Why do cameras struggle to take a shot in dark locations?

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January 3, 2011 at 6:00 am  •  Posted in Q&A by  •  0 Comments

“I just got my first digital-SLR camera and I am having difficulties already. I was trying to take photos of my son’s Birthday party in our fairly dark basement using the telephoto zoom lens that came in the kit. It would take a long time before the camera would actually take a shot so I missed some photo opps. And sometimes it blinked a signal in the viewfinder and refused to take a picture. What is the solution to the problem?” —M. Dominguez

This is certainly a common complaint with many D SLRs. The problem is typically due to the same primary cause: inadequate light reaching the autofocus sensor. This is worsened by the small maximum apertures of affordable zoom lenses since they do not transmit much light. When the AF sensor gets less than the optimal amount of light, the camera does take longer to focus and yes, sometimes it will fail. And it will not take a photo until sharp focus has been confirmed.

Every D-SLR manufacturer employs its own proprietary technology but most cameras provide faster autofocus when Live View is not used. As well, AF is always the most reliable when the autofocus sensor receives plenty of light.

Here’s one simple solution. If you are using Live View (to compose photos on the LCD screen) turn that feature off. Use the optical viewfinder instead. Many D SLRs will then provide more reliable autofocus, employing phase-detection AF instead of the contrast-detection AF technology that’s used in Live View. Switch from AUTO mode to P mode and set the autofocus sensor to the central focus detection point instead of using multi-point autofocus. (This is not possible in AUTO mode but is useful because the single AF point will provide faster AF in low light.)

Should that not solve the problem, you could try focusing manually, rotating the narrow ring on the front of the lens.  Stop rotating when the subject appears to be sharply focused in the viewfinder. Granted, that can be difficult in low light because the subject will appear quite dark in the viewfinder. If you do succeed with manual focusing—and if the camera-to-subject distance will not change while you’re taking a series of photos—this may be the best solution.

For the fastest/most reliable autofocus during this performance in low light, I used the camera's optical viewfinder instead of Live View, activated only the central AF sensor and moved closer to the subject. The latter allowed me to use the lens' widest maximum aperture and ensured that the subject was within range of the flash unit's focus-assist beam projector. © 2010 Peter K. Burian

But if you do want to use autofocus move closer to the subject—3 meters or less— so it’s within range of the focus-assist lamp or the pre-flash burst of light. Or buy a large external flash unit with a more powerful focus-assist beam projector;  the range might then be four to five meters. Use a shorter zoom setting such as 70 mm instead of 150 mm, where the lens’ maximum aperture is wider. That will allow more light to reach the AF sensor. Also turn on more lights in the area where you’re taking photos and you’ll probably experience less frustration.

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