[Rexlawyer]

Curiosity drives me. I was wondering if anyone had ever experimented with trying to increase the efficiency of their auto focus in poor light by using a very light yellow filter, like a y = 0.75, on their camera to see if it helped the AF lock-in better/quicker. It works for our eyes when driving in poor light by increasing the visual contrast. So, why not our cameras? Sure it would tint the picture, but couldn't that then be negated in post-processing? That brings up side question #1: "Does AF work better when taking pictures in B&W?" Side question #2: "If the camera were programed to identify a particular geometric shape with good contrast, like a black boomerang looking sports logo for instance, wouldn't it lock its focus on that quicker?" Curious minds want to know.

[BigMackCam]

Moved to General Photography forum...

[Alex645]

I was wondering if anyone had ever experimented with trying to increase the efficiency of their auto focus in poor light by using a very light yellow filter, like a y = 0.75, on their camera to see if it helped the AF lock-in better/quicker. It works for our eyes when driving in poor light by increasing the visual contrast. So, why not our cameras?

For manual focusing, this would help but the main problem with AF at night is the lack of light, and thus the AF assist light. So then the question might be, why are some AF assist lights white, green, or yellow? This all has to do with that AF system fine tuned for a certain wavelength. The trade-off for a yellow filter is that you would lose about 1 EV and that light loss won't compensate for any benefits in the filter for AF.

Sure it would tint the picture, but couldn't that then be negated in post-processing?

For a monochromatic image, yes. For a full spectrum color image, no. A yellow filter filters out all of the spectrum except yellow. So any blue is absent; cyan and magenta would also be compromised. You can't recover what isn't there.

That brings up side question #1: "Does AF work better when taking pictures in B&W?"

No. Except for the Leica Monochrom which only shoot RAW sans color, all cameras shoot RAW in color, and then converts them to B&W as a jpeg.

Side question #2: "If the camera were programed to identify a particular geometric shape with good contrast, like a black boomerang looking sports logo for instance, wouldn't it lock its focus on that quicker?" Curious minds want to know.

OVF use phase-detect and EVF use contrast-detect. When considering the ideal AF system, you have to also consider accuracy, lock-on speed, missed focus time, noise, durability, ability to quick shift, minimum light level required and cost. Isolating specific shapes like curves on a boomerang would be like programming cars to avoid colliding with slow old pedestrians or faster humans on a skateboard. Regardless of the object, we want it focused or to avoid hitting it, so it has to use a more generic parameter like contrasting edges or in the case of cars, quickly approaching objects while the brakes are not applied.

[photoptimist]

Maybe.... A yellow filter helps human eyes because it reduces blue light that tends to scatter off the haze in an aging pre-cataract human eyeball lens. Perhaps it might also help in the case of an ancient hazy Takumar.

Jokes aside, anything that increases the contrast of the scene (without losing too much light) will help both PDAF and CDAF.

As for the side question #1 about B&W AF: to the best of my knowledge, the AF sensors in a DSLR are B&W and certainly worked just fine with B&W film back in the day.

As for the side question #2 about geometric shapes: It could help CDAF slightly but not PDAF. PDAF does not seen the scene at all and cannot recognize objects or shapes. Instead, PDAF sees linear slices of the image as seen from different parts of the lens aperture and measures the misalignments between the zig-zag patterns of brightnesses in the slices.

Programmed objects would only be useful for AF systems that use an area sensor and which the photographer knows they want to focus on the that specific object. Eye-AF is a good example of that. Programming in a "black boomerang looking sports logo" would only be useful for photographers on Nike's advertising team or for foot fetishists.

[Rexlawyer]

Entertaining response. TY