Blackmagic Forum

BMD support and hive mind:
for technical reasons, I have to remove the IR on my URSA 12k. after removing it, i see two different effects:
1. EF lenses don't have infinity focus anymore, and generally look pretty bad (soft)
2. massive WB shift (>2000K and 50 magenta tint) is necessary to get the image to look more normal, but overall, colors look generally pretty awfully and off.

questions:
1. has anybody else experienced this, and found a solution for it? (other than reinstalling the IR which is not an option with the Entania lens due to interference with the lens element)
2. does removing the IR change the back focus (flange distance projection) drastically?
3. does BMD sell IR for the URSA 12k separately? I may try to be adventurous and grind it into a 25mm round filter for the entaniya lens (if it doesn't cost $500 like the RAWlite filter)

UPDATE:
The BMD IR filter is very close to a ROSCO 4330 (30 Cyan), which brings the whole color science gen 5 back to normal

The Ursa 12K doesn't have an OLPF. It has an IR filter. If you remove it you need IR filtration somewhere in the optical pipe or you're going to get IR contamination. It's a common misconception that BM cameras don't have an IR filter, in actuality they have a historically weak IR filter.

Yes the back focus is going to change. If optical elements are removed from the optical pathway the back focus will change just like any other imaging system.

Good Luck

Hi Howard,
thanks for the clarification that it is indeed an IR cut filter, i did edit my post.
never the less, the entaniya has an IR cut filter as the last element, and I even doubled it up. but on average, IR cut filters have a strait transmission for the visible spectrum, but the BMD IR filter is very very green. and seems to affect the internal WB calibration a great deal.
also not sure if a filter should be considered an "optical element" but I guess there could me a minimum of refraction happening on the two parallel surfaces.

There is no optical filter that blocks all IR and has no influence on the visible part of the red side in the spectrum. Take the OLPF out of a Red camera and it'll have a similar effect. That's why every manufacturer has to keep a balance between blocking IR and damaging far red and that's why we need extra IR filtering with ND (which the URSA has).
Finally, even the thickness of the flat glass is influencing back focus to some degree, that's a known fact.

joergengeerds wrote:also not sure if a filter should be considered an "optical element" but I guess there could me a minimum of refraction happening on the two parallel surfaces.

Removing the filter will shorten the flange focal distance and adding glass, even flats, will extend it. When adding an OLPF over the IR filter, shims need to be added to the mount, if the filter is removed entirely then remove shims to compensate.



Good Luck

Hi Howard, thx for the explanation, makes sense.

I got to ask. What are you shooting with a lens that can see behind the camera?

Anytime you remove something from the optical path behind the lens yes, for sure you will change the FFD and need to compensate on the mount (or the lens)

The sensor cover glass also acts as both an IR filter and dust cover for the sensor.

Removing it means you’re drastically altering the sensors view of the world and changing the FFD.

JB

joergengeerds wrote:2. massive WB shift (>2000K and 50 magenta tint) is necessary to get the image to look more normal,

Wait, add 50 magenta tint after taking out the IR filter in front of the sensor?

That doesn’t sound right – you’d need to take out the magenta, and still be nowhere near the normative color scale, after the consequently immense IR contamination following the removal of the IR filter.

joergengeerds wrote:never the less, the entaniya has an IR cut filter as the last element, and I even doubled it up. but on average, IR cut filters have a strait transmission for the visible spectrum, but the BMD IR filter is very very green

Do you mean the way the actual filter coating looks green (which it’s supposed to) or the green that it imparts the footage? If you mean the latter, then it’s quite the contrary – BMD IR filtration tends to transmit significant near-IR wavelengths, and the resulting red–magenta needs to be balanced in post.

I’m guessing the 50 magenta tint you needed to add was because, as you say, the Entaniya lens you’re using has IR-cut filter as the last element, which you also doubled up.

Did you evaluate the IR filtration of the lens’s last element before doubling it up? If it’s sufficient, then you may not need to use an extra IR filter, thereby eliminating the need to add the extra magenta tint.

However, even if you could mitigate the color issues, it’s likely you’ll still have difficulties with the hyperfocal distance without the sensor’s IR-cut filter.

lost_soul wrote:I got to ask. What are you shooting with a lens that can see behind the camera?

I am a long time (2011) VR filmmaker. the HAL250/BMD12k is just our latest rig combo. it's kinda of a niche of a niche, and there are less than probably 5 people on the planet that use/have this combo. previously, the HAL250 was attached to a GH5 (two actually as a back-to-back rig)

Bromine 18 wrote:Do you mean the way the actual filter coating looks green (which it’s supposed to) or the green that it imparts the footage? If you mean the latter, then it’s quite the contrary – BMD IR filtration tends to transmit significant near-IR wavelengths, and the resulting red–magenta needs to be balanced in post.

However, even if you could mitigate the color issues, it’s likely you’ll still have difficulties with the hyperfocal distance without the sensor’s IR-cut filter.

My main concern at this point is not the IR contamination yet, but the massive WB/color shift the removal of the BMD IR filter is causing. I am used to dial in WB by numbers for years, and now a scene that is nominally a 5500K/0 scene gets a 3000k/-30 from the camera AWB. a 4000k scene gets me 2500k/-12 AWB. This makes color critical scenes a bit difficult, and I have not checked the color chart matching in DR yet. it also makes it difficult to shoot scenes with less than 4000K lights, as the camera settings bottom out at 2500K.

focus with this lens is easy/tricky. it doesn't have internal focus gears, and the focus is only controlled via the back-focus distance (turning the whole lens which has a M45x0.5 thread in the custom EF mount.) which makes the shims irrelevant fortunately. but it is the main reason why I need to remove the BMD IR filter, the lens would crush the filter otherwise.

Doubling up the IR cut filters as the last optical element in the lens increases the perceived sharpness of the lens (needs to be tested/verified with res charts)

The 12k only shoots BRAW, therefore adjust your white balance in post for your 'color critical' scenes.

UPDATE:
The BMD IR filter is very close to a ROSCO 4330 (30 Cyan), which brings the whole color science gen 5 back to normal

we did tests with the ROSCO installed on the back of the lens, and the WB and color science is where it is supposed to be, no more funky colors, focus works, and the lens doesn't make contact with anything it shouldn't (IR filter is removed)

Did you check IR? You may need an additional IR filter.

Even with the stock sensor glass intact, I've found that additional IR cut is necessary in some lighting conditions with the 12K UMP. With the sensor glass removed, I'm sure it would be required to add an IR cut filter.

On my other BMD cameras I've had good results from Hoya's IR cut filter. Unfortunately for my setup on the 12K with matte box, Hoya doesn't make it in a 4x5.65 size. If anyone has recommendations for a 4x5.65 IR cut filter that works well with BMD sensors, please let me know.

I'm using the RAWlite OLPF on our UMP 4.6K. Together with the built-in NDs it's doing a good job filtering IR.

Uli Plank wrote:Did you check IR? You may need an additional IR filter.

I have a good IR cut filter at the back of the lens. plus the ND2 + have some good additional IR (and possibly UV) filtering.

Uli Plank wrote:I'm using the RAWlite OLPF on our UMP 4.6K. Together with the built-in NDs it's doing a good job filtering IR.

As stated above from helpful people in this forum, the 12k does not have an OLPF, just an IR cut. But the hole point of this post was about removing the main IR cut filter...

Jamie LeJeune wrote:Even with the stock sensor glass intact, I've found that additional IR cut is necessary in some lighting conditions with the 12K UMP. With the sensor glass removed, I'm sure it would be required to add an IR cut filter.

On my other BMD cameras I've had good results from Hoya's IR cut filter. Unfortunately for my setup on the 12K with matte box, Hoya doesn't make it in a 4x5.65 size. If anyone has recommendations for a 4x5.65 IR cut filter that works well with BMD sensors, please let me know.

NISI makes exceptionally good filters, IR+ND cost around $200 at 4x6.65in (1 stop ND is the clearest i could find)

Jamie was asking about clear IR (not ND)

I’m not aware of any that are food with the 12k in PSR size

JB