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jaapv

M8-Focus on focussing

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Since M went digital we have had dozens of threads on rangefinder accuracy, diopters, magnifiers, etc., undoubtedly set off by the pixel-peeping possibilities. There have been some disinformative opinions on RF accuracy, recently there have been doubts about the suitability of the venerable mechansm for full format sensors, a real snakepit without hard facts.

So here are the numbers:

 

Focussing acuracy is in any system is determined by triangulation. The measuring base is the determining parameter for accuracy.

On a rangefinder it is quite simple. The measuring base is the distance between the windows of the camera, multiplied by the magnification.

The values are:

 

M8 47.1 mm

0.72 VF 49.9 mm

0.58 VF 40.2 mm

 

On an SLR it is different. The measuring base is determined mainly by the focal length.

The values are (for an R series camera):

 

21 mm 1.63 mm

35 mm 4.43 mm

50 mm 9.82 mm

90 mm 29.20 mm

135mm 65.70 mm

 

The cross-over point between the systems is approx. 110 mm. (0.72)

 

 

Now for magnifiers:

M8: 1.25x = 58.4 mm

1.4x = 65.9 mm

 

0.72: 1.25x = 62.0 mm

1.4x = 79.8 mm

 

This clearly shows that there is no reason to change the rangefinder mechanism for anything "better" to compete with a DSLR. At 35 mm it is a full magnitude more accurate.

 

Often the tolerances in the roller/cam mechanism are quoted as a drawback. That may be true in an absolute sense, but it must be offset against the tolerances in mirror placement and screen positioning in an SLR. I would be hard pushed to say which system introduces more inaccuracy.

Focus shift is just as disturbing on an SLR as on an RF because an SLR will measure the distance wide open.

 

Now to Autofocus.

The same remark about tolerances in the mirror and AF sensor apply, added to which the AF motor in the lens adds more inaccuracy. The accuracy is measured at optimal contrast and falls below the factory specification at low contrast and/or low light.

 

The best AF systems have a tolerance of "within 1/3rd DOF".

That means on an 1.4 50 mm lens plus or minus 5 cm at 3 meter.

For an f 4.0 zoom at 50 it would be a whopping plus or minus 15 cm at 3 m.

Values that would have us return our M8 to Solms with the first UPS and set off an angry thread here.

 

The conclusions are:

 

A. If you feel you M8 is less accurate at 90 mm or lower than your SLR there are just three possibilities:

1. the system is out of calibration and needs to be adjusted

2. there is an eyesight problem. Consider a diopter.

3. (sorry) simple user error.

 

B. The M8 is acurate enough to beat any other system.

 

C. A full-format DRF with a 0.72 viewfinder will be more accurate than the M8 - no need to change anything in the rangefinder

 

D. To equal SLR and AF accuracy on a 135 lens one may need a magnifier. 1.25x on an 0.72 viewfinder, 1.4x on an M8.

 

E. The optical RF is not stressed by modern technology and does not need to be improved as far as measuring ability is concerned.

 

source for the numbers: Günther Osterloh - Leica M - Advanced Photo School and Johnathan Eastland - Leica M Compendium

Edited by jaapv

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Yes, thanks this is a very useful summary. It is worth noting that a rangefinder is able to distinguish a ridiculously small error in triangulation, about +/- 5 cm at 5 metres (or better?), which gives a change in angle of 10^-4 radians or 6/1000° or 20 arcseconds. This is high tech engineering.

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How did you get your SLR figures Jaap?

I may be wrong of course but you give me the feeling to compare the effective base length of a rangefinder (47.09mm for the M8 for inst.) to the length necessary to focus such or such lens accurately. For instance about 60mm to focus a 135/4 lens on the M8 without magnifier.

Would you mind to elaborate a bit on this point?

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I too am a little puzzled regarding the SLR figures and triangulation. My understanding of SLR (Manual) focussing, is that you are looking at the image on a ground glass screen that is equidistant from the film/sensor plane.

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Nicole,yes- but it is still triangulation. The image can only be sharp if the angles of the incoming light match. Look at the diagrams of lenses, focussed and defocussed. With the microprisms and central wedge the situation is even more clear. Günther Osterloh adresses just this in his analysis. pp 42 ff. I can look up the exact formula for the SLR triangulation tonight and I will post it, unless somebody else has the book in his reach right now. It is on page 44. He mentions btw that the figures for a ground glass are arrived at slightly differently from the wedge and prisms, but that the resulting figures are very close.

Edited by jaapv

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How did you get your SLR figures Jaap?

I may be wrong of course but you give me the feeling to compare the effective base length of a rangefinder (47.09mm for the M8 for inst.) to the length necessary to focus such or such lens accurately. For instance about 60mm to focus a 135/4 lens on the M8 without magnifier.

Would you mind to elaborate a bit on this point?

I copied the figures from the Osterloh book.

No, the base of a 135 mm lens at 65.70 is the one produced by an R camera. That is not to say that everybody needs this length to obtain accurate focus. Many of us will focus an 135 accurately on the M8 without problems, but the SLR is easier. To equal that ease one would need the magnifier.

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I copied the figures from the Osterloh book.

No, the base of a 135 mm lens at 65.70 is the one produced by an R camera. That is not to say that everybody needs this length to obtain accurate focus. Many of us will focus an 135 accurately on the M8 without problems, but the SLR is easier. To equal that ease one would need the magnifier.

 

Interesting. I had always thought that the aperture of the lens came into the formula, but since you don't mention it I presume that Osterloh says it doesn't.

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Interesting. I had always thought that the aperture of the lens came into the formula, but since you don't mention it I presume that Osterloh says it doesn't.
He does mention that, but he discounts it .

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Interesting post Jaap. I enjoyed reading it.

 

In my experience, critical focus from a high performance DSLR AF system is much easier to operate in a dynamic environment than the RF. It's easy to split a vertical of a building with an RF for a super sharp image, but follow a human subject through six hours of a wedding, or street work, and those verticals are difficult to find.

 

No doubt, it's for the reasons you mentioned and one you didn't - speed of operation. An RF takes time to focus (presume there is no room for zone focus in critical focus situations) and that time slows the work down.

 

I take thousands of photos of people in a month and would love to switch to RF for all it's other advantages, but easily pinning down critical focus won't allow the switch.

 

As evidence, I've many hundreds of images I could post where the eyelashes, or even the veins in the eye can be clearly seen, not in a single photo, but in a sequence of ten different images, say, in a four minute window.

 

I'm not suggesting that all DSLR AF systems are equal, but mine has a separate computer dedicated to the AF system that will track minute movements in the subject, or the shooter. It's absolutely accurate, very quick, quite reliable and works well in low light. I'm confident that RF focussing can get nowhere near it in the applications I use it for and that's a real pain because I'd love to dispose of a complete system.

 

I cherish my 3 M's, but if I need critical focus, it's not the right camera for me to take .

Edited by Rolo

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Nicole,yes- but it is still triangulation. The image can only be sharp if the angles of the incoming light match. Look at the diagrams of lenses, focussed and defocussed. With the microprisms and central wedge the situation is even more clear. Günther Osterloh adresses just this in his analysis. pp 42 ff. I can look up the exact formula for the SLR triangulation tonight and I will post it, unless somebody else has the book in his reach right now. It is on page 44. He mentions btw that the figures for a ground glass are arrived at slightly differently from the wedge and prisms, but that the resulting figures are very close.

 

Thanks Jaap, but surely in this case the only triangulation is concerned with the lens focussing elements, and as such would vary between different lens designs and indeed the aperture in use. I still fail to see how this could be compared to rangefinder accuracy. Maybe I'm just being dense here.

:confused:

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Of course, Rolo, that is the practical approach, and nothing against it, quite the contrary. My post was theoretical and an answer to the charges that the rangefinder was antiquated and inaccurate.

However, my figure of "within 1/3rd DOF" is the official Nikon and Canon accuracy span for their top-end AF systems, and the precise focussing points at that. Their consumer level AF is, according to themselves, accurate to "within DOF" .

 

http://doug.kerr.home.att.net/pumpkin/AF_accuracy.pdf

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Thanks Jaap, but surely in this case the only triangulation is concerned with the lens focussing elements, and as such would vary between different lens designs and indeed the aperture in use. I still fail to see how this could be compared to rangefinder accuracy. Maybe I'm just being dense here. :confused:

I'll copy the exact wording of Günther on the SLR computation later, to eliminate all misunderstanding

Edited by jaapv

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Thanks again Jaap. One small but possibly very significant point though. When you talk about SLR focussing, I presume that you mean manual focussing (As you mentioned 'R' lenses.)? If you are talking about SLR AF systems, then I accept that triangulation is involved in the majority of systems.

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As you see I split it up. The numbers refer to manual focussing on an R series camera, the next paragraph takes the tolerances of Nikon and Canon for their own top end "precise"f ocussing points. I assume other makes are the same or worse.

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Of course, Rolo, that is the practical approach, and nothing against it, quite the contrary. My post was theoretical and an answer to the charges that the rangefinder was antiquated and inaccurate.

However, my figure of "within 1/3rd DOF" is the official Nikon and Canon accuracy span for their top-end AF systems, and the precise focussing points at that. Their consumer level AF is, according to themselves, accurate to "within DOF" .

 

http://doug.kerr.home.att.net/pumpkin/AF_accuracy.pdf

 

Thanks Jaap. As I wrote, do not doubt the theory put forward.

 

With regards to Can/Nikon, that's not my experience, if I focus on the eyeball at f2.8 I see at least sharp eyelashes.

 

Another point is that I never focus and re-compose with my DSLR. The rear thumb-wheel is set to move the the focus point and then check against surrounding points.

 

Here's a heavy crop from a manually focussed image where there's no way to see the veins ! Ha.

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Whatever the accuracy of the equipment we all know there is a difference. Indirect focusing using a RF linked to a lens and direct focusing through a lens using a mirror/focusing screen and now of course live-view.

 

Jeff

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The point about focus-recompose is an important one, Rolo.

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Slightly at a tangent to this thread - on Sunday evening after my daughter's wedding, I was trying to take photos of people dancing in a marquee, lit only by by "fairy" lights. Focusing the M8 was very difficult. I wonder if the SF58 flash should have incorporated a focusing light, activated by a half push of the shutter release. An accurate focusing mechanism is only effective if you can actually see what you are trying to focus on with a reasonable amount of contrast. In the end, I had to focus on a fairy light bulb at an estimated equivalent distance and reframe. BTW, the SF58 performed very well and I am delighted with it, although the blink reflex problem is still there with the pre-flash of GNC, albeit less than the SF24 - see attached.

 

Wilson

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