Focus Help

[JimP]

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Hello,

Just bought a CV 28mm/1.9 today... love the contrast of the lens (or lesser amount), but I think that the focus is off a bit. After taking a number of images that are out of focus, I set up this jig and focused on the target. Looks like the lens is sharp behind the plane of focus.

Infinity focus also appears to be off as well. Is this consistent? Is there a way to tweak the lens a bit, or is it better to return?

FYI... Lens was set to f 8.0,

Thanks in advance

[gwelland]

The thing you need to determine is whether it's the lens or the camera. How do your other lenses perform?

 

I have two M8's and each has different focus biases - one slightly front focuses and the other back focuses. I only really noticed this until I got a Noctilux and found that it was basically unusable on one and works fine on the other. Subsequent testing with a similar target set to you under controlled circumstances was very revealing about the cameras and lens collection.

 

The good news is that if the RF is off and back-focussing it's relatively easy to adjust out.

[JimP]

Hi.

The other lenses that I have focus quite well. Two Zeiss lenses as well as the CV35 2.5. This seems to be the odd man out. I REALLY like the color and contrast of this lens, so I would like to find a remedy. One question that I have is if the adapter (screw to bayonet) could be the issue somehow. Looks to be seated tightly against the lens.

I have read that some people are putting a shim on the focusing cam... is this symptomatic of this problem?

Thanks

[Shootist]

Focus the lens on infinity at a star. If the image in the viewfinder/rangefinder patch do not line up then the lens is either NOT going far enough in (the spit/overlayed image is to the right in the rangefinder patch) or going in to far (split/overlay image it to the left).

If it's to the right then the adapter is to thick. If it's to the left the adapter is to thin.

Only if the adapter is to thin will a shim help, moving the lens out slightly.

Otherwise you need to mill the adapter down, make it thinner, or find one that is thinner.

You should also try that adapter on another LTM lens and see if the same thing happens on the other lens. If it does them the adapter should be replaced. If it doesn't change the other lens then the new lens you have is out of adjustment and the adapter is fine.

[JimP]

Thanks, Cloudy night, so I will have to wait to do this till tomorrow. I did do a test where I put a paper shim between the lens and the adapter. Focus is now spot on at least for the 5 foot range (actually quite good!). However, the "top" of the lens, now resides at 8 o'clock looking from the front. Again, further testing tomorrow. So appears the adapter is too thin (is a Voigtlander adapter)?? Still wondering if it is the adapter or the lens. If the adapter, I plan on getting a Milich adapter for coding purposes anyway. So maybe get the JM adapter first if the patch is misaligned, otherwise replace the lens? Is there a history of adapters being an issue, or more likely the lens?

Thanks

[orjanf]

Hi,

This was the problem with mine and also with the same lens for couple of other forum users.

My final solution was to put a thin (0.08 mm) shim between the bayonet adaptor and the mount to move the lens out a bit so it focuses correctly at the infinity stop and then add two coats of paint on the part of the focusing cam that touches the cam follower to get the viewfinder focusing accurate.

A couple of users have just put a layer or two of tape on the cam but I worry about it coming off. It gets pretty hot and humid here.

 

Incidentally my cv35/2.5 suffered the same problem although to alesser extent (0.04 mm)

[orjanf]

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P.S. if you remove the flange of the lens you can shift the infinity mark into one of three positions to align the mark with the 12 o'clock position - or very near

[JimP]

Hello All,

Well, tried the lens outside and the focus is definitely off. If I focus on an object and then back off a few degrees on the focus barrel, I get a nice sharp image. At infinity, the patch is lined up, so it will not be something as simple as putting a shim between the adapter and lens (wish it were that simple). Since it is a new lens, I think I will return it to the dealer and get another sample. Before I do that, is there an adjustment that can be done in the lens to make the correction that is permanent... in other words, a physical adjustment to the lens, not a shim and then tape/paint the cam? I would even consider shipping it off to someone to do the adjustment if the fee were reasonable in order to really get the focus nailed.

Thanks!

Jim