Where is infinity for different focal length?

[Mylek]

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Hello,

 

i'm wondering where is the infinity for each focal length? I read somewhere that this distance is not the same for a 28 or 90mm.

Lately, i found my 75mm to have double vertical lines at at infinity but also realize that it could only be the way i place my eye in the viewfinder as i'm wearing progressive spectacle.

I want to be able to check it properly before sending it for adjustment.

 

Regards!

[cbretteville]

As far as I can remember a general rule is that infinity is considered to be from 1000*focal length and beyond.

Carl

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[Michael Geschlecht]

Hello Mylek,

 

Welcome to the Forum.

 

The Moon, Planets & the Stars make a reliable Infinity target for all focal lengths. They are available to everyone Worldwide & they are free.

 

Best Regards,

 

Michael

[IWC Doppel]

I am pretty sure you will find something between 75m and the moon that might be a little easier to shoot during ggod light

[Mylek]

Thanks!

 

Did anybody notice these double vertical lines at infinity wearing progressive or other prescription glasses when moving laterally?

Edited August 3, 2012 by Mylek

[01af]

As far as I can remember a general rule is that infinity is considered to be from 1000 × focal length and beyond.

1,000 × focal length is way too short ... or maybe good enough for low quality demands.

 

I'd say for medium quality demands, infinity may considered to start at about 10,000 × f. For high demands, make that 40,000× or 100,000× ... i. e. 2 - 5 km (approx. 1 - 3 miles) for a 50 mm lens on 35-mm format.

[Michael Geschlecht]

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Hello IWC Doppel,

 

But: Why not use the Moon, etc? They are universally available, reliable subjects of known & repeatable value. A readily available standard usable equally well w/ all lenses.

 

If someone chooses the Moon as a target they will even be able to differentiate between lenses of different focal lengths w/o making reference to notes, etc because of differences in image size.

 

Why struggle to find a suitable alternative which may or may not be usable under certain circumstances when an unquestinably appropriate subject is readily available?

 

Best Regards,

 

Michael

[Michael Geschlecht]

Hello Mylek,

 

Are the double vertical lines because:

 

1. W/ glasses on the image is sharper & clearer in the range/viewfinder. What was 1 reasonably sharp line w/o glasses is now 2 reasonably sharp lines w/ glasses.

 

Or:

 

2. The double image is always there w & w/o glasses. It is more clearly visible w/ glasses.

 

Or:

 

3. The image w/o glasses is so unsharp it is not possible to determine whether it is 1 or 2 lines w/o glasses.

 

Or:

 

4. The image in the range/viewfinder is sharper & clearer w/o glasses.

 

Or:

 

5. There is never a really sharp image w or w/o glasses.

 

Best Regards,

 

Michael

[thighslapper]

If you are trying to assess if a lens is accurately focussed you need to refer to the images the camera takes.

 

My moon photography with an M9 has been uniformly useless for this purpose

 

You need a distant pylon or building ... at least several kilometres away.... which is invariably on the skyline....

 

You then run into issues with testing fast lenses wide open as you end up with shutter speeds faster than 1/4000.... and if you do it deliberately in dull light the contrast is poor and focussing difficult and innaccurate.... stopping down increases DOF so much that results can be misleading....

 

Checking lenses and the rangefinder mechanism for 'miscalibration' needs to be approached with caution and a logical methodology....

 

Otherwise you will end up sending gear back to Solms that is actually perfectly acceptably adjusted for 'normal' usage

 

and.......

 

as you will find in other threads, not all Leica lenses 'rangefinder' at full extension or the infinity mark on the barrel...... some coincide images before full extension and the odd few never coincide 'infinity' images fully extended. If photos taken are sharp from the camera when used at mid and long distance this 'error' is irrelevant and can be ignored.

Edited August 3, 2012 by thighslapper

[IWC Doppel]

Hello IWC Doppel,

 

But: Why not use the Moon, etc? They are universally available, reliable subjects of known & repeatable value. A readily available standard usable equally well w/ all lenses.

 

If someone chooses the Moon as a target they will even be able to differentiate between lenses of different focal lengths w/o making reference to notes, etc because of differences in image size.

 

Why struggle to find a suitable alternative which may or may not be usable under certain circumstances when an unquestinably appropriate subject is readily available?

 

Best Regards,

 

Michael

 

Fair point, but it's been very cloudy here for a while !

[andyedward]

Does atmospheric haze reduce the perception of sharpness with subjects several kilometers away?

[Mylek]

Michael,

 

w/o glasses everything is soft!

w/glasses, i see a sharp image but if i turn my head slightly toward the left, i see that double image one sharp and the other soft.

It does the same with a few different lenses but easier to see on 75 and 90mm.

Maybe it is why i never really notice it before because my 35mm is almost all the time on the M9.

Probably it have to do with the curve of my glasses...

[wda]

Does atmospheric haze reduce the perception of sharpness with subjects several kilometers away?

I recall discovering this from an elevated apartment in a foreign city centre. I had good clear targets but was puzzled why my negatives were less sharp then expected. The cause was heat haze rising from hot buildings in winter causing a pronounced shimmering of the air mass and distorting perception of sharpness; much like a mirage effect.

[tobey bilek]

Bifocals do not cause a doubling of image at infinity. That would be under or over corrected astigmatism or improper axis of the correction in your glasses.

[Michael Geschlecht]

Hello Mylek,

 

Progressive lenses are generally for use @ a distance @ the top. They are generally for close up, ie: reading, @ the bottom. Grading from 1 to the other visibly seamlessly.

 

They are still:

 

Distance - top

 

Close Up - below

 

Between top & bottom - ?

 

M range/viewfinders are optically distant.

 

I know the projected frames are @ a different distance optically than the viewfinder image & the line that does not move is part of the viewfinder image. It may be possible that the moving rangefinder image is @ a different optical distance than the stationary image in the viewfinder. You might ask Leica.

 

If so: It is possibile that when you turn your head more of the "?" part of the seamless transition zone of the glass's lenses comes into your field of view & that creates a composite image more out of focus on 1 plane than another.

 

Do you have another pair of glasses you might try which are only for distance or have a clear line delineating the bi-focal portion? Even if it is an older pair w/ a slightly different prescription.

 

Best Regards,

 

Michael

[SJP]

Using some rule of thumb for infinity like > 500m for a 50mm lens (factor 10.000) sounds sensible enough. Choose a factor that you feel happy with, a few kilometers is certainly enough.

 

But what I think you are asking is that for some of your lenses the maximum barrel rotation is slightly more (or slightly less) than "infinite in the rangefinder". The only way I can think of to check this is by using a star or a planet or the moon and focus with the rangefinder - the resulting image should then be sharp. If the lens can be rotated a bit, very slightly, further than "infinity" is not really important, in fact that is as it should be. Less than infinity is not really OK.

[Mylek]

Michael,

 

i tried it with my old glasses which were for distance and i have the same result.

I can see double from 10m to infinity.

I guess, i'll have to learn to squeeze less my nose against the body...

At less, my rangefinder and the lens seem ok as far as i can see.

 

Yes, atmospheric haze reduce the perception of sharpness with subjects several kilometers away? As i leave close to the sea, i can see this effect specially on windy or hot days.

[Michael Geschlecht]

Hello Mylek,

 

If the softer line is the line that moves then it is possible the softness is not your turning your head but rather that your eye is not in the center of the eyepiece. M's need a more or less centered view of the frame for the vertical lines in the moving image to look as sharp as the vertical lines in the stationary image.

 

Also: W/ or w/o glasses: When you focus w/ a longer lens @ longer distance & then move your head laterally: The lines in objects representing things in front of or behind the point of focus appear softer & go into & out of your vision.

 

If you center your eye in the eyepiece @ Infinity & there are still 2 images when using a longer lens then it might be an issue of rangefinder alignment. To test you might try photographing the Moon w/ the camera on a tripod w/ lens hood & cable release. Wait 1 minute after winding & focussing before taking each exposure. Press cable release gently.. Perhaps 4 photos of each,. Refocussing each time: 4 photos @ rangefinder alignment. 4 photos marked Infinity on lens. Racking back & forth each of the 4 times w/ both.

 

Best Regards,

 

Michael

Edited August 4, 2012 by Michael Geschlecht