This thread is directed to the L21b focus expert @PiercingTheDarkness but will benefit the whole community.
I finally bit the bullet to get a dedicated thrower - an L21b sft40 6500 k 8 amp driver. Reading some threads on here @PiercingTheDarkness has had to modify the LED gasket to perfect the focus.
Here is a ceiling bounce. Is this optimally focused?
The above was done with a 2.2 mm black round gasket. FYI I also have from Simon (on request) a white square gasket that is 2.2 mm thick. Below are pictures of them both:
It’s hard to see but looking edge wise I think the black gasket will lift the reflector slightly higher above the led.
If I read the comments correctly, I should add some extra thickness to raise the reflector slightly. The square gasket seems preferred as it’s less likely to shed particles from the reflector interference during assembly and disassembly.
So am I good on focus now and no optimization is possible?
Will I benefit by shimming the gasket base to raise the reflector? If so which gasket should I start with, how thick of a shim, and what material should I use to create a shim?
Only on BLF could I get an answer to this!
BTW once optimized on focus I hope to try the convoy 10 amp driver. FYI right now at 1 meter using my Sekonic L-478D-U I get 290,000 lux.
First of all if my focus is already very good/close to optimal I will just leave it. Waiting for a consensus opinion on whether that’s true.
The below assumes the gasket is installed with the wide diameter portion against the mcpcb. That’s how it was assembled by Convoy. This allows the smaller diameter circle to fit perfectly inside the reflector hole.
Sanding down the narrow diameter portion of the gasket does nothing to the focus because it doesn’t change the reflector height.
Sanding down the wide diameter portion lowers the reflector. However that’s what the white gasket does too - and that made it worse.
@PiercingTheDarkness said you have to add thickness not take away by sanding. Adding would be done to the wide diameter portion of the gasket as that’s the part that would result in standing the reflector higher.
Yup. Leave it.
But if you get a little neurotic about it, add layers of hole-punched index card (thicker) or perhaps, polyimide tape (thinner) to the ‘shoulder’ of the gasket, where the reflector sits atop it.
Good luck
Yeah, by increasing it by 0.1 - 0.2mm there was only a 40m increase. Main reduction now is the low flux bin Simon is selling at 1600-1700 ANSI. Simon said it’s meant to be the highest bin, but not by my measurements. I have an SFT40 from the previous batch and turn on was close to 2200. Maybe the reel he purchased was mislabled
With such a large thrower, measuring at 1m is unfortunately misleading and favors a prematurely convergent beam over an optimally focused one. For this light one needs 10m+ for reliable measurements.
But the SFT40 has a large enough die size that it is tolerant of minor imprecisions in focus, your beamshot looks fine.
I never really understood the gaskets with circular opening, the square ones are better (or can be slightly redesigned to be better) every time.
I have read about the short distances but it’s just so convenient since the arm of my couch is exactly 1 meter from the wall and I can hold the light in one hand and the light meter in the other.
Also at one meter distance no calculation necessary to convert lux to candela.
Glad that knowledgeable people say my beamshot looks good. I was not sure what a perfect beam shot would look like. There is a center hot spot, then a corona around it approx 3x diameter of hot spot, and then the spill. I was hoping that with a large reflector you could get the corona light into the hotspot. Oh well….
I would be more cautious about short distances because the error may be more than you think. The more close to parallel a beam is, the more distance is required for reliable mesurement. I just ran the math: if you measure a large diameter LEP like Astrolux WP3 at 1m, the intensity is thrown off not by a few percentages, not even a factor of 2 or 5, but a factor of 15!
That sounds exactly like how a well focused beam should look from a distance. A corona is just a part of a reflector’s beam, there is no getting rid of it. Not having a corona in a reflector beam would indicate an excessively short observation distance, or a badly focused hotspot whose center is dimmer than the boundary.
For spacing/padding a gasket, I’ve had great luck using vinyl electrical tape, or plastic glow tape. The layers are thicker than most types of tape but still thin enough to offer enough granularity for adjustment, and the material is more durable under pressure than duct tape. Just cut out 2 small pieces and stick them onto the MCPCB next to the emitter, on 2 opposing edges.
The glow tape is a bit better, more durable and offers some extra bling.
I’ve found the glue in vinyl tape to be pretty stable, unlike duct tape which does squeeze out. Though vinyl tape is a bit soft and is compressible over time, especially if you are layering it, which is why I think glow tape is a bit better–I could layer them pretty much however I like, absolutely no glue leakage or compression.
