FOCUS RECALIBRATION WHEN WORKING ON LENSES

[David Lyga]

Be honest, when you work on lenses for SLRs, cleaning, focus recalibration, don't you set the infinity focus just a wee bit further than infinity, barely perceptible, just to make certain that, no matter the distance or camera, you will always be covered? I always do this and feel that there is nothing wrong here and experience a feeling of reassurance. Am I being too paranoid here? - David Lyga

 

[BrianShaw]

Nope; I don’t.

 

[John Koehrer]

That's fully justified especially with longer or zoom lenses where part of the design allow for expansion of the glass.
I think it's less important shith shorter lenses because of a less magnified image on the GG.

 

[David Lyga]

That's fully justified especially with longer or zoom lenses where part of the design allow for expansion of the glass.
I think it's less important shith shorter lenses because of a less magnified image on the GG.

There is nothing more annoying to me than purchasing a cheap wide angle lens (prime or zoom) whereby infinity is 'not quite' there. It is almost as if the manufacturer is depending upon depth of field to make up for the lack. I go to work, immediately, and have never had regrets in doing so. Let's face it, we NEVER shoot at infinity, anyway, Not even the moon is infinity. - David Lyga

 

[takilmaboxer]

I've done this with Zeiss folders I've repaired and it works. A word of warning, though, just because the indicator for infinity is set, doesn't mean that the closer markings are accurate. With the folders I've fixed, I also shoot a roll to test at ten feet focus; set a target accurately from the camera and shoot at several different markings near 10 feet on the focus ring, then examine the negs at 20X. There can be a significant difference. I make a mark on the ring if needed.
Mess Ikontas can be even worse, I always test for 10' focus, then set the uncoupled rangefinder to that setting, regardless of where the focus ring is.
Never had that problem with my coupled Super Ikontas.

 

[David Lyga]

I've done this with Zeiss folders I've repaired and it works. A word of warning, though, just because the indicator for infinity is set, doesn't mean that the closer markings are accurate. With the folders I've fixed, I also shoot a roll to test at ten feet focus; set a target accurately from the camera and shoot at several different markings near 10 feet on the focus ring, then examine the negs at 20X. There can be a significant difference. I make a mark on the ring if needed.
Mess Ikontas can be even worse, I always test for 10' focus, then set the uncoupled rangefinder to that setting, regardless of where the focus ring is.
Never had that problem with my coupled Super Ikontas.

Excellent and needed point. With SLRs, if it is in focus at one distance, it is in focus at ALL distances. That is the beauty of "Single Lens" reflex technology. Not necessarily so with rangefinders, which give you theoretical information, not real time information. The RF mechanism is quite accurate, but short of the real time info given on an SLR. To compound matters, a range focus camera, where one sets the focus as per the focus scale, can be quite inaccurate, especially at the closest distances.

Bottom line is this: with either RF cameras or focus scale cameras, test for absolute best infinity setting. Mark it on the focus scale (It just might not be quite at the infinity setting and, heaven forbid, if there is not enough room to turn to for your actual infinity setting, then the lens must be recalibrated). Then focus at the closest possible distance. Mark it on the focus scale. Visually compare the two settings: there are many examples whereby the written scale settings are actually a bit incorrect. This is why I like SLRs more than RF systems.

Of course, SLRs' focus can be off, as well, but the 'off' pertains to ALL distances. The correction comes from perfect alignment correction of that mirror's precise angle.The front of the mirror rests upon a (post, set screw, bracket?) and the mirror must be either raised or lowered (an imperceptible amount) in order to achieve dead accurate focus. But, when achieved for one distance, it is accurate for ALL distances. - David Lyga

 

[ic-racer]

Focus past infinity is the worst thing one could possible do. It will ensure absolutely NOTHING is in focus.

 

[Dan Fromm]

Hmm. Cine lenses' focusing scales are precise. Exact. Not fudged in either direction. They're sometimes focused by tape measure instead of through the lens.

David, I think you equivocated. Setting the far focusing distance stop so that a lens can be focused through infinity is one thing. Adjusting the focusing scale so that it is inaccurate is another. My 700/8 Questar's far focusing stop is past marked infinity. When the lens hasn't sat baking in the sun, the infinity mark and all closer distance marks are right.

 

[David Lyga]

Focus past infinity is the worst thing one could possible do. It will ensure absolutely NOTHING is in focus.

No, you might have misinterpreted. If I recalibrate a lens so that it focuses a tiny bit past infinity, there will be no misfocus because the viewfinder of an SLR will indicate such misfocus just as readily as if I had not done that. Maybe you are thinking of an rangefinder camera, but even still, the RF mechanism would indicate that the rear element was a tiny bit too close to the film plane to be focused properly at infinity. The mechanism would indicate, by closeness to the film plane, whether the image is being recorded in focus or not. - David Lyga

 

[David Lyga]

error posting

 

[David Lyga]

Hmm. Cine lenses' focusing scales are precise. Exact. Not fudged in either direction. They're sometimes focused by tape measure instead of through the lens.

David, I think you equivocated. Setting the far focusing distance stop so that a lens can be focused through infinity is one thing. Adjusting the focusing scale so that it is inaccurate is another. My 700/8 Questar's far focusing stop is past marked infinity. When the lens hasn't sat baking in the sun, the infinity mark and all closer distance marks are right.

With an accurate SLR, its focusing scale is of little importance. With a dead accurate RF camera its focusing scale is of little importance (because the viewfinder will tell you what is in focus). With a scale-focus based camera, it pays to check that scale at both infinity and at closest distance. Why? Because if there is an error at either or both ends, one must superimpose an accurate scale onto the pre-existing one so that future errors will not manifest.

Actually ALL cameras should be tested at both ends. In the case of the SLR, a superimposition onto the existing scale is not necessary because all that need be accurate is that precise mirror angle. In the case of a RF camera, both ends should be tested to confirm that the indication in the VF is accurate at both ends. A real problem comes from only one end being accurate in a RF camera; that means that the cam guiding that focus indication in the VF is not consistent. Then, the scale has to be manually overridden because changing the internal calibration will then upset the 'correct' end focus test. In this worst case RF test, the VF cannot be used throughout the focus scale. It can be used only in the distance region where the VF indicates accurate focus. - David Lyga

 

[ic-racer]

No, you might have misinterpreted. If I recalibrate a lens so that it focuses a tiny bit past infinity, there will be no misfocus because the viewfinder of an SLR will indicate such misfocus just as readily as if I had not done that. Maybe you are thinking of an rangefinder camera, but even still, the RF mechanism would indicate that the rear element was a tiny bit too close to the film plane to be focused properly at infinity. The mechanism would indicate, by closeness to the film plane, whether the image is being recorded in focus or not. - David Lyga

I collimate the other way. Just barely in front of infinity by a micron or so. If I collimated past infinity, I'd be afraid to accidentally bump the focus ring to the far stop or not see the focus correctly, in which case NOTHING will be in focus. There cannot be a focused image when the lens is closer to the film than it's focal length, and that is what one would be doing when focusing beyond infinity.

 

[David Lyga]

I collimate the other way. Just barely in front of infinity by a micron or so. If I collimated past infinity, I'd be afraid to accidentally bump the focus ring to the far stop or not see the focus correctly, in which case NOTHING will be in focus. There cannot be a focused image when the lens is closer to the film than it's focal length, and that is what one would be doing when focusing beyond infinity.

Your explanation is for SCALE FOCUS cameras. In the case of the SLR, a tiny bit past infinity is actually a slight improvement, as oftentimes we focus most accurately by a 'back and forth' jockeying of the focus ring, zeroing onto the precise focus. And, if a RF camera's rangefinder is dead accurate, the same thing can be handily done. However, with SCALE ONLY cameras, we MUST depend upon a dead accurate scale focus, and it is immaterial whether that scale is the manufacturer's or one we have superimposed onto the manufacturer's scale (in order to increase focus accuracy). In THAT case, with scale focus only cameras, it does not pay to attempt a focus point that is slightly beyond infinity. There would be no purpose, only confusion. Indeed, a precise infinity should be the ultimate stop on that scale focus focus ring. - David Lyga

 

[ic-racer]

There is no focused image beyond infinity. That is physical property of a lens and is independent of the lens purpose. Film can have two states, flush to the pressure plate or a bulge forward. So any film movement coupled with focus beyond infinity would be Double Indemnity!!

Furthermore when focusing a view camera with no infinity stop, there could be as much as a ten degree span on the focus knob over which an image at infinity might appear to be in focus. One would be best served by using the closer focal point. For all the reasons stated above.

Focus slightly in front of infinity you get 'everything.' Focus slightly behind infinity and you lose 'everythng'....

 

[David Lyga]

True, there is no 'image' beyond infinity. I posited creating a stop slightly beyond infinity (with SLR lenses) for the sole purpose of having a reference point when jockeying into focus position. That is all; I am not trying to create another focal entity out of thin air. - David Lyga