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I use a Sony NEX-5R. I find that the autofocus often fails during low light, as in produces out of focus photos.

Sometimes it's clearly out of focus, and I can see that on the LCD, in which case I move the camera a bit to the side and back to get the autofocus mechanism to re-evaluate the scene, and then half press the shutter button to confirm the focus.

But sometimes, it looks fine on the LCD, but the result is disappointing.

Is this expected? Or is something wrong with my camera?

Here's a sample photo of the scene I'm referring to. This one was autofocused and is blurry:

Auto-focused and blurry

while this was was manually focused:

manually focused and clear

Both photos were shot with an F2.8 19mm lens at ISO 100 and in Program mode. The camera ended up choosing f/2.8 and a 2-second exposure. I enabled long exposure NR and electronic front curtain shutter. They were shot in RAW and exported as JPEG in Lightroom, with no post-processing other than the "sharpen for screen" present while exporting to JPEG. I used a tripod in both cases, with a ten second timer to eliminate camera shake.

Notice that there's plenty of contrast in this photo, and it's not anywhere as dark as the sample photo Esa posted below.

Hyperfocal distance for the F2.8 lens on my 1.5 crop factor camera is 21 meters, and everything in the photo is farther, so it's not a problem of DoF or too wide an aperture. Am I missing something?

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2  
What exactly is "low light"? Most camera AF systems are rated for operation down to a certain EV level. The NEX-5R is rated down to 0 EV. In the grand scheme of things, that isn't really all that dark. To contrast, many DSLR dedicated AF units support EV -1 (twice as dark) or EV -2 (four times as dark). Some newer DSLR AF units support EV -3 (eight times as dark!) If you are trying to AF below EV 0, then you are bound to have some issues. You might try illuminating the scene to AF, then expose. –  jrista Dec 15 '13 at 16:44
    
From info in a comment to answer, this question seems related to Why are these light trails large blobs instead of thin lines? –  Esa Paulasto Dec 15 '13 at 20:25
    
jrista, Please see the updated question, which has a sample photo and more information. This is an open space, which I cannot illuminate. –  Kartick Vaddadi Dec 16 '13 at 1:06
    
@KartickVaddadi: From the sample photo you have shared, I can't tell that there is any focus problem. Everything looks sharp to me, at least, at this downsampled size. Is the photo you linked blurry at 100%? You say you use a wide aperture, yet the scene you are photographing has a VERY deep depth of field. I would expect that there would be a fairly thin plane of focus in this scene, and that anything nearer or farther than that would appear blurry. For this kind of depth of scene, you need to be using f/11 or f/16, maybe even f/22...and a MUCH longer exposure, or a higher ISO (i.e. ISO 800.) –  jrista Dec 16 '13 at 2:12
    
Let's discuss this in the comments to your answer below. –  Kartick Vaddadi Dec 16 '13 at 4:42

6 Answers 6

up vote 3 down vote accepted

Low light, long exposure and "landscape" photography like your example is typically where you want to use your manual focus, live view assisted by zooming in. AF points need contrast, and you should point your camera at the edges of the white illuminated walls to preselect focus, confirm the focus is correct, and then place your camera. In concert fotography I have to focus/recompose all the time, to even get AF right 3 out of 4 times. A lot of cameras require F2.8 or faster lenses to have accurate AF under normal light, so in dim light it needs even faster lenses.

But in a static situation like this you might as well go manual, as you need to confirm the AF result anyway. You might as well stop down, too, to around F5.6-F8 where your lens is sharpest (even without DOF considerations). Your dof is infinite at this distance even wide open, assuming normal focal ranges (17-55mm, as you dont mention it).

So the direct answer to your simple question is, yes, it is normal that lack of light leads to inaccurate AF performance, considering F2.8/F5.6 thresholds for AF performance in normal light, and also the type of scene in question, where you need to place the high contrast edges under AF points yourself. And your own experience that manual is more reliable is confirmed, since the hassle of setting up lock focus/recompose with the tri[od setup outweighs the manual focus process with liveview zoom.

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The focal length is 19mm, with a crop factor of 1.5. Updated the question with that information. Thanks. –  Kartick Vaddadi Dec 16 '13 at 17:15

If your autofocus is relying on contrast then in low light - when there is low contrast - the autofocus is likely to struggle.

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Please see the updated question -- it answers this, and I've added a sample photo, and provided more details. Thanks. –  Kartick Vaddadi Dec 16 '13 at 1:04
    
Short version: there's a lot of contrast here, it seems. –  Kartick Vaddadi Dec 16 '13 at 4:53
    
Lots AFTER how long an exposure - if you had to wait 30 secs for autofocus to work you would soon complain. –  Steve Barnes Dec 16 '13 at 6:28
    
Hmm... correct me if I'm wrong, but shorter exposures result in high noise, but still lots of contrast between the lights and the dark areas. –  Kartick Vaddadi Dec 16 '13 at 6:40
    
And then the focus may be on the noise or may be fooled by the noise and when really long exposures are needed the sensor will not saturate properly. –  Steve Barnes Dec 16 '13 at 8:19

Loss of sharpness in the dark is not all about focus.

Notice that in the dark your camera uses higher ISO and longer exposure time. Motion blur happens and low-light turns so easily into noise. And then you get unsatisfying photos because of the noise, de-noising, and motion. Noise makes for fuzzy outlook, denoising eats details, and motionblur is motionblur. Plus the bleeding of bright lights into neighbouring photosites.

Here a nighttime photo, a landscape illuminated by 3/4 moon only. In the lower lefthand corner you see a house with lights on in one room. The light was very dim, barely a reading light for someone reading a book in bed. Still the light was so bright that it bled all out of the actual window boundaries. Partly this was due to non-perfect focus, but more from the light bleeding out onto neighbouring photosites on the image sensor.

nightscape

14mm - 30 sec - f/4.0 - ISO 800

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Thanks, Esa. I'm actually shooting at ISO 100, with a tripod, with a 2-second timer, and a long exposure (multiple seconds). And I'm not shooting anything moving. I get good results when I manually focus and sometimes with auto-focus, but sometimes not, so it's just a focus issue. It's the same kind of blur I get when I manually mis-focus it, so it's not any of the other factors. Basically point sources of light become big circles. –  Kartick Vaddadi Dec 15 '13 at 10:11
1  
@KartickVaddadi - Count motionblur out then, but noise is not related to ISO only. A lot depends on the way you expose and postprocess. –  Esa Paulasto Dec 15 '13 at 10:30
    
As I said, I'm shooting with a tripod. I've enabled electronic front curtain shutter, and long exposure NR. I'm also shooting RAW and not post-processing it in Lightroom at all. –  Kartick Vaddadi Dec 16 '13 at 0:57
    
In this example image, if the aperture is f4, I'd suggest that the land is out of focus because the lens is focusing to infinity or somewhere close to get the stars, and the land is much nearer so is out of the dof for this aperture –  laurencemadill Dec 16 '13 at 13:44
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@laurencemadill - My picture is here to show how even a dim light bleeds to neighbouring pixels. The crop in the middle is 1,6% of the whole image area (600x440 crop from 16MP). The OP's question was originally very sparing with details, leading to answers not finding the key really. Dropping further details one by one after first posting a quick question is not the ideal way to get good answers. –  Esa Paulasto Dec 16 '13 at 14:26

Based on your explanation and sample photo, the only conclusion I can come to is that you are trying to photograph a scene with great depth with a lens at a very wide aperture that will only allow a narrow depth to be in focus. I am not sure you necessarily have a focus problem...rather, you have a depth of focus problem.

You mentioned you are using a tripod, a long exposure, and using a fast aperture (f/2 or f/2.8). Assuming the image you posted is indicative of the type of scene you are generally working with, an f/2-f/2.8 aperture is going to be very thin, even at those distances. Distance scene content is very likely going to be out of focus, and if you are focusing on something relatively "central" in the frame, like that building, it is also likely that near scene content will be somewhat out of focus.

To capture a scene with such great depth, you really need to use a narrower aperture...and a MUCH narrower aperture at that. I would offer that f/11 or f/16 is probably REQUIRED to get the entire scene, from the near foreground trees to the distant buildings in acceptable focus. You may even need to use f/22, which would add some diffraction blurring across the whole scene, but likely LESS blurring overall than the areas that are currently out of focus (it appears that the distant buildings are actually rather considerably out of focus.)

You will need to increase your exposure time to do this. We are talking about many stops difference in aperture here, anywhere from 6 to 8 stops! You are going to need to increase your exposure time (reduce your shutter speed) by a similar number of stops. If you are currently exposing for say two seconds, then you would need to expose for 128 to 512 seconds! I assume that exposures that long are out of the question, so the only other option is to increase the ISO setting as well. We are probably talking ISO 800 to ISO 1600 at least, which takes care of three to four stops of the aperture difference. That would mean you need a shutter speed between eight and sixteen seconds (assuming you are currently exposing for around two seconds.)

If you want your entire scene in focus, then there really isn't much of a way around this. There are tradeoffs for each setting that affects exposure. Wider apertures reduce the depth of field, resulting in a non-linear blur curve as you move away from your plane of focus (in other words, a type of blur that is practically impossible to fix with post processing.) Longer shutter increases the chance that you will experience blur from camera shake, or result in motion blur for moving subjects in the frame. Higher ISO allows shorter exposures, but the reduction in true exposure (total light gathered at the sensor plane) and an increase in gain results in more noise. The trick is to balance these three settings to achieve the most acceptable result. For example, you may be able to get away with f/8, along with ISO 800 and an 8s shutter, at the cost of some additional blurring in the deepest depths of your scene.

I mentioned that you might even need to use f/22 if you are ok with very long (multi-minute) exposure times, and require the entire depth of scene to be in clear focus. I would also point out that linear and evenly distributed diffraction blurring is FAR preferable to non-linear and unevenly distributed iris blurring. Why? Because diffraction is a well understood convolution process, and correcting it in post is relatively easy...a moderate application of a decent sharpening algorithm will usually reverse most the effect of diffraction blurring.

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Thanks, jrista. I will try a smaller aperture. BTW, this particular photo has no issue. But other photos of this scene do, and I wanted to give you all an idea of what the scene is that I'm photographing, and the brightness and contrast there. ISO 800 makes me a little queasy. I avoid even ISO 400, because it makes the black sky look a little noisy and more gray than black compared to ISO 200, on my 30-inch monitor (in the Fit mode, not 100%). I try to shoot at ISO 100 or 200. –  Kartick Vaddadi Dec 16 '13 at 4:49
    
From cambridgeincolour.com/tutorials/hyperfocal-distance.htm it seems that the hyperfocal distance at F2.8 for my 1.5 crop factor camera is 21m (if I'm using my 35mm lens, and much lower if I'm using my 19mm). Everything in this photo is farther away, so I shouldn't be having a depth of focus problem, should I? And in the photo I claim is not correctly focused, everything is blurry, while in the correctly focused ones, everything is clear on first glance. So it can't be a depth of focus problem, can it? I will try to post a sample photo that's misfocused. –  Kartick Vaddadi Dec 16 '13 at 4:52
    
It could be a DOF problem if the camera focused on something you did not expect it to. With only 21 meters DOF, if the camera focuses too near or too far, then the vast majority of the photo could appear out of focus. Again, I don't know how long your exposures are, you never gave any of that information. If you are exposing for a second or longer, camera shake could very well be a culprit. Fear of high ISO is only going to hold you back and limit your work. Noise is easy to clean up until you are REALLY pushing it (i.e. top two tiers.) Don't fear ISO. –  jrista Dec 16 '13 at 5:27
    
If you can't expose for several minutes (which would be required even if you are currently at 1/30s or around there), then f/16 and f/22 are going to necessitate using a higher ISO. No way around that. If you do use a higher ISO, you should look at Topaz DeNoise 5 as a companion tool to reduce noise. It has debanding and DR recovery, so it can not only help reduce noise but also recover those blacks that end up looking gray. –  jrista Dec 16 '13 at 5:29
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All my exposures are > 1 s. Regarding camera shake, I use a tripod, and set a timer of 2 seconds (I've made it 10 now, after I read elsewhere on photo.SE that it's not enough), and enable electronic front curtain shutter. Before I used some of these techniques, like a timer, I had camera shake. I don't think I do any more, at least not that I can tell. I will try taking a photo this evening with both auto and manual focus and see if I can reproduce this issue. –  Kartick Vaddadi Dec 16 '13 at 6:31

I'm guessing you were shooting the scene through a window? A window much closer to you than anything in the scene perhaps? It appears your camera focused on the window rather than the scene beyond the window. The red AF Illuminator probably bounced off the window and back to the camera. Try turning off the AF Illuminator as discussed on page 130 of the NEX-5 User Manual.

As other have already mentioned, in a low light situation where you are using a tripod and a long exposure via timer you will probably get better results by using Manual focus anyway.

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There's no window or anything else between the camera and the scene. I was standing on the balcony. Thanks -- I guess I'll stick with manual focus. –  Kartick Vaddadi Dec 17 '13 at 2:30
    
If there was no window then your camera appeared to focus at the minimum focus distance for some other reason after it could not determine proper focus using the AF routine. –  Michael Clark Dec 17 '13 at 3:18

I would bet it's the camera's autofocus system that's causing you grief. I don't think you can find any camera or camera system that'll give you amazing autofocus under all conditions.

After looking at the operating manual (which I highly recommend) it seems you have 3 options for the focus area - Multi, Centre, and Flexible Spot.

If your camera is set to 'Centre,' it's likely the AF will struggle even if there's plenty of contrast around the centre and into the edges.

Setting it to 'Multi' would mean the camera will focus on the parts of the scene that contrast the most, so this would be best for the scene in your example photo. Note, however, that using this feature at very large apertures could throw the foreground or background out of focus.

Finally, if I'm not mistaken, 'Flexible Spot' allows you to choose the focus area yourself. I use this feature on my D7100 to auto-focus on on off-centre subject after I've composed the shot.

Having said all that, if you're using a (good) tripod and long exposure, focus manually and decrease the aperture to 5.6, 8 or 11. It should help sharpen things up. If you must use AF, then try fiddling with the focus area options and see if that helps.

I'll also add that the longer you expose your shots (at small apertures and low ISO) the better you'd want your tripod to be. The heavier the tripod is, the less vibration there'll be to blue your photos.

Good luck

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Aleks, I updated the question with some more details and sample images showing a good and a bad photo. Do you mind taking a look and confirming that it's indeed an AF problem? I'm using a 10-second timer, so I can't have camera shake, can I? And if that were the problem, why is the manually focused photo clear? The camera is in the default 'Multi' focus mode, so that shouldn't be a problem, either. –  Kartick Vaddadi Dec 16 '13 at 16:01
    
Thanks for posting that image. It certainly looks out of focus so I guess camera shake can be ruled out. It looks to me like your camera has focused on a distance before the scene, and this can happen when it struggles to deal with a certain scene, especially dark scenes. I don't think there is an issue with your kits, so I recommend manual focus to overcome this problem. –  Aleks Danger Dec 16 '13 at 19:41
    
From page 81 of the NEX User manual: "When the AF Illuminator is used, the setting of [Autofocus Area] is invalid and the AF area is indicated with a dotted line. AF operates with priority in and around the center area." –  Michael Clark Dec 16 '13 at 19:45

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