seeking some advice regarding a purchase

Started Sep 23, 2012 | Discussions thread
awaldram
awaldram Forum Pro • Posts: 13,258
Re: Wich "science" please? ;-)

antoineb wrote:

let's talk science then: you are probably aware than on a DSLR:

You are not talking science but your view of the world.
Your stated facts are basically wrong.
http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/5700/AUTO-FOCUS/Auto-Focus.html

  • the AF chip receives ONLY light that goes through a narrow semi transparent area in the main mirror, and then needs to be reflected by a secondary tiny mirror

  • as a result the incoming light goes through an f8 aperture - it makes no difference for the AF whether you're using a lens with a large aperture (such as my superb Nikkor 85mm f1.4) or a cheap lens with a small aperture

This is not true at all, your confusing the reflecting mirror and baseline aperture willy nilly here.
This very simplified explanation may help you differentiate the two

http://graphics.stanford.edu/courses/cs178/applets/autofocusPD.html

  • the AF chip itself is tiny, no larger than then tiny imaging sensors in compact cameras

What does sensor size have todo with anything.

  • finally for the AF to work, you need for two DIFFERENT optical paths (the one from lens to semi-transparent area of mirror, to 2nd smaller mirror, to AF chip - and the one from lens to imaging sensor) to be made to coincide exactly

on the other hand on a mirrorless system, whether compact or interchangeable lens:

  • the AF is done with the main imaging sensor, which in the cheapest compacts is about the size of the AF chips on DSLRs, and in more expensive compacts or mirrorless systems, is MUCH larger than the AF chips on DSLRs

Again your point? (even though your statement is not true )

  • the AF is done using ALL the incoming light vs. just whatever part manages to get it via a tiny semi-transparent are

Again your very confused as to how either system works seeing inconsequential as weakness and missing the obvious advantages of either system.
http://www.lensrentals.com/blog/2010/07/how-autofocus-often-works

  • there is no need to make two different optical paths coincide

At last a true statement but again missing the massive issues in CD in that it does not know which way to turn (literally) cannot detect camera shake, has no inkling of DoF and cannot implement trap in focus.

If you want to be scientific and care to read reviews, you'll find that many DSLRs, apart from the most expensive ones, will cease to be able to AF in light levels where compacts still AF fine.

In the case of my Nikon D7k:

  • its Live View AF will cease to function in still decent light levels, where any of my compacts still does great

  • its PDAF will fail in low light levels, unless the very strong AF-assist light is on

Your camera is faulty

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