Focus accuracy

[NZDavid]

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Algrove, yes. Shortsighted, minus 4 and 5. I don't find rangefinder focusing a problem at all. The difficulty is looking at the LCD and shutter and aperture settings with my glasses on. I can perch them on my nose or take them off to see up close. From others' experience, I am not keen on progressives.

 

Despite its age and limitations, rangefinder focusing is still highly accurate and convenient. It is very likely we shall see more EVF cameras with AF, including from Leica. But it won't necessarily beat or even match RF in all situations.

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[algrove]

Algrove, yes. Shortsighted, minus 4 and 5. I don't find rangefinder focusing a problem at all. The difficulty is looking at the LCD and shutter and aperture settings with my glasses on. I can perch them on my nose or take them off to see up close. From others' experience, I am not keen on progressives.

 

Despite its age and limitations, rangefinder focusing is still highly accurate and convenient. It is very likely we shall see more EVF cameras with AF, including from Leica. But it won't necessarily beat or even match RF in all situations.

 

I only asked since I found after getting the Walter eye piece my hit rate is much much better and I guess that's due to it correcting my astigmatism and minus 5.75. Actually I use progressives and am now contemplating getting some regular glasses where one side or the other can be rotated up and away when I need to use that eye piece.

 

I see the company that markets the "Hoodman" also sells this type of frame, but the buyer then has to find an optical store to fit lenses to them. Incidently, the Hoodman has helped me tremendously in seeing the LCD in bright sunlight and it has a diopter adjustment built-in. Like someone said now they needed a glasses cord to hold them around the neck, but then if I were to use that approach, I would loose my view of whats happening around me with the other eye which BY FAR my worst eye.

[Sp12]

I don't follow you there. Contrast AF is a mechanised version of eyeball focusing on an all matte screen. You turn the focus and see if the image gets any sharper, if it gets fuzzyer you turn the other way, continue until the image gets fuzzier again, change direction, turn until it gets fuzzier ... repeat until you are satisfied. Hopefully, every 'stroke' will be shorter than the previous one.

 

Sometimes it isn't. What then happens is called "AF hunting". It can go on for some time. We have all seen cases when it never stopped.

 

That's exactly what makes it accurate. It's a closed-loop system. You never hear people say "my Sigma 50/1.4 doesn't AF properly/backfocuses in contrast-detect mode". It uses actual sensor data to pick the best position for the lens. I haven't seen AF hunting (yes, it will rack back and forth, that's not hunting endlessly) on any reasonably useful camera in CD mode, and this is the only mode able to provide pixel-level results consistently.

 

Phase comparison AF on the other hand is technically similar to both a rangefinder, and to a split image aid on a screen, which is in effect a simulated mini-rangefinder, as it simultaneously compares two images. Like the split-prism, the AF system's base length is limited by the aperture of the lens which is hopefully wide open. But like a RF, the separate AF sensor and its associated electronics can decide both in what direction focus is to be changed, and also how much. So it can get it right in the first try.

Yes and no. No because phase-AF is an open loop system. It tells the lens what distance to focus to. It does not check the focus after it sends that signal. If your lens backfocused, it won't notice. If you're looking at MFD blur, it won't notice. Even if all goes well it's limited in accuracy by the calibration of the sensors with respect to the THE sensor, as well as the DoF of the lens. Even high-end AF sensors don't claim (or provide) accuracy beyond F2.8 (at any FL)

 

The great advantage of the RF is that it is independent of the lens. It delivers one precise level of accuracy irrespective of what lens you mount on the camera, as long as both the RF and the lens are correctly adjusted, while short focal lengths and slow speeds result in lower accuaracy with 'TTL focusing'. With longer forcal lengths however the accuracy requirement increases, until at around 150mm it does no longer suffice. But TTL focusing, being lens dependent, delivers greater accuracy with increasing focal length.

Agreed with this (though if you're after critical sharpness the Leica rangefinder will not do for 50/1.4 and faster/longer).

 

L.B.