Tips to Help with Focusing

Can anyone provide any solutions to help with focusing? I have a Pi HQ and am trying to focus the camera enough to see noticeable film grain on 16mm film. However, while the focus looks OK, I still see no noticeable grain; the soundtrack also looks soft, causing the extracted audio from AEO Light to not sound great. I tried focusing with graphing paper, but I got confused when doing this; not sure what the purpose of that is, I assume to help figure out how far away the camera needs to be from the film? I believe I’m using a 30mm and a 5mm extension tube, totaling 35mm. Would adding another tube help with focusing or is the focusing issue only related to the camera mount? I’ll add that the mount I’m using is only temporary and does not allow for precise focusing. I plan to construct a better mount soon.

Well, first of all, the sharpness of your image will depend on the specific lens you are using. So make sure to employ one from a high-quality manufacturer.

Now, the depth of field will mostly depend on the aperture (f-stop) you are operating with. For focussing, you want to use the largest opening your lens is capable of. Adjust the lens in such a way that the whole field of the camera is in focus. If you can not achieve that goal, your camera might either be decentered/tilted against the film gate or: your lens is not good enough.

With a decentered lens, you will be able to achieve good focus only on a small part of the frame while the rest is blurry. Slightly refocussing will move the spot of maximal focus to another place of the frame.

If your lens is of the less than perfect variaty, you might notice additional blur and color fringes towards the edges of the frame. If this is the case, search for a better lens.

Anyway. Once you have achieved optimal focus with the f-stop wide open, it’s time to set it to a sensible value for the actual scannig. Most lenses display the following behaviour: there is an optimal f-stop were the lens achieves the best results, and it is usually located in the mid-range of the available f-stops. So for the Schneider Componon S 50 for example, the sharpest images are obtained in the f-stop range around 5.6. If you are lucky, you might find information or even measurements for that on the internet.

In any case, once you focused with wide open aperature, dial in the optimal f-stop, making sure you do not move either film gate or camera or lens relative to each other during that operation. Start scanning.

Note that the HQ camera has several modes, some of which are using binning. These binning modes reduce noticeably the achievable image quality. Make sure you operate your camera in a non-binning mode.

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The lens I am using is a EL-Nikkor 50mm. I’m thinking that the camera is not centered correctly because when focusing with graph paper, I noticed some parts were in focus while others were not. I never thought about opening the aperture to its maximum value, which is f/2.8. I have kept the aperture on f/11 the entire time when adjusting focus. I’ll say again that the camera mount I am currently using won’t allow for precise focusing since it’s only temporary.

Wouldn’t an f-stop of 5.6 make the image too bright? From personal experience, when pointing the camera at film, anything lower than f/11 makes the film too bright.

Well, the f-stop of a lens is not really for making an image brighter or darker. Most of the time you would use instead different exposure times for adjusting image brightness.

Given, there are cases where you want to work with a fixed and given exposure time (fast movin action/light painting/screen shots of CRT-displays, to name a few), but that is not the case with our film scanning application.

The f-stop is primarily used for defining the depth of field. That is: a large aperature will give you a shallow depth of field. That shallow depth helps of course with focussing. Because even the slightest misfocus will immediately show up.

On the contrary, if you close your f-stop all the way to 22 or so, your depth of field will expand (get larger). So small defocus errors will not matter. That is the reason why you never should focus with the lens set to a small aperature. Turning the focus ring will simply not make a too much difference.

So remember: focus at the maximal aperature your lens can support, capture with the optimal aperture of your lens.

So what is the “optimal aperature” of a lens? Well, it is definitely not the smallest aperature your lens is capable of. Why? The explanation is a little bit complicated and has its roots in classical optics.

Here’s the story: lenses are traditionally made out of optical elements with spherical surfaces. These optical elements are easy to manufacture, but they have a problem. They only approximate the “perfect” lens. Especially in the outer areas of a lens, rays of light are focussed onto a different spot than the rays falling through the center of the lens. And it is here that an aperature helps, because the outer layers of the lenses are blocked by the aperature. So closing the aperture (higher f-numbers) makes your image sharper.

(Lately, people have started to develop aspheric manufacturing technologies, or even so-called freeform optics. Typically, such lenses are made with single point diamond turing machines. Fascinating possibilities, but slightly expensive…)

So why than not using the smallest aperature of your lens? Well, because light is not a ray, but a an electromagnetic wave (I drop the particle part here, introduced by quantum theory :wink: ). And any wave has the property of diffraction. If you are interested in this, you can look up the details by googling “Airy disc”. In essence, the smaller the aperture of your lens gets, the blurrier your image is going to get.

Summarizing, large apertures (small f-stops) are bad because your lens is manufactured less than perfect, small aperatures (large f-stops) are bad because of the wave-nature of light.

So there will be an optimal aperture for optimal resolution, located somewhere in the middle of the f-stops your lens is supporting. In fact, you can measure that for a given lens, and for some lenses, an internet search will give you the perfect f-stop. From my experience, I think an f-stop of 11 or so might already be too much on “Airy disk” side. Best approach is to capture a single fixed frame with f-stops of 5.6/8/11 and compare the results by enlarging the captures.

[A nice way to do so: stack all captures in an image processing program (for example gimp) as layers into a single image. By activating or deactivating single layers you can quickly compare the same image region (possibly significantly enlarged).]

That is a strong indication that your optical alignment is less than perfect. The center of your sensor and the center of your lens define an optical axis. Both sensor and lens have to be aligned with that optical axis (not tilted or twisted; exact 90°). That needs also be the case for the film you are about to scan. It needs to be exactly aligned 90° toward the optical axis. If you get the blurry areas in sharp focus when changing the focus while the areas which were sharp are simlutaniously getting blurry, your alignment needs to be improved. But note that (graph) “paper” might not be perfectly planar either and in the optical arrangement you are working with (HQ camera, 50mm Nikkor lens) even fractions of a millimeter are making a difference.

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I use the EL-Nikkor 50mm. There are two versions of the lens, this website has a lot of information, and it depicts the new version.

The practical way to focus is actually to open the lens to 2.8, peform the best focus you can, and then close to the desired aperture. From the data -and from my experience- it is best to operate it at f5.6.

@cpixip points are spot on and all applicable.
Any misalignment of the sensor/lens/film, no matter how small, would be noticeable. I had a below that would introduce a small error, and had to replace for a tube.

The EL does not have the best sharpness away from the center. I also have/used the Schneider Componon-S 50mm f/2.8 and the performance away from the center is slightly better. Having said that, the EL should produce good definition, just a bit softer on the corners.

To see grain, you need properly collimated light, like that employed in projectors and enlargers. If the light is diffused, then the grain particles will be illuminated from several directions, making them less visible.
As an example of this, consider a sunny day with no clouds - shadows will be pretty well defined, assuming no local reflections. If there is some haze, or light cloud, then shadows will be soft.
The sun is so far away, that light from it on a clear day is almost totally collimated.
On the moon, where there is no atmosphere, shadows from the sun are highly pronounced.
Enlargers sometimes used diffusers to produce ‘soft’ images for portrait photography.

I’ve been using a diffuser for the light instead of collimating it. How would I collimate the light source? This 16mm projector I’ve been working with had two convex optical lenses in front of the projector’s lamp held together with a bracket. Would this work or would I need to find another way to properly collimate the light source?

I think you can get good results with a diffuser. But in my experience it has to be as close to the film as possible to avoid loss of sharpness.

I reinstalled the mount that has the two convex lenses to use that to collimate the light. It works, but the light is way too bright. With the lens set to f/16, which I think is the highest f-stop on the EL-Nikkor I have, there’s nothing but white light in the preview; you can’t make out the film’s frame. How can I focus the lens if the light is too bright when on f/16? This was the question I had initially. When I set the f-stop to f/2.8, you can see the film frame a lot better (you can see what’s on the film’s frame), but it’s still too bright. I had better results with a diffuser, but wasn’t getting proper focus; focus looked sharp, but I couldn’t see film grain. Any ideas?

Well, generally, you should expect a brighter image with f/2.8 than with f/16 - so if your image is too bright at f/16, it will blown out for sure with f/2.8. What you describe is the other way around?

Also, note that the proper exposure of an image is a factor of several variables. Depending on your needs, you will choose the various variables optimally for your use case. In the case of film scanning, the following should give you good results:

  1. f-stop: set the f-stop to the value your lens has an optimal sharpness. The value for this is usually between f/4 and f/8. Try to look up the optimal value on the internet. If you can’t find one, use f/5.6. Leave it at that value for the following steps.
  2. ISO/Gain(s)/EV-compensation: do not fiddle with any of these. ISO should be the lowest value your camera can operate on (this reduces noise). The same goes for the gains (analog or digital). Set these values to the lowest values possible, do not touch them afterwards. Forget about EV or similar values.
  3. Exposure time: if your picture is too bright, use a shorter exposure time. This should get you darker pictures. If your image is too dark, increase the exposure time. Find the optimal value for your material and leave it fixed through all your scanning. If you are lost here, try to use autoexposure (if available) and take a note on the exposure time the automatic chooses. Be sure to have ISO and gain settings at the absolute minimum.

To summarize: set all values to optimal values and leave them there. Vary at most a single camera parameter, exposure time, to get the correct exposure.

Well, that status used to be the holy grail of small format movies - at least during the last century: sharp image, no grain. Obviously, things have changed nowadays…

Joking aside: if you want to “enhance” the appearance of film grain, use a condensor-based setup. This is exactly what you described here:

A diffusor-based illumination will attenuate small structures on the film, film grain for example, compared to a condensor-based one, as @VitalSparks described above.

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