Mounting filters smaller than lenses

[JMC]

I recently got hold of some 25mm filters for my work looking at camera sensor sensitivity, and as I side project I thought about making a filter holder to use them with the Rayfact 105mm lens (which has a 52mm filter thread). This is what I came up with. I got a 52mm-55mm filter adapter and a 25.5mm filter holder. I then cut a thin piece of metal to fit inside the 52-55mm adapter, and a cut a hole in the middle of that so the 25.5mm filter holder could fit inside. I then JB welded it all together and painted the bare metal with some of the black paint I've used before which has very low reflectivity. Here's how it looks with a Dichroic blue filter from UQG Optics in the UK.

 

I mounted it to my Rayfact 105mm and took a picture in the garden with my Multispectral EOS 5DSR (f11, ISO400);

 

This was white balanced to 25000k and 2.326 tint in Darktable, and this is the full frame image (no cropping). The good thing is that there doesn't look to be any significant vignetting at the corners, so these smaller 1" filters will be fine for this lens (and I am hoping for my 85mm UAT too, although I have yet to check that).

 

The filter is an odd one - it's a dichroic blue filter from UQG, link here: https://www.uqgoptics.com/materials_filters_dichroic_blue.aspx

 

I got it because the spectra on the website didn't go down below 400nm and I wondered if it went into the UV at all. Here's the spectra from their website;

 

I measured it on my Ocean Optics setup, using two settings - both lamps on, and with the deuterium lamp only which results in less stray light for the UV reading, and I got this;

 

My measured spectra don't look exactly like the one on the website, especially between 500 and 700nm. Interesting that it says this on the website - WHILE EVERY ATTEMPT HAS BEEN MADE TO VERIFY THE SOURCE OF THE INFORMATION, NO RESPONSIBILITY IS ACCEPTED FOR ACCURACY OF DATA.

 

So, always check your filters. Unless there is some very odd quirk of measuring dichroic filter properties mine looks a bit different to what i though it would be. It does however let quite a bit of UV through as well as blue.

 

I'm not sharing this as a potential UV filter, but to share the concept of making smaller filter holders for the lenses. Obviously it's mainly for longer focal length and smaller maximum aperture lenses - not something for wide angles - but may make it easier to try out filters which would be too expensive in larger sizes.

[Andy Perrin]

Nobody makes a step-down to 25.5mm then?

[enricosavazzi]

That is one way of doing it. For another way that allows one to also use lens shades of different sizes than the filters, see http://savazzi.net/photography/mounting_filters.html . See also examples at http://savazzi.net/photography/coastalopt_60_hotspot.html . There are additional ways that use adapters without modifying them, for example for some of my 2" astronomy filters I use a 52 to 48 mm step-down ring at the rear of the filter, and a 48 to 52 mm step-up filter at its front to make the filter fully compatible with the rest of my 52 mm equipment.

 

I generally solve the problem of "odd" filter sizes like 25.5 filter mounts by re-mounting the filter's glass in the filter rings of cheap UV filters of more common sizes from eBay. The shoulder of these mounts is often wide enough to keep safely filters 1-2 mm smaller in diameter. If necessary a ring of paper, tape or plastic around the edge of the glass can keep the glass centered in the mount. One more thing to remember is not to overtighten the retaining ring. In some cases I epoxied a mounted filter or other optics with odd mounts into a step-up or step-down filter of standard size wide enough to contain them (but too wide to engage with the threads of the mount. Black silicone is also handy for these things, if you are extra careful not to get it on optical surfaces (it is almost impossible to clean it up if you do).

[Dmitry]

I found a cheap source of frames for thick glass. It is close-up macro lens.

[Cadmium]

A couple minor thoughts.

One could do something like this with a screw in lens cap, or a 'stacker' cap.

 

Also, one could mount the filter on the inside of the 'cap', if it fits between the inner surface of the cap and the front lens element,

then the field of view would be slightly larger than if the filter is mounted on the outside which positions it slightly further from the lens element, increasing vignetting.

[Andy Perrin]

I usually just rear mount those tiny ones with putty because it avoids some of the dichroic artifacts but that makes it hard to change filters.

[dabateman]

I have been playing a lot with filters and trying to house them. What I have learned is not all photos on ebay are correct and most of the chinese sellers don't know the English word "retaining ring".

I got a couple filters with or without metal rings. For mine which are 24.5 in diameter, They will fit a Chinese sourced 28mm uv filter. The Chinese 28mm seem to have a smaller rear window at just under 24.5, possibly 24.2mm and do have retaining rings to remove the glass.

I also tested the kenko 28mm uv filter with built in hood to allow 37mm accessories. The rear window on these is 26mm, so most of the small filters will fall through, all of mine do.

For me what I am doing is using 25mm 8 point star filters. My 330wb80Improved, is 24.5mm by 2mm thick. This was not in a metal frame. So I carefully sanded down the glass until it drops in to the 25mm filter. The opening is about 24.2mm on these star filters. For my 390bp25 filter in a metal ring, which is 4.5mm thick and 24.5mm in diameter. Instead of cracking the metal ring and droping in, as the thickness would just not allow the retaining ring to fully tighten. I instead sanded down the inner threads on the filter housing and while still hot from sanding with a dremel tool, carefully pushed the filter in place. This not only snuggly fits but the hot ring when cool down offers a cold weld. So the filter will not pop out. This has the size advantage of not being too thick.

Now the reason why I like the 25mm size, is there is a Russian ebay seller, I think Rafcamera, whom sells 52mm to 25mm step down rings. These are expensive, about $30, but have male 52mm on one side, female 52mm in the other side and a 25mm thread in the middle. So you can front or reverse mount the filters.

The smaller size filters has been easier to solve than my 50mm filters. I bought a blind lot of microscope uv bandpass filters. Since my background was in microscopy, I roughly was expecting ug1 and ug5 glass. This was there with some surprises. I got some BG12 at 1.5, 3 and 5mm thickness. Ug1 at 1 and 2mm thickness. The 0.5mm ug1 was not included. Ug5 at 1.2mm and 2.5mm thickness. BG38 at 4mm thickness and 2% and 0.2% neutral density filters. The 5mm and 4mm filters I have now mounted using a 52mm to 55mm stepup ring and Chinese 55mm uv filter housing. The retaining ring holds the glass and the stepup ring offers smaller hole so not to fall through. I will probably add 55mm to 52mm step down ring. The 3mm and 2.5mm fit in Quantaray CPL 52mm filters, which do have retaining rings and are thick enough. The thinner filters fit in Cheap Chinese 52mm filters. I was lucky to finally find ones with rings. Most just have snap in springs to hold the glass.

That has been my filter journey.