Nettar focus measurement

[Daire Quinlan]

I just put a roll through a rather nice Nettar I got last week. Novar f/4.5. Focus is not looking good. In the normal course of things I'm not the best estimator of distance but I did do some measured tests from what I figured was the focal plane of the camera out to the minimum marked distance on the lens (1.2m) and the results were ... poor. However I've read intimations that the focus distance should be measured from the front element of the lens and not the focal plane? Extended the camera is a good 9cm or so from the focal plane to the front element so that would certainly be enough to throw the focus out.

TLR; Should I measure from the focal plane or the front lens element on my Nettar for precise focus.

 

[Rick A]

http://www.cameramanuals.org/zeiss_ikon/zeiss_ikon_nettar.pdf

 

[Daire Quinlan]

http://www.cameramanuals.org/zeiss_ikon/zeiss_ikon_nettar.pdf

Yes, thanks for that, the manual was the first thing I consulted. It's silent on the matter :-D

 

[shutterfinger]

I've yet to see a camera that did not set its focus at the film plane.
Make a piece of ground glass, Scotch Magic Transparent Tape on a piece of glass or clear acrylic, non over lapping, tape side toward the lens, that sits in the camera as the film does not the backing paper and check the marked distance on the focus scale to the actual measured distance. Focus at marked infinity should match infinity in sharp focus at the film plane. Use a target at least 5000 feet/1524 meters away for infinity testing. Celestial objects make good infinity targets. Do not look directly at the sun.
Set up/test with the aperture wide open.

 

[Daire Quinlan]

Yeah I've done variations for a bunch of cameras in the past, I was wondering if it's actually NECESSARY here i.e. if the focal distance is measured from the front of the lens it's probably right on. I guess I should just buckle down and check.

 

[Nick Merritt]

Do what shutterfinger describes. Does the shutter have a T setting? Otherwise, set it to B and use a release cable to lock the shutter open. You may not be able to find an object 5,000 feet away, but the farther the better, obviously. I assume focusing is done by turning the front ring of the lens, right? Turn it until you find the sharpest point -- use a normal lens from an SLR (becauses it's small, and with a wide maximum aperture) to magnify the image on the Scotch tape to see the sharpest focus. The Nettar should have three small grub screws around the edge of the lens ring. Loosen those, and carefully turn the lens ring (it should turn freely without changing the focus of the front element) until it's set at infinity. Then tighten the screws. Once infinity focus is set, focus will be dialed in for all distances.

 

[DWThomas]

Yes, thanks for that, the manual was the first thing I consulted. It's silent on the matter :-D

On page 18 of the manual under the tables for using supplementary lenses it says "The distance of the object must be measured from the front edge of the supplementary lens mount." I do seem to recall some inconsistencies about the base of the focal distance measurement between different cameras of the past. Not really sure what the story is without a supplementary lens. A +1 lens on a lens focused at infinity should give sharp focus at 1 meter and the chart shows 39.25 inches which sounds close enough. Haven't analyzed whether one could walk that chart backwards and figure out what they are assuming. Once the object is 20 or 30 times the lens focal length away, the starting point for measurement wouldn't be all that critical for most work.

 

[Alan9940]

As Nick said, I'd recommend doing what shutterfinger describes. However, I will tell you that this effort may only confirm that you cannot properly adjust the focus, if off. I did this very thing with a Zeiss Super Ikonta III that I have and, after a couple days of messing around with it, I sent it off for repair. Turns out the lens assembly was missing some shims that should have been present, and the front lens assembly (standard?) was ever so slightly twisted in relation to the film plane. Bottom line...no amount of me adjusting the focus setting would have fixed my issue.

Good luck, though. These old cameras are so fun to play around with!

 

[John Koehrer]

The idea that testing a lenses infinity setting has to be determined by looking at something
5000 feet away or the moon and stars is way beyond what's needed.
Granted the moon, stars and 5000 feet will do but a PRACTICAL infinity to set up
a lens is only about 500 X the focal length. Use something a block or two away
and you're good. 50mm lens

FWIW, if you're in town, finding someplace to set up where you can see a target
at 5000' isn't usually feasible.
Think about it. one mile. Flat(?) land, no obstructions and through a 50mm lens you
may not even find something to focus on.

 

[shutterfinger]

Granted the moon, stars and 5000 feet will do but a PRACTICAL infinity to set up
a lens is only about 500 X the focal length. Use something a block or two away
and you're good. 50mm lens

More wives tales or shade tree corner cutting.
The inland coastal hills visible out my front window measure 3/4 mile (3960 feet) away when used as an infinity target will produce a soft image of the moon or similar distant subject and the lens has to be stopped down at least 2 stops to get the further distant object in focus.
A 10 story building on the edge of the bay also visible through the same window measures to 1 mile (5280 feet) away. When this is used as an infinity target then objects at further distances are sharp with the lens wide open.
I routinely set up rangefinders, Large Format, and Medium Format camera lens and infinity stops.
The 500X the lens focal length may be good enough for 35mm but it sure is not for larger formats especially if you prefer accuracy.

 

[Daire Quinlan]

Honestly, my bugbear when calibrating cameras is almost always determining the precise point of sharp focus on whatever it is I'm using. I'd ordinarily use a big chunk of GG and a 24mm lens as a loupe. Trying to check the Nettar last night was more complicated as, so far as I can see, the film is stretched tight between the rollers, it doesn't sit on the rails. I had to stretch a bit of developed TMAX between the two rollers, I could make out enough grain on the emulsion side that I could semi-reliably focus on it. Trying my target though I kept on losing the grain on the strip of film and and focusing on the aerial image instead. I eventually gave it my best shot and put the distance ring back on. OTOH I did verify that the WataMeter I got in the post yesterday is pretty much spot on :-D

 

[John Wiegerink]

Many Agfa folder directions call for focus to be measure from the lens and not the film plane. Also, placing the ground glass on the film plane while the shutter is open and then focusing on a far distant subject is not always correct. I remember (barely) reading in a Modern Photography magazine about this very topic. The writer had a terrible time setting focus on a Zeiss folding camera. I believe it was an Ikonta, but could be wrong on that. He then contacted a retire Zeiss tech and they told him that they had a special GG that actually focused slightly passed the film rails. He said it allowed for the slight bow-out of the film emulsion. I know, it didn't sound right to me either, but the writer followed his advice. Well, he lived happily ever after, end of story. I never worried about or tried this until years later when I bought a Agfa Record that had frozen grease on the focus helical. I freed the focus and relubed it. Now, to set focus. I did the GG on the rails and ran a roll through the camera. Not in focus!!! Start over again! I then looked at the pressure plate and noticed two ridges, one on top and the other on the bottom. So, I put the GG acros on the rollers which are at a different height than the film rails. Tested another roll. Still not 100%, but close. If I focused with the GG on the higher rollers and moved/rotated the lens 1/8th inch past INF it was right on the money. That meant it was focusing past what I thought was INF when the GG was on the film rails. I don't know if all large roll film cameras are like that, but the Agfa sure was. Just out of curiosity I'm going to get my Super Ikonta C out and check that for focus.

 

[John Koehrer]

As they say "YMMV"
It must be nice to live where you have a clear view of something a mile away but most of us don't.

You're experienced and your opinion is respected but When I worked in a shop, we
used the warning lights on a water tower two blocks away.
With no customer complaints.

 

[John Wiegerink]

As they say "YMMV"
It must be nice to live where you have a clear view of something a mile away but most of us don't.

You're experienced and your opinion is respected but When I worked in a shop, we
used the warning lights on a water tower two blocks away.
With no customer complaints.

John,
I'm not doubting what you say one bit. I've done the same with Rolleiflex and other cameras. I have done the same with my Hasselblad and then inserted my GG adapter back on it and it was the same. All I'm saying is "not all cameras are created equal" when it comes to finding the proper focus. Many of the simple and older cameras are different. Like I said, just my experience and I have the results to prove it on the old Agfa Record camera. Actually I wouldn't have believed it myself if I hadn't remembered that 40+ year old article in Modern Photography. Errr, maybe it was Popular Photography? One of those damn photo mags anyway.

 

[shutterfinger]

Kalart rangefinders from the mid 1930's through the 1940's used a movable prism and a fixed beam splitter mirror, in the early 1950's they changed the prism to a mirror. Graphic rangefinders use a movable mirror and a fixed beam splitter as do hand held/hot shoe mount accessory rangefinders. There is no DOF to carry any focus error, its like shooting an f1 lens wide open.
A circle has 360°; a clock/watch has 60 second minute marks; a second/minute mark is 6° of arc. Most lens, if not all, focus 100 feet 6° to 12° of arc from infinity and a TLR might turn 18°.
A 75mm f2.8 lens found on some TLR'S at 2500 feet may show infinity adequately sharp.
I have seen city blocks 500 feet or less and others that were 1/4 to 3/8 miles long (1320 feet to 1980 feet).
A #14 welder glass is strong enough to safely view the sun through and are available at hardware stores. There is also special metalized mylar or glass filters for use on camera lens, binoculars or telescope.
A newcomer is this filter on Amazon https://www.amazon.com/Solar-Filter-Telescopes-Binoculars-Cameras/dp/B00DS7IFQS
The sun when viewed through one of these filters on a lens or rangefinder gives a small circular spot that is easy to get sharp with the focus of a camera or calibration screw of a rangefinder.
75mm=.246 feet*500=123 feet.

 

[Denverdad]

Honestly, my bugbear when calibrating cameras is almost always determining the precise point of sharp focus on whatever it is I'm using. I'd ordinarily use a big chunk of GG and a 24mm lens as a loupe. Trying to check the Nettar last night was more complicated as, so far as I can see, the film is stretched tight between the rollers, it doesn't sit on the rails. [snip]

That's really the key - verifying exactly how the film registers in your particular camera so you can define the focal plane location accurately. There is more variation in this among the different manufacturers and models than some might think. Over on the rangefinder forum I found this thread to be a particularly interesting discussion of all this, so you might want to check that out.

I can't speak to the Nettar specifically since I don't own one, but on my Ikonta 524/2 at least, the film registers not to the rollers, but against the two raised serrated rows just to the top and bottom of the film opening. The rollers actually sit slightly proud of these surfaces according to my measurements, and the pressure plate actually nests within them when the back is closed. So at least for my camera the rollers can't be the registration surface.

There was also a suggestion made in the thread linked above that one should establish focus not at the center of the frame, but rather at some distance between the center and edge so as to account for possible field curvature of the lens. It's not something I've ever done before but now that I think about it, it makes sense and could make a difference. At least in some cases.

 

[Daire Quinlan]

That's really the key - verifying exactly how the film registers in your particular camera so you can define the focal plane location accurately. There is more variation in this among the different manufacturers and models than some might think. Over on the rangefinder forum I found this thread to be a particularly interesting discussion of all this, so you might want to check that out.

Oh man I wish I hadn't read that thread. I measured the LENGTH of the pressure plate all right, enough to confirm that it stretched across the rolllers at both ends. I never really checked to see if it sits inside those two outer rails or not, I had assumed that it did. I have film in it at the moment but I'll have to check when I get through the roll. If nothing else I can shoot a few actual distance shots using the film and re-calibrate from there :-D

The thread above concerns an Ikonta 521-16 but the interior layout looks identical to (what I can remember of) my Nettar.

Has anyone a clear idea what's meant by the following "As said before , I always use my auto-collimator on a film loaded in the camera." Is that possible? I.E. a clearly marked bit of film would through the film gate or something ? Both lenses set to infinity ? And you should be able to see the mark on the film in the target camera ? I guess it should be possible if you can get some light in there as well.

 

[Denverdad]

Has anyone a clear idea what's meant by the following "As said before , I always use my auto-collimator on a film loaded in the camera." Is that possible? I.E. a clearly marked bit of film would through the film gate or something ? Both lenses set to infinity ? And you should be able to see the mark on the film in the target camera ? I guess it should be possible if you can get some light in there as well.

You have the right idea - that by using an actual roll of film instead of some surrogate target, you eliminate any uncertainty as to the proper location of the focal plane. Basically, it is where it is! No guesswork. The technique in that case would be the same as is done with ground glass or tape or any other target - i.e., view into the camera under test with another, digital camera focused at infinity, and adjust for the sharpest image. But as you said, the tricky part would be to find a way to get enough light in there to be able to see the marks made on the film. The method he is using with an autocollimator is essentially the same as this I think, except that is a device which projects it's own light source.