"Why you can't take a good picture of a rainbow"

[ameerat42]

After wondering what title to give this thread, I used the title of the Youtube video
It's 14 mins long, but well illustrated...

Now look here!

[bobt]

Now look here![/URL]

I did ... but my brain doesn't contain enough brain cells or memory cells to absorb it all! After a while I find my brain just starts to dissolve.

[ameerat42]

...After a while...my brain just starts to dissolve.

--Ah, and then you can't see the rainbow

[nardes]

Interesting viewing Am; although I "know" this stuff, it was a useful knowledge recalibration exercise.

Cheers

Dennis

[arthurking83]

No only was he 'wrong'(originally) ... but he's wrong as to why he's wrong!

He said that the violet spectrum was out of the gamut of the RGB pixels(on the camera sensor), but this is wrong.
The blue can record violet and ultra violet, but it's pretty weak(much less sensitive).
But most electronic sensors can record violet and ultra violet too.
As we all know, manufacturers use UV filter over the sensor to cut UV contamination, even more than the low sensitivity that the sensor naturally has anyhow.
But they do so as it creates slightly higher contrast in outdoor scenes.
So the sensor itself is simply less sensitive AND the filters used by manufacturers reduces this even further.

The reason that the GoPro doesn't record violet isn't because its sensor cant' record violet(or ultra violet) .. it's (most likely)because a stronger UV filter was used by GoPro in the design of the device.
Which kind'a makes sense as it's an action camera and by definition the most likely use for such a camera will be outdoors with minimal accessory filtration(ie. lens filters, like UV cut and or polarisers .. etc.)
So the images look cleaner to please the masses.

You learn all this stuff if you're interested in full spectrum imaging.
ie. if you want to take photos of pure UV light, you can get a clear filter pack for your cameras sensor, where you remove the UV and or IR filter(all have IR filtration too, that one has a transparent blue tint to it).
This then allows you to shoot straight UV images if you use a UV pass filter. UV pass filtration blocks visible and IR(to a degree) and only lets UV pass through to the camera sensor.
That I know of, you can get UV wavelengths down to the high 200nm(I think around 280nm, maybe) with the right combination.
Exposure times are very long without UV lighting assistance tho. 10x or more.
Other issue is that almost all lenses also cut UV transmission through to the sensor too. If you want UV pass lenses you need to look at the really really old stuff .. back in the days when UV filters actually did something.
From about the 60's tho, very few lenses allow UV transmission, but folks still thought that there was a need for UV filters

I guess people just hold onto 'old ways' without looking into it in depth.

So, it now begs the question .. does two wrongs make a right?