November 13, 2024 -> Full Review was published.

***Reading Time: 26 minutes***

Intro

One of the most commonly used lens types for starscape astrophotography is the ultra-wide-angle lens. This type of lens allows the photographer not only to capture an impressive night scene along with the vast, beautiful sky but also extends the shutter speed times due to its wide field of view. (By the way, you can download a free NPF rule calculator in Excel format here.)

However, most of these lenses, particularly those with bright apertures (i.e., f/2.8, f/1.8), have a bulbous front element, which prevents the attachment of standard filters.

The Kenko Rear Prosofton filter is a recent addition to Kenko’s soft-focus lineup, specifically designed for astrophotography. Unlike traditional front-mounted filters, this filter attaches to the rear mount of the lens.
In this review, I’ll share my thoughts and experiences while testing the Kenko Rear Prosofton Filter set.

Why Use a Softening Filter in Astrophotography?

While it may seem unusual to use a softening filter for astrophotography—since maximum sharpness and detail are typically prioritized—these filters can be used intentionally to create artistic effects:

• Enhancing Star Prominence: Softening filters add a gentle glow around bright stars, making them appear more prominent and emphasizing certain celestial objects.
• Highlighting Nebulosity: For wide-field astrophotography, softening filters can accentuate faint nebulous areas, making interstellar clouds more visible.
• Creating an Ethereal Effect: The subtle diffusion effect adds a dreamy, painterly quality to images, softening the harshness of pinpoint stars.
• Balancing High-Contrast Scenes: Softening filters help smooth transitions between high-contrast elements, like the bright moon or illuminated landscapes.
• Reducing Comatic Aberration: These filters can also help reduce coma (comatic aberration) near the edges of the frame, a common issue with ultra-wide-angle lenses.

Kenko Rear Prosofton Filter Set

The Rear Prosofton filter set includes three (3) filter sheets made of polyester material, with different softening strengths: weak (050), medium (100), and strong (150). Each sheet measures 57x60mm and has a thickness of 0.18mm.

Softening Effect: Weak (050)

Softening Effect: Middle (100)

Softening Effect: Strong (150)

Advantages of Rear Mounting

Attaching the filter to the rear of the lens provides several benefits:

• Minimized Vignetting: The rear placement reduces the risk of vignetting, especially on ultra-wide-angle lenses.
• Compatibility with Bulbous Front Elements: It allows the use of filters on lenses that don’t support front filters.
• Protection from Flare and Damage: The rear position protects the filter from accidental damage and reduces flare.

Testing Notes

The camera used in this review was the Nikon D750 with the Tokina opera 16-28mm F2.8 F-Mount lens.

*Disclaimer*

I’ve been a Global Ambassador for Tokina lenses since 2017; however, this is not a sponsored post.
The opinions and experiences I share about the Kenko Rear Prosofton Filters are entirely my own.

How to attach (DIY)

Basically, the user has to cut the filter with a cutter or scissors, based on the size of the lens mount that wishes to use it  and then attach it to the back of the lens.

If the lens offers a dedicated filter holder on the lens mount (very few lenses offers this option these days) it can be attached there, or in all other cases, use a double-sided tape to attach the filter sheet on the rear element of the lens.

Easy? Well, yes… and no. Allow me to explain.

First of all, it is very important to note that this product is very sensitive. It is very easy to scratch, so please handle it with care.
It is highly advised to use gloves when cutting and/or attaching the filter to the lens.

From my experience, I found that the easiest way to cut the filter in the proper size for your lens is to use a pen to make some markings while having the filter inside its protective case, like in the shot below.

I just placed the filter on top of the rear mount, with its plastic protective case, and made some rough markings with a pen above the rear element.
Obviously, it will require some trimming after that, but basically with this way you are like half way done.

And now that you have successfully cut the filter in the proper size, its time to attach it.
Well, here is the difficult part because you have to find a material that can hold the filter in its place without blocking the mount or the electric contacts.

I used paper tape since it doesn’t leave any glue behind, its strong enough to hold the filter in place and relatively easy to cut it properly, while at the same time it is so easy to remove it when needed.

User Experience Out in the Field

Just a few days ago, I returned from my autumn Landscape and Astrophotography Workshop in the Tzoumerka mountains of Greece.
I had the opportunity to properly use the filters out in the field and form an opinion, because in the safe environment of the lab everything look great.

Basically my biggest issue was not the performance or the optical quality, but the attaching procedure. I simply could not attach or change the filter while out in the field, because it was too dark, windy and cold.
For some this might not be the case, however and although I’m a very experienced and skilled (in technical terms) user, I think that it is better to have the filter attached to the lens before going out in the field, and simply stay with the selected filter for the whole shooting, or at least until you get in a well lit environment, with no wind or cold.

Optical Performance

I’m familiar with the quality of Kenko products, and the Rear Prosofton doesn’t fall short on this aspect.
I couldn’t find any artifacts or aberrations with this filter, or even vignetting. Of course the most important thing is the filter to be attached to the lens properly and not having any visible “gaps” that would create issues.

Examples

Softening Effect: Weak (050)

EXIF: F2.8 | 15sec | ISO 3200 @ 16mm
Nikon D750 & Tokina opera 16-28mm

Softening Effect: Middle (100)

EXIF: F2.8 | 15sec | ISO 3200 @ 16mm
Nikon D750 & Tokina opera 16-28mm

Softening Effect: Strong (150)

EXIF: F2.8 | 20sec | ISO 3200 @ 16mm
Nikon D750 & Tokina opera 16-28mm

Comparison between Weak (050) and Strong (150)

Personal Thoughts

As a user of ultra wide angle lenses in the majority of my photography career, it is good to see filters coming in this form.
Ideally I would love the filters to have another mechanism of attachment to the lens, and also to be less sensitive to scratches, but I suspect that if that was the case, then it would be way more expensive and also it wouldn’t be so easy to cut with a simple scissor.

I really enjoyed the effect of the low strength, as it is very subtle and can create interesting shots, while I found the strong effect “too strong” for my taste, but that is totally subjective to each one.

I will definitely use this filter on some occasions, especially when I want to isolate specific constellations in the night sky.

Conclusion

“The Kenko Rear Prosofton filter is a valuable tool for astrophotographers looking to add a soft, artistic touch to their images. While installation can be challenging, the optical performance and creative effects make it worth the effort.”