What is a tube lens…

… you may be wondering. Then again, probably not! If you are reading an article on my website, you probably know more about relay lenses than I do. By the way, a couple of the links in this article are affiliate links and, as an Amazon Affiliate, I may make a small commission from qualified sales through these links - just so you know!

A tube lens is an intermediary optical element that is required to focus the flux of parallel rays that is formed as light passes through an infinity corrected microscope objective. Without a tube lens it would not be possible to form an image, either on the eyepiece of a microscope, or the sensor of your camera.

There are a number of ways that light traveling through the “infinity space” of an extreme macro photography setup can be captured and focused onto the sensor of the camera. You can use a camera lens, a closeup filter, or even a dedicated tube lens from a microscope. But whichever lens you select, it will need to have a focal length close to that of the tube lens in the microscope for which your objective was designed. Some companies make objectives that require focal lengths that are different (Zeiss uses 165mm and Olympus 180mm, for example) from the 200mm used by Nikon, Mitutoyo, and Leica. It just so happens that the very best objectives for photography all use the 200mm reference focal length.

I plan to test two closeup filters as tube lenses, even though one of them actually has a focal length of 208mm. That lens is the famous Raynox DCR-150. It is a very good closeup lens, though not the best, and it is tricky to use given its small size, plastic body, and clunky mounting adapter. But optically, it is a superb three-element optic that has long been prized as an amazingly sharp, and extremely affordable relay lens for extreme macro photography.

Raynox actually makes several closeup lenses, including the more powerful DCR-250, which has a shorter focal length of 135mm, and allows you to capture incredibly crisp 5X photographs when it is used with a 10X infinity corrected objective. Every serious macro photographer has at least one of each somewhere in the studio.

Today, though, we are going to be testing a relative newcomer to the closeup filter lineup. This lens is a very nice closeup lens and I was quite impressed with its performance when I reviewed it a few weeks ago. It is a +5 diopter lens with a focal length of 200mm. It is very solidly built, all metal and glass, and is considerably larger in diameter than the Raynox. It claims to be apochromatically corrected, despite only having two glass elements in its construction. You would normally expect to see at least three elements in a true apochromat lens. But that is something that we will be looking at in this test. If you are looking for a high-quality closeup filter/lens for your macro, you will be very happy with this one…

But how will this lens perform as a tube lens, and up against the tried and true Raynox DCR-150? Well to find out, I encourage you to watch this video. I go through the entire testing procedure and explain in detail what we are looking for and how the test images need to be captured to yield a realistic comparison of the two. Everything else I am getting ready to say will be a spoiler for the video, but if you want to get straight to the results, read on…

In a nutshell, the Raynox DCR-150, once again proved itself to be one of the best tube lenses on the market. When you consider the price ($60!), it is really unbeatable. And it was not beaten by the NiSi 58mm size closeup lens, I’m afraid to say. I had high hopes for the NiSi after some promising early trials using it as a relay lens and while some of the test images were perfectly respectable, it failed to keep up with the sharp, virtually aberration free images from the DCR-150.

In the image shown above, taken with the NiSi at 200mm, there is clear evidence of field curvature. The center of the image is quite sharp, but the image softens dramatically as you move away from the center. I recognize that my setup was not ideal for field curvature testing as it was impossible to get my test subjects completely perpendicular to my lens axis, and while that explains the out of focus areas on one side of the image, it can’t explain the softness on the opposite side. It is not possible to determine just how much curvature is introduced by the lens, but it is definitely present. Close inspection of the entire image will reveal the longitudinal and lateral chromatic aberrations that this lens produced in most of the test images. By comparison, the Raynox also shows some very slight field curvature but no chromatic aberrations were visible when viewed at 100%. To be fair, there were some very faint aberrations in the out of focus areas, but they could only be appreciated at 300%+.

Check out the video to see examples from all the test setups.

I would point out that while most of the NiSi images had at least some chromatic aberration, much of it could be reduced or eliminated by the CA removal tool in Lightroom. This is an important point to consider, as this simple processing step can make a huge difference in your images. Unfortunately, the lack of sharpness was not as easy to correct, even with aggressive sharpening of the RAW images.

In closing, I intend to do some more testing of the NiSi, specifically to find out if there is better short-focusing extension than the rather arbitrary 144mm I chose for this test. If I find anything to change my overall impression of this lens, you will be the first to know!