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Kern Switar RX 10mm focus issues


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Hello all, 

Sorry for the slightly silly question but I am new(ish) to 16mm cameras and lenses and I cannot seem to find a satisfying answer elsewhere. I recently came across a Bolex Rex 1 with a 25mm and a Kern Switar 10mm. When the 10mm is wide-open at 1.6 and set to infinity, the objects in the distance are noticeably blurry. However, when I begin to stop down the lens, the distant objects are in focus. I have noticed a similar thing with my K3, stopping down the lens set to infinity appears to render the distant objects clearer. My question is, is this normal? I was told that wide-open the 10mm will be soft around the edges, but this seems to be soft all over. For the record, the 25mm does not appear to suffer the same issues so I do not believe it is a problem with the camera. 

 

Cheers

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Stopping down any lens will increase the depth of field (making slightly out of focus things get sharper) and will also reduce the lens aberrations (which could be both inherent and introduced by age or wear). So yes, it's normal that stopping down will make a lens appear sharper or more contrasty, some lenses more than others. 

However there could well be issues making your 10mm perform worse than it should. 

Firstly, is it an RX lens? For reflex Bolexes, RX lenses are recommended for focal lengths of 50mm and under. If it's not labelled RX, the reflex prism in your Bolex will introduce more abberations when your aperture is wide open. Stopping down past around f/3.5 makes them diminish.

Secondly, the lens itself may need back-focus adjustment. Perhaps it is not quite reaching infinity focus before hitting the infinity stop. Check the focus at closer distances using your eye, not the marks. (Make sure to properly set the eyepiece diopter). See if the focus mark matches the distance to subject. A Bolex tech can adjust this for you if necessary.

Thirdly, the lens may have coating damage or internal fog causing the image to appear muddy. Usually you can see this by holding the lens up to a strong backlight. If so, it may be repairable by a lens technician.

Lastly, the camera viewfinder may need cleaning to give you the clearest image possible. Wide angle lenses in particular can be hard to judge focus in small viewfinders like the one in a REX 1, and any internal fog or fungus will only make that harder.

The easiest way to sort all this out is to have the camera kit checked and serviced by a good Bolex technician, which I would heartily recommend to anyone who has just obtained a Bolex and wants to use it.

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5 hours ago, Dom Jaeger said:

Stopping down any lens will increase the depth of field (making slightly out of focus things get sharper) and will also reduce the lens aberrations (which could be both inherent and introduced by age or wear). So yes, it's normal that stopping down will make a lens appear sharper or more contrasty, some lenses more than others. 

However there could well be issues making your 10mm perform worse than it should. 

Firstly, is it an RX lens? For reflex Bolexes, RX lenses are recommended for focal lengths of 50mm and under. If it's not labelled RX, the reflex prism in your Bolex will introduce more abberations when your aperture is wide open. Stopping down past around f/3.5 makes them diminish.

Secondly, the lens itself may need back-focus adjustment. Perhaps it is not quite reaching infinity focus before hitting the infinity stop. Check the focus at closer distances using your eye, not the marks. (Make sure to properly set the eyepiece diopter). See if the focus mark matches the distance to subject. A Bolex tech can adjust this for you if necessary.

Thirdly, the lens may have coating damage or internal fog causing the image to appear muddy. Usually you can see this by holding the lens up to a strong backlight. If so, it may be repairable by a lens technician.

Lastly, the camera viewfinder may need cleaning to give you the clearest image possible. Wide angle lenses in particular can be hard to judge focus in small viewfinders like the one in a REX 1, and any internal fog or fungus will only make that harder.

The easiest way to sort all this out is to have the camera kit checked and serviced by a good Bolex technician, which I would heartily recommend to anyone who has just obtained a Bolex and wants to use it.

Hey Dom, thanks for the response. The lens is indeed marked RX, so I do not think that is the issue. It also does not appear to be fogged or have any mould on the inside. And since things seem nice with the 25mm and the diopter has been adjusted, I do not think it is the camera either. The camera was also serviced in 2021. 

I will try using the focus method you suggested. However, this leads me to another question that might be more difficult to fully explain. For whatever reason, the marks do no align. The aperture marks match up with the white/orange centre line (between the aperture ring and focus ring) but the only focal distance marking that matches the centre line is the infinity marking. From there I can only turn the focus ring counter clockwise, meaning none of the distance markings match. This had me really puzzled because it renders all the D0F preview dots completely useless. Again, no idea if that makes sense or is normal, but I cannot measure any distance and match it up with the lenses because I have no index on the lens as to what the distance is. 

I do not want to sound too conspiratorially with this suggestion, but could someone conceivable place an RX cutter ring on a non RX lens and try and pass it off? Is there any way of distinguishing the difference beyond the RX markings? 

Thanks

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9 minutes ago, Andrew Hamilton Watts said:

.the only focal distance marking that matches the centre line is the infinity marking. From there I can only turn the focus ring counter clockwise, meaning none of the distance markings match. This had me really puzzled because it renders all the D0F preview dots completely useless. Again, no idea if that makes sense or is normal, but I cannot measure any distance and match it up with the lenses because I have no index on the lens as to what the distance is. 

Ok well that sounds like the focus barrel is on the wrong side of the stop. So regardless of the marks, can you focus at any distance closer than infinity? How far does the barrel turn, only a small amount?

The stop screw is the large screw in the knurled band around the focus barrel. There will be three smaller screws as well - don't touch them as they hold the focus calibration - but you could try undoing that large stop screw and turning the focus barrel so the distance marks are in the range of the index line, then put the screw back. Make sure you don't screw the focus out to the point where it looks like it might unscrew completely, go back the other way if so. The focus barrel should travel from infinity down through all the marks and stop past the minimum distance mark. 

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Posted (edited)
27 minutes ago, Dom Jaeger said:

Ok well that sounds like the focus barrel is on the wrong side of the stop. So regardless of the marks, can you focus at any distance closer than infinity? How far does the barrel turn, only a small amount?

The stop screw is the large screw in the knurled band around the focus barrel. There will be three smaller screws as well - don't touch them as they hold the focus calibration - but you could try undoing that large stop screw and turning the focus barrel so the distance marks are in the range of the index line, then put the screw back. Make sure you don't screw the focus out to the point where it looks like it might unscrew completely, go back the other way if so. The focus barrel should travel from infinity down through all the marks and stop past the minimum distance mark. 

I was able to focus at closer distances yes, and the barrel turned the full amount. However, in my haste and curiosity I did as you suggested and now I believe I have made a royal mistake by that. I was able to shift the barrel but now a gap as opened up between knurled ring which fits flush with the camera body and the smooth ring between that and the focus knurled area...which is not good. And I cannot seem to get them back flush again. Also, it no longer turns as it did, it is pretty much jammed up. Looks like I am going to need a crash course on lens repair here. 

Edited by Andrew Hamilton Watts
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