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Are there huge differences in the ability to focus in low light between different brands and models?

I have a Pentax Kx with a Sigma 30mm 1.4. My problem is, that the camera's autofocus does not work in low light, even when a picture with manual focus turns out ok*. I accepted that and mostly use manual focus at night now.

But last week I was in a bar and a girl had a Nikon and was shooting all the time. It did look like she was using autofocus and I assume she wouldn't have continued if the photos did not turn out. Unfortunately I did not get to see the exact model of the camera or the lens. Is it possible that here autofocus is so much better?

[update]

As I understood the first answers, my options are to upgrade to K5 or K7 for 800$ or sell my Pentax stuff and buy a Nikon or Canon with new lenses for 2000+ $? Changing the options in my camera or buying a different lense will not help in my case.

[updateII]

Would it have been wiser to buy an entry level Canon/Nikon and then 400$ on a lens with a good autofocus?

* I just tried in a unlit room of my house with 1.4, 1/30s, 1600 ISO, which produced a photo with EV=-3, but autofocus would not work.

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2  
Autofocus will rarely work at EV -3. Most AF units are designed to work down to EV -1, maybe EV -2 (usually only in professional models, and then, only the more recent.) It is not surprising that your camera couldn't focus in light as dim as EV -3. –  jrista May 12 '12 at 19:58
    
Regarding your second update...the AF motor in a lens is only half (probably less than half) of the story. The actual AF system lives in the camera. A special sensor embedded in a special light-splitting unit, embedded in the bottom of the sensor cavity, is responsible for actually detecting a focus discrepancy and correcting it. There is advanced software in the camera itself that drives the AF system, and the more expensive the body, the better the AF system tends to be. If you want the best AF, you'll need to spend more, not less...to the tune of $7000 or so. –  jrista May 13 '12 at 0:15
    
The best AF system available today seems to be the Canon 61pt AF system. To get the most of of it, you would need the as yet unreleased Canon 1D X with its full-color metering sensor ($6800). In lieu of that, the Canon 5D III has the same 61pt AF system, however it is linked with a less capable metering sensor and is therefor a bit less effective. It only runs for a mere $3500. Nikon offers several cameras with a 51pt AF system that is just a tad behind Canon's latest offering, and you can find those cameras for a couple grand or more. –  jrista May 13 '12 at 0:17
    
For what it's worth, the K-5ii has an even better low light autofocus system, specified to work down to -3EV, which is pretty dark –  mattdm Dec 16 '12 at 23:46
    
Note that if it is really dark, even the most expensive DSLR will have a very tough time focusing. That's why some flashes come with infra-red focusing beams allowing you to focus in pitch black very very quickly. –  Gapton Dec 17 '12 at 2:39

4 Answers 4

Yes, there are huge differences. Things are different between brands but there are even more differences between models.

The higher-end the model, the faster it focuses and conversely. Just this morning I was commenting to @rfusca who answered you already how slow the Nikon D3200 focuses. This model sits at the low-end of Nikon's lineup, just like your K-x in the Pentax lineup. The thing is that with Pentax you can only go one level up to a K-5 (or K-7) which improves things considerably. However, with Nikon or Canon, there are multiple levels to go up and they get considerably faster at autofocus.

Furthermore, lenses focus at different speeds. Worse even, they focus at different speeds on different bodies. Pentax, Nikon (except low-end Nikon DSLRs) and Sony support lenses which focus motors in the lens or using one in the body.

Generally but not always, brand lenses with in-focus motors are faster than those without. Off brand lenses have slower in-focus motors as far as I can tell but my sample is limited to 30 or so Sigma and Tamron ones (Only tried 1 Tokina).

More importantly, focus is always done with the aperture wide-open, so lenses with wider apertures have an advantage since the focus system has more light to work with.

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So, what would you recommend me? Buy a different lens? Buy I higher end Pentax model? Switch to different brand? –  Framester May 12 '12 at 19:59
1  
You gave too little information for me to recommend anything. Is AF-speed your only requirement? In low-light the most sensitive camera I know is the Nikon D4 and the fastest is the Canon 5D Mark III (along with the 1D X) which you could pair with an F/1.4 (or wider for Canon) lens. If those are above your budget or you have an investment in Pentax lenses, you have to compromise. –  Itai May 12 '12 at 20:04
    
Thanks for your information so far, I can't +1 yet. I tried to qualify my post by adding two more questions. I would be grateful to hear your opinion on these, too. –  Framester May 12 '12 at 20:22
3  
Maybe I did not say it so directly, but an entry-level Nikon (or Canon) and a $400 lens will not buy you improved performance. Going to a K-5 or K-7 will but probably not at the light levels you tried, I think even the $7000 Nikon D4 can't, so you have to set your expectations to a realistic level. –  Itai May 12 '12 at 20:40
2  
Maybe she was focusing manually or prefocusing on a brightly lit object based on distance and recomposing. She would have had to dissociate exposure and focus lock which you can do on all Nikon and most other brands. Maybe its time to put one of those change encounter ads I was in a bar while you were focusing with such ease, I really want to ask you ... :) –  Itai May 12 '12 at 21:09

Buying a $7000 system is not a helpful answer. I sugest following:

1) have a small LED flashlight with you. light helps your camera to focus. after focusing with half-pressed shutter button turn of the flashlight and take your picture. cost: $5,-

2) try pre-focus methods: focus on something better lit at same distance and then with half-pressed shutter recompose to your subject

3) buy a prime lens. these let in much more light. the price is that you have to use your feet for zooming. 2nd good effect is to get blured backgrounds with large apertures which makes your picture look "better" (the viewer is not distracted by the backgrounds and looks more to your subject). For Pentax this could be the model: SMC Pentax-DA 35mm F2.4 AL ($180)

4) buy a "good" flash with AF-Ilumination. Better flashes cast a (infra) red pattern onto the subject which helps the AF to focus even in completly dark. a good flash starts at $300. not always it has to be the original brand (pentax), also good enough are: metz 48 or sigma xyz super

5) if you already have a prime lens and a flash buy a semi-pro body like the K5-II. But remember: Lenses last for years and decades, every body gets outdated after 2-4 years. So investing in lenses is "cheaper". the K5 also has a built in LED to assist AF. But a $5 flashlight is the cheaper way for iluminating the subject :-)

Maybe the girl photographing with nikon had the advantage, that nikons have a built in LED that iluminates the subject and helps the camera focusing

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2) Do it with the focus-lock button on the camera. Page 114 in the pentax Kx manual. –  Michael Nielsen Dec 17 '12 at 9:35

Autofocus can differ drastically in low light situations. I have a Nikon camera with a Sigma 50mm f/1.4 and a Nikon 85mm f/1.8.

The Sigma 50mm lens often hunts in low light - my house's living room lit by a single lamp for example. When its bright out, it locks on quickly and accurately.

The Nikon 85mm lens locks quickly and accurately at that same low light. Its near instant and dead on most of the time.

Can there be a big difference? Can it be 'so much better'? Absolutely.

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For Nikon the autofocus is in the lens, am I right? And so did I buy the wrong camera? –  Framester May 12 '12 at 19:34
1  
Nikon lenses with the AF-S designation in their alphabet soup have the focus motor in the lens, but the AF decisions are made by the body. –  Blrfl May 12 '12 at 20:25

There are several things to consider when trying to compare AF across different brands, models, and lenses, and this is especially difficult because you don't know exact details to compare.

The lens certainly makes a difference. The maximum aperture of the lens can make a big difference, and you're well-off by starting with an f1.4 lens. My own experience with Canon and Nikon would suggest that third party manufacturer lenses always perform a little worse than an original manufacturer's equivalent lens, though by how much exactly is debatable.

The camera model is certainly going to impact this. Basically, you get what you pay for. It's typically buried but manufacturers usually supply a range for the AF sensitivity, usually somewhere between +2 and -1 EV, though the newest Nikon's are rated to -2 EV.

Be sure you're taking advantage of your camera's best AF sensor to get the best results. That usually means selecting the center AF point, which is most sensitive. Also make sure you're trying to focus on something that the AF can lock on to: high contrast and textured. Focusing on a white wall will always be a challenge regardless of the light levels; focusing on a brick wall will always be easier because of the varying horizontal and vertical lines and color difference.

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