From left to right: Focus On Stars, SharpStar2, and Reveal Focus. All three go over the front of lenses to add diffraction spikes to bright stars as a visual focus aid. Credit: Alan Dyer

Each promises to make focusing at night precise and easy. Do they work? Yes. And no. 

Plus: The Focus On Stars filter works best on wide-angle lenses; the other two work great on longer lenses

Minus: SharpStar2 and Focus On Stars requires a square filter holder system, an additional expense

Summary: These three over-the-lens filters add diffraction spikes to stars to facilitate precise focusing. The two Bahtinov mask variations (the SharpStar2 and Reveal Focus) don’t work on wide-angle lenses, while the Focus On Stars filter doesn’t work well on longer lenses. 

Who Are They For? Astrophotographers shooting nightscapes and tracked starfields using camera lenses. 

While aspiring astrophotographers worry about the correct exposure settings, achieving sharp focus is often the most difficult task at night. Poor exposure can often be fixed in processing; poor focus cannot. 

These three aids are designed to take the guesswork out of focusing nightscape scenes and tracked starfields.

The usual focusing method is to use the camera’s Live View function — DSLR cameras require this be activated; mirrorless cameras always focus with Live View. You aim at a bright star or planet, then manually focus the lens to make the image as pinpoint as possible. Magnifying the Live View image by 10x (or more if the camera allows it) helps to reveal the point of best focus. 

The challenge is that with the wide-angle lenses we typically use for nightscapes and Milky Way scenes, it can be hard to tell when the tiny star image is in best focus, even in a magnified view. These devices promise to make precise focusing easy.

As you focus, the middle diffraction spike shifts. A lens is focused when the middle spike is centered, as at right. This is the Focus On Stars filter on a 24mm lens. Credit: Alan Dyer

But first, a caveat — they might not work at all with your camera. In many older DSLRs, especially budget cropped frame cameras, the Live View image is so dim it is hard to see anything at night. If that’s your situation, none of these filters is going to be of value, as they each make stars even dimmer.

There is a trial-and-error method that requires taking a succession of exposures to iteratively home in on focus. One of these filters might make that process more precise, but won’t make it any easier or quicker. 

The secret to easy, yet accurate, focusing is a bright Live View image, a requirement for all these filters to function as advertised, where the filter is typically shown focusing in real time, as I do in my video linked to below.

The Focus Aids Trio

The SharpStar2 comes from influential astrophotographer and blogger Ian Norman (LonelySpeck.com), while the Reveal Focus comes from photographer David Lane (davidlaneastrophotography.com), renowned for his nightscape panoramas.

Both filters are variations of a Bahtinov mask, a popular device for use on telescopes. Rather than use a metal grid, as most telescope masks do, Norman’s and Lane’s filters use etched Plexiglas to add a diffraction pattern on stars. A similar 100mm filter, not tested here, is available from Kase as their Night Focus Tool. 

The new made-in-Hungary Focus On Stars filter is different. Its finely etched glass is a diffraction grating that disperses the star image into a pattern of colorful spectra, but still with a trio of spikes on either side of the star. 

The Focus On Stars (left) and SharpStar2 (middle) require a user-supplied square filter mounting system. The Reveal Focus (right) can fit over a lens on its own. Credit: Alan Dyer

The SharpStar2 and Focus On Stars devices come as unmounted square filters compatible with any filter holder capable of accepting 85mm (an option only for the SharpStar), or 100mm and 150mm square filters, with the larger size better for use on ultra wide lenses with large front elements. Indeed, a 150mm filter could simply be held in front of a lens for focusing. 

The Reveal Focus filter is also square (available in 85mm, 100mm and 150mm sizes), and has three adjustable plastic posts. These make the filter awkward to store, but are sufficient for clamping it loosely over a lens without a filter holder. Remember, the filter is used only briefly when setting up a shot; it is not used to shoot through. 

The glass Focus On Stars filters each include a protective pouch; it is an option with the SharpStar2.

Focusing Tests

I found that the Bahtinov mask style of aids (the SharpStar2 and Reveal Focus) don’t work on the very lenses you are likely to have the most difficulty focusing: wide lenses under 50mm focal length. The diffraction spikes both filters create are too small and faint to be discernible, even when magnified in Live View. 

The Focus On Stars is advertised as overcoming that problem, and indeed does. Its diffraction spectra are obvious, even with a 14mm lens, making it easy to tell when the middle spike is centered. 

With a 14mm lens wide open at f/2.5 only the Focus On Stars provided a usable image to focus on. With a 24mm lens at f/1.4, the Focus On Stars worked well, while the SharpStar2 was just beginning to be usable. The Reveal Focus was not. Credit: Alan Dyer

I tested all the filters on a 14mm f/2.5 Rokinon lens, a 24mm f/1.4 Canon lens (both shown above), and on an 85mm f/1.4 Rokinon lens (shown below). All were on a Canon 6D MkII, which has a good but not very bright Live View function. 

To make things easy the target I used was Sirius, the brightest star in the night sky. I shot the Live View screen by aiming another camera at the 6D MkII, which was on a tracker. 

This compares all three filters to images taken without any focusing aid (at top). The SharpStar2 and Reveal Focus both worked well at 85mm. With the Focus On Stars the middle spike no longer shifted — it just expanded when out of focus. Credit: Alan Dyer

When testing the Focus On Stars on a modest 85mm telephoto lens, its spectral spikes were certainly obvious, but the center spike no longer moved back and forth. All the spectra simply went in and out of focus, as did the star, making focusing no more precise than with Live View on its own. You focused until images looked sharp.  Having the filter in place provided little visual assistance. This proved to be the case even when stopping the lens down, as the developer, Gabor Takacs, advised me to do. 

By contrast, the SharpStar2 and Reveal Focus now worked great on the 85mm lens, presenting obvious spikes that shifted back and forth, making it easy to focus exactly. They performed just as a traditional Bahtinov mask does on a telescope, but only when used on longer focal length lenses. 

In short, none of the focusing aid filters was a complete solution. Having to fuss with different filters and holders for different lenses might become too much of a bother. Not to mention the expense and kit bag clutter.  But if you are looking for assistance when focusing wide-angle lenses, the Focus On Stars filter will do the job. It works as advertised, for lenses under 85mm focal length. 

Buy one of the other two filters only if you need help focusing normal and telephoto lenses, where they each work very well. 

MSRP: Focus On Stars

$113 (100mm) and $154 (150mm), includes pouch

Website: https://focusonstars.com 

MSRP: Lonely Speck SharpStar2

$64 (85mm), $69 (100mm), $94 (150mm)

Website: https://www.lonelyspeck.com/sharpstar/

David Lane Reveal Focus Filter 

$40 (85mm), $45 (100mm), $85 (150mm)

Website: http://www.davelaneastrophotography.com/product-category/products/reveal-milky-way-focus-filters/  

About Alan Dyer

Alan Dyer is an astrophotographer and astronomy author based in Alberta, Canada. His website at www.amazingsky.com has galleries of his images, plus links to his product review blog posts, video tutorials, and ebooks on astrophotography.

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