Test Focus shift

[JL2006]

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I would like to test focus shift ( if there is any) on my M8 with 35/2, 50/1.4 and 75/2.

Would someone explain the proper procedure ?

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[marknorton]

This is how I do it.

 

This is not something to do hand-held, you have to mount on a tripod, you have to use a 2 second self-timer, preferably with a cable release. Use a focus magnifier if you have one.

 

Mount the camera looking down at 45 degrees onto a piece of graph paper with a contrasty line drawn on it; that line is a focussing aid and should be across the shorter direction of the viewfinder so that the image of it shifts as you focus. I tend to use the camera in portrait orientation so that the line crosses left to right in my view through the finder.

 

The line should be in the middle of the viewfinder image and everything should be "square" and properly aligned.

 

Focus critically on the line with the lens wide open and take a shot; the graph grid will show you where the true focus is; that will show the inherent level of backfocus. Then stop the lens down and change nothing else; take another shot, and look to see where the true focus is, that will show you how much the focus is shifting as you stop down.

 

Using graph paper is preferable IMHO to using a measuring tape because it allows you to evaluate the sharpness across the whole image, not just where the tape measure it.

 

Importantly, I've been able to confirm with this technique that some of my lenses, which I thought were back-focussing, are not. The problem with focussing on the tape measure is that you get slightly different perspective for the two images - viewfinder and rangefinder - which makes it more difficult to determine when they are truly coincident.

[marknorton]

Here's a full frame image when I tested my 75mm/1.4...

 

[ATTACH]48980[/ATTACH]

 

and the centre of the image which suggests the level of focussing error to be around 1 cm or less - the true focus is a fraction closer to the camera.

 

[ATTACH]48982[/ATTACH]

[TimF]

and the centre of the image which suggests the level of focussing error to be around 1 cm or less - the true focus is a fraction closer to the camera.

And that could be down simply to human error. The magnification of the M8 finder is somewhere between those of the standard 0.72 and 0.58 film cameras IIRC. I had an 0.58 camera for around a year, but eventually traded it because I wasn't satisfied of getting consistently accurate focusing at wide apertures.

 

In his "Lens Book" (David & Charles 1994), Roger Hicks noted that "a 50mm f/1, for example, cannot hold an eyelash in focus from front to back at 1m", so critical focusing with fast lenses at close range is vital. Even breathing in and out can cause small movement, and could throw your focusing out I suspect.

 

This subject is referred to briefly in the Eastland article I posted about yesterday (and which still isn't up, sadly). The writer suggests that greater care in focusing is the true solution. That's not to say there aren't some defective lenses out there of course, but this isn't something I can recall from film days.

[stunsworth]

Tim, I think that's something that often seems to be overlooked with all this backfocussing test. Like yourself I'm not saying there aren't lenses with problems, but focussing with a Nocti at 1 metre and f1 on a low magnification viewfinder is very, very difficult.

[JL2006]

Thanks Mark for the info. May I know whether it will suffice to do the test only at minimum focusing distance or the result will differ at various distance ?

[j. borger]

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My take on this .... if you are not encountering problems with real pictures (read: a serious amount of unexplainable out of focus pictures) ... forget the test ... it will bring nothing.

The human error in real life shooting can compensate the slight of focus or make it worse ....

So if you are happy with your pictures there is no need to test..

The whole story about lenses not focussing well with the M8 is way out of proportions.............. you will losse far more pictures due to human error!

 

just my 2 cents of course!!

[Maarten]

In the latest LFI issue from August you will find an excellent article in which the back-focussing issue is tackled. The effect is very well explained. Also why the older aspheric designs suffer more than the later ones. The article shows some test pics that were taken with the M8 + 1,4/35 Asph from bottle labels and where you can see the effect at different apertures very clearly.

Furthermore I liked the explanation why Zeiss can claim less back-focussing issues with their latest designs.

 

Maarten Zuiderduin

[carstenw]

Maarten, the article does explain all this, but it doesn't say what to do about it, so I guess there is room for another...

[TimF]

In the latest LFI issue from August you will find an excellent article in which the back-focussing issue is tackled.

Maarten - any chance of a precis for those who don't have the magazine?

[jaapv]

Maarten, the article does explain all this, but it doesn't say what to do about it, so I guess there is room for another...

 

 

Well, actually it does, Carsten. It suggests two things

1. Adapt your focussing technique, i.e. misfocus deliberately

2.Leica has to revise the design by incorporating a floating element.

[carstenw]

Right, that is what I mean Memorising the frameline positions is a really bad solution, more a way to adapt to the lack of one. For Leica to redesign the lenses doesn't fix the lens in anyone's bag. Btw, a floating element is a part of the 50 Lux Asph design, yet this is one of the lenses with the highest rate of problems, so there is clearly more needed. The floating element typically helps improve close-up performance, AFAIK, so I am not sure why they mentioned that anyway. I found the LFI article well written and informative, but also a little aloof, not helping users with existing problems, and pointing more to Leica's historical excellence and so on. A bit weird.

[charlesphoto99]

As far as focus tests go I find stacking a number of books on top of each other and then moving the spines back and forward say 1/4 - 1/2inch or so from each other to be an easier way to test. That way you can shoot from straight on, and pick a book in the middle with nice large crisp easy to read/focus type as your main target.

 

And yes, in real life situations human error is the biggest problem, but it's nice to rule out mechanical error first.

[woodyspedden]

Well, actually it does, Carsten. It suggests two things

1. Adapt your focussing technique, i.e. misfocus deliberately

2.Leica has to revise the design by incorporating a floating element.

 

A third approach is to send the affected lens to Leica and ask them to shim the mount for perfect focus. However it will still be in focus only at the minimum focus distance (as Leica designed it) but at least you know your lens will be as good as is possible to make it.

 

Woody Spedden