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Do faster lenses mean better autofocus?


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#1 nathanm

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Posted 04 April 2012 - 09:28 AM

Most autofocus systems need a fair amount of light to operate, and in general they focus faster with a lens that is faster - i.e. has a wider maximum aperature. Sports photographers will use a 300mm f/2.8 lens even if they are shooting at f/5.6 to f/8 because the faster lens acquires focus faster than a 300 mm f/4 or slower lens. Of course a 300mm f/2.8 lens is also typically built for the task, with special focus motors, image stabilization and other features which contribute, but the raw light gathering power is an important factor.

My question is whether this effect works out for the lenses we typically use underwater?

In general the answer would seem to be yes, but the raw light gathering power might be balanced by other factors. Wide angle lenses may not have the same sort of focusing motors as say a 300 mm f/2.8.

A case in point in the Canon 85mm f/1.2 lens. It isn't wide angle, and isn't really suitable for UW use but it is a fast lens. The focusing motor on that lense is ridiculously slow. Canon came out with a second version of the lens that improves the focusing speed, but it is seems to be slower (at least anecdotally) than the 70-200mm IS lens at 85mm.

An as example consider the Canon 35mm f/1.4 and 24mm f/1.4 II lenses. They are two stops faster than other ways to reach those focal lengths which are at f/2.8 (say, the 16-35mm f/2.8 zoom, or f/2.8 prime lenses). Do those two stops matter for either speed of acquiring focus, or the ability to use autofocus in lower light levels? They certainly ought to, but I have not tried them yet. I wonder if anybody else has?

Focusing speed is one issue, ability to autofocus at all is probably more important for UW use. However it is governed by similar factors.

In principle this should also depend a bit on the camera. The forthcoming Nikon D4 and Canon 1DX are supposed to have greatly improved autofocus.

#2 nathanm

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Posted 05 April 2012 - 08:27 AM

Canon has this autofocus white paper about autofocus settings on the EOS 1DX and 5D Mark III.

It divides lenses into various groups (see page 37 onward) depending on number of autofocus points that the two cameras support. Group A lenses support all of the 61 autofocus points that the camear has. It is made up of lenses that are f/2.8 or faster. However, not all f/2.8 lenses are in Group A, some are in Group B, C, D, which can use 41 autofocus points.

Group A includes a couple lenses that are popular underwater (16-35mm f/2.8) and some that are not widely use but could be (24mm f/1.4 and a few others).

Note this is only one measure of autofocus quality. It relates to how many focus poitns are used to acquire focus, and what flexibility you have in choice of focus point. It isn't everything. Some of the lenses in Group A - like the older 85mm f/1.2 lens are notoriously slow at autofocus. So while it may support 61 autofocus points, it doesn't mean that it is a great performer. Meanwhile other lenses like the 600mm f/4 are very fast at acquiring focus, but are not in group A.

I bet that Nikon is similar - there may be a similar white paper on D4 focus system.

#3 Stewart L. Sy

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Posted 05 April 2012 - 08:39 AM

A lens' maximum aperture doesn't necessarily mean it will AF faster. Compare the 85 1.2L vs the 85 f1.8 non-L. The 1.8 version runs circles around the 1.2, but if you want ultimate DOF control then you go for the 1.2. Also, given the super shallow FOF that the 85 f/1.2 can produce, a slower AF might be beneficial as a fast AF motor might go past the focus point...kinda like not braking in time for a stop light. ;-) Fisheye lenses like the Tokina 10-17, Sigma & Canon 15mm's don't have USM motors since the massive DOF and short throw of the lenses don't require fast motors.

Some lenses are meant for sports/fast action, they have very quick AF response, like the 300 f/2.8, 500 f/4 etc. The speed at which those lenses go from minimum focus to Infinity is truly awesome. Then some lenses just don't need fast AF. The 50 f/2.5 Macro is one. Stunningly sharp and distortion free, but it uses the old style focus motor that not USM.

All lenses AF at their maximum aperture to allow the most light onto the AF sensor, it's only when you take the shot does the iris close down to your selected size.

Most cameras focus fast enough, it's the accuracy on how it maintains or acquires focus that's important (kinda like the 1dmk3 controversy)

My 2 cents...

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#4 Paul Kay

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Posted 05 April 2012 - 09:06 AM

I use both the Canon 24/1.4 and 35/1.4 lenses underwater. But first, the 85/1.2 is a bad example of fast lens and focus speed for various reasons not least of which is because it is not an easy lens to focus at all. I sold mine because I found it rarely got used and was not always spot on focus - a common problem apparently and largely down to depth of field which is incredibly shallow. Its a big bulky lens with powered focus even in manual focus mode because of the design and large amount of glass to be moved around.

The 24 and 35 are certainly fast AF lenses - I rarely have problems with either underwater or above and I shoot in the low contrast and light levels found in temperate waters. They also have another useful feature which I really like - the image in the viewfinder is as bright as it gets and added to a Seacam Sportsfinder gives as bright an underwater viewfinder image as you are likely to get. This helps when viewing and assessing what point to use for AF, etc., so indirectly probably helps 'AF' speed as well.

Lastly, comparing the AF of long lenses with a purpose designed, lightweight, internal focus lens group with shorter lenses which usually have a lot more glass to move around (which takes time) is not a fair comparison. To make short focal length lenses focus faster will probably require radical new optical designs or far more effective AF motors. Also FWIW the Canon 50/1.2 will focus much faster than the 1.4 which is slow and noisy (I have one which I use at times but its pretty rough sounding - the 1.2 rarely got used at 1.2 so went too).
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#5 divegypsy

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Posted 06 April 2012 - 09:28 AM

One downside to using the fastest lenses is that a lens's sharpest apertures are usually 2-4 apertures closed down from wide open. In the case of an f1.4 lens this is usually about f4 or f5.6, which might give less depth-of-field than you would like, particularly when using a wider fast lens behind a dome port. The slower shorter focal length macro lenses, both Canon's and Nikon's, are usually sharper at apertures like f8 or f11. So you might find that using a 50mm or 60mm f2.8 macro lens might give you better overall results than a 50mm f1.4 since the sharpest apertures would inherently have more depth-of-field. These shorter focal length macro lenses also perform very well behind a small dome port which increases the angle of coverage over the more frequently used flat ports. I often use the Nikon 70-180mm Micro-nikkor behind a small dome with excellent results.

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#6 Paul Kay

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Posted 06 April 2012 - 10:32 AM

One downside to using the fastest lenses is that a lens's sharpest apertures are usually 2-4 apertures closed down from wide open. In the case of an f1.4 lens this is usually about f4 or f5.6, which might give less depth-of-field than you would like, particularly when using a wider fast lens behind a dome port.

This certainly used to be the case but is no longer as accurate as it once was. Modern fast designs are optimised to work extremely well wide open but still usually perform somewhat better as they are stopped down, and perform extremely well until they hit much smaller apertures and become diffraction limited. I very, very much doubt that the difference in image quality between shots taken at 4, 5.6 and 8, and even at 11, will be significant, but below 11 there may well be degradation due to diffraction. As domes will limit corner quality of fast wides its more a matter of using the fast aperture for viewing and helping AF as opposed to actually shooting. FWIW I have Canon 35/1.4 and 35/2 lenses and the cheaper f/2 is not as good at any aperture. Some modern zoom designs perform extremely well - but they do distort significantly (not much of a problem underwater) and this still seems to be the trade-off of such designs - and they only have maximum apertures of around f/2.8. Whilst slower macro lenses optimise at 8 to 11 they are offering little better performance here than other quality modern designed faster lenses such as Canon's L series.
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#7 pKai

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Posted 06 April 2012 - 01:00 PM

Yes, faster lenses AF faster.... to a point..... Somewhere at about F4-5.6 and under, you won't see any difference... That said... some lenses AF faster than others.... see Canon chart from other poster.

What this means in practice is that if you have one of those variable F-stop zooms where the max is (for example) like F3.5~5.6, you will see a difference in AF speed with the lens on wide (max F-stop 3.5) to Tele (max F-stop 5.6)....

On many cameras/lenses a focus light will speed up AF dramatically. That's true with my Oly E-pl2 and was also true with my former Canon S90. Is it worth $500 for a Sola? Not in my opinion. I rigged up a 4AA LED Princeton Tec ($65 -- the one meant to strap to a mask) to a cold-shoe ball mount and it works as good as a Sola...... doesn't flash or have the "red" option, but who cares. I get 5-6 dives out of a charge from the 2000ma AAs.

#8 Drew

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Posted 06 April 2012 - 07:03 PM

Nathan
As the others have said, fast lens doesn't mean necessarily mean faster but, as you say, it does help with achieving focus at all, since bottom line , there has to be contrast of some kind for the AF to work. At f1.4-2.8 on the super precision X-type AF points, it does help to detect contrast that can't be seen @ say f4. The less precise X-type (I forget the Canon vernacular) work well up to F4.
I've tested this many times underwater and even more times topside.
With the Nikon system, low light AF tends to be quicker. I found the D3s to achieve AF a tad faster than the 1D4 once it gets very dim on f2.8 lenses. I don't have any of the 1.4 lenses but even the specs support this (-2 EV vs 0 EV, Nikon vs Canon respectively) Once there's enough light, the Canons do achieve focus faster, in my experience. I'm not getting into a another Nikon vs Canon AF tracking debate! :lol:

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#9 nathanm

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Posted 07 April 2012 - 07:11 AM

One of the reasons that I am asking this is that a UW photographer can have situations where a wide angle lens is an action lens - say for sharks or other big animals. Wrecks are another case where you want to acquire focus but there isn't much light - in that case it is usually more of an issue of getting focus at all rather than getting focus fast.

Fisheye lenses, and more generally very wide angle lenses (like 14mm) are generally not the ones that a sports photographer uses for action so there has been less reason for manufacturers to design them to be fast. The widest lens that is likely to be optimized for focus would a 16-35mm zoom since it is the go-to lens for a lot of news and photojournalism.

As it turns out, I already have a 24mm f/1.4, but haven't used it underwater. I'll buy a port extension ring and try it.

Canon just announced, but has not shipped, a 24mm f/2.8 lens with image stabilization. It would be interesting to see how that compares. Image stabilazation is another thing that has been lacking on wide angles. Modern IS systems claim 3-4 stops worth of stability. So, will it be better for low light shooting than 24 f/1.4 - it might be by 1-2 stops which would be big. Howeve, autofocus performance may not be as good.

Low light autofocus is a question begged by the fact that Canon and Nikon are both shipping cameras that have extremely high ISO. I just got the 5D Mark III, and from my initial tests ISO 25800 actually looks pretty usable. 1DX supports that too, but that hasn't shipped yet. Nikon D4 ought to be similar. Of course no housings exist yet so it will be a while before we see these underwater. High ISO may be interesting for wrecks or other big UW subjects that you can never light with strobes.

#10 Drew

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Posted 07 April 2012 - 07:40 AM

Nathan,
If it's action you're shooting IS won't do much since the shutter speed will have to be in the higher range of the X-sync to freeze action, to prevent ghosting. It may be useful with slower shutter speeds. I've used the 24-105 IS as a general dive lens. I tried it with IS a few times but since I like to shoot video as well, IS is turned off for me.

High ISO performance is definitely going to be a boon for action photography in crappy light. Some of my diving is shooting within 1-3 hours of dawn in the southern hemisphere winter with fast predator action and I usually go up to 1250ISO with the 5D2 so I can maintain f7.1 or F8 and more importantly shutter speed of 1/200 or more.

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#11 Glasseye Snapper

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Posted 07 April 2012 - 12:46 PM

What is a better investment, buying the fastest glass for all your lenses (at hundreds of dollars more than a lens that is one or two stops slower), or getting a good set of strobes and focus light. After all, people take perfectly good shots on night dives with zero ambient light. The only situation where I see this break down is if you are shooting objects from such a large distance that strobe and focus light power falls off too much. Low light large predator action may be one of them.

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#12 divegypsy

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Posted 16 April 2012 - 02:55 AM

As I mentioned previously, most lenses reach their highest resolution 2-4 apertures from wide open. Paul Kay says this is no longer so. Here is a short review of some very new Zeiss lenses and what Zeiss reported about the resolution of those lenses. One lens had reached a resolution of 320 lp (line pairs) per mm, which was the highest any Zeiss lens had ever achieved. This resolution was reached at apertures between f2.8 and f5.6 with their new 50mm f1.4. This best resolution, at f2.8 to f5.6 is exactly as I suggested, 2-4 stops from wide open.

Copied from http://www.photodo.com/topic_96.html

Details from the (Zeiss) newsletter include:
The new ZEISS ZF lenses went to test for resolving power recently. Attached to a Nikon F6, which was mounted on a Sachtler heavy duty tripod, we exposed our Eastman resolution test chart onto Kodak Imagelink HQ film. The best we had ever achieved before with any SLR lenses was 250 lp/mm.

The new Planar T* 1.4/85 ZF achieved that same resolution at f/5.6, and even down to f/2.

The new Planar T* 1,4/50 ZF went even further: It reached 320 lp/mm in the aperture range from f/5.6 to f/2.8, and 250 lp/mm at f/2.

NOTE THAT ZEISS IS NOT CLAIMING EQUALLY HIGH RESOLUTIONS AT F8 OR F11, which would be 5 & 6 apertures from wide open. Jim Brandenberg, when talking about his testing of the Nikon D800 prototypes also talked about using fast lenses like Nikon's 24mm f1.4 and 35mm f1.4. He also said that to get best sharpness he had to use apertures like f5.6 and f8, and occasionally f11, but that beyond that the lenses lost too much sharpness (to diffraction}. In almost all shooting with almost all lenses there is a hobson's choice of resolution vs depth-of-field

When Paul says his Canon 35mm f2 is not as good at any aperture as his 35mm f1.4, I would say, " You get what you pay for. Do you really expect a $330 lens to be better than a $1750 lens (US B&H prices)?

Particularly with the very fast lenses, the manufacturers know that many of the buyers of such lenses want and expect to use them frequently at the very wide apertures. And so the manufacturers try to maximize lens performance at those faster apertures. This is not say that these lenses are not good at smaller apertures, just that the resolution will not be as great. And that maybe a quality f2.8 lens will provide better sharpness at f8 or f11 than a quality f1.4 lens. Compare the Canon 50mm f2.8 macro lens at f11 vs their 50mm f1.2. This is a good reason to look at lens reviews and to test lenses you may want to buy. Faster apertures always provide less depth-of-field, and depth-of-field is necessary to offset the focus disparity between the center and edge of the image a dome port produces with a rectilinear lens.

Edited by divegypsy, 16 April 2012 - 02:56 AM.


#13 divegypsy

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Posted 16 April 2012 - 03:59 AM

A second response is necessary to answer the original question posed as to whether faster lenses focus faster or work in lower light.

The specifications of most auto-focus cameras give a light range, usually in EV's, in which the camera will be able to auto-focus. An f1.4 lens delivers the same light level to the camera's auto-focus sensors in 4 times (2 f-stops) less light than does an f2.8 lens. So the answer is yes, a faster lens can auto-focus in lower light. Because the lens with the faster maximum aperture also exhibits less depth-of-field at that wider aperture which is when auto-focusing takes place, the f1.4 lens (vs an f2.8 lens of the same focal length) should also be able to provide greater focusing accuracy if the lens does not shift focus as it is stopped down to the shooting aperture.

Will the focusing be faster depends more on the focusing motor and the weight and size of the lens elements being moved during focusing. And while faster focusing might seem to be an advantage, it can be a disadvantage if the focusing is so fast that the lens whizzes right past the correct focus before detecting that it is there. This very problem has been discussed on other Wetpixel threads concerning the newer AF-S VR version of Nikon's 105mm f2.8 Micro-nikkor vs the older 105mm f2.8D lens with body driven auto-focus, which does focus slower, but has far less "searching" problems, particularly at higher magnifications. For this reason I prefer the older 105 for most underwater shooting.

Questions were also during the discussions about the old 105 vs the new version as to which version was sharper and there was no definite conclusion. Perhaps now, with the advent of the 36 Mp D800, comparison shots might show a difference that was not visible with a 12 Mp D700. That is something that I hope to check for myself in the near future. Though, I may still prefer the older lens even if it is a tiny bit less sharp.

Fred

#14 Paul Kay

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Posted 17 April 2012 - 05:58 AM

As I mentioned previously, most lenses reach their highest resolution 2-4 apertures from wide open. Paul Kay says this is no longer so.

When Paul says his Canon 35mm f2 is not as good at any aperture as his 35mm f1.4, I would say, " You get what you pay for. Do you really expect a $330 lens to be better than a $1750 lens?

First point is that not all designers/manufacturers design and build in the same way - what I said is probably not so for all manufacturers but certainly is for some.

Second point. Yes a very well understood lens like a 35mm f/2 should be extremely good today - If Canon can produce stunningly good lenses like the 100 macro (not the L) then they should be able to produce a very good 35/2 pretty cheaply. Slow designs are easier to compute and manufacture well even using conventional lens technology.

I'm not going to get embroiled in a pointless argument over lenses - but things do change.....
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