How did the old school film autofocus mechanism work? (the 2 "eye" system)

PhoBoKho · May 20, 2020

So I'm looking at an old point-and-shoot of my wife's, a Canon Sureshot Supreme. It has 2 small lenses diagonally opposite the main lens. Other point and shoots had similar lenses, some like "eyes" above the lens.

1) how did this work to get things in focus? It's certainly not like mirrorless cameras with their on-sensor focusing pixels. Nor DSLRs (though I'm not quite sure how they work).

2) some cameras, like my wife's Canon, had the "eyes" about as far apart as possible. Others, like another camera we found lying around the house (a Pentax Espio 115M), had the "eyes" literally right next to each other. I would have thought a greater distance between them would be better for "range finding", but then again I don't know how these work in the first place.

jim10219 · May 20, 2020

I can't say for sure, but it looks to me like the Canon Sureshot Supreme used and active AF system. That's basically where it sends out an IR beam of light through one of the eyes, and uses the other eye to read it's reflection and measures the distance of the subject by measuring the time it takes for the light to bounce back from it.

PhoBoKho · May 20, 2020

jim10219 said:
I can't say for sure, but it looks to me like the Canon Sureshot Supreme used and active AF system. That's basically where it sends out an IR beam of light through one of the eyes, and uses the other eye to read it's reflection and measures the distance of the subject by measuring the time it takes for the light to bounce back from it.

Sounds plausible. But wouldn't the IR beam get completely drowned out in bright sunlight?

John Bragg · May 20, 2020

PhoBoKho said:
So I'm looking at an old point-and-shoot of my wife's, a Canon Sureshot Supreme. It has 2 small lenses diagonally opposite the main lens. Other point and shoots had similar lenses, some like "eyes" above the lens.

1) how did this work to get things in focus? It's certainly not like mirrorless cameras with their on-sensor focusing pixels. Nor DSLRs (though I'm not quite sure how they work).

2) some cameras, like my wife's Canon, had the "eyes" about as far apart as possible. Others, like another camera we found lying around the house (a Pentax Espio 115M), had the "eyes" literally right next to each other. I would have thought a greater distance between them would be better for "range finding", but then again I don't know how these work in the first place.

Fantastically well is the short answer. I like mine and it can be relied upon to nail focus accurately even in total darkness. A link here to a description from Canon.
https://global.canon/en/c-museum/product/film120.html

I have owned a number of these and they are becoming desirable as folks are seeking out credible alternatives to the Mju series etc.

A favorite shot from one of mine. Available window light.

Emily and Elephant by E.J. Bragg, on Flickr

John Koehrer · May 20, 2020

jim10219 said:
I can't say for sure, but it looks to me like the Canon Sureshot Supreme used and active AF system. That's basically where it sends out an IR beam of light through one of the eyes, and uses the other eye to read it's reflection and measures the distance of the subject by measuring the time it takes for the light to bounce back from it.

Exactly. Minolta held the patents and when Nikon & Canon came out, guess who got a chunk of change from them.

johnha · May 21, 2020

The 'Active' IR systems had limited range (1 to 2 meters ish) and didn't work through glass, thus many had an 'infinity' or landscape mode for when this would be a problem. The AF resolution usually had fixed focus distances and the camera picked the most correct one (some magazine reviews stated the number of steps available).

The Pentax Espio most likely has 'Passive' phase detection just like an AF SLR but using a separate sensor outside of the lens (the two eyes). This will work across longer distances and through glass. The best compacts used this system, my Pentax Espio Mini is a fabulous compact with a 32mm lens, uses this system.

AgX · May 21, 2020

The "old" AF focus is called the Honeywell system. Two lenses, laterally apart, project two images on a line sensor. One of the lenses is swung during focusing of the taking lens until highest contrast is read by the sensor.

radiant · May 21, 2020

PhoBoKho said:
Sounds plausible. But wouldn't the IR beam get completely drowned out in bright sunlight?

Of course bright sun can drown the IR light. However the IR light is pulsed and therefore it is not so vulnerable to other static lights.

I'm pretty sure this form which can be seen in many IR distance sensors is some kind of standard.. https://www.pololu.com/product/136

Maybe hyperfocal focusing comes into help when the sensors maximum distance is reached. It is enough to get focus distance to for some amount but after that cameras just focus to hyperfocal distance based on aperture?

foc · May 21, 2020

All you need to know about Autofocus, but were afraid to ask.

https://en.wikipedia.org/wiki/Autofocus

PhoBoKho · May 22, 2020

Thanks for the answers everyone. Even the one referring me to the Wikipedia article. I usually find Wikipedia articles very good. Sometimes dry. This one on AF was on the arid side... I enjoyed reading the member's thoughts more.

PhoBoKho · May 22, 2020

John Bragg said:
Fantastically well is the short answer. I like mine and it can be relied upon to nail focus accurately even in total darkness. A link here to a description from Canon.
https://global.canon/en/c-museum/product/film120.html

I have owned a number of these and they are becoming desirable as folks are seeking out credible alternatives to the Mju series etc.

A favorite shot from one of mine. Available window light.

Emily and Elephant by E.J. Bragg, on Flickr

Cute child!

My wife's Canon Sureshot Supreme has been sitting for many years. When I pulled it out the other day, I was surprised to see that it has an f/2.8 lens. Fairly fast. I see that the Olympus MJU II has a similar lens. Though they go for big money these days. I'm happy that should I go out shooting with my wife's Canon, I could presumably get some decent shots with it.

John Bragg · May 22, 2020

PhoBoKho said:
Cute child!

My wife's Canon Sureshot Supreme has been sitting for many years. When I pulled it out the other day, I was surprised to see that it has an f/2.8 lens. Fairly fast. I see that the Olympus MJU II has a similar lens. Though they go for big money these days. I'm happy that should I go out shooting with my wife's Canon, I could presumably get some decent shots with it.

Indeed you can. One point to remember is that it locks focus on the first pressure on the shutter release, but doesn't lock the AE, unlike the Mju and others. It is still a capable machine and focusses more reliable than the Mju ii.

How did the old school film autofocus mechanism work? (the 2 "eye" system)

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