How does AF perform?

[justin_ngo]

Hi,

 

A person in another photo forum mentioned that the AF depend only on physical length of the lens.So he thinks

Nikkor AF-D focuses much faster than the 70-200 AF-S or even HSM Sigma because the lens is very short! Is this

correct???

 

Since my D80 has AF motor, so if I use a AF-S, HSM, or BIM lens, then which one do the AF: the AF motors inside

the lens or inside my D80 body? And which one focuses faster?

 

Please advice.Thanks!

Sorry if this question is asked before.

[ronald_moravec1]

I am sure it depends on how heavy the lens component is and how they have to gear it so as not to stress the in camera motor.

 

The more that has to move, the lower the gear and the slower the focus. It has nothing to do with focal length, except

coincidentally long lens have more mass to move.

 

Motors inside the lens should be faster as they can be made lens specific and as strong or lightweight as required.

[Matt Laur]

If it helps, Justin, I can tell you that in practical, every-day use (not deliberately trying to run the focus from nearest distance out to infinity in some sort of horse race), the screw-driven focus on something as small as my D-class 50/1.8 is NOT as fast my AF-S 70-200, no matter what focal length I have that zoom set to. Likewise, I have a Sigma 30/1.4 HSM, which has the focus motor onboard the lens. It's not unreasonable to compare that to a the non-AF-S 50/1.8. The HSM is much quicker on its feet... and (I find this very important in some social or animal-related settings, since I do work with dogs) the onboard motor lenses are quite a bit quieter. You can really tell the difference.

[justin_ngo]

Thanks!

Yes, he said that 50mm 1.8 AF-D focuses faster 70-200 AF-S because the lens has much shorter physical length.But before posting this question, in my shallow DSLR knowledge, I think the how AF performs ( speed, quiety...) depends on how/where the AF motor made.

[ShunCheung]

If you are talking about screwdriver AF lenses that have no built-in AF motor in the lens, AF speed varies depending on the strength of the motor inside the body. In that case a consumer body such as a D50 is going to be quite different from a high-end body such as an F6, D3, etc. The AF motor inside a D3 is going to be much stronger and faster. Therefore, you cannot generalize this discussion; you need to specify which body + lens couple you are referring to.

[justin_ngo]

Sorry for less detail OP, Shun.

 

He mentions the AF speed of the Nikkor 50mm 1.8 AF-D would be obviously faster than the AF-S/ HSM/ BIM 70-200 on the SAME BODY with built-in-AF ( like my D80) because the lens is much shorter physically.

 

I feel my Tamron 70-200 BIM focuses in low light condition faster, finishes sooner the job, than the Nikkor 50 1.8 does but he does not believe so because the short physical length.

Anyway, I think it also depends on what brand name of the lens' motors, is that correct?

Thanks.

[justin_ngo]

AF-S > HSM > BIM > screw-driver AF... is this order correct?

[richard_driscoll]

You'll also find among the "screwdriver" lenses those with internal focusing tend to be faster than those

without. The internal focus system means that only a few lighweight elements are moved; without IF the whole lens

may have to be moved in its mount.

 

In fact my "screwdriver" 28-105 lens with IF focuses faster on my F90x (N90s) than the little flyweight kit 18-55

AFS does on my D40 - some of the kit AFS lenses are not actually all that fast.

[ShunCheung]

<I>AF-S > HSM > BIM > screw-driver AF... is this order correct?</I>

<P>

Again, you are making a generalization that is very questionable.

<P>

As Richard points out, not all AF-S are created equal. The "AF-S" motors on some of the very inexpensive lenses are inferior. For example, you cannot manually override the focus as you could do on the more expensive, traditional AF-S lenses. Those cheap AF-S motors are slower, but they are also typically on very light and slow plastic lenses that do not have big elements to move around.

[wpahnelas]

i think it is safe to say that your friend's assertion that AF speed depends on the physical length of the lens is wrong. it is not the case that a short lens will always focus more quickly than a longer one. your friend may have it fixed in his mind that less travel during the focus operation equals faster focusing. but to put it plainly, that is not the case.

[justin_ngo]

" i think it is safe to say that your friend's assertion that AF speed depends on the physical length of the lens is wrong."

 

William, that's what exactly I want to know.

Thanks.

[justin_ngo]

Shun, Richard,

 

Now I learn a $1,700 AF-S would be different from the $170 AF-S. I should had known this :-)) Thanks!

[alex_lofquist]

Pretty well!

[eric_arnold]

hi justin,

it's possible you might be taking the statement out of context. but shun is correct that different bodies also factor into the equation, i.e., a d80 vs. a d300.

 

nevertheless, the micromotor versions of the tamron 17-50 and 28-75 are said to have slower focus than the screwdriver versions. i've never tried the micromotor tamrons, but i've seen this mentioned several times on nikonians. it's normal to assume that a lens with a built-in motor would be faster than one without, but this appears to not always be the case.

 

in regards to the 50/1.8, that lens has no motor, but it seems to focus about as fast as my HSM-enabled sigma 30/1.4 and 50-150/2.8. there may be some incremental differences in speed, but i haven't noticed it. it's definitely noisier, though.

 

obviously, the amount of light factors into the equation as well. any lens can 'hunt' in poor light, and going from 50mm to 150mm or 70 to 200mm may not be as instantaneous in terms of acquiring focus as focusing on two different subjects about the same physical distance from each other with a 50mm lens.

 

i have noticed that the tamron 17-50 focuses faster than the 28-75, which isn't due to the physical length of the lens per se, but rather the 'throw' across the focal plane. focus is acquired faster because the lens has less far to travel. it happens that the 17-50 is smaller in length than the 28-75, but it has more to do with the internal dynamics than merely how far the lens extends.

[kohanmike]

Just a quick correction, it's screw-drive, not screwdriver.

[justin_ngo]

"nevertheless, the micromotor versions of the tamron 17-50 and 28-75 are said to have slower focus than the screwdriver versions. I've never tried the micromotor tamrons, but i've seen this mentioned several times on nikonians"

 

Yes, I read a lots about these reports. But I have Tamron 28-75 screw-driver and now Tam 70-200 micro-motor. The new Tam 70-200 built-in-motor focuses much better: quieter, faster, and accurate than the old 28-75.

In this case, I know what I'm having. Thanks.