Focusing issues with digital cameras


We've pulled together some internet forum comments regarding the focusing issues people are experiencing with another digital video camera - the Red - and some possible explanations and work arounds:

ISSUE: "I had a fairly serious shoot a couple of weeks ago with two Red cameras - we took a back up camera along - and we had real difficulty getting our sharps - with both cameras. We were using a just-before-checked set of master primes. We checked back focus (which was about 6 microns out) - and were helped in this by an extremely helpful lens technician from Arri. There were two issues:

Firstly, we could really only get the centre of the frame sharp - it got very soft as soon as you moved away from dead centre.

Secondly, the depth of field appeared to be a matter of a few inches when it should have been several feet. We took focus at T1.3 by eye (zoomed in on an HD monitor) and closed down to 5.6 to shoot - that alone should have given us enough leeway to be sharp - but obviously we also set focus with a tape. Eye and tape seemed to match what the lens said they should.

We then looked at the 4k tiffs and they were soft - and I'm not talking about a lack of sharpening. I've shot handheld, pulling focus by eye, using the Red (a different one) on a doc with super speeds wide open and it was all pin sharp."

EXPLANATION 1: "There may be a small focus shift with the lens stop used when there is an OLPF and sensor cover glass+micro lens array behind the lens, even if you adjust for the Paraxial Ray path length.

There may also be a small focus shift when the angle of the light varies from parallel, some lenses have their ray crossing point closer to the sensor than others, so the distance through the glass plates will be longer or shorter for some lenses or others. The focus shift is about 1/3 the path distance difference or maybe a few 1/10000" or just maybe enoough to be a little soft in the corners.

With f/1.2 lenses the sides of the rays cone take a longer path through the glass plate than the center rays, or in the corners one side is shorther than the center and the other side, so that makes the "best focus" distance shift not only from center to corner, but from full open to stopped down.

To avoid this shift only use f/8 and remark the lenses. Otherwise there will be a small shift, not much for a film camera, but at 4K small shifts might be a little soft.

The mount should be set for your zoom since the zoom cannot be remarked, then remark your prime lenses.

Anamorphic lenses are VERY fussy about the backfocus and cannot be remarked and get right focus, so if you shoot scope, set the mount for the lens at the stop you will be shooting at.

The lens and f/stop used when the mount is setup may not be the same as what you are shooting at, so you should check the backfocus at the stop and with the lens you will shoot with. The error is small, and if you focus on the monitor at 1:1 pixel, the marks do not matter (except for INF), the focus will just be a little soft at large stops, which is what the OLPF is meAnt to do anyway, blur the image over four pixels so that two green and one blue+red pixel are always exposed to avoid colored spots. If the corners go softer than the center, it does not matter except on charts since those parts of the frame do not have in focus things most of the time anyway.

I should add, that if you use lenses that were aligined to their focus mark on a lens projector or collimator, those do not have the optical plates (OLPF and coverglass+microlensarray) so the alignment could show a slight shift when the "Best focus" is checked on the camera. If you adjust the lens mount for one lens aligined on a lens projector or collimator, then put another on that is opticaly at a different distance for the rays to cross, the "Best focus" can be a little different ON THE CAMERA even though the lenses were right on the lens projector or collimator, due to the shift in optical path length with angle through the OLPF and coverglass+microlensarray. So if you rent lenses, rather than have your own, the rental house would probably not have aligined the lenses focus marks on a RED ONE (tm) of the same serial number as yours, and therefore you would need to remark the lenses for each stop if you are going to focus by tape, and hope that you can go to INF when you need to.

The reverse can also happen, if you mark the lenses for use on a RED ONE (tm) and then put them on a film camera, you might get some soft shots on the film camera, but with film being what it is you might not notice so much. Anyway, if you are going to a filmout the lens in the projector will probably be far enough out of focus that the camera lens issues will not be visible from the back of the theater(?)."

EXPLANATION 2: "What some people seem to get confused about is that they seem to think there is one setting of the backfocus that will fix this problem for all lenses that are made for a film camera, there is no single setting of the backfocus that will correct for this plate thickness since the length of the rays through the plate varies with the f/stop, the angle of the lens (ray crossing point), and from the center to the edge of the frame introducing curvature of field as well as negative spherical aberration.

The "plates" do three things: 1) there is no longer a single "best focus" for all stops (not that there was, its just worse now). The lens needs a new focus mark for each stop. 2) the "best focus" error is larger more for larger stops. (unless lens had positive sperical aberation before). 3) the "best focus" for the center and corner are not the same, i.e. the lens is no longer "flat field" (not that it ever was, just maybe worse now, although it might be better for some odd lens.)

The thicker the "plates" behind the lens the more this shows up. The faster the lens stop, the more this shows up.

If the "plates" are less than 0.01" total its not so bad, maybe just a little shift of the "best focus" on the different lens stops, but thicker you might see something in the end result. Does anyone know the total thickness and index of the OLPF and coverglass+microlens array?

How much shift in backfocus are people seeing between lenses in 1/10000" (I guess they check wide open)? Most lenses are retro-focus or tele-photo so the point where the rays cross is not as different as when "short" focus wide angle and "long" focus lenses were used."

EXPLANATION 3: "At 4K there is almost no DOF, being just a little off focus means that you are not getting a 4K image of what you want in focus, something may be in focus like the actors ear, but you might be focusing in front of his noise as well.

To get a 4K image, you need the subject in focus. The point of the OLPF is not not get 4K with a high MTF, but to fuzz the image up so that the high MTF comes below 2K. None the less, some detail comes through in the LUMA from the De-Bayer near 4K, and that is what makes the image look in-focus or out-of-focus.

When shooting at f/1.2 using a 75mm at 4K DOF is very short, the charts made for film cameras relate to something closer to a 1K image, check to see what size spot the DOF table was made using.

When you look at the circle of confusion at high magnification you can see the shift in "best focus" for the stops, for film cameras a lens stop shift of 0.001" might just look like the follow focus guy was a little off, but at 4K if you see the image with all its glory 0.001" may be enough shift to notice.

The simple fix is to set the backfocus short so that all lenses will go to INF, then focus by the 1:1 pixel display, and only use prime lenses that focus by moving the whole lens in and out. You cannot set the backfocus too short or you will run out of travel on wide angle lenses. If you have a fixed focus wide angle lens you would need to fiddle with the mount, so try to only use lenses that move and focus on the image not the marks, or tape a new mark on for the stop/lens on the camera...

The thickness of the OLPF can only be properly compensated for if the light is coming telecentric through the filter. In most cases an OLPF has about a thickness of 3mm! So you a talking about a significant amount of glass here. It has about the same dimensions as a regular Tiffen, it just sits in the optical path behind the lens (I heard of DPs who are concerned about this amount of glass in front of the lens). I haven't really understood the details of the OLPF but it seems that it is hard to design a filter which is much thinner than this. It's almost impossible to judge the thickness of the filter in the Red without demounting it, but I would assume it is also in the range of about 3mm.

In addition those filters are often sitting rather close to the sensor. In a Red the OLPF is pretty far out. It's hard to guess, but I would assume it is about 8-10mm away from the sensor.

This helps to not see dust, but the downside to this is, it seems to increase focus shifts and chromatic abberations. If the exit pupil of a lens is rather short the light is entering and exiting the filter on an angle. As there is a significant increase of length (from what I recall about x1.5) of the optical path if light travels thru glass rather then air, the distance to the sensor is not the same if the light is going thru the filter at 90° or on an angle. This could also explain a backfocus shift when stopping down, as the rays of light are more bundled.

As film lenses are always designed for a film plane with no glass in between focus issues and also chromatic abberations will be much more critical when such a heavy amount of glass is introduced in the optical path. I've had very ugly magenta and green edges on highlights, which I have never seen on film before. On 16mm lenses this is much more obvious as they have a very very short exit pupil.

It is very disappointing but it looks like these are the options: 1) Only work with telecentric lenses (get a set of Uniqoptics) 2) Only do focus by eye 3) Collimate each lens with shims to match to each other and create a Red set (which wouldn't be good for film anymore) 4) Use a much much thinner OLPF (if only it would be available) 5) Make different marks on the adjustable mount for different lenses. 6) Mark the scales of your lenses individually if needed."

EXPLANATION 4: "It may hurt some of you to hear this, as you glance over at your cases of Nikons or antique PL mount lenses. An OLPF is necessary in front of a digital sensor. These filters are made of layers of crystal, alternately oriented north/south & east/west, so that they do not astigmatically reduce sharpness (meaning that they do reduce sharpness evenly). That's fine & dandy for the center of the image, but as one moves off to the corners, the path of light on non-telecentric lenses becomes increasingly oblique, or at an angle. This means that the light is passing through the OLPF at an angle and therefore passing through more of the filter. How much more does this diffuse the light?

Depends on the grade of OLPF needed for the sensor size and design and of course the angle of said light. But this is what RED is referring to with their "optimized for digital" lens designs.

I can tell you from my experience with RED and other Digital Cinema cameras that generally this is really not an issue or an incredibly minor one at best. And this is with testing on optical benches and various metrics charts. And I can also tell you that it should have little to no bearing on why popping one lens on comes up soft while the next appears soft. This would effect corner to corner (edge fall off) sharpness only."