Reflex lenses for Astrophotography

Lenses that can collect light are, to be more precise, convex lenses. However, it is not only convex lenses that can collect light; concave mirrors can also achieve this. Most photographic lenses in the world are based on the application of convex lenses, and in actual photography, multiple lenses are combined to correct aberrations and fulfill the role of convex lenses.

Many people might be aware that during the film era, reflex-type lenses were released by various camera and lens manufacturers. Let's revise the reflex lenses that use concave mirrors.

Reflex lenses are particularly suited for super tele photography and have several major features:

• Very compact: Since light passes through the barrel three times in reflex lenses, their overall length is very short, making them lighter and easier for handheld shooting.
• Budget-friendly: Examining the cost of convex lenses and concave mirrors, as the size increases, concave mirrors can be manufactured at a lower cost, making it possible to produce them at a lower price.
• Very little chromatic aberration: Chromatic aberration, a major enemy of telephoto lenses, can be minimized. While a telescope purely made of mirrors has zero chromatic aberration, photographic lenses combine lenses to balance out aberrations, so a slight amount of chromatic aberration still occurs.

On the other hand, the drawbacks include:

• No diaphragm: Due to its structure, incorporating a diaphragm isn't feasible, so most reflex lenses have a fixed aperture, and light is adjusted using ND filters.
• Donut-shaped bokeh: The secondary mirror obstructs the light, causing the bokeh to be donut-shaped. This donut-shaped blur often divides opinion and is frequently disliked.
• Darker aperture: Also due to the presence of the secondary mirror, the actual brightness is darker than the numerical F-value, leading to increased exposure compensation.

In the past, when fluorite glass and ED glass were not common, super telephoto lenses inevitably had chromatic aberration. Therefore, having very little chromatic aberration and being compact were very attractive features, and many manufacturers included reflex lenses in their lineups. However, today, these advantages have diminished and the drawbacks have become more apparent, leading to reflex lenses becoming less common.

Taking the SZ 600mm PRO Reflex F8 MF CF to the United States

In this context, the Tokina reflex lens lineup, which has made a comeback, refocuses attention on compactness and low cost, filling a gap that camera manufacturers haven't addressed. After using it, I found it indeed very compact and felt I could take it anywhere without hesitation. I took the SZ 600mm PRO Reflex F8 MF CF on a trip last year to photograph the annular solar eclipse that occurred in North America on October 14th (October 15th in Japan). My primary setup was a combination of a telescope and a dedicated Hα filter for solar photography, and the compactness of this lens made it easy to carry along. Moreover, for astronomical photography, there's no issue with out-of-focus or blurred rings, and there's no need to stop down the lens. Even though it's manual focus only, there's no inconvenience, as manual focus is typical for telescope photography. Furthermore, since all astronomy enthusiasts are familiar with reflex telescopes, reflex lenses are welcome.

Going even further by photographing the Moon, nebulae, and star clusters

Pleased with the results from photographing the annular solar eclipse, I decided to try photographing the Moon, nebulae, and star clusters after returning to Japan. The compactness is a major advantage, allowing me to use a smaller equatorial mount for both the camera and lens. However, with a focal length of 600mm (equivalent to 900mm with the Fujifilm X-T4), a small mount can still suffer from vibrations in strong winds and requires high precision in polar alignment and tracking for long exposures. Ideally, a large, high-precision (and very expensive) mount would be best, but a small portable equatorial mount can still be practical with careful use. Specifically, by significantly increasing the camera's ISO sensitivity and taking multiple exposures just enough to prevent star trails, you can later combine these images with software to achieve high quality. This technique of enhancing image quality by combining multiple shots has been used in astrophotography since the film era. Moreover, with a super-telephoto setup equivalent to 900mm, precise composition and focus are critical, and even a slight touch of the focus ring can cause the subject to shift, making it challenging to focus. However, astronomy enthusiasts are accustomed to these challenges with telescopes, so with practice, you can master these techniques.

Thus, while reflex lenses have their drawbacks, they work exceptionally well for astrophotography without many of their downsides being noticeable. For those who only use autofocus lenses regularly, using a reflex lens can offer a unique experience combining ease of use with precision. I'm considering using the lens not only for astrophotography but also for other genres like bird and airplane photography. I believe that for those used to modern autofocus lenses, the experience of using a reflex lens will be quite invigorating, so I encourage everyone to give it a try and enjoy super-telephoto photography.

Hoping to see a brighter reflex lens

Lastly, reflex lenses used to come in brighter versions like F4 or F5. While the compact F8 reflex lens is certainly good, as an astronomy enthusiast, I find brighter lenses more appealing. I hope Tokina will consider developing a brighter, low-cost reflex lens.

Currently, with the F8 model, faint nebulae and star clusters cannot be seen through the viewfinder or monitor, so you need to locate bright stars to guide the setup or use an automatic goto feature if your equatorial mount has one. For beginners, I particularly recommend using a slightly larger equatorial mount with an automatic goto system.

Reflex lens light path diagram

This is a diagram from Kenko Tokina's website, showing how light passes through the mirror barrel three times in a reflex lens, making it very compact. Thanks to this compact design and minimal chromatic aberration, reflex lenses are well-suited for super telephoto use. However, this mirror system is not ideal for lenses with focal lengths under 200mm due to difficulty in widening the field of view.

Comparison of reflex lens and telescope with nearly identical specifications

Here is a comparison of a reflex lens and a telescope with nearly the same aperture and focal length. You can clearly tell how compact the reflex lens is. Despite having the same F-value on paper, the telescope is actually brighter and has higher resolution on the optical axis.

Tokina SZ 600mm PRO Reflex F8 MF CF and a 50mm F1.2 lens

This shows the Tokina SZ 600mm PRO Reflex F8 MF CF compared to the 50mm F1.2 standard lens. While the 50mm F1.2 is quite large for a standard lens, the reflex lens is surprisingly slimmer and lighter. This makes it easy to take anywhere without hesitation.

Equipment I used to photograph the annular solar eclipse

This is the equipment I used for photographing the annular solar eclipse in North America on October 14th last year, in New Mexico. The telescope with an Hα filter shown next to it has an aperture of 60mm and a focal length of 900mm. This comparison highlights just how compact the reflex lens, which has the same 900mm equivalent focal length, is.

The annular solar eclipse as seen through the Tokina SZ 600mm PRO Reflex F8 MF CF

I took photographs at approximately 5-minute intervals from the start to the end of the annular solar eclipse, with a few shots taken around the annular phase included here. The lens delivers sharp images with no noticeable chromatic aberration, though focusing requires a fair amount of precision. The images shown have been cropped to an equivalent focal length of 1800mm.

I used a D5 filter (ND filter with a 100,000x exposure multiplier).

Fujifilm X-T4, ISO200, shutter speed 1/250 sec.

Example of Tokina SZ 600mm PRO Reflex F8 MF CF on a portable equatorial mount

For astrophotography at an equivalent focal length of 900mm, an equatorial mount is necessary rather than a regular camera tripod. Ideally, a large, high-precision equatorial mount would be used, but a small portable mount like the Kenko SkyMemo S can also be practical with some adjustments. Using optional accessories like micro-adjustment platforms, balance weights, and compact wedges makes polar alignment and composition easier. Choosing days with little wind is also key to successful shooting.

A 4-day-old Moon as seen through the Tokina SZ 600mm PRO Reflex F8 MF CF

The Moon with earthshine was beautifully visible in the evening sky, so I decided to take some test shots. When exposing for the bright part of the Moon, the earthshine did not appear at all, while exposing enough to capture the earthshine resulted in overexposure of the Moon. Accordingly, I used bracketed exposures and HDR processing to approximate the image seen through the telescope. Some stars are also visible near the Moon.

Tokina SZ series

In addition to the 600mm model I used, the SZ series includes an impressively compact 300mm model and an even longer 900mm model. If you set aside preconceived notions about reflex lenses and give them a try, you're sure to have a novel shooting experience.

Example of an equatorial mount with automatic goto

Here's an example of a slightly larger equatorial mount with an automatic goto system. This is the "NEW Sky Explorer SE II-J" model by Kenko. Although it is somewhat expensive, this class of equatorial mount provides reliable performance. It is also possible to attach an auto-guider for accurate tracking.

Examples of nebulae and star clusters

Open star cluster in Taurus, M45 (Pleiades)

Feeling encouraged by the annular solar eclipse, I decided to photograph the Pleiades star cluster, known as "Subaru" in Japanese. While Subaru is well-known from songs and car names, it also refers to this stunning open star cluster surrounded by a blue nebula. Although F8 is a bit dim for photographing nebulae and star clusters, I performed a series of 1-minute exposures totaling 30 minutes, followed by additive averaging processing for high image quality.

Spiral galaxy in Canes Venatici, M51

This is an image of the M51 spiral galaxy in Canes Venatici, also known as the "Whirlpool Galaxy." Spiral galaxies are often faint, and face-on galaxies, where the spiral structure is oriented towards Earth, are especially faint. Therefore, I increased the contrast through retouching.

Globular cluster in Hercules, M13

This is the brightest and largest globular cluster in the northern sky. Globular clusters are collections of stars, making them relatively easy to photograph. However, they are not identifiable in the camera's viewfinder, so it is helpful to locate them from nearby stars or rely on the equatorial mount's automatic goto function.

Emission nebula in Sagittarius, Lagoon Nebula (M8)

The Lagoon Nebula (M8) is a bright emission nebula that is visible to the naked eye and represents summer's prominent nebulae. However, due to its low altitude at the meridian, it's prone to light pollution and atmospheric transparency issues. It's generally better to photograph it before dawn in the spring. By the way, Fujifilm cameras are known for capturing red emission nebulae well.

About the author

Akio Nakanishi

• Astrophotographer and engineer of low-light imaging equipment.
• Leading Japanese astrophotographer and engineer of low-light imaging equipment.
• President of Nakanishi Image Lab Ltd.

Born and raised in Itabashi, Tokyo, a town of optics and printing in 1964, the year of the Tokyo Olympics. He became interested in space when he was in the fifth grade of elementary school, and started astrophotography in the first grade of junior high school, developing and printing black-and-white photographs. During his engineering school days, he worked as a commercial photographer's assistant and a bridal photographer for three years, while learning photography other than astrophotography.