Focus Confirmation in RF

[pico]

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From Jaapv''s other thread: "And this is the reason a little focus confirmation LED, linked to the existing RF mechanism, would be useful for us old codgers"

 

How would such a gizmo work, and can it be incorporated into the current M without external dimensional changes. But mainly, how would it work?

 

EDIT: I'm sure there are a few old timers who remember the Focuspot. It was a light that was directed into the rangefinder and projected an image

of its light source (element of small bulb) through both of the front RF windows of a Graflex (and some Super Technikas). The photographer would

focus until the two sources co-incided. I have two and they actually work. I tried replacing the bulb with a laser but the laser just went through

the half-silvered mirror. An LED might be better. See: http://lommen9.home.xs4all.nl/kalart/page9.html

 

Now, using far more engineering smarts than I will ever have, maybe Leica could come up with something similar but with an invisible beam.

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[Xmas]

How would such a gizmo work, and can it be incorporated into the current M without external dimensional changes. But mainly, how would it work?

 

If you can do a better rangefinder then a M3 you are good, if you cant use an M3 you need a white stick and a dog.

 

Noel

[IkarusJohn]

Thanks Noel. Helpful.

 

The technology has been around for a while, though I have no idea how it works. Putting an manual focus lens onto my F5, I got focus confirmation lights on the righthand side of the view finder. I could focus where I liked, do what I liked, but the lights told me whether or not what was in the centre spot was in focus.

 

Having said that, the focussing screen on the F5 (when used for manual focus) was pretty hopeless. The parallax split image of the Leica view finder is the best I've used. My first camera (that I actually owned) was a Canonet, so I'm used to split image focussing.

 

The only benefit I see in focus confirmation on an M camera is through the lens confirmation, thereby addressing focus-shift. From reading this forum, it seems that while written about, the issue is small, and the benefit of TTL focus confirmation slight.

 

I'll get my white stick now.

 

Cheers

John

[Xmas]

The parallax split image of the Leica view finder is the best I've used.

Hi John

 

It is the best there is, unless you want to try a ZM...

 

If you want to be annoyed try a Contax G1 or G2.

 

The prescription for my specs is real bad...

 

Noel

[giordano]

There's no way of adding a focus confirmation signal to the M viewfinder (other than the existing signal, the coincident images:)).

 

A fundamental reason is that the only place in the optics where real images (of the views through the viewfinder and rangefinder windows) exist and can be compared is on the user's retina.

 

In previous threads we've discussed the possibility of adding a phase-detection focus sensor in front of the shutter. Because the focus sensor must effectively be in the film plane or digital equivalent, this means using a mirror to reflect light onto the sensor SLR style. With the very limited space behind the M lens mount, it turns out to be somewhere between impossible and extremely difficult to do this, especially as there's no point in doing it unless can be trusted to be more precise than the rangefinder.

[adan]

if you cant use an M3 you need a white stick and a dog.

 

No sympathy, eh? http://www.l-camera-forum.com/leica-forum/customer-forum/190269-too-old-m-camera.html

 

Anyway....

 

The problem with the FocuSpot for the M cameras is that the light source has to cover the rangefinder eyepiece. For the 4x5 Graphics, that was OK because the VF and RF were separate windows. It would shut down the combined M viewfinder/rangefinder completely, though.

 

The FocuSpot is similar to ACTIVE AF systems in that it triangulates actual distance rather than reading image blur. Other active systems include the Polaroid Sonar, and Contax G infrared-triangulating systems. Active systems have the plus of working in complete darkness, and the defect of not usually working through glass - they measure to the glass, not through the glass to a subject beyond.

 

Active systems also need to know where the lens is focused, in distance, since they are not actually "looking at" the projected image. The Contax-G had a spinning disk scribed with dark lines and geared to the AF motor, that rotated between an LED and light cell to precisely count how many 100ths of a rotation the motor had applied ("manual" focus with confirmation in the G cameras actually used the AF motor to move the lens.) The camera had look-up tables stored in a ROM to translate rotations into distance ("90mm lens: 2.47 rotations = 3.6 meters" and the like). Which is why it always had to reset to infinity before each shot - it always had to start over from "zero" to get the count right.

 

Active systems also usually have variable accuracy, being very good at close range but losing precision beyond 20-30 feet. The Contax G had a "dead zone" between 35 feet and infinity - anything beyond 35 feet was just "infinity" to the Gs. Which was OK for the DoF of a 45mm @ f/2, but not their 90mm @ f/2.8. The 5-inch base of the FocuSpot likely did a bit better - but where would one put a 5-inch RF in an M body?

 

Passive systems:

 

In previous discussions of MF-assist/confirmation, it has been pointed out that off-the-sensor "contrast-detect" AF can't do this - it doesn't know "which way" the image is out of focus. So a Live-view AF system can focus, but cannot tell YOU which way to focus.

 

It requires phase-detect AF, SLR-style off the mirror. Which would mean adding at least a small movable mirror somewhere between the lens and shutter, to redirect focused light to a phase-detect AF sensor.

 

The real bottom line is that the Leica M rangefinder already IS a focus-confirmation system. If the two images are aligned, the lens is focused. Aligned = "Focus confirmation light" Not aligned = "no focus confirmation light." If it's too hard to see, get a magnifier.

 

Both the M body and the RF/VF itself are jam-packed already, so there really is no room for any additional doodads. Attached is a diagram of the M RF/VF - anyone who wants to can sketch in where in that tight space to put sensors, wires and lights to allow an additional FC system.

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[lct]

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...the Leica M rangefinder already IS a focus-confirmation system. If the two images are aligned, the lens is focused. Aligned = "Focus confirmation light" Not aligned = "no focus confirmation light." If it's too hard to see, get a magnifier...

Or Noel's white stick . Seriously, the RF patch is more contrasty when the two images are aligned. Wouldn't it be possible to detect that contrast instead of using an AF device? Just curious.

[adan]

Take a look at the diagram - the only place where the two images are coincident (and contrast could be detected) is the cm in front of the eyepiece where the red and blue arrows overlap.

 

Where would you place a sensor/detector - within that cm of red/blue arrow - to measure the contrast of the dual image. And not block the viewfinder?

[SJP]

Put in a 4-way splitter and measure contrast to the left.

[giordano]

Put in a 4-way splitter and measure contrast to the left.

 

It's not that simple. Even if it's possible to divert part of the beams to the left, you have to

 

1) Add optical elements to focus the beams into overlapping images on the sensor. I.e. you can't just stick the sensor on the side of the 4-way splitter.

 

2) Provide a mechanism to keep the image on the sensor in focus at any subject distance between infinity and 70cm. If the images on the sensor aren't sharp there won't be a distinct peak in contrast when they coincide and the focus confirmation will be less accurate than just looking at the RF patch.

 

3) Cope with the large distance the RF patch moves in relation to the fixed elements of the viewfinder as you focus between infinity and 70cm. This might mean a mechanism to physically move the sensor or an oversized sensor with a physical or electronic mask (either of which means bigger lenses and beam splitters), or a swinging prism between the beam splitter and the sensor. None of them easy to do, and all of them impossible in the current M bodies.

 

OK: can anyone prove me wrong? No wishful thinking allowed.

[WigglePig]

Why would anyone wish to do this?

[lct]

...2) Provide a mechanism to keep the image on the sensor in focus at any subject distance between infinity and 70cm... 3) Cope with the large distance the RF patch moves in relation to the fixed elements of the viewfinder as you focus between infinity and 70cm...

Why so if i may ask? My eye doesn't need any additional device when i'm focusing so why would a sensor need one (apart from a pellicle mirror i guess). Just curious again as i'm not techie at all sorry.

[giordano]

Why so if i may ask? My eye doesn't need any additional device when i'm focusing so why would a sensor need one (apart from a pellicle mirror i guess). Just curious again as i'm not techie at all sorry.

 

Your eye is focusing without your noticing. My accommodation is pretty limited, and if I'm wearing my distance glasses the image in the finder is pin sharp at most distances but begins to blur closer than about 0.9m. At 0.7m I need to use my mid-range glasses (or tilt my head to use a different part of a varifocal lens).

[Xmas]

Hi

 

When you focus on a near eye ball mid eyelash you know where the contrast enhancement is ... any auto focus system may want to pick a different eyelash or eyeball.

 

Although my perscription is real bad I dont have real difficulty focusing on where I want the sharp plane. My left eye is a lot better but I still use the right eye.

 

My Contax G never focuses on what I want unless I want a lamp post to be in focus or something similar. If the lamp posts are echoloned who knows...

 

It is either a M3 or a ZM take your pick, maybe a M8 or M9...

 

Noel

[pico]

No sympathy, eh? http://www.l-camera-forum.com/leica-forum/customer-forum/190269-too-old-m-camera.html

 

Anyway....

 

The problem with the FocuSpot for the M cameras is that the light source has to cover the rangefinder eyepiece. For the 4x5 Graphics, that was OK because the VF and RF were separate windows. It would shut down the combined M viewfinder/rangefinder completely, though.

 

It is true that the VF and RF were separate but some people could still estimate well enough to shoot using only the RF. The Focuspot did not cover the RF window. It was placed in a separate port at the top of the rangefinder body, as an option of course.

 

Of course one cannot just add such a port to the Leica. It would blind the user.

[....]The real bottom line is that the Leica M rangefinder already IS a focus-confirmation system.[...]

 

That's what I was thinking, too. I thought people would image the patch turning red or something when two objects coincided. So much for selective focusing on the near eye or bridge of the nose, and almost anything at a distance. I'm sure we aren't surprised by how very many out-of-focus images we find coming from the Leica when people think Infinity is 'just kinda far away' with say, a 35mm lens. It still must be focused.

[epand56]

My Contax G never focuses on what I want unless I want a lamp post to be in focus or something similar.

 

I shot several years with Contax GI an GII and looking at my old film pictures its auto-focus was quite spot on. And remember they had a focus confirmation when used in manual focus. I loved those cameras and waited to the last wishing for a digital back.