The central AF sensor is almost always the most accurate

Started Jun 24, 2012 | Discussions thread
ThePaleRider Regular Member • Posts: 251
Re: The central AF sensor is almost always the most accurate

May I politely suggest that you direct that question towards somebody qualified to comment within Nikon? You may be disappointed to discover that such a person is difficult to find amongst the salesmen whom Nikon dress up as technically competent reps though.

I did not specify and design this system, so maybe you should be asking the people who did. Especially if you are not happy with their decisions.

primeshooter wrote:

ThePaleRider wrote:

For many (if not most) cameras, my own included, the standard deviation of the normal distribution about the correct focal distance may be very small, yielding very good results.

Discontentment arises because for some cameras, that standard deviation will be much larger (it is another aspect which is influenced by manufacturing tolerances) giving a much greater spread of actual focus results around the intended focus distance.

Extreme discontentment arises when the camera decides to plonk the mean focus distance nowhere near to the intended point, far behind it as some new camera users are reporting.

primeshooter wrote:

ThePaleRider wrote:

The outer AF points are specified as being able to place the subject within the DOF when the lens is stopped down to f/5.6.

Technically, it is anywhere within that DOF, with results roughly fitting a normal(ish) distriubtion around the precise point of focus. This is with a theoretical lens with no focus shift.

It does seem rather laughable that the spec is this lax, but that is how it is. If the point of focus is routinely outside of this rather wide region, the you may have an issue which Nikon will take seriously. If it is just imprecise, Nikon will likely say that it is within spec.

Whilst that might be the case, when I display good technique I am getting perfect focus at 24mm on both centre and far left and right points at only 2.8. And with a 50mm 1.4 wide open the exact same, so maybe I have the most exceptional D800 in the world, who knows?

Also to the OP, you cannot focus and recompose at f/ need those outer points if your subjects eye's are up there. I often use 5:4 mode now.

Let me ask you a question. Why do you think we have outer focus points like we do in these cameras? Nikon know that to focus a fast prime at 1.4 for example wide open on a subjects eye's at close distance, that if you used centre and recomposed you'd be out (pythagoras theorem) in terms of focus distance you would see backfocus as the hypotenuse of a triangle is the longest side. I cannot believe that 5.6 is it. The reason they are there is to pull focus accurately, granted they are much slower (expected).

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