Back focus, collimation film cameras

[Stephen Perera]

....so im gonna start shooting a very important to me, self-funded personal project on 16mm and I'm getting the normal pre-shoot camera owner 'is the equipment ok' jitters...and specifically back focus/collimation issues as I'm thinking of shooting with adapted lenses......so I get to the point when ever the most obvious doesn't seem obvious anymore......specifically I've tested Hasselblad lenses on Nikon digital via adapters and had infinity focus issues when looking at images on screen......now im asking.......and forgive the utter stupidity of this......but I've never adapted a lens before on a 16mm film camera so........regardless of 'warnings' or whatever they say about infinity focus when adapting a lens......is what you see through the rangefinder always what you get on the film....i.e. if its in focus when you frame and shoot its in focus on the film......haha forgive the stupidity
I say this because I bought an M42 to Nikon adapter and GOBE told me I would have infinity focus issues if thinking of adapting an M42 lens to Nikon F mount to Aaton mount on the camera itself....

Edited December 16, 2019 by Stephen Perera

[Bruce Greene]

Yes, what you see in the viewfinder should be accurate to what will focus on the film.  The hard part is seeing focus clearly through the viewfinder on wider lenses which have a larger depth of field.  With wider lenses, it's easier to see focus on close subjects, but as you get near infinity, it's more difficult, and one usually then relies on the footage scale on the lens.  But, in your case with adapters and such, that might not be accurate.

In your case of using lens adapters, I'd be tempted to bring the lens and adapter to a lens technician who can then test the lens and adapters on the the lens projector for accuracy.  You might also want to have your camera lens flange distance checked by a camera technician to make sure the mount is accurate as well.

And then of course, shoot tests and project or scan the footage to make sure all is good.

[Daniel Klockenkemper]

As Bruce says, wider lenses are most susceptible to flange focal distance issues - the depth of focus (the depth of field behind the lens) is most critical for short focal lengths.  Long focal length lenses have a much greater depth of focus, so if you're adapting Hasselblad lenses you shouldn't encounter any significant issues.  This bit me on a project many years ago - footage from the 75mm turned out fine, but footage from the 20mm was soft.

This assumes that the ground glass and gate both are an equal distance from the lens mount.  A good technician should be able to check this for you easily. 

The other issue to look out for when stacking adapters is that one or more of the adapters might not be up to the task of holding the weight of a heavier lens.  This could introduce some tilt, which would also affect focus.  If you're adapting some of the longer Hasselblad lenses, I'd highly recommend a lens support if you don't have one already. 

[Aapo Lettinen]

Cheaper adapters can have varying tolerances in manufacturing quality. A good example is the NikonF to EOS adapters where the springs pressure and the locking lever and ffd etc can vary a little from adapter to adapter. 

It helps if you use one adapter per lens so that every lens can be semi permanently fitted with its own adapter. Then you will need to worry only about the nikon to aaton adapter and not multiple possible adapter combinations. Then it is also possible to fine tune adapters for correct ffd.

If one of your adapters is not good quality it is best to just replace it. 

It might also help to have another aaton to nikon adapter. Then you could fit and adjust the hasselblad adapter to work with it permanently and only change the hasselblad lenses. It would be more stable when only one of the connections would be opened and closed when changing lenses, not the both 

[Aapo Lettinen]

M42 has shorter ffd than nikon f so it would not be possible to adapt it with infinity focusing unless using an adapter with corrective lens(that may lower the image quality).

[Aapo Lettinen]

It is pretty easy to develop b/w negative at home so you could do camera tests very easily and quickly if needed. Just use the cheapest Lomo tank which accepts 16mm and shoot short lengths of 7222 for tests. Develop it in the bathroom with coffenol or rodinal and inspect with microscope to verify the results

[Stephen Perera]

Some great replies much appreciated as always......Bruce...Aapo...Daniel et al.....I actually have a LOMO tank....a beautiful condition one I bought early this year but haven't used yet.....for both Super8 and 16mm developing.....and I have a Hasselblad Flextight 646 scanner I bought off a friend of mine a couple of years ago for £1,200.....and I have the negative holder for 16mm for that scanner which I bought in eBay so I could certainly conduct the test and scan the frames myself 

[Dom Jaeger]

Hi Stephen,

if this is a project that's important to you, I would really encourage taking your gear to a technician for a check-up. Make sure they have experience with film cameras, and an auto-collimator. A lens projector would be handy but not essential.

It takes very little time to check the important camera settings of flange depth and ground glass focus, and with the lenses mounted via adapters, to check infinity focus and if the lens focus scales are accurate. Once done, you can feel confident that your gear is set up properly for not just this job but future jobs also.

As Bruce said, you can usually trust what you see in the viewfinder. It does require the ground glass to be set at exactly the same depth from the lens mount (reflected off the mirror) as the film in the gate, but as it's an internal setting that is hard to fiddle with it tends to stay good. But for whatever reason I have (very occasionally) had to adjust the ground glass depth slightly in cameras over the years, so getting a tech to check it with a collimator and a calibrated test lens will take any doubts away. (If you have shot footage previously by eye focussing and it was always sharp you can probably trust that the ground glass depth is good).

The camera flange depth is also usually pretty reliable, but it can be fiddled with or altered because of heavy lenses/knocks etc, so definitely worth checking. 

The next factor is the lens back-focus, which in my experience is the most variable, especially when you're using adapters. Doubling up with two adapters will almost certainly introduce some error (especially if any lenses are heavy, as Daniel mentioned). You can do your own focus tests with film, but if it simply shows you an error you won't know how much shimming you need to remedy it or which direction to shim or whether it's the lens, the camera or the adapters. A good tech can work that out very quickly.

If the Hasselblad focal lengths are longer than say 50mm, the back-focus has more room for error (as Daniel and Bruce mentioned) and you will most likely just rely on eye focus anyway which bypasses the need for an accurate focus scale and what you see will be what's on the film regardless of errors in back-focus. The only issue might be if the lens doesn't reach infinity. 

 

[Tyler Purcell]

I mean the solution is to simply do a lot of testing before shooting. Setup a bunch of different scenarios where you can measure focus to different lengths and shoot it on film with a log. Then when you get the film back, make sure it all works fine. It's not that difficult OR expensive as you only need a second of each setting. A 100ft roll of film could probably do ya fine and the cost would be negligible. Sending the camera out to get collimated can be expensive if your tech isn't near by. Plus you can introduce a lot of other issues when you ship a camera. 

So I'd do a test and see what happens.