Focus

[wheelie52]

I'm a bit new to the rangefinder system and the behaviour of my M cameras in

regards to the variation in focus confuses me! The same camera with different

lenses mounted and set to infinity will behave differently. My 28mm

voigtlander mounted on my M7 and set to infinity on the lens scale will have a

slightly misaligned image in the rangefinder .. mount a 50mm summicron and the

result is the same .. still not a perfect alignment. Two other lenses I have

however, when set at infinity align the rangefinder images perfectly. It seems

to change from lens to lens so I can only assume that there is a variataion in

the infinity setting of the individual lenses and it is not necesarily a

problem with the camera's rangefinder!

 

Can someone tell me why this is so and should I worry about it and am I right

in thinking that this is just a slight variation in lens construction and where

they actually stop at infinity!

 

Keith

 

ps.....this seems to happen on all my M series cameras!

[SCL]

Probably the lenses not properly moving the rangefinder cam. Other possible causes...include the focusing ring on the lens not aligned with infinity focus, or even the rangefinder in the camera needing alignment. Best thing to do is to put camera on a tripod, and check each lens at infinity with a groundglass and loupe (or the more expensive option is to shoot a series of pix with the different lenses and check the negatives). Probably, when all is said and done, the DOF will take care of the issue...but is not generally a variation in lenses which are properly collimated and adjusted.

[bob haight]

Some lenses have a bit of overfocus or underfocus at the infinity detent. Overfocus is preferable and some lenses are intentionally designed that way. I would have the most offensive lens checked and then retest. I doubt its the rangefinder if some lenses align correctly.

[EricBoehm]

I wouldn't worry so much about what the scale says, either. Instead, check the rangefinder against an object at least 1 mile away (maybe a radio tower or tall building) and make sure that you are able to get the rangefinder to coincide on the object at or near the infinity mark on the lens. Shoot wide open if possible and check that the image is reasonably sharp. Remember, it's not what the scale says, it's what's recorded on the film that matters.

[frederick_muller]

Based on your post, one thing I would confirm is that you are in fact testing at optical infinity. Repeat your experiment by focusing on the moon, or a bright star. If that's not possible, focus on a vertical mast at least half a mile away. Then see which camera and lens combinations line up and which don't.

 

I don't buy the overfocus and underfocus argument. I think a properly adjusted lens should focus bang onto infinity with a properly adjusted rangefinder.

[wheelie52]

Yes I forgot to mention that the object I was focusing on was a good kilometer away and was vertical and clearly defined. I think the idea of taking a photo with each lens set to infinity and then checking the individual results post developing is excellent and I will do that!

 

The other way I thought of checking was to focus on an object a measured distance away from the focal plane and see if it matches the scale on the lens.

 

Keith

[fjords]

Keith, to check infinity, I use a long white picket fence. Tie a colored scarf to a picket 10

meters from the film plane, place the lens at the 10m mark. Now check the range finder,

if it's on take a pic @ f/2. If everything is correct, the photo should be right on @ the 10m

scarf. The depth of field @ infinity is so great even @ f/2 that it's not a very good photo

test. The 10m mark is adjusted by the infinity screw-cam on the roller. I have a 35/50/

75 and the infinity marks are slightly different on each, but a 10m they all agree.