Higher ISO's and the need for fast glass?

Started Dec 27, 2012 | Discussions thread
Ferguson Senior Member • Posts: 1,188
Re: Higher ISO's and the need for fast glass?

John Motts wrote

Also, having a fast prime, such as a 1.4 doesn't actually give you a noticeably brighter viewfinder.

Well, depends I suppose on "noticeably", but the f-stop (or more properly the T-Stop) is exactly that, an indication of how much light you have for a given focal length, and so whether you notice it or not the light is there in the viewfinder and is available for the auto-focus system.

The speed and accuracy of focus of the lens is a combination of many things, but in low light having a lower f-stop improves focus all other things being equal.

That doesn't mean (for example) that the 50/1.4 will focus faster than the 70-200/2.8.  The mechanism in the 70-200 is faster (or so I am told, I do not have the 50/1.4).

But it does mean that as light decreases the 70-200/2.8 will stop focusing entirely a long time before the 50/1.4 will on the same camera.  And your eyes will be able to see in the view finder longer as well, as light decreases.  Try it.

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