Toronto Photography Meetup Group

Jamesy and I have been discussing this and thought it would be useful to get some input from other members. With all other things being equal will a lens faster than F2.8 have better auto focus ability in dim light conditions. For example will a 85 1.2, 50 1.2/1.4, 35 1.4 etc be able to auto focus on subjects where a 2.8 maximum aperture lens wouldn't be able to?

From what I have heard, those lenses can hunt just as much because of the depth of field being to shallow.

James (Seren Dipity) found a really good article while researching the topic here:
http://community.the-digital-picture.co ... /2012.aspx

Aside from the pure technical discussion behind this, I would be interested in techniques in getting better focus lock in low light situations. I shoot with a 40D/17-55 F2.8 and 580 EX2 in these situations.

The problem is the phrase "all other things being equal". That simply doesn't happen.

For example, in Canonland the 85/1.2L is slower than the 85/1.8 and the 50/1.2L is slower than the 50/1.4, but the 35/1.4L is much faster than the 35/2 and the 24/1.4L is faster than the 24/2.8.

It's all to do with the physical design of the lens, USM vs non-USM or simply having to move much larger, heavier elements around. There seems to be a "sweet spot" around f1.4 or f1.8 for the short primes.

At the same time, by virtue of simply having a larger aperture and allowing more light in, the 50/1.2L or 85/1.2L can AF in light conditions where a slower lens cannot, however, the slower lens will AF _faster_ in better light. The 35/1.4L and 24/1.4L will be faster and focus in lower light than the consumer equiv's.

This is all in Canonland, but Sony, Nikon and Pentax will also have their own little quirks, since their lenses also can be the old body driven type, or micro-motor, or Ultrasonic/HSM/SilentWave/etc type.

Use the AF assist light/grid on the flash, and keep the camera out of AI-Servo mode - the assist light does not work in AI-Servo, it's One-Shot only. Typically in low-light, I usually switch to the center-point AF as well.

Its not how fast the lens is.... its how fast the focus system is! AFS or silent wave motor's are usually faster to focus than body driven motor systems....

Thanks for the input Kin.
+1 for your recommendations.
All of the above was already in place, I was in one shot mode, center focus only, I had the focus assist on the 580 turned on. I just found I needed to stop down the lens well beyond F2.8 into the F8-F16 range to get reasonable focus. I also was using the AF-On button for focusing and the shutter release to meter and release the shutter.

That is exactly why I mentioned "all other things being equal" because I don't want the physical design of the lens to come into play.



That was my thought initially - due to light gathering ability the larger aperture lenses would be able to AF better in dim conditions. But apparently this is only true up to F2.8 - see link that Jamesy posted. What are your thoughts on this?

Thanks lxdesign.

The issue I find is not hunting, although I have a nifty-fifty that does that alot (non USM). I find the focus system will lock into what it thinks is the right spot when in fact it is wrong. I am wondering what the wedding phtogs do in these situations, they routinely shoot in similar conditions, dim rooms with competing lights from flickering candles and other sources, etc...

At f2.8, and close-up, you don't have much DOF to play with, and if you and or the subject is moving, then you don't have the option of "focus and re-frame". I pretty well don't re-frame in that situation. I just use the center point with AF linked to the shutter release and often crop later.

Understood. Neither I nor the subjects were moving and I starting stopping things down to 3.2 then 4.5 then 5.0 and finally stayed up at F16 with focus back on the shutter release button. I feel this is a bit of a gludge though and a waste of fast(er) glass. Like anything else, perhaps I need more practice in these situations.

In that case, you'll have to separate the two scenarios. AF speed and ability to AF in low light are often separate issues. With AF speed, you _must_ take the physical design into consideration.



I do find a big difference in this regard btwn my Sigma 30/1.4 and 18-50/2.8. Once you're down to "reception/dance_floor level" lighting, I'm often using the 30/1.4 instead. It does AF much faster and at lower light levels with my 20D. My new 7D has cross-type AF points everywhere, so I should be able to use any AF point instead of just the center.

Once we're beyond the "1st dance", I sometimes use a manual focus Zenitar 16/2.8 fisheye for dance floor shots. Depending on how crowded the dance floor is, I can set it for 3ft or 6ft and just fire away.

So does everyone think this is bunk? http://community.the-digital-picture.co ... /2012.aspx

Larger max aperture lenses will allow better focus lock in low light conditions? Or only up to F2.8?

I'd like to hear from the "gear geeks" - since this is more of a theoretical question rather than a practical one.

The OP (Daniel Browning) is making conclusions without any facts to back them up... his illustration of the tubes is not valid.

I don't know the technical answer to this however here are my thoughts:

Optimal AF ability (fast speed AND accurate - fast AF that isn't accurate or accurate AF that is slow are both problematic) is determined more by the AF sensors and motors in the body, rather than the speed of the lens. Therefore all else being equal (camera, focal length, amount of light and contrast in the scene being shot) then it comes down to whether or not the AF sensors in any given camera body, perform optimally with lenses with a max aperture of f2.8 OR faster.

I suspect that since most of the Nikon pro zooms max out a f2.8, Nikon's AF design is optimized for lens with f2.8 (doesn't mean that a AF won't be marginally faster or more accurate with an f2 or f1.4 or f1.2 lens but that any improvements won't be noticeable MOST of the time). Also consider that most AF sensors are based on contrast detection and, up to a certain point, a lens that collects more light will probably only provide a marginal advantage as the contrast in a particular scene doesn't improve or change because you're using a faster lens. Also consider that the likelihood of error increases the closer the plane of focus and the faster the lens, so even if a faster lens offers a focus speed advantage, it might be at the expense of accuracy. I've used a D200 for the last couple of years and find that when I need focus accuracy it's best to us MF lenses - the AF system is quite fast but not that accurate - especially when using wider lenses.

Anecdotally (I didn't conduct any technical tests), I've also used the D3 and D200 with the Nikkor 28/1.4 AF (Nikon's fastest wide angle lens) and found that the AF wasn't noticeably more accurate on the D3 although focus speed was a bit faster (no doubt because the D3's AF motor is considerably more robust and the 28/1.4 is a big piece of glass). I also used the both the D3 and the D200 with the Nikkor 24/2.8 AFD (one Nikon's sharpest WA primes) and found that the D3 focus speed and accuracy were not noticeably better than the D200 when used wide open. I've also used the F4 with the 24/2.8AFD (primitive 80s AF technology) and found that neither the D200 or D3 were noticeably faster when shooting static subjects.

The two main developments in AF technology are multi-point AF sensors and continuous focus mode (both contribute to significantly improved focus tracking ability and therefore speed) - but not much has been improved from the perspective of accuracy.

I don't have experience with the current generation of Canon dSLRs so I won't speculate.

Hope that helps.

The problem with fast lenses such as 85 1.2 is that you need to stand still without moving. If you move slightly while focusing, you can have your subject completely out of focus.
AF focusing system plays a huge role as well. Cheap(well...not really) 85 1.8 lens has a much faster focusing ability than the all mighty 85 1.2

I've used some lenses that aren't fully open when focusing, but will open up the aperture when you take the picture...I just found it interesting.

Some quick back of the napkin thoughts.

Assuming 1.5 crop factor body, 85 mm body focusing on a face at 2 meters distance.


f...... DOF (front)*.. Normalized Light Intensity


1.2.........1.2 cm.....................8

1.4.........1.4 cm.....................4

1.8.........1.8 cm.....................2

2.8.........2.8 cm.....................1

* online dof calculator (don't ask me why f = DOF)

1)
If there is enough light or a focus assist light for the AF to work at 2.8 them 8 times more light is not going to make the focus work that much better.

If there isn't enough light then the faster lenses may be able to focus where the f2.8 can't. The light has to be well above the noise level of the AF circuitry

2)
At f2.8 the AF just has to get within 1.4cm to be in focus, like shooting fish in the proverbially barrel.

At f1.2 the AF has to get within 0.6cm, pretty fine focus indeed.

Now the the actual focusing is done through a servo mechanism that determine focus first coarsely and then with fine precision. It does this by going through and past focus and then moving back through focus at a slower pace and finally to exactly to where the algorithm picks as the position with maximum contrast (focus). If it doesn't find this position it hunts until it does.

To make a long story short: the faster lenses have to move slower to find the focus within the shallow DOF then they then have to position the lens's focus with more accuracy or else they might miss the correct focus point. Correct focus point in this case may not be what you think you are focusing on.

Precision in this case comes at the expense of slower focusing.

Thanks for the input everyone. Very good information here.

Through my experience, I've found that my primes focus better in low light levels than my f/2.8 zooms. More importantly to me though is that the viewfinder is brighter so that I can actually see what is going on.

Kevin - better as in faster AND more accurately or just faster or more accurately?

How dim of a light are you talking about? Because depending on what you are trying to shoot, it may not matter on the light condition.

Doesn't matter how fast your AF or lenses speed are, if you're trying to capture a fast moving object in low light, it will most likely end up being a blur. Except if you're using a new Nikon D3s and shooting at 102000 ISO.

Now, if it is something that is not moving fast at all, then does speed really come into play?

What the marketing department sells us and real shooting experience are two different animals.

I think it depends on what focusing screen you're using too, as most of the stock screens are optimized for slower lenses and doesn't get any brighter beyond f2.8.

To get the benefits of DOF on the viewfinder and faster AF, you may need to get one of those old-school "precision matte" focusing screens but the viewfinder will get dark progressively at around f4.

Unless you're talking about live-view or those early in-lens AF systems (those you put batteries into the lens barrel), the focusing screen is upstream to (i.e. "goes before") the AF sensors on a SLR.