Anybody believe that fast glass, ie f/2.8 is better for the AF module, making it work better, than f/6.5 glass on your nikon?

EDIT: To clarify, I mean AF in very low lighting conditions, not daylight or well lighted shots.

2nd edit** Do you believe that there are low light levels in which fast glass, ie f/1.4 will make AF work, when an f/5.6 lens will not?

Sorry for not stating it that precisely in the beginning.

Yes it is... for that reason, some points only operate with f/2.8 or faster lenses. The larger the maximum aperture is, the more light it lets in for the focusing process. The more light let in, the easier it is for the camera to focus.

Taylor Barrett wrote:
Yes it is... for that reason, some points only operate with f/2.8 or faster lenses. The larger the maximum aperture is, the more light it lets in for the focusing process. The more light let in, the easier it is for the camera to focus.


What? What body are you referring to? I believe that is a Canon thing unless i am mistaken. All of my af points work perfectly weather I have a slow lens or a fast lens on. I have never noticed a difference..

That's right, Nikon doesn't distinguish the AF sensors that way. They are guaranteed to work with f/5.6 or faster glass. In Nikon-land, only the orientation of the sensor (vertical vs horizontal vs cross-type) will affect AF.

In fact, f/6.7 (a f/4 lens with a 1.7 TC) will also AF well into fairly poor light using cross-type, as I found out in my trip to the Toronto Zoo.

90 5.0 wrote:
Taylor Barrett wrote:
Yes it is... for that reason, some points only operate with f/2.8 or faster lenses. The larger the maximum aperture is, the more light it lets in for the focusing process. The more light let in, the easier it is for the camera to focus.


What? What body are you referring to? I believe that is a Canon thing unless i am mistaken. All of my af points work perfectly weather I have a slow lens or a fast lens on. I have never noticed a difference..

Yes, that's a Canon "shortcoming" but its relevant to this point. Light and function are the same in principle regardless of brand.

The more light you get, the easier time your autofocus motor has. The wider your aperture is, the more light you've got.

Taylor, the sensors will work well as long as there's f/5.6 worth of light. Certainly, more light isn't going to hurt, but it's not crippled if you only have f/5.6 aperture That is to say, that you get fully functioning AF at f/5.6

Taylor Barrett wrote:
90 5.0 wrote:
Taylor Barrett wrote:
Yes it is... for that reason, some points only operate with f/2.8 or faster lenses. The larger the maximum aperture is, the more light it lets in for the focusing process. The more light let in, the easier it is for the camera to focus.


What? What body are you referring to? I believe that is a Canon thing unless i am mistaken. All of my af points work perfectly weather I have a slow lens or a fast lens on. I have never noticed a difference..

Yes, that's a Canon "shortcoming" but its relevant to this point. Light and function are the same in principle regardless of brand.

The more light you get, the easier time your autofocus motor has. The wider your aperture is, the more light you've got.

yes but you said" saome points will only operate at 2.8 or faster " which is false

i have plenty of situations with long lenses where using a TC was the only way to get enough magnification but the loss in light meant i had to switch to manual focus even on static subjects. dusk owls in flight are about as hard as it gets for ordinary AF situations and even f4 is marginal.

Herb....

I've seen this even with my 70-200 and 17-55.

More light to tell what to focus on, how can the AF NOT work better?

i believe there is no gain any faster than f2.

I noticed that I can focus with 2.8 lenses till I have to use the flash, but my 80-400 stops focusing before that. Sometimes I use my SB800, with the flash part turned, as focus help.

Yes it works better. People don't buy fast glass just for faster shutter speeds and a shallower depth of field.

For shear focus lock speed in decent light, I so absolutely NO difference whatsoever between say a 24-70 f2.8 and 70-300 AF-S F4-5.6. The so called slower lens is instant to focus. Instant is instant and I mean NO hesitation, no waiting and no delay. Lower the light and it's another story entirely. You can throw the background out of focus with the subject easier on a "faster" lens. Otherwise on a current D body they all do a great job.

the AF module needs contrast to determine focus. A brighter scene may offer that greater contrast. A brighter lens may present a brighter scene to the camera. A large aperture lens is brighter. The AF works at maximum aperture. Therefore in general a large-aperture lens can offer better focus opportunities in poor light. In really good light the AF system is happy with whatever lens it has but for technical reasons the aperture must be f/5.6 or bigger no matter how much or how little light is available. If the aperture was allowed to be too small then the AF system would see everything sufficiently well focused and not be able to determine the correct focus.

There can be problems with too little DOF if the camera has initiated a lens search to attain focus, especially if the contrast is too low. That's because the camera may not recognise the focus before the lens has trundled on past the focus point, even though you might recognise it in the viewfinder. No such problem in good light.

Nikon AF sensors seem to be either cross type (better) or linear (not as good) but each one seems to offer a single grade of precision regardless of lens aperture. Canon allows a higher precision with some sensors at larger apertures.


I'm assuming that Nikon AF is similar to Canon in that the camera see the light, determines the actual focus and estimates the required focus change, tells the lens where to go, and lets the lens go there. A good camera can speed up the first parts of the system. A good lens can speed up the last part by having faster AF motors, better gearing, etc. and with a larger aperture it can speed up the first part by providing more contrast for the camera to work with. A good combination works best.

- Alan

Alan321 wrote:
AF system is happy with whatever lens it has but for technical reasons the aperture must be f/5.6 or bigger no matter how much or how little light is available. If the aperture was allowed to be too small then the AF system would see everything sufficiently well focused and not be able to determine the correct focus.



- Alan


Not true exactly. my 70-300 is 5.6 at 300 and it will af with a tamron pro 1.4 tc in decent light and even indoors with good light. This combo would be greater than 5.6 at 300/450 equivalent...

In low light, my 300 f/4 will slow down quicker than my 70-200 f/2.8. Throw on a TC and this is amplified. Therefore I would say that faster glass has the potential to focus faster in low light.

David

As of now, only 2 people, other than myself, voted no, which is about what I expected. Until now, I've always believed that faster glass was better for AF than slow glass, in low light. I suppose it is a holdover from my MF days, where fast glass certainly made a difference to the prism and to the viewfinder in general.

I recently did some tests, on static subjects, with the d200, d300 and d700, using f/5.6 glass and several other lenses, down to f/1.4 glass, in very dark conditions. In no case, did AF fail with f/5.6 that also failed with any other lens. In every case, AF worked at f/5.6 when it worked with faster glass. This included cross type and single type sensors, so that also didn't matter.

I was really surprised to see the f/5.6 lens, the 24-120, lock on targets that I could barely, and I mean barely, see in the viewfinder.

The only difference that I could discern, in about 2 hours of testing, was that the viewfinder was brighter with the faster glass.

My conclusion is that the AF module sensor is very, very sensitive and doesn't need much light at all, to work and work reliably. All of my tests were done in a large room lighted by a single, shaded 60 watt bulb and I deliberately targeted very low contrast, low light objects far from the lamp. The testing conditions were such that no "normal" photographer would even consider trying to shoot without benefit of flash, focus assist or other added light source, (long exposure night landscape scenes excepted).

I did not test TCs, which I tend to think goof AF more than some others seem to think, nor did I do any tracking tests.

no comments? Nobody telling me I'm FOS?

ok, you're FOS No, not really. I'd have expected the AF to be slower in poor light / poor contrast situations and that would be made worse by dark lenses. Did you not notice that ?

There is a difference between accuracy and speed, as there is between accuracy and precision. As the speed of AF determination by the camera slows down there is less chance of successfully tracking moving subjects. Some cameras offer you the choice of focus priority (accuracy) vs speed priority (frame rate).

Another factor is that if you shoot with smaller apertures (whether or not they are the maximum aperture of the lens) you get more DOF and hence there is more scope to disguise focus errors. Technically, focus precision is related to depth of focus and that is related to the optical component of depth of field, which in turn is related to aperture.

Still another factor is that a softer wider-aperture lens may offer poorer contrast for the AF system than a sharper narrower-aperture lens.

- Alan