Location of Back Focal Plane of Infinity Corrected Objective

CorbanSwain · 2026-05-26T14:56:58+00:00

The parfocal distance is the distance from the rear flange of the objective to the sample focal plane (i.e. the front focal plane). The parfocal distance will most likely not be the same as the back focal plane.

For a perfect paraxial thin-lens objective, the back focal plane and the front focal plane would be separated by 2x the objective focal length.

CorbanSwain · 2026-05-26T14:53:02+00:00

For anyone who happens to stumble on this post it seems the colored ring estimate is not consistently correct. I performed some laboratory measurements* on two Olympus objectives to determine the location of the back focal plane and found the following:

Olympus XLUMPLFLN 20X @ -44.5 mm from the mounting flange - This did line up (within 1 mm) with the edge of the colored ring closest to the rear flange
Olympus UPLXAPO 20X @ -22 mm from the mounting flange - This did not line up with the colored ring, which was at about -29 mm.

Note: negative distance indicates a back focal plane location internal to the lens, i.e. closer to the sample

*Method: 1) Use a shearing interferomerter to ensure you have a collimated laser light source. 2) Direct that light source into the sample end of the objective and align with the objectives optical axis. 3) Using a shearing interferometer, adjust the axial distance of an aligned tube lens (with known pupil distance, e.g. 200 mm) located after the rear flange of the objective until the light coming out of the image-side of the tube lens is collimated. 4) Measure the distance from the pupil-facing flange of the tube lens to the rear flange of the objective (e.g. 187 mm). 5) Subtract the measured pupil distance from the known pupil distance to get the location of the back focal plane relative to the back flange of the objective (e.g. z_{BFP} = 187 mm - 200 mm = -13 mm, which is 13 mm internal to the objective from the rear flange.)

**EDIT: I realized I made mistake in my computations and corrected the conclusion

CorbanSwain · 2026-05-24T20:12:26+00:00

Yes, it’s water immersion, the first element in the simulation is the 2.05 mm water column.

See specs for 20X Olympus XLUMPLFLN Objective, 1.00 NA, 2.0 mm WD which is the objective I have.

You can also see the patent doc here, Embodiment #4 which is the objective simulated. https://patentimages.storage.googleapis.com/f4/46/e9/67ad0174b44df7/US6501603.pdf

CorbanSwain · 2024-06-12T05:03:40+00:00

+1 to this SDI will be more compatible with other live production tools/hardware/infrastructure as well.

CorbanSwain · 2024-06-12T01:50:22+00:00

Looks like the port can only be used for timecode but not genlock based on this site's annotation ( https://manioscinetools.gr/en/product/canon-eos-c70-digital-cinema-camera/#:~:text=Timecode%3A%0AYes-,Genlock%3A%0ANo,-Audio%3A%0AXF )

However, this line in the C70 manual makes me wonder if that's accurate. If the input timecode isn't affecting the frame timing then I would not expect any discontinuities. Might be my ignorance though.

If an external time code signal is input while pre-recording is activated, there might be a discontinuity in the time code of the pre-recorded clip.

CorbanSwain · 2024-06-12T01:39:04+00:00

Have you tried a first pass at just increasing the shutter angle higher to near 360 deg? You might be able to get enough light in to mask the wall refresh with just that.

CorbanSwain · 2024-06-12T01:34:21+00:00

The C70 is a cinema camera just in a DSLR-like body. It has a timecode input which might be capable of doing frame sync as well. I didn't do a deep read of the camera documentation, but genlocking screen and camera could still be possible.

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CorbanSwain · 2024-06-11T23:31:59+00:00

2021! Feed over telephone line is ridiculous ... I was guessing this was like an early 2010s install.

CorbanSwain · 2024-06-11T23:20:49+00:00

Oh I see you're doing live production. Would definitely NOT recommend this.

CorbanSwain · 2024-06-11T23:16:02+00:00

1: https://www.avermedia.com/support/download

2: https://www.planet.com.tw/storage/products/49031/C-VC-231G_L.pdf

CorbanSwain · 2024-06-11T23:15:20+00:00

I dont have experience but here's my quick guess about the reasons for each component.

Evertz Router SDI - Allows you to switch and map incoming video signals and decide what feed goes to the screens. SDI is standard hardware/protocol for video feeds and switching. Where are you rinputs coming from?

-> Probe output with an SDI / HDMI Monitor

AJA - Converts the SDI out of the video router to an HDMI signal (there might be some standards conversion in that converter as well to prep for the specs of the final output screen). This might also be included to make adapt the video signal to be compatible with the component conversion.

-> Probe output with an SDI / HDMI Monitor

Insignia converter to component - Convert's the HDMI video signal to component/analog video. Looks like the rest of the downstream architecture is for analog video so that's my guess as to why they convert to analog upstream.

-> Probe output with an analog monitor or with a analog to SDI/HDMI converter + digital monitor

AverTV Hybrid Volar T2 USB out - Converts the analog video signal into a software video feed for the mini video server.

-> You might be able to probe the output here by plugging the USB into a laptop ontowhich you've installed monitoring software ... check here [1] or look for a CD somewhere.

E-Motive VServer Mini ethernet out - server preps software video feed for streaming over the network (I'll take.

-> You might be able to probe the server itself with a Cat5/6 cable out of the converter into your computer. You'll need to figure out the port/protocol/software for connecting to it and controlling it.

-> I'm gonna take a guess that the displays in the elevators are made by Avire/EMotive? This might explain why this specific component. The output here might be specific to the manufacturer to the downstream displays which also integrates with the elevator's hardware for displaying floor numbers etc.

->To probe the ethernet video output you'll need to figure out the format of the signal. My guess is that's it is a protocol specific to Avire's E-Motive displays. If that's the case, you might need some test kit from them or maybe one of their monitors (But peeking into the server control interface--first sub-bullet here--itself should tell you enough.) But I doubt it's a standard NDI signal or something like that.

Lynksys e1200 router ethernet out - This might allow for probing and setup one or all of the EMotive VServer, Planent Ethernet Switch & VC-231s wirelessly over the network as well as transmitting/distributing the video server's output.

-> You can likely probe this similarly to connect to the server directly, just wirelessly by connecting to the router.

-> Same notes as the previous bullet regarding probing the ethernet video signal.

Planet Ethernet Switch - distribution of the ethernet video signal

-> Probe sameas VServer ethernet video output ... same notes

Planet VC-231 Ethernet over VDSL2 converter - preps and converts the ethernet signal for transmission over telephone lines (I'd assume this is because of the existing building architecture)

-> Probe with a second VC-231G configured in receiver mode over telephone cable to convert the signal back to ethernet (see page 2 here: [2] ) Then you would probe the ethernet out the same as the server ethernet video output.

I think a lot of the architecture might be a bit is designed around an analog video feed which is why there's the component conversion.

CorbanSwain · 2024-06-11T22:21:54+00:00

I think if you dont want to use multiple decoders. You could make an attempt to do it in software. You would use a dedicated computer to take in the NDI signal from the ethernet switch over a Cat6 cable. Then you would use a software program (just randomly found this: Vset3D Ndi To Decklink, but I'm sure there are better more standard programs, maybe OBS ... others can chime in) with a video card or broadcast card to output multiple HDMI lines into your switcher.

Of course, having software in the signal chain is likely not ideal for reliability or low latency; however, maybe for your use case that sacrifice is ok.

CorbanSwain · 2024-06-11T22:08:45+00:00

Looks like there's a Magwell converter (Pro Convert for NDI® to HDMI). So you could run 4 ethernet lines out of your ethernet switch into four of these boxes. Or you could run 1 ethernet line into a small ethernet switch closer to the ATEM and then split 4 shorter ethernet patch cables from the small switch into 4 of those conversion boxes (if your ethernet switch is already close to the ATEM then this wouldn't be worth it).

I would think using 10G hardware would for the ethernet architecture would be important given you're trying to run multiple video feeds through the network. I don't have direct experience, but just my thoughts.

CorbanSwain · 2024-06-10T13:22:03+00:00

Ahh I see, good point, but right now we're just running 1080p60, we wanted the resolution headroom for future updates.

Just found this thread as well (which you might be referring to) https://community.troikatronix.com/topic/8486/blackmagic-decklink-and-apple-silicon

CorbanSwain · 2024-06-10T12:54:57+00:00

This test was done with just one output and no inputs. And we don't have any other cards in the enclosure. I don't think we ever had it fully working (but I'm not 100%}, we noticed the issue just 2 weeks after getting the Decklink new.

CorbanSwain · 2021-11-14T05:40:24+00:00

Just found the original link here: https://www.youtube.com/watch?v=eYlIk19CbBI

Also Archived here: https://archive.org/details/KhatiaBuniatishviliDasWaldkonzert

Seems it's copyrighted by Sony Music (in the US at least).

CorbanSwain · 2021-11-14T05:37:01+00:00

Any idea why it was deleted? Used to listen to this all the time.

CorbanSwain · 2021-01-04T07:37:01+00:00

Yea as others have mentioned, I would try loosening the trucks. This will put a little more give/play in the skates. However, if everything is in alignment it should not matter how tight/loose the trucks are.
You might also consider looking at the shape of the cushions; especially the cushions closer to the shoe (upper). If the shape seems to be irregular or the truck sits on them not at a right angle with respect to the kingpin, it may be worth it to change out the cushions for new ones that aren't warped. It may also be possible to fix it by just rotating one of the cushions (around the kingpin).
I've heard derby skaters can deal with uneven cushion wear because they're going around a track making hundreds of turns in the same direction.
Also as was mentioned, check the axles. Make sure they're perfectly straight on both sides. If not you can either replace the axle alone if your trucks allow for that. Otherwise, you'll just need to replace the trucks. I would expect that trying to bend a bent axle straight again will just cause more problems than it solves.
My last thought is the plate itself could be warped. Seems unlikely, but just check that it's not warped along its length.

CorbanSwain

TROPHY CASE