OLPF change affecting back focus?

[Dan Hudgins]

Works if actors hit their marks for primes

Works if actors hit their marks for primes

What about focusing by eye with the aid of a bare maglite bulb? Set the focus marks on the follow-focus wheel that way and just pull focus. Nevermind the marks.

This is a good idea for prime lenses without floating elements since you are seeing the exact focus at the stop used, lens used, focus distance used...

But it does leave the problem of childern, animals, and actors that can't hit their marks... and also nature, sports, and other times when you cannot take the time to mark, so the FF guy needs to look at the scale and use his mental rangefinder to guess the focus distance.

Focusing on the monitor while shooting tends have the focus go past the right spot and come back, but you have a good idea, make a maglite with the reflector panted black and take out the lens maybe, or make a better device with the maglite bulb so you see it from the side. I find focusing on the coils of the lamp filament under high optical magnification in the camera the best way to get the focus right on. If you use a bulb though, you might want to put an IR filter on it since the sensor may show a IR flare around the filament image and you DON'T want to focus the lens on the IR rays!

A green LED might work also to avoid IR focus, you could make a device with green LEDs a diffuser, and a black and white transparency of a face or something plus a clear X. Just having a high contrast X may not find the "Best Focus" as well as a transparency since the low contrast detail would have you set the focus a little off the high contrast target if the lens is soft since the lens rays in the center of the light cone give detail, and the ones at the edge give brightness and contrast. A dimmer on the LEDs could help keep the brightness of the target in range.

This mark it option, still leaves some issues,

1) The backfocus should be checked for each true zoom lens change. The focus of true zoom lenses cannot be touched up with the focus ring if you are going to zoom, unless you do it in sync with the zoom.

2) DSLR lenses with moving prime groups might drift "best focus" over their zoom range. If used as a "variable prime" this might not be a big issue since you can touch up the focus with the slightly off focus ring.

3) Scope lenses that have moving prime groups might need to be stopped down more than for film use and have their backfocus checked on each lens change. Scope lenses cannot have their focus touched up with their focus ring, the backfocus needs to be exact right on to get full resolution.

4) It may take a little longer to set the marks when focusing with the monitor and a lamp target.

Does anyone know if it is possable to have the RED ONE (tm) output 1:1 pixel display on one monitor, and full frame view on a second monitor?

Edit: Oops, did not see the posts while I was typing, NO 1:1 pixel mode!!!!, how do you check the focus while shooting? Guess you need to download and look at the 4K frames in 1:1 pixel mode on a crop on a monitor... not good for shooting is it? Something to add to Epic (tm) and later RED ONE (tm) dual monitor 1:1 + fill frame at the same time.

 

[jcirbes]

see post #39 & #40

 

[George A.]

I usually take the cap off a regular mini maglite and focus on the bare bulb from the side (when shooting 35mm) for focus marks on the wheel. Never had a problem. Didn't try it on the RED, though.

Do you really think you have to go through all that (as far as the bulb itself, IR blocking etc) when transferring this approach to RED?

 

[George A.]

Also, speaking of focus, it's not such a good idea to be so gung-ho about getting minimal DOF. Shooting wide open is not easy on anybody - actors, focus puller etc. Closing up 2 stops from the maximum open setting gives best optical results on most lenses, anyway.

This way you also give yourself some room as fas as DOF is concerned, you're not always on the edge (especially with RED and its back-focus problem, if it really exists). I open up all the way only when I really need it (minimal light levels), or when selective focus is truly important to the story. Plus the audience gets to see the nice set, production design etc

I don't understand why some people are so obsessed with extremely minimal DOF even in normal conditions.

 

[Dan Hudgins]

IR and other issues with DOF

IR and other issues with DOF

Also, speaking of focus, it's not such a good idea to be so gung-ho about getting minimal DOF. Shooting wide open is not easy on anybody - actors, focus puller etc. Closing up 2 stops from the maximum open setting gives best optical results on most lenses, anyway.

This way you also give yourself some room as fas as DOF is concerned, you're not always on the edge (especially with RED and its back-focus problem, if it really exists). I open up all the way only when I really need it (minimal light levels), or when selective focus is truly important to the story. Plus the audience gets to see the nice set, production design etc

I don't understand why some people are so obsessed with extremely minimal DOF even in normal conditions.

Many lenses have bad focus in the Red end, so I assume that if your sensor has IR also it can only get worse. When you focus by eye there is not IR issue and you can ignore the red halo around the filament. Blue halo is bad in film images sometimes because the film is UV sensitive, so when you focus by eye the UV can be out of focus, best to shoot through a UV and IR filter on Digital to avoid halo on both ends.

One of the big selling points of the RED ONE (tm) was the ability to shoot 4K with 35mm cinema lenses, to get the shallow DOF of today's cinema to make in not look so much like a SONY (tm) camcorder.

People associate degraded images with grain, "bad lab" color, and shallow DOF as being like high production value films, sort of the "world turned upsidedown".

Now "video" is crisper than "professional" film. Now the wide color gambit of Digital is feed through a 3D look-up table to get the "bad lab" look. Now people shoot "movies without movie lights (tm)", like Amateurs (that was a Kodak (tm) slogan for the people too young to remember it).

I think one reason people have an upsidedown view of what has value, is that the yonger generation never saw good films in the theatre like fresh IB prints, EDTA bleach came in in the mid-60s because of the EPA and film labs had problems getting the silver and stain out, so films made after about 1968 have a washed out bleach bypass look, and the videos and DVDs of those now faded films make young filmmakers think that all films of the past looked like that when new, they did not, what they are seeing are bad dupes and faded copies of bad dupes on todays higher contrast print stocks which make the grain look worse than it did in the original prints. That look was emulated when the filmmakers got there chance, so it has become a style, shoot without lights, shoot without filters, shoot out of focus, get flare in the lens, all things that would have gotten you fired before 1968, maybe.

Before the do it cheep came in, color negatives were printed onto three seperation master positives and then three seperation dup-negatives then onto color print stock. Today, people can still make color seperation negatives in their film recorder for striking prints, but do not want to add some cost to get better results.

Do you need to fuss to get 4K images, well maybe, you can look at your 4K images at 1:1 pixel crop on your 2K monitor and see how soft you want them.

Having shallow DOF on a cheap set can be a big help in avoiding the cheapness showing, it also concentrates the audience focus to where the director wants it to be.

The odd thing now is that almost every production is desigined to have its own "look" since technical perfection has passed out of style, but one thing that still prevails is that soft footage with any "look" is not as appealing as having the subject in focus.

The sharpness of a 4K image has an appeal of its own, like 65mm/70mm did. But if the focal plane is not on the actor's eye etc. the audiences attention will be forced off the subject to the sharp details elsewhere, hence the preoccupation on getting the focal plane where you want it.

I cannot figure why the RED ONE (tm) would not have a 1:1 pixel display, that would have been the 1st feature I would add after full frame view finder. How can you be sure the focus is right without full zoom mode?

 

[George A.]

I totally agree with the 1:1 zoom for focus. I can't understand why it is not in the works (I even remember Jarred saying that we should not even expect it soon).

 

[Dan Hudgins]

No De-Bayer needed for 1:1 view...

No De-Bayer needed for 1:1 view...

I totally agree with the 1:1 zoom for focus. I can't understand why it is not in the works (I even remember Jarred saying that we should not even expect it soon).

It would be simple, no De-Bayer would be needed, I think almost everyone here would accept a monochrome display for the 1:1 focus mode, even if it has a little checker board in it as well, just something that is BIG enough to see exact focus on each pixel of the sensor. Even 1:2 (one sensor pixel to a square of four pixels on the monitor) would be useful since the single pixels on the monitor are blurred with the color dots.

I would like a two monitor setup, with one for full view and another for 1:1 display. The 1:1 display should be able to joy-stick around the frame. Another option could be picture-in-picture for the 1:1 zoom with window placement and "pan-n'-scan" .

Even if you have a 4K monitor, you cannot see well to focus unless the monitor is 24"-36" or bigger, at least not at my age (and because of the dot pitch artifacts), so a 1:1 pixel view on a 8-12" monitor can be better than a 4K monitor since a 8-12" monitor is easer to use in the field...

 

[Shawn Bannon]

Chamfer

Chamfer

Hopefully this will be useful to some.

PROBLEM
I have several older PL MOUNT lenses, the lenses themselves were collimated at Clairmont camera and were at perfect focus on the projector. HOWEVER, when I put them on the RED some were perfect and others would be totally out of focus.

When I told my tech about the problem he took a look at the lenses mounted on my Red and knew exactly what was going on. the CHAMFER on the Red PL Mount wasn't angled as much as some older PL MOUNTS or PL MOUNT adapters.

SOLUTION
he just machined down the CHAMFER on my old older PL MOUNTS.

NOW EVERYTHING IS PERFECT.

 

[Mike Prevette]

Shawn,

I've seen that same problem a bunch of times as well. The machining down of the chamfer is the best solution. I wish red would have changed their mount a bit, but it looks like they couldn't because of the proximity to the mount screws.

However I do think there is merit to the OLPF effecting the flange depth differently to differing lenses. It makes perfect sense when you think about how the light passes through it.

Light traveling at a steep angle from the exit pupil of the lens as to pass through more of the OLPF VS light traveling straight through. The old calculation has always been adjust for 1/3 the thickness of the behind the lens filter.

So for example say the OLPF is 1mm thick. Light coming out of a lens that has a exit pupil near the PL flange would only be offset .33mm but a lens that with a very deep exit pupil could be passing offset as much as 1mm or 2mm depending.

I don't have any exact numbers from the mount so I can't do any real calculations, but in my mind I can see it can be a significant offset.

 

[Mike Prevette]

********DISCLAIMER: These distances are a pure guess!

This is the basic idea. Notice Lens 2 has to send it's light through a lot more OLPF than Lens 1. At least it's enough at 1/3 the thickness to throw of the flange depth enough to effect wider lenses.

 

[Evin Grant]

I really don't think it's the OLPF, on the first shoot I did with my Red I had checked the mount at the prep with a set of S4s and it was fine. The next day at the shoot the mount was out at lunch. I re-collimated it and it seemed to hold. I took the cam directly down to Red, they re-lubed and re-collimated the mount and it has held ever since. Granted I have #199 so it's definitely the early series you are talking about but this may be as simple as some dry mounts that are shifting slightly.

 

[Mike Prevette]

After thinking about it on the drive home, there is a big hole in my theory. The light passing through the dead center of the OLPF would always be the same regardless of lens design.

I'm just trying to account for some of the voodoo I have seen between lenses, and some of the comments I've heard from ac's about not getting reliable marks from lens to lens.

An 85mm Zeiss prime is a hugely different lens than an Canon 6.6-63mm It stands to reason that there could be some hocus pocus going on somewhere.


I'm sure that 90% of the problems people are seeing are due to simple incorrect flange depth.

 

[David Groundwater]

hi guys,

has this issue been resolved?

i had a fairly serious shoot a couple of weeks ago - 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 (and i really don't get this), 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 leway 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.

any thoughts??? is there something about master primes that is in someway incompatible with red because of the olpf or whatever?

david

 

[Sebastian Cramer]

The thickness of the OLPF can only be properly compensated for if the light is coming telecentric thru the filter. In most cases an OLPF is 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 filteres 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.

It looks like this is one of the reasons why I heaven't heard a lot of mismatched focus scales with MPs as they are rated to be telecentric. But with 35 Superspeeds and 16mm lenses it can get difficult. From my understanding matching exit pupils should require similar backfocus setups.

As film lenses are always designed for a film plane with no glas 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 Master Primes)
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.

Sebastian Cramer

 

[David Groundwater]

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.
Sebastian Cramer

thanks sebastian, this is very interesting - though i'm not quite sure where it leaves us...

when i shot with super speeds, i was focusing by eye and it was fine - even at t1.3. as i was operating whilst shooting/pulling focus i didn't look at the focus marks so they could well have been out.

with the master primes, we could not get parts of the image in focus when we stopped down - period.

this would be explained by the back focus shifting when we stopped down.

is there any other possible explanation - possibly the sensor not being square to the lens or not being flat for example?? on two cameras....

cheers,

david groundwater

 

[Michael Lindsay]

1) Only work with telecentric lenses (get a set of Master Primes)

Are you sure that MP are all telecentric?

thanks

Michael

 

[Michael Lindsay]

I tested a range of lenses on a SI-2k the other day. The OLPF causes problems. When using a Zoom that is very retrofocal (to the point that the light out the back is very telecentric) you get very good results.

If using a wide prime lens I saw CA and sizable backfocus problems. The back focus pproblems are solvable on the SI-2k by using per lens PL mounts. The CA?

I believe the above may to a lesser extent be applicable to Red.

reagrds

Michael

At the time of posting this, and a few defending 'what I saw' counter posts, there had not been allot of bad focus comments pointing to OLPF problems.

I will check all our Ultra primes and see if I can see anything. However if anyone has some S16 distagons with a PL mount that would be the way to go. They result in huge focus problems as you get wider with PS SI 2K's 2mm OLPF.

regards

Michael

 

[Sebastian Cramer]

thanks sebastian, this is very interesting - though i'm not quite sure where it leaves us...

when i shot with super speeds, i was focusing by eye and it was fine - even at t1.3. as i was operating whilst shooting/pulling focus i didn't look at the focus marks so they could well have been out.

with the master primes, we could not get parts of the image in focus when we stopped down - period.

this would be explained by the back focus shifting when we stopped down.

is there any other possible explanation - possibly the sensor not being square to the lens or not being flat for example?? on two cameras....

cheers,

david groundwater

David,
if you focus by eye you will always have a super sharp image as long as your monitoring is good enough. At this point I trust the monitor more than the scale. But it's not something you can rely on in a production shoot. You must be 100% sure the scale is right so the focus assist can do his job.

I'm not familiar enough with the master primes to understand if their design shows no issues. I heard of the set of MPs working very good, but just the MP zoom was off in the scale.

 

[Sebastian Cramer]

Are you sure that MP are all telecentric?

thanks

Michael

Hi Michael,
this is what I heard, that Mater Primes are designed to be telecentric to work with digital cameras. As far as I know this basically means they have a certain fairly long distance for the exit pupil. To my knowledge there is no such thing as true telecentric, lenses are concidered to be telecentric when they show a certain distance here.

Please keep me updated on the tests of the UPs.

 

[David Groundwater]

David,
if you focus by eye you will always have a super sharp image as long as your monitoring is good enough. At this point I trust the monitor more than the scale. But it's not something you can rely on in a production shoot. You must be 100% sure the scale is right so the focus assist can do his job.

I'm not familiar enough with the master primes to understand if their design shows no issues. I heard of the set of MPs working very good, but just the MP zoom was off in the scale.

hi again,

thanks for your thoughts.

focusing by eye will NOT always give you a super-sharp image - however good your monitoring is.

until we get one to one zoom and can look at the image pixel for pixel on a monitor, you cannot be sure that the image is sharp by using a monitor. i've found that shots can look sharp on an HD monitor at 720p, but not sharp when you look at the 4k tiffs.

now this wouldn't be problem if you were staying at 720p, but in this particular instance we want to retain the full 4k resolution for a number of reasons.

usually, if an image is sharp to the eye at 720p on a 17" HD monitor with the lens wide open, by the time you've stopped down to 5.6 there will be sufficient DOF to insure that it's pin sharp even at 4k.

the great problem in this case, however, was this business of the depth of field apparently not increasing when stopping down the lens - and the image not being universally sharp throughout.

cheers,

david