AF Sensitivity and Function

Started Oct 14, 2008 | Discussions thread
Marianne Oelund Veteran Member • Posts: 7,788
Kerry's Experiment #3: Simulated AF Optics

Picking up where we left off in Part 2, we again have the lens at maximum aperture, projecting a focused spot of light onto the screen.

Cut out a rectangle of paper, about 1.5" by 3", and punch a hole in the center of about 1/16" (1.5mm) diameter. Hold this paper across the back of the lens, with the small hole inside the light cone. The projected spot on the screen will become much dimmer, but note that you can move the hole to any position inside the light cone, without the spot disappearing, and in fact, without its brightness changing.

Now punch a second small hole in the 1.5" by 3" paper, centered 12mm away from the first hole, and in line with the length of the paper. Hold the paper so that both holes are within the light cone, and align it so that both holes are on a diameter and evenly spaced from the center (i.e., evenly spaced from the edge of the circle - you want the holes positioned symmetrically). The projected light spot will now be a bit brighter, having the illumination from two holes.

Here's the fun part. You may need a second person to assist. While keeping the paper with the two holes aligned at the back of the lens, rotate the focus ring and observe what happens to the projected spot of light. It will split into two spots, and you control the distance between them by turning the lens focus ring. What you are seeing now, is exactly what is projected onto the AF sensors in the camera. The AF system compares the two small images, and adjusts the lens focus until they line up.

Now, what happens when the lens aperture is adjusted? With the two holes in the paper centered 12mm apart, they have been set up to work with apertures of f/2.8 or faster. If you set the lens to f/1.4, f/2, or f/2.8, the projected spot(s) will appear the same. But, if you try to stop down smaller than f/2.8, the spots will extinguish because the light cone becomes too small to pass light through both holes at the same time (you need to hold the alignment of the paper rather accurately to see both spots go out at the same time).

Of course, there is no such mask with small holes in our cameras, but the mask does simulate the AF optics, which samples a small pair of spots at the back of the lens, for each AF sensor.

There are several more things you can try:

Second light source

Add a second light source to the side of the original lamp. Note that you will have two projected spots on the screen, which you can split or merge with the lens focus ring.

Try moving the second light source closer. Now you will see that the split spots will merge at different focus settings, for each light source.

Combination horizontal/vertical AF optics

You can add a second pair of holes to the paper. If you place these at right angles to the original holes, you will form a combination horizontal/vertical AF optics set. For added interest, make the second set of holes closer together; if they are spaced 5mm between centers, then they will work down to lens apertures of f/5.6. Note, however, that the pair of light spots which they project will be closer together, i.e., the AF sensor requires more precision to use the closer holes.

Additional variations: Up to your own imagination!

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