AF, fast vs slow glass

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Archive 2008 · AF, fast vs slow glass

Kerry Pierce
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p.1 #1 · AF, fast vs slow glass

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.

Edited on Nov 19, 2008 at 01:42 AM · View previous versions

Nov 19, 2008 at 12:33 AM

Taylor Barrett
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p.1 #2 · AF, fast vs slow glass

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.

Nov 19, 2008 at 12:34 AM

90 5.0
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p.1 #3 · AF, fast vs slow glass

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

Nov 19, 2008 at 01:09 AM

Avi B
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p.1 #4 · AF, fast vs slow glass

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.

Edited on Nov 19, 2008 at 01:34 AM · View previous versions

Nov 19, 2008 at 01:31 AM

Taylor Barrett
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p.1 #5 · AF, fast vs slow glass

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.

Nov 19, 2008 at 01:32 AM

Avi B
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p.1 #6 · AF, fast vs slow glass

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

Nov 19, 2008 at 01:37 AM

90 5.0
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p.1 #7 · AF, fast vs slow glass

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

Nov 19, 2008 at 01:40 AM

HerbChong
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p.1 #8 · AF, fast vs slow glass

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....

Nov 19, 2008 at 03:36 AM

Andre Labonte
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p.1 #9 · AF, fast vs slow glass

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

Nov 19, 2008 at 01:20 PM

BenV
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p.1 #10 · AF, fast vs slow glass

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

Nov 19, 2008 at 01:35 PM

lou f
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p.1 #11 · AF, fast vs slow glass

i believe there is no gain any faster than f2.

Nov 19, 2008 at 01:59 PM

hjanssen
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p.1 #12 · AF, fast vs slow glass

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.

Nov 19, 2008 at 01:59 PM

gman1339
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p.1 #13 · AF, fast vs slow glass

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

Nov 19, 2008 at 02:42 PM

traveler
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p.1 #14 · AF, fast vs slow glass

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.

Nov 19, 2008 at 03:56 PM

Alan321
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p.1 #15 · AF, fast vs slow glass

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

Nov 19, 2008 at 04:01 PM

90 5.0
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p.1 #16 · AF, fast vs slow glass

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...

Nov 19, 2008 at 09:02 PM

eaglewolf
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p.1 #17 · AF, fast vs slow glass

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

Nov 19, 2008 at 09:33 PM

Kerry Pierce
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p.1 #18 · AF, fast vs slow glass

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.

Nov 19, 2008 at 11:05 PM

Kerry Pierce
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p.1 #19 · AF, fast vs slow glass

no comments? Nobody telling me I'm FOS?

Nov 20, 2008 at 02:26 AM

Alan321
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p.1 #20 · AF, fast vs slow glass

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

Nov 20, 2008 at 05:59 AM

Makten
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p.1 #21 · AF, fast vs slow glass

AF speed has nothing to do with aperture. The fact that fast lenses often has good and fast AF is because they are more expensive (only applies to AF-S lenses) or that the optical design is free from spherical aberration that can confuse the AF module.

Kerry Pierce wrote:
no comments? Nobody telling me I'm FOS?

You are right. The AF module does not care for light at angles greater than ~f/5.6 or so (probably a bit slower to be on the safe side).

Nov 20, 2008 at 06:02 AM

Alan321
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p.1 #22 · AF, fast vs slow glass

90 5.0 wrote:
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...

OK, perhaps not true exactly. Still there is a limit and Nikon have chosen to make f/5.6 the notional cut-off. Some Canon cameras work at f/8 quite ok. In general, and as you seem to have noticed judging by your references to "good light", there is a noticeable deterioration in AF speed and/or ability at smaller apertures especially when non-reporting TCs are used or when small lenses are used that lie about the maximum aperture to trick the AF into working (e.g f/6.3 reporting as f/5.6).

- Alan

Nov 20, 2008 at 06:05 AM

R. Francois
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p.1 #23 · AF, fast vs slow glass

I have no idea really. My fast lenses (2.8 or faster) seem to focus really well (AF-s or not). But they better be! sheesh they are expensive

Nov 20, 2008 at 06:27 AM

Alan321
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p.1 #24 · AF, fast vs slow glass

I just thought of something else that may seem a bit contrary to what I've been saying. A lens that has a large aperture presents a narrow DOF and so increases the chance that what you want to focus on cannot be detected at all the camera with the lens at its present focus position. More DOF would give the camera a better chance of measuring focus in cases of extreme defocus. Unless the camera has a focus sensor set up for that role (as some Canons do, but I'm too new to Nikon to whether or not they do) then the camera must initiate a focus search. That will be way slower than if it could determine focus without such a search.

- Alan

Nov 20, 2008 at 11:48 AM

Kerry Pierce
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p.1 #25 · AF, fast vs slow glass

Alan321 wrote:
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 ?

Had no objective way to discern that, really. My only short f/5.6 lens is 24-120vr AF-S. All of my faster glass, other than 28-70, is screwdriver. My inclination though, is to believe that there is no difference with respect to max aperture. The only difference, IMO, would be the actual focus speed of the lens, which would be there in daylight too.

I also had no way to objectively judge "critical" focus. I was shooting hand held with ISO maxed out on all 3 cameras and still getting some very long shutter speeds. Noise was pretty nasty too.... But, again, my inclination is to believe that the AF wouldn't care about max aperture for that either. When the AF hunted, it hunted about the same for f/1.4, f/2, f/2.8, as it did for f/5.6. On targets with "good" contrast in those test conditions, AF just worked pretty much the same on all 3 bodies.

Nov 20, 2008 at 02:55 PM


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