Blur with focus on infinity - Scottish Landscape filming

Chloe Charlton · August 25, 2022

Hi everyone,

I am currently in production of a new moving image work on 16mm and have received my first few rolls back from the lab. Many of the images have a soft focus despite the lens' focus being set to infinity, I have attached an example. Does anyone know what the cause of this might be? I'm wondering whether its: (1) a fault with the lens or (2) due to there being so much humidity in the air and the generally flat lighting conditions, as its on location in Scotland.. or (3) just because the mountain is very very far away (is infinity really infinity?)

Appreciate any advice or comments from previous experience.

Thank you,

Chloe

Phil Rhodes · August 26, 2022

That looks far enough away to be effectively infinity. What sort of camera and lens was this? If it's a reflex viewfinder, is the problem visible in the viewfinder? Is any part of the viewfinder image sharp?

This can arise from a number of sources:

- The flange focal distance is perhaps not correct. Simply put, the lens has to be an extremely precise distance away from the film for focus to work as expected (to within something like the width of a human hair). The mounting surface on the back of the lens and the lens mount on the camera must be very accurately positioned. Is there anything stuck to the mount, any damage? Did anything get stuck between the lens and the mount? What sort of mount is it? Was it properly and firmly attached? You can test this to an extent by verifying that the lens accurately (and I mean accurately) focuses to the expected distance otherwise. This will require a measuring tape to establish. Do it at a wide aperture to minimise depth of field and measure a few distances. Make sure you're measuring from the right point, which is where the film is inside the camera body.

- Is this a zoom lens? Some zoom lenses have internal controls to set up tracking, which ensures the lens stays in focus as you zoom in and out. Normally that's a problem with broadcast TV lenses, though. Zooms can break in other ways which cause focus problems. If it's a zoom, does the focus appear to change as you zoom in and out? It shouldn't. Again, test.

- The lens may be allowing the focus to move beyond infinity, which means nothing will be in focus. If you look at the markings on the lens barrel, do they appear to let things move beyond infinity? That can be an issue; infinity is not necessarily just a matter of turning the ring as far as it goes. In an ideal world, the lens should mechanically prevent this from happening, but sometimes they don't.

- Was this shot at a very high f-stop? I see it's a day exterior presumably with lots of light. What film stock did you use, and what sort of filters, if any? Lenses have a diffraction limit which can mean fuzzy pictures at very high f-numbers. That does have a fairly specific sort of look to it, though, and this does look more like a conventional focus problem.

- Did you shoot anything else with similar problems, whether set to infinity or not?

- Did you use any diopters (close-up lenses) on any part of the shoot? Is it possible one of them was left in place? That might cause this.

Notice several of these problems would imply a problem with the lens, so if you can only send one thing off to be looked at, send the lens off. I know it's ruinously expensive but if you can do some tests on film to make sure someone didn't make some sort of daft mistake on the day, test again to make sure this is really happening and is a repeatable problem.

Once you're convinced it is, it'll have to go off to be fixed. Has this camera and lens been maintained recently?

Phil

Dom Jaeger · August 26, 2022

We need more information.

Is it a zoom? More than one lens being used? Reflex camera where you can eye-focus? Are closer shots in focus? Does the lens look clear looking through it?

It actually looks softer on the right than the left. And the distant mountains seem quite blurry. I would probably suspect the lens has issues without any more info.

Either you need to do some testing to determine if it's the lens or the camera - ie shoot some charts with different lenses, comparing eye-focus to focus marks etc - or just send the kit to a cine service technician. I could check your camera flange depth and lens collimation in under an hour if you lived in Australia, and possibly project the lens to assess the image if it was a common mount like Arri or C mount. But I have no idea what services you have in Scotland.

Eric Eader · August 26, 2022

Hello,

My first question is this: Is the camera a reflex camera?

Second question: When you say you were focused on infinity, do you mean the lens was rotated to its full end stop or focused by eye and short of its focus end point?

Infinity is "variable." The moon is much farther from any mountain on earth yet with a properly set up lens and camera combination both can be brought into focus (in separate shots), BEFORE the infinity stop is reached.

Just setting the lens to its end point will not guarantee you are actually in focus for the scene in front of you.

Also, the lens' infinity mark on the barrel usually is slightly short of the end point so that on cool days and warm days infinity will vary for each lens differently. (Expansion and contraction of internal components).

All of the above responses figure in after settling whether or not focus was simply placed on a mark, at the end stop, (if using a non reflex camera) or focused by eye (if using a reflex camera).

Hope this helps.

Eric

Tyler Purcell · August 26, 2022

The only real issue I see is the right side. That could be a lot of things, but the aperture plate gap to the film seems to be fine across the frame. So that would mean the flange distance is way out of whack on the left side of the gate. This could be a lens issue, it could be a flange distance issue, could be a lens mount issue. I've seen many lenses where the mount is held on with screws and they come loose. I'd examine all your components, I'm sure the answer is there if you look hard enough.

Robert Hart · August 27, 2022

I am wondering about whether the camera/lens combination is off at all.

Is there something else going on in the film scan itself? The frame edges are also soft. The sprocket edges are sharp. It is almost as if someone has set the scan focus on the back of the film, not the emulsion face.

Edited August 27, 2022 by Robert Hart
added text

Eric Eader · August 27, 2022

Perhaps it's time we saw a moving image, not just a still shot.

Eric Eader · August 27, 2022

Three blind men and an elephant, what??

Phil Rhodes · August 27, 2022

2 hours ago, Robert Hart said:

I am wondering about whether the camera/lens combination is off at all.

Is there something else going on in the film scan itself? The frame edges are also soft. The sprocket edges are sharp. It is almost as if someone has set the scan focus on the back of the film, not the emulsion face.

Ah, yes. I tried very hard not to miss what I was missing, but I missed that. Very good spot.

Doug Palmer · August 28, 2022

13 hours ago, Robert Hart said:

I am wondering about whether the camera/lens combination is off at all.

Is there something else going on in the film scan itself? The frame edges are also soft. The sprocket edges are sharp. It is almost as if someone has set the scan focus on the back of the film, not the emulsion face.

That's how it looks to me. So maybe if you use a good loupe (which could be an old projection lens) over a lightbox, you'll see if there's a sharp image there or not.