 1/4000 in my Canon XTI.
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Old 09-17-2007, 03:00 PM
AdrianW AdrianW is offline
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Default Re: How does a fixed focal length lens work?

If the lens were of a simple design, then yes, the focal length would change; in reality most lenses these days correct for this by moving several elements within the lens at the same time. Rest assured you don't need to walk backwards and forwards to adjust the focus :)

The biggest advantage of the 1.4 is that it has USM with full-time manual focussing; that means you can have the lens set to autofocus, but adjust the focus manually if the need arises without switching to MF mode. Whether that's worth the price difference to you is a tough call.

The 1.8 also has some advantages too, have a look at this comparison on

Remember that a lens doesn't have to be used at its maximum aperture, you can stop down to any other aperture that the lens supports; obviously you'll lose the shallow DOF though. If you want to use that shallow DOF in bright light, invest in an ND (neutral density) filter; they're pretty cheap. Note I mean a non-graduated type.
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Old 09-17-2007, 04:45 PM
kinginexile kinginexile is offline
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Default Re: How does a fixed focal length lens work?

f/1.4 at night or inside will rarely have you shoot at 1/4000th, unless you aim at the source light, or a firetorch blower spits a huge flame. I think the 1.4 is better built than the 1.8, which is liked by many, but chases the focus easily in difficult light situations. Not sure the other does the same thing, maybe not.

When I want to refresh my info about a lens or a zoom, this is the site I always come back to, it will tell you a lot about a lens, and if not interested in tech like me, his conclusions sum up very well:

( has also a good hand-on "review" section by members)
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Old 09-17-2007, 08:33 PM
greg greg is offline
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Default Re: How does a fixed focal length lens work?

The focal length, by definition, is the distance between the lens and image plane when focused on an object at infinity. If you're focussing on something closer, the distance between lens and image plane may vary, but not the focal length.
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