Focus Point Sensitivity?

[Playdo]

Can someone explain focus point sensitivity to me. I understand the difference between cross/horizontal and vertical types but I'm not quite sure about the sensitivity.

If a crosspoint is f2.8 sensitive does this mean that it only works if the aperature is set to f2.8 or wider? A f5.6 sensitivity is better than a 2.8 point because it is sensitive to f5.6 and wider?

 

[Garbz]

Yes and no.

There are focus points which are sensitive at only f/2.8 and wider. These are *better* than the f/5.6 ones but only in contributing final accuracy in well lit circumstances. They are disabled (which I think is a bad) with max apertures smaller than f/2.8. I think it may be due to the narrower depth of field that f/2.8 gives, but I'm just guessing here.

I think this is all marketing crap holding our cameras back. It's not aperture that should goven when a camera stops focusing but light. There's no reason a camera can't focus with an f/8 max aperture lens in broad daylight. But Nikon / Canon / whoever say no.

 

[Playdo]

So it refers to the lens' max aperture? ie a 50mm f1.4 lens could focus accurately at f3.5 (on a 2.8 sensitive focus point) whereas an 18-55mm f3.5 lens at f3.5 couldn't?

 

[marp]

Yes, all cameras (afaik) focus with the aperture wide open, so the max aperture is what's important. (I am speaking about lenses without aperture ring).

 

[Derrel]

Garbz' statement, "there are focus points which are sensitive at only f/2.8 and wider. These are *better* than the f/5.6 ones" is in error. In the newer Nikon focusing modules, there are AF sensors which are CROSS-TYPE CAPABLE AT f/5.6 and LARGER apertures, and the f/5.6 ones are, therefore, "better".

If an AF sensor is cross-type capable at f/5.6, that means it is cross-type capable with two full f/stops' LESS light-admitting power than one that requires a lens aperture of f/2.8; the reason Nikon has introduced a 51 point AF system with so many cross-type capable sensors at f/5.6 is that there are sooooo many slow,consumer kit lenses and superzooms that drop to a maximum aperture of f/5.6 that it truly makes sense to equip the 300, D3, D3x, and D700 with a pro-class AF system that only requires f/5.6 to bring the benefits of cross-type sensor capability to the game.

The statement that this is all marketing crap is in error as well. There IS A REASON why a camera can not autofocus with an aperture of f/8 in daylight; phase detction autofocusing systems work based on differentiating between "in" focus and "out" focus on an aerial image. At f/8 with many lenses, like short lenses, once the lens is focused even close to the hyperfocal distance, the differentiation between "in" and "out" with the lens actually stopped down to f/8 is almost nil; at the current state of technology and price, f/5.6 is about the smallest amount of admitted light that will allow a phase detection AF system to actually "function". Anybody who has enough experience with lenses and telephoto converters will tell you that once the effective maximum aperture drops below f/5.6 and into the f/6.3 to f/7.1 range, autofocusing becomes MUCH,much less-accurate, slower, and on many lenses, very inaccurate and or simply un-reliable. Or even inoperative.

It would be "nice" if we could autofocus lenses that had f/8 maximum apertures, but there are only a handful of lenses that actually have that pathetic of a maximum aperture. However, there are those who will put a 2.x tele-converter onto a slow,pokey kit lens, and wonder why it will not focus. It would also be nice if we had cars that could get 250 miles per gallon of gasoline, but at the current state of technology we cannot achieve that desired goal, just as a phase-detection (slr-style) AF system that functions even reasonably well at f/8 is simply not possible.

 

[Garbz]

Hence the *s around better. I was referring to those specific f/2.8 points being used to enhance the accuracy of the focus thus providing a *better* result. I believe this is how it works in many cameras.

Derrel if the reason for the limit was so clear cut then you'd also need to accept that a 400mm f/8 mirror lens would probably give you better depth of field than a 10mm f/2.8 Yet the AF is disabled at f/8 why? And again f/5.6 is the smallest light emitted? What light emitted? Why does my camera autofocus work very well after dusk at f/4.5 but then not in the bright sunny day at f/8? More light is definitely emitted in the latter.

At the moment what you have said still indicates to me an arbitrary line drawn for two highly variable features (DOF and available light).

Not all pro lenses are large aperture too. The 70-200 f/4 from Canon produces some stunning results with a 1.3x and 1.7x multiplier.

 

[Playdo]

Is that true that most cameras (which have f5.6 sensitive focus points) cannot focus if the aperture is set smaller than 5.6?

I'm relatively new to photography but have taken many photos at f8 using autofocus. So what is happening there - Is the camera autofocusing but not accurately?

 

[Garbz]

No just some models specifically disable AF if the MAX aperture is smaller than f/5.6

Cameras focus, meter light, and provide the viewfinder at the maximum aperture, and Derrel is right there are a very very limited number of lenses that would actually have such a small aperture as their max aperture.

When you take a photo at f/8, a split second before the shutter opens, as the mirror is moving out of the way the camera closes your lens aperture to f/8 At this point all autofocus is already finished, as is the calculation of exposure.

 

[Playdo]

Ok that makes things clearer. The camera autofocuses by opening up the aperture to it's widest setting and focusing before stopping down to the set aperture.

So the only problem this would cause is if the lens' widest aperture was smaller than 5.6 or if teleconverters were used (producing an aperture smaller than 5.6).

I feel clear headed now.