What's the easiest and most affordable way for obtaining UV reflectance images?

ProfHankD wrote:

Johannes Zander wrote:

ProfHankD wrote:

petrochemist wrote:

Not surprisingly focus afterwards was way off.

This is actually my #1 issue with full-spectrum conversions. Generally, if you remove a 1mm thick piece of glass typically with index around 1.5, you've basically pushed the sensor about 0.5mm closer to the flange. MFT uses a 4mm cover glass, so we're talking about a potential shift of 2mm!

Could you explain it to me please?

What I have learned from the internet is that the filter stack increases the focal distance by about 1/3rd of its thickness. Leaving the sensor at its position would mean to lose the ability to focus to infinite.

My question: Must the sensor without the filter stack be moved nearer to the lens mount, or further away in order to be able to focus to infinite für visible light?

The precise distance depends on the index of refraction. Basically, the index is a ratio of the speed of light in a vacuum divided by the speed of light in the material... and optical distance to the focus plane is really determined by how long it takes light to get there. Thus, a 1.5 index 4mm glass behaves like 1.5*4mm=6mm of vacuum... and speed of light in air is essentially same as in a vacuum (ok, air's index is actually about 1.000293). Thus, removing the glass effectively makes the sensor closer to the rear of the lens by 2mm.

To correct for removing the cover glass, you need the sensor to be farther from the rear of the lens (by 2mm in my example). Thus, you'd need to add 2mm of shims to the lens mount.

I think to correct for removing the filter stack in front of the sensor, you need to bring the sensor nearer to the lens.

from: http://www.ir-photo.net/ir_mod.html

From the formula given on the above mentioned website you would bring the sensor round about 1,3mm forward to the lens. No shimming of the lens needed.

Yes, the speed of light is reduced in glass, but the light is refracted. The angle of refraction in the glass is less than the angle of incidence resulting in a focus point further away from the exit papilla of the lens than without the glass filter.

That's how I understand Snell's law.

BTW, this is also one of the reasons MFT doesn't really do so well with legacy lenses (in addition to having rather small pixels that really want higher resolution). You see, the index of refraction is actually somewhat dependent on wavelength of the light: that's what dispersionis about. Thus, the 4mm glass actually has slightly different optical thicknesses for different wavelengths, causing CA -- unless the lens is designed taking that into account. MFT lenses can be, but adapted lenses aren't... although the Metabones MFT focal reducers do correct for it (Brian Caldwell, designer of these focal reducers, if you see this please feel free to jump in with a better explanation ).

The reason MFT went with such a thick glass isn't documented, but likely has something to do with the fact that a thicker glass keeps dust particles farther from the sensor (effectively, about 6mm for MFT), so they cast less obvious shadows. Basically, if the dust is close to the sensor, it can block light from all angles coming to the pixel behind it; if the dust is further away, some ray angles to the pixel behind it aren't blocked.

One more odd note: If you had an adapted lens that didn't quite reach infinity focus, you might be able to make it reach simply by installing a glass flat behind it. However, you'd really want that flat close to the sensor to minimize the loss of image quality, so this isn't a great way to solve the too-long-flange-distance adapting problem.