A review of focus magnification aids, including two Bahtinov (mask) filter products to achieve precise infinity focus for astrophotography or nightscapes.
Obtaining a sharp infinity focus is the keystone to good astro-landscape photography. If your stars are not in focus, your shot is ruined, no matter how beautiful and well planned! I cannot tell you how many times in my early nightscape adventures that I've come back from an amazing night photography shoot only to be disappointed when I enlarged my photos on the computer screen—discovering that the stars were slightly out of focus, and that I'd never be able to make a decent size print out of my efforts.
What do out-of-focus stars look light? These two beautiful photos were taken last month by R. Craig Lefebvre in Arches National Park (left) and Dead Horse Point (right), Utah. Because the Arches image was taken with a 15mm lens, the four stars shown below Jupiter, in the Sagittarius constellation, should be about 40% smaller than the same stars shot with the 24mm lens, yet they are actually larger. And, the reason they are bigger is because they are slightly out of focus. Unfortunately, it is almost impossible to see this problem until the images are projected to 100% on a computer monitor—or by using some of the advanced techniques below. Note: This image is only very slightly out of focus. It still can be used for social media and possibly for prints up to 16x20 inches. Out of focus stars that are twice this large (even blurrier "circles of confusion") are often not detected until it is too late. In these cases, the image is worthless for any practical use.
Start with You Camera's Live View: Miss-focus still happens to me occasionally, and I continue to see and hear about this disappointment and frustration from my fellow nightscapers, especially those who are just starting out. On page 53 of my Milky Way NightScapes eBook I use three paragraphs on how to manually focus and find your true infinity with the camera's Live View function. I then end with, "Once you’ve found your lens’ true infinity, mark it for future reference, or tape down the focus with a piece of gaffer’s tape. You’ll have one less thing to worry about in the dark."
Temperature Affects Focus: That advice works most of the time, but I've found that you can't always rely on it because the tape can loosen or the focus mark become meaningless due temperature changes. Changes in temperature can cause considerable focus shifts in your lens, so it is best to redo your focus each time you go out shooting at night. And, if temperatures are considerably different than the last time you were shooting, wait several minutes for your lens to adjust to the new outside temperature.
Zoom lenses often have a different infinity focus for each focal length, so no single infinity focus setting will give you accurate results throughout the zoom range of the lens.
Additional magnification to your Live View image: As we get older, it becomes much harder to see if a star is sharply focused on a 3-inch LCD camera monitor, even when the image has been magnified to 10X. For this reason, I carry an inexpensive pair of +3.0 diopter (power) reading glasses from my local drug store. This, or a Hoodman Loupe (with the Hoodman Cinema Strap) allows for hands-free, enlarged views of your little camera monitor.
Aaron King of Photog Adventures continues to promote his favorite focusing aid: The $5 Carson Lumiloupe 10X magnifier (shown in the above middle photo). Here's his 13-minute video tutorial. I've used this method for years, and I highly recommend it!
Bahtinov mask limitations and new product solutions: The problem with most Bahtinov masks is that they work well for small telescopes and medium to long telephoto lenses on cameras, but their use with wide angle lenses that are often used in astro-landscape photography is not as decisive.
Reviewing the SharpStar2 and the Focus on Stars filters
Improvements on the Bahtinov mask design: These two commercially available Bahtinov mask "filters" attempt to provide a more accurate and easier-to-use focusing tool for nightscape photographers. Unlike the typical Bahtinov grid or mask, these are more like true photographic filters. In the case of the SharpStar2 filter, the Bahtinov mask pattern is finely etched by laser into the clear plastic filter. The Focus on Stars filter goes even further with multiple Bahtinov mask patterns contained in a film that is sandwiched between tempered glass sheets.
Is a star focusing filter really necessary? Some may think there is no need for gadgets like this to aid photographers in getting a sharp focus on their stars, considering the available focusing tools I've already mentioned. I admit, that I was one of these people (I don't need more stuff cluttering my camera bag). However, as I've helped hundreds of students in my nightscape workshops over the years and seen them struggle to get a correct focus on the stars, even with these other tools, I'm beginning to think there is a need for a better focusing tool, and one or both filters of these might be it—especially as we get older and we struggle reading the small print (the stars on your LCD screen are even smaller, even at 10X!
Consider this: Many professional photographers resisted auto focus when it first appeared on pro SLR cameras back in the 80's—now, few of us would think of being without it! When articulating LCD camera screens first appeared a few years ago on low-end digital cameras, many scoffed at these as this gimmick. Now, they are fast becoming a "necessary"' feature in professional cameras.
PRODUCT PREVIEWS...
SharpStar2 by Lonely Speck
SharpStar uses the properties of optical diffraction to create a set of three fine spike lines around a bright star. These diffraction spikes will shift position as you focus your camera. Adjusting focus so the central diffraction spike sits evenly between the other two diffraction spikes will ensure perfect focus. Once you’ve verified the focus pattern on your test shot, you can simply remove the SharpStar and shoot like normal — knowing that you have perfect focus. The diffraction spike pattern that SharpStar creates is a precise visual reference of focus success. If the central spike is off to the left, focus is too far, if it’s off to the right, focus is too near. If it’s in the center, it’s just right. Here's a 3-minute video tutorial.
Lonely Speck’s SharpStar2 is made of precision laser etched optical plastic and comes in a square filter shape, made to fit into the most popular square filter systems. SharpStar2 requires a square filter holder and the appropriately-sized system adapter ring to fit your lens (both items sold separately). The SharpStar2 works with the Lee, Cokin, Formatt-Hitech and other similar filter holders that come in 85mm, 100mm and 150mm sizes. If you don’t already have a filter system, Lonely Speck personally recommends the Formatt Hitech system because of its price and its modular design that works well with wide angle lenses. October 9 update from Lonely Speck: "The SharpStar2 is made in the USA. We offer a 100% satisfaction guarantee (refund or replacement if the customer isn't satisfied for any reason). All they need to do is email us." ~ Ian and Diana
PRICING (filter only - filter holder not included):
- 85mm $64.00
- 100mm $69.00
- 150mm $94.00
Filter Pouches are optional (soft-lined semi-rigid pouch with magnetic closure)
~ 85mm $12.00 • 100mm $14.00 • 150mm $20.00 ~
The Focus On Stars high precision device fits into standard size square filter holders available on the market. At first glance, it looks like a filter.
Focus On Stars developer, Gabor Takacs, claims that his filter works better with the wide angle lenses most commonly used in astro landscape photography: "During my research, I found a promising device that the manufacturer recommended also for lenses with shorter focal length. It is a plastic sheet in which the grid lines of the Bahtinov mask known from astronomy were densely etched with laser. I have tested it. Using wide-angle lenses common in astro landscape photography, I have found that this device gives me a diffraction pattern that is too faint and too small to ensure a sharp focus. ...After more than a year of research and development, I develop a device that works well even with ultra-wide-angle lenses."
PRICING (filter only - filter holder not included):
- 100mm $113.00
- 150mm $154.00
Each filter comes with a protective, hand-sewn pouch with velcro closure :
My OUT OF THE BOX Experience
SharpStar2
I ordered the 100mm SharpStar2 for $69.00, with economy shipping from California (U.S.A.) to Utah (U.S.A.) for a cost of $3. I received a notification the next day that the product was out of stock. Two days later, the product was available and I received another notification that it was shipping. The package arrived three days later, and included a filter pouch (a $14 value) that I didn't order. When I questioned (via email) the inclusion of the pouch I didn't order, they said it was free because my order had been delayed due to stocking problems (nice gesture!). Total cost for filter with shipping to the U.S.A. was $72. However, if I lived in Australia, my USPS Priority Mail Express International shipping cost would be $82.75, for a total cost of $151.75.
All-in-all, the products were just as described, and nicely packaged. The filter was enclosed in a clear plastic envelope (blue product description printed on the outside), which could be used a storage pouch for a while, but the real filter pouch (made of suede leatherette material), with its magnetic enclosure, was much nicer and more substantial. The filter seems to be finely etched with the Bahtinov mask pattern into a sheet of clear plastic.
Focus On Stars
I ordered the 100mm Focus On Stars for $113.00, with DHL Express shipping from Hungary to Utah (U.S.A.) for a cost of $32. The FOS filter comes with its own padded pouch. I received a notification the next day that the product was shipping. The package arrived four days later. Total cost for filter with pouch and shipping to the U.S.A. was $145 — about twice as much as the SharpStar2 (however, you do get a pouch include). I noticed that high international shipping cost outside of the United States for the SharpStar2 tend to raise its total costs closer to that of the FOS filter. For instance, if I lived in Australia, my DHL Express shipping cost for the FOS filter would be $42, for a total cost of $155 (making it only about $2 more in total cost than the SharpStar2).
All-in-all, the FOS filter and pouch were just as described, and nicely packaged. Unlike the SharpStar2, the Focus On Stars filter is made of tempered Gorilla 3 Glass, not plastic, and claims to be very scratch resistant. It was also much easier to clean off my fingerprints and dust, due to a special coating that is applied to the glass.
150mm versions of SharpStar2 and Focus On Stars
I was also able to obtain the larger 150mm versions of these two filters, which I've hung on a clothes line for a visual comparison so you can quickly see the density differences of these two filters. Because of its complex mosaic pattern, the FOS filter is about one to two stops darker than the SharpStar2 (about 0.3 to 0.6 optical density). Will this extra density hinder in your ability to see its star focusing pattern at night, when compared to the more transparent SharpStar2 filter? It didn't seem to be a problem in my field tests (see below).
Actual Field Tests
Preparation
In order to use the SharpStar2 or the Focus On Stars filters, you need to have a filter holder. There are filter holder systems for 85mm, 100mm and 150mm square filters. The most common square filter size is the 100mm. I use the popular LEE100 Filter Holder. You'll also need a lens adapter ring. Many of my Canon and Rokinon astro lens take a 77mm filter size, so I use LEE's 77mm Wide-Angle Lens Adapter Ring for the 100mm System Holder (here are links for 82mm, 72mm, 67mm, 62mm, 58mm and 55mm filter sizes). The Formatt Hitech 100mm Aluminum Modular Filter Holder is also a good system, and is slightly less expensive than the LEE system.
Unless you already have a Cokin P or Formatt-Hitech filter holder system for 85mm square filters, I would avoid this system size, since it has limitations for the more serious amateur or professional photographer; and the Focus On Stars filter is also not available in this size.
Ultra wide-angle lenses in the 14mm, 15mm and 16mm focal lengths typically do not have filter threads because of their huge front elements. These lenses usually need 150mm square filters and special square filter holder systems like the LEE Filters SW150 Mark II Filter System Holder. Another popular solution for super-wide lenses is the Haida 150mm Filter Holder (which is often backordered). Here are Haida holders for the Tamron 15-30mm, Nikon 14-24mm and the Samyang-Rokinon 14mm.
Focusing Procedures with the Filters
The following steps were used in the field with both the SharpStar2 and Focus On Stars filters:
- Turned off the autofocus and rotated the lens focusing ring to the infinity mark.
- With the camera on a tripod, pointed it at the brightest star in the sky, so that it was in the center of the camera's field of view. The brightest "star" was often the planet Jupiter.
- Turn on the camera's "live view" image and magnify the star to the maximum.
- Insert the focusing filter into the filter holder.
- Manually adjust the focus to cause the middle diffraction spike or diffraction spots (in the case of the Focus on Stars filter) to center between the outer spikes or spots. The symmetry of the pattern ensures perfect focus.
- Take a "confirmation" test shot and review the image under magnification.
- Make focus adjustments until perfect pattern symmetry is achieved.
- When the center spikes or spots are evenly centered, remove the focusing filter and point the camera optics at the scene you want to capture.
These steps worked fine with my 70mm, 50mm, 35mm and 24mm lenses; however, with the 15mm and 14mm ultra wide-angle lenses, even the brightest stars were so small, I had to take these additional steps to get the best pattern symmetry:
- I often had to rotate the camera a few degrees horizontally on the tripod head until I saw at least two pair of spikes or two rows of dots.
- I then had to arbitrarily slide the focusing filter, in the holder, up and down until all three pairs of spikes or spots became visible.
Using these additional steps, with the 14mm and 15mm lenses, gave me beautiful diffraction pattern symmetry and perfect focus with the Focus On Stars filter. Seeing and achieving a good diffraction symmetry pattern with the SharpStar2 filter was much more difficult when using ultra wide-angle lenses, as you'll see in the following images.
Field Test Comparisons
Both the SharpStar2 and the Focus On Stars filters produced excellent symmetrical diffraction focusing patterns when using a 70mm lens (a 24-70mm zoom @ 70mm). They both produced good patterns using a 50mm lens (a Samyang/Rokinon 50mm f/1.4). The colorful Focus On Stars pattern is more distinct to my eyes, and easier to judge when it has reached symmetry (perfect focus). However, both products performed well in the 35mm through 70mm range.
Using a 24mm lens (Samyang/Rokinon 24mm f/1.4), symmetry was much easier to see with the Focus on Stars filter than the SharpStar2; however, the SharpStar2 was acceptable as long as you used a very bright star, i.e. the planet Jupiter, and you used a loupe to help you magnify the camera's LCD screen.
The Focus On Stars easily out performed the SharpStar2 when using an ultra wide-angle 14mm lens (Samyang/Rokinon 14mm f/2.8). Even with a 10-power loupe to magnify the back of the camera's LCD screen (already magnified to 10X), it was difficult to see when there was spike pattern symmetry using the SharpStar2. Several image confirmations and slight focus adjustments had to be made in order to get the best pattern. I had virtually the same frustrating results using a 15mm ultra wide-angle lens (Irix 15mm f/2.5).
You'll notice that with the same confirmation exposure, the Focus On Stars image is about a stop darker than the SharpStar2 image. As I mentioned earlier, the Focus On Stars filter is darker due to its complex mosaic pattern. I didn't find that this extra density hindered being able to see or obtain a good symmetrical focusing pattern. Besides, the filter is removed before starting your creative astro photography.
A Work-Around Method to Improve Your Results
The brighter the star (or planet) you use for your Live View focus, the better will be your results with both filters, especially when using the SharpStar2. Even though the SharpStar2 performed poorly with ultra wide-angle lenses like the 14mm, I discovered that using a brighter, man-made "star," such as a Lume Cube 2.0 light, allowed me to produce more acceptable results with the SharpStar2.
My procedure was to place the Lume Cube light about 100 paces from the camera (and aim the light back towards the camera). You can place the light on a rock or use a light stand. This creates a distance of at least 200 feet (61 meters) from the lens—more than enough for any wide angle lenses to optically be at an "infinity" distance. I use a diffuser filter over the light, so that it is no longer a focused light, and I reduce its intensity down to about 10% (go higher, if you still have trouble getting a good focusing pattern).
Because this artificial, man-made "star" is such a higher "magnitude" than real stars, your exposure will only need to be a few seconds, using a low ISO of about 200. This exposure will typically give you greater contrast (a darker background) than real stars in the sky, which should also produce a larger and crisper focusing pattern, even with the Focus On Stars filter—albeit this extra effort is something I find is rarely needed with the FOS filter.
My Conclusions
It may seem ridiculous to spend an average of $108 for a focusing aid (and even more if you don't already have a square filter holder system), but missing one great Milky Way shot due to inaccurate focus will cure you of this opinion, especially when you consider the travel costs you spent to get to a great location.
SharpStar2
PROS: Less expensive than the Focus On Stars filter. A good focusing aid for all lenses up through as wide as 24mm (or 16mm when using an APS-C cropped sensor).
CONS: It is difficult to achieve good results with ultra wide-angle lenses i.e. the 14mm and 15mm lenses (I suspect that 20mm and 18mm lens users will also have some difficulties). Plastic construction makes the filter more susceptible to scratches, and is harder to keep clean. A protective filter pouch will cost you an additional $12 to $20.
Focus On Stars
PROS: It is easy to obtain a decisive symmetrical focusing pattern with this focusing filter (apparently due to its more complex mosaic mask), especially with ultra wide-angle lenses in the 14-16mm range. Construction quality is excellent. The filter pattern is sandwiched and sealed between tempered Gorilla 3 glass, making it easier to resist scratches and to clean. A protective filter pouch is included.
CONS: More expensive than the SharpStar2 filter (U.S.A. purchasers will spend almost twice as much for the FOS, with shipping costs, than buyers of the SS2); however, pricing becomes closer for many international purchasers due to less expensive shipping costs (Australian FOS purchasers will pay almost the same total price as SS2 purchasers and European FOS buyers will actually spend less). The inclusion of filter pouch also helps to even out the price difference.
Disclosure: I am NOT receiving any income from the sales of the SharpStar2 or Focus On Stars filters. The links to the LEE filter holder systems are being monetized through B&H. Other than that, this blog post is purely for educational purposes.
Excellent thorough review - many thanks!
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