Bolex H16 SBM Focus Issues

James Hoare · January 2

Hi everyone!

I recently shot on 16mm for the first time using a Bolex H16 SBM with a Vario Switar 16-100mm zoom lens.

After getting the scans back I noticed that it was severely out of focus in a lot of the takes. It would often be sharp for the first couple of frames of the take and then shift focus considerably even though I was not shifting focus via the lens. I was wondering if this might be an issue with the pressure plate in the Bolex, where it might be slightly moving backwards as the film travels through it, affecting the focal plane. I’m certain I had the pressure plate locked into place correctly when shooting. I used a tape measure from the focal plane of the Bolex to measure the distance each time to the subject and then set the lens accordingly.

Here are some examples of the film shifting in and out of focus.
Footage Examples

And here are some stills:

Frame 1 of Clip 1

A few frames later from Clip 1

Frame 1 of Clip 2

A few frames later from Clip 2.

I was wondering if anyone had any advice on what might cause this and if it is a pressure plate issue? I’ve also attached a videos showing the condition of the pressure plate and how the motor runs etc.
Bolex Motor Running

Here are is a photo of the pressure plate.

I should also note that when the focal length of the zoom lens was set to 100mm it seemed as if the focus didn’t shift for whatever reason.

Simon Wyss · January 2

Hi, James

When the first frames are in focus technical sharpness is given at the time the corresponding length of film passes by the gate. Focus going off can be caused by

the film moving away from the focal plane either by not being held flat or by buckling due to dryness and or age;
the lens losing the setting, either at the front or as a whole relative to the film; this would imply a mechanical weakness somewhere between lens, mount, and film;
the reflex double prism moving in some fashion.

Check whether the prism holder is held onto its seat by spring force, it must spontaneously fall back after being lifted only the slightest amount. When swung back and pushed upon it must not clapper. This is an uncommon issue.

I suspect the climate’s influence. You were having temperatures up to 40 degrees Celsius in December. Not so fresh film can warp heavily in warm air. The effect is typical like you’re describing: film at rest is pushed flat(ter), after starting a run the open length between gate and spool is getting exposed. How old is the stock?

Don H Marks · January 2

The film would indeed have to move behind the normal pressure plate location to shift the focus closer than intended. The pressure plate is spring loaded and it can move backwards. One way this can happen is if the upper loop is not formed or something happens to the upper loop. In that case the film can pull on the little extended upper lip of the pressure plate and can move the pressure plate backwards.

The above happend to me once but the effect was more pronounced than you show.

Stiff film stock, as Simon suggested might be more likely.

Edited January 2 by Don H Marks

Dom Jaeger · January 2

Hi James,

Hmm, that’s an interesting fault.

A loose pressure plate is hard to do on a Bolex because the lid doesn’t close properly if the pressure plate is open, but sometimes it can be not quite latched, or the plate with the hole that it latches into can be bent or damaged.

A bad loop can sometimes cause focus issues, but it’s usually accompanied by jitter and instability. The condition and age of the stock is probably worth looking at. If you have a prime lens maybe shoot your next roll with that to rule out any issues with the zoom.

I service and calibrate Bolex cameras and lenses in Australia if you ever need your kit looked down the track. Feel free to email me anytime via my cinetinker gmail.

James Hoare · January 3

Thanks for all the replies everybody!

15 hours ago, Simon Wyss said:

Check whether the prism holder is held onto its seat by spring force, it must spontaneously fall back after being lifted only the slightest amount. When swung back and pushed upon it must not clapper. This is an uncommon issue.

I suspect the climate’s influence. You were having temperatures up to 40 degrees Celsius in December. Not so fresh film can warp heavily in warm air. The effect is typical like you’re describing: film at rest is pushed flat(ter), after starting a run the open length between gate and spool is getting exposed. How old is the stock?

Hi Simon!

Is the prism holder the little pressure plate holding the film? The little plate is held on by spring force as it snaps back when pushed slightly against and released. By not "clapper" do you mean snap back loudly?

Link to a video of the spring in action

I shot the film in October and looking back at the weather it was a max of 26°C on the shoot day. The stock itself was fresh as far as I recall. I purchased it new from a lab which posted it over and then I stored it in a fridge for about a month prior to shooting. On the shoot day I took it out of the fridge and let it acclimatise to room temperature for about 1 and a half hours prior to loading. I'm not sure if that could have potentially caused the stock to be stiff? I tried to follow Kodak's guide for warm-up times.

The stock itself looked to be in good condition, however as this is my first time shooting in 16mm I could be mistaken. But I didn't notice anything out of the ordinary.

8 hours ago, Dom Jaeger said:

A loose pressure plate is hard to do on a Bolex because the lid doesn’t close properly if the pressure plate is open, but sometimes it can be not quite latched, or the plate with the hole that it latches into can be bent or damaged.

A bad loop can sometimes cause focus issues, but it’s usually accompanied by jitter and instability. The condition and age of the stock is probably worth looking at. If you have a prime lens maybe shoot your next roll with that to rule out any issues with the zoom.

I service and calibrate Bolex cameras and lenses in Australia if you ever need your kit looked down the track. Feel free to email me anytime via my cinetinker gmail.

Hi Dom!

Here is a photo of the inside of the bolex without the plate. I didn't notice anything too wrong with the latch hole.

The loop seemed alright when I loaded the film and it looked stable after I released the loop formers. The footage itself was a touch jittery at times but nothing crazy. Unfortunately I don't have any lenses that fit the bayonet mount on this Bolex nor a bayonet to c-mount adapter so at this point in time I can't test any primes. I do know the lens I was using did have some issues as I noticed one of the aperture blades was stuck and remained in place when adjusting the aperture. So it is possible there is a fault elsewhere in the lens.

Good to know you do servicing of the Bolex! I might be in contact when I can save up a bit more to send it over if I can't identify the issue! Luckily this was just a test roll to see the functionality of the camera so I'm very glad I checked.

Simon Wyss · January 3

There’s a number of issues. Steadiness is over the hill, you have a ligh flash in a scene, sharpness is not good overall. When you state that the lens’ iris is defunct and mistake the prism block for the pressure plate I can only say: Dom, Melbourne.

Dom Jaeger · January 3

The prism Simon is talking about is the piece of glass behind the lens port, the beamsplitter that sends some of the light to the viewfinder. Very, very occasionally the spring that holds it in place is misaligned (by bad servicing I always assume) and the prism can be loose. It is designed so you can flip it out and clean the back surface and field lens on top (in the case of an SBM), and then snap it back in place.

If you haven’t already done so, you should get hold of a manual and read it through, knowing your camera is quite essential. Here’s a free copy of an SBM manual:

http://www.vintagecameras.fr/images/MonSite/BOLEX/H16_Reflex/_Doc/H16_SBM_Manuel_en.pdf

Your pressure plate and the plate beneath it seem ok. If the film stock was also ok and the loop seemed good then I’m perhaps beginning to suspect the zoom, though it’s an unusual fault if it always seems sharp at the opening frame. But maybe a loose optic is shifting around, or there is play in the zoom mechanics. It’s usually better to use the viewfinder for focussing, unless you know for sure that the distance marks on the zoom are correct. They should both agree at any rate. With cine zooms, you generally focus zoomed all the way in, and then zoom out to the view you want and the lens should stay in focus. If the focus drifts, then the back-focus is out and needs to be calibrated. Or the lens has a fault within.

Honestly I think your kit needs to be examined in person to determine the issue, perhaps there’s something an experienced technician will pick up immediately that you can’t see, or will find with tools like a collimator.

The cost of a quick professional check is less than the cost of one spool of film, processing and scanning, and can save you a lot of headaches. Ultimately anything bought off eBay or the like in unserviced condition should be at minimum checked if not serviced if you want to ensure a good experience shooting film.