For D.Kerr: about split prism...

[Bogdan Hrastnik]

Hi Doug (and greetings to other members as well),

I've read "Principle of the Split Image Focusing Aid.." article several times and I find it as THE reference for that topic. The point being: because of the way, how You have explained thing there, I was able to understand what's all about.
What I'm missing in this article and I simply "need to know" is: what's the reason, that AF points are positioned in middle of the frame? What prevents them to work at very edge of the frame? May be, that answer can be found, if studying your article carefully enough, but I failed on that.

Thank you for taking time to answer.

Wish you the best,
Bogdan

 

[Doug Kerr]

Hi, Bogdan,

What I'm missing in this article and I simply "need to know" is: what's the reason, that AF points are positioned in middle of the frame? What prevents them to work at very edge of the frame? May be, that answer can be found, if studying your article carefully enough, but I failed on that.

I'm not sure I know whether your question is "why do the manufacturers only provide AF points that work at the center of the frame" or "what makes the AF points only sensitive to a small region of the frame (such as at the center)".

Regarding the former: many digital cameras have multiple AF points working on various portions of the frame, not only at the very center. Usually none of these are in the outermost portion of the frame, I suppose because the manufacturers feel that it would not be generally useful to have them there.

Regarding the latter: the area of the frame to which a particular AF detector responds is controlled by the dimensions of the little sensor array for the detector along with the "aim" of the lenses in the AF detector (just as the portion of the scene to which the basic camera responds is determined by the dimensions of the image sensor, of course taking into consideration the focal length of the lens).

I hope I have answered the right questions. If not, let me know.

Best regards,

Doug

 

[Bogdan Hrastnik]

Hi Doug,

Thanks for answering -even I'm not sure if that's the answer I've expected. Knowing you, I should knew I need to be precise when asking
What I am curious about is somehow both of above questions you mentioned:
-Is there a physical/optical reason (or limitation) why all manufacturers choose to cover the same frame area with AF points, or
-They actually could place AF points wherever they want (wouldn't affect AF precission), but they simply decided not to (because there's no need to have some AF points near the edge of the frame).

Thank you for taking time,
Bogdan

 

[Bart_van_der_Wolf]

Hi Doug,

Thanks for answering -even I'm not sure if that's the answer I've expected. Knowing you, I should knew I need to be precise when asking
What I am curious about is somehow both of above questions you mentioned:
-Is there a physical/optical reason (or limitation) why all manufacturers choose to cover the same frame area with AF points, or
-They actually could place AF points wherever they want (wouldn't affect AF precission), but they simply decided not to (because there's no need to have some AF points near the edge of the frame).

Hi Bogdan,

I think the reason is that the light for the AF sensors has to be deflected from the mirror down towards the AF sensors through some specialized optics. Increasing the size would increase the mass of the mirror (=more wear and noise or need for damping, and increase the size (and thus cost) of the optics that focus the partial images on the AF sensors.

Also, increasing the size of the AF optics would only work with lenses with large(r) exit pupils and it might require a repositioning of the segmented AF optics in the bottom of the mirror box, so it would potentially not work with lenses that have a small exit pupil.

I think the cost issue is the most restrictive factor (manufacturers of mass consumer goods try to save every cent of material cost they can).

Cheers,
Bart

 

[Bogdan Hrastnik]

Hi Bart,

Thank you for comming by to cure my curiosity
Yes, you're right: size/weight of second mirror is probably the main reason for this. It's not that I needed AF points closer to frame edges. It's just, that when reading "Split prism" article, I thought, there might be some "optical" reason as well.

Greetings,
Bogdan