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Old Jan 27, 2004, 2:18 AM   #1
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Default 5700 AF Area Mode Off Test

There have been so many discussions of the low light and low contrast AF problem on this board ever since the first days when the 5700 was released. Some believe the sluggish AF speed of the 5700 is a minor problem mainly due to incompetence, while some others consider it is a major setback. Some pointed out that the sluggish AF speed can be improved (significantly as some suggested) by turning off the AF Area mode. According to 5700's manual (page 112), center focus area is used if AF Area Mode is Off. This goal of this post is to show that this is not always the case, and the validity of this claim depends on the illumination of the center area. I did not test the claimed possible AF speed-up, because it is the subject of another post. What I intend to show is that what the Nikon manual claims is misleading at best. Some NTF'ers also showed similar example; but, I intend to provide a repeatable DIY experiment.

First, here is the testing environment. See the diagram below. A 5700 is placed 17 feet away from a wall and zoomed in all the way in to 280mm. The aperture is set to F4.2, the largest aperture at 280mm. Exposure is not important (because we only focus), and, hence, no intention to make exposure accurate. I just let the camera to make a decision. On the wall, there is a color comic page, which covers the center portion of the image. Five feet away from the 5700 is a white sheet with black bars, which covers less than 1/4 of the image frame. Thus, the 5-ft subject is of very high contrast (i.e., white-black bars). One 250W studio hot light is used to illuminate each subject (i.e., comic page and white-black bars). These studio lights have variable power capability so that I can fine tune the light intensity. An incident light meter was used to measure the EV on each subject.

Here is the first image. The measured EV's of the white/black bars and the comic page are 7 2/3 and 2 2/3, respectively. In fact, the light used to illuminate the comic page was turned off, and, this 2 2/3 EV actually came from the light used to illuminate the white/black bar subject. Therefore, there is a 5-stop difference between the two subjects. When the 5700 AF Area Mode was set to Off, the 5700 focused on the white-black bar subject rather than the comic page at the center! So, what the Nikon manual says is, unfortunately, incorrect.

In the second image, the second light is turned on and measured to have 5.0 EV on the comic page subject. The difference is now reduced to 2 2/3 stop. The 5700 still focused on the white-black bar subject.

If the power of the second light is increased to 6 EV to make a 1 2/3 stop difference, the 5700 focused on the comic page at the center, and the Nikon manual becomes correct.

Now, if both lights are tuned on so that both subjects receive 7 2/3 EV, there is no doubt that the 5700 will lock on the center subject.

What does this test imply? Simple. In the AF Area Mode Off mode, the 5700 does not always lock on the center area. In fact, the 5700 may lock on any area with very high contrast. However, if the center contrast is not significantly different from the contrast of other areas, the center portion may be used. This explains why in some images the human subjects are out of focus but the background with flashy and/or contrasty textures is in focus, because human face has low contrast in low light condition.

Hope this will settle the AF Area Mode Off confusion once for all. More detailed and more complete tests will appear in my 5700 user guide in the future.

Nikon Coolpix 950/990/995/2500/4500/5700 User Guide
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Old Jan 27, 2004, 12:06 PM   #2
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Thanks. I will run some tests and see how it works.

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