Why do you buy fast lenses/cameras?

Started 3 months ago | Polls thread
tony field Forum Pro • Posts: 10,284
Re: Why do you buy fast lenses/cameras?

Michael Fryd wrote:

tony field wrote:

Michael Fryd wrote:

Don't forget:

  • Brighter Viewfinder
  • More accurate auto focus

If you are using a camera with a padf system, why would an F 1.4 lens provide more accuracy then an F 4 lens or F 2.8 ??

Take the Canon 5D 4 for instance. There are 61 Autofocus points, but not all are created equal. Each point has from one to four PDAF sensors in various orientations; horizontal, vertical, right diagonal, and left diagonal.

Some of the sensors work with lenses that's f/8 or faster, some work with lenses that are f/5.6 or faster.

A few of them are slightly higher precision, but only work with lenses that are f/4 or faster.

Five AF points near the center offer the highest precision, but only work with lenses f/2.8 or faster.

Take the center AF point. With an f/8 lens the horizontal and vertical PDAF sensor will do a good job with most lenses. However, the right and left diagonal sensors can provide higher precision results with lenses faster than f/2.8.

As the camera focuses with the lens wide open, you get the benefits of the high precision focus points with an f/2.8 lens, even if you are shooting at f/8.

Thus the f/2.8 version of the 70-200 lens may yield more accurate focus than the f/4 version, even if you are shooting at f/4 or slower.

with the Canon systems you do not gain more focus precision with a 1.4 lens than you do with the 2.8 lens. As Marianne Oelund pointed out years ago the Canon F 2.8 focusing system does not seem to provide more precision than the Nikon F 5.6 focus system.

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