Low light Focusing

cjdesu6

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Hi I currently have a Nikon D3200 which has 11 focus points and one cross type in the very center. In low light however only the center focus point will give usuable photos with the other points giving blurry photos (camera was on a tripod and triggered remotely). AF assist will only work when set to the center point or when set to auto point. So my question is in low light situations should I set the the point to the center and recompose or should I set to auto point. (Don't like the idea of recomposing, but would also like to select my own focus point for portrait shooting.) Just wondering how everyone else gets sharp pictures in low light areas. Thanks.
 

Dao

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In some cases, a fast lens help in low light focus situation.
 

Derrel

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If the light is so low that focusing is very challenging, it's usually time to put on a FAST prime lens and shoot HIGH-ISO available light OR to bring in supplemental light in the form of flash. I think one of the single biggest areas where the d-slr is superior to film cameras is the ability to shoot ON-camera bounce flash with the head of the flash zoomed and the bounce angled off of various surfaces, so the lighting effects can be previewed, evaluated, and monitored in more or less real-time.

Slowish zoom lenses in the f/5.6 max apertur range in dimmer light on many subjects are going to be tricky to focus except on the MOST-capable Nikon bodies with the best AF performance. If the target the AF bracket is on is "low contrast", a lot of times the AF module can have some difficulty when the lens is slowish. And...when just focusing is an issue, that means the light is probably pretty sucky on its own...
 
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cjdesu6

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Thanks for the responses
I have been hearing a lot about the "better lens" point, but the lens actually affects the focus accuracy?! It just seems strange since the brains are in the body's processor.
 

Robin_Usagani

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what the camera sees (and your eyes see) while focusing is the largest aperture of the lens. The aperture only closes down to whatever aperture you set it at when you hit the shutter button. So basically if you put a kit lens and you look at the viewfinder, it will look darker if you compare it when you look at it through a 1.4 prime.
 

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Thanks for the responses
I have been hearing a lot about the "better lens" point, but the lens actually affects the focus accuracy?! It just seems strange since the brains are in the body's processor.

Yes, the light-gathering ability of the lens is a factor at the lower end, in two main ways. First, a wider maximum aperture means MORE light comes in. BUT, in a more important way, it also means that the depth of field is shallower, so a typical phase detection AF system has better "data" to make the focus decision. In simple terms, phase detect AF, like most d-slr's use in normal mode, is a series of questions the processor asks,and answers: is this IN-focus or OUT? That question is asked and answered very fast, and over and over, until AF is locked on...the AF system makes corrections, and evaluates them...when the system determines the image is IN-focus, to an acceptable degree, the AF lock-on light lights up...

Now, with short-length lenses with low image magnification, and deeper DOF as a result of low magnification and small maximum aperture value, like say 18-55mm kit zoom, at 55mm the max f/stop is only f/5.6...

The depth of field at f/5.6 at 55mm at 10 feet is soooo deep that a very discrete IN-focus and a very concrete OUT-of-Focus Yes/No situation does not exist to the same degree as with say, a 50mm f/1.4 lens. The "faster" f/1.4 lens offers an AF system's phase detection system images that go from out-out-out-to IN-FOCUS, bang-dead on, very distinctly. But at f/5.6 at 10 feet, the in-or-out detection is much less "distinct".
 

Tailgunner

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Thanks for the responses
I have been hearing a lot about the "better lens" point, but the lens actually affects the focus accuracy?! It just seems strange since the brains are in the body's processor.

Try renting a faster lens and you will see the difference. In the mean time, you could focus on something relatively the same distance as your subject and then switch to Manual on your lens. Even Fast Glass has troubling focusing in dark enough environments and this helps.
 

bratkinson

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It took me a while to understand how the 'kit glass' zoom, in my case, an EF-S 18-135 f3.5-5.6 would take so long to focus in dim light situations. I could even hear it 'hunting' to focus, on occassion. I'd often be zoomed in the 100-135mm end of the range and at f5.6, the maximum aperture at that focal length, very little light was passing through the lens for focusing.

When we humans, or any animals, go from a brighter lit area to dimly lit area, the pupils of our eyes get larger to let in more light so we can see as best as possible. Digital cameras, on the other hand, have their aperture (pupil) 'wide open' all the time, until we press the shutter, when it will instantaneously 'stop down' to whatever smaller size it is commanded to do by the camera. But when 'wide open' is comparatively small, such as f5.6, it's like wearing sunglasses outdoors at night with only a street light for lighting. Our eyes 'struggle' to see (and to focus) what is out there. The focus mechanism of a camera likewise has to struggle to not only select what to focus on, but to check/adjust/check/adjust that focus until it is right. "Faster" lenses (small f-stop numbers = larger aperture (pupil) effectively removes the 'sun glasses' and the camera can focus more easily.

Faster glass, lenses with f2.8 or smaller f-stop numbers, let in more more light making focusing easier for the camera. As noted above, the wider apertures also give a smaller depth of field, so it's made even easier to focus. The downside, however, is fast glass gets expensive.
 

KmH

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The center focus point is a cross type focus point.

Cross-type focus points can detect both vertical and horizontal edges.
The other 10 focus points in your detect vertical or horizontal edges depending on their orientation.

I recommend using the recompose technique rather than focus AF-A mode.

Scroll down to the section that describes, and has illustrations of, the way SIR phase-detection auto focus is done in most DSLR cameras - Autofocus - Wikipedia, the free encyclopedia.
This tutorial includes info on AF point types - Understanding Camera Autofocus
 

slow231

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i saw a wedding photographer carry around a small pocket LED flashlight to help illuminate for AF (non-flash, dark dancefloor type shots). it was just a pushbutton light that'd he used momentarily to get the af to lock. wasn't really any more distracting than the fact that there was a photographer there taking your picture anyways, and it seemed to work well for him. granted that's not ideal, and might not work for all situations/subjects, but it is a short term option until you get faster glass, flash, or better af body.

also make sure to watch out for motion blur, it could be that your shutter is just simply too slow in these dark situations.
 
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Mach0

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Fast glass, higher ISO, and a flash with AF assist. I've never had an issue with focusing in low light unless it was damn near no light . That's when I used flash.
 

cynicaster

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A decent speedlight (like the 580EX-II) will have a built in AF assist light. Using that, I can quickly lock focus using a kit lens in a virtually pitch dark room.

You can probably set up your pop-up flash to rapidly strobe light to assist in focusing, but in my experience that doesn't compare to the assist beam on the speedlight.
 

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