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A member of a digital photo forum, to which I subscribe, stated that when she was photographing some badgers at night, the auto focus wouldn't work due to the lack of light.

She then said she upped the ISO, and then the camera did auto focus allowing here to take some shots.

Is this possible?

This is the exif from one of her shots:

Exposure Time (1 / Shutter Speed) = 1/125 second ===> 0.008 second
Lens F-Number / F-Stop = 28/10 ===> ƒ/2.8
ISO Speed Ratings = 3200
Original Date/Time = 2014:04:13 21:16:52
Digitization Date/Time = 2014:04:13 21:16:52
Exposure Bias (EV) = 0/1 ===> 0
Metering Mode = pattern / multi-segment (5)
Flash = Flash fired, compulsory flash mode
Focal Length = 200/1 mm ===> 200 mm
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    \$\begingroup\$ What camera? Phase detection or Contrast detection Auto Focus? \$\endgroup\$
    – Michael C
    Feb 9, 2015 at 1:45
  • \$\begingroup\$ i would expect a camera with contrast AF to use the best ISO for the job automatically, but i may be too optimistic. \$\endgroup\$
    – ths
    Feb 9, 2015 at 15:51

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It would really depend if the ISO-boost in the A/D conversion is applied at AF-time or just for recording of the image. For a regular Phase-Detection Autofocus (What Canon EOS Digital cameras, Nikon DSLR's and I would assume most other DSLR's based on the modern design) I do not believe this is possible. It would help if we knew the camera that your friend was shooting with so that the method the camera utilizes for autofocusing could be explained and examined further

For more information on Phase-detection and why it is the standard in DSLR's, see http://www.digitalphotopro.com/technique/camera-technique/autofocus-evolution.html

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    \$\begingroup\$ So if a citation is needed (it is), could you please provide one? \$\endgroup\$
    – Saaru Lindestøkke
    Feb 11, 2015 at 14:25
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The ISO enhances the light sensitivity in each pixel sensor - that is, each pixel registers a higher amount of radiance than it normally would. Hence, it's an inherently digital procedure. Auto-focusing, on the other hand, has existed since film cameras.

There are many auto-focusing systems, but two are most common today: contrast detection, and phase detection. Contrast detection gets the image from your sensor and moves the focus point farther or nearer, then compares it to the new image, rinse and repeat. The one that has the sharpest contrast on your focus region of choice (the options spot, center-weight or matrix in the camera menu) is elected the optimum focus point. Since this method works with the image out of the sensor, it should be affected by ISO.

Phase detection, on the other hand, splits the light beam in two, as a rangefinder would, and tries to stitch them back together. The point in which they stitch perfectly is the optimum focus point. This is a physical procedure, and it may or may not be affected by ISO settings, depending on the camera system. Some cameras also have a hybrid of contrast and phase detection, so it's really hard to say if her trick worked without knowing her camera model, though it is possible.

A couple of notes: higher ISO may help autofocusing, but also has its shortcomings. Depending on the sensor, a high ISO may generate too much noise, and that can mess up the autofocus. Also, some autofocusing systems use an IR beam to help make contrast, and while the beam off the camera may be weak, you can use an external IR lantern to help.

On top of it all: if the lens does not have autofocus capability (that is, a motor), all will be for naught.

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