New Lens aberration/ focusing quality test question

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WhistlerNorth Regular Member • Posts: 444
New Lens aberration/ focusing quality test question

BACK IN DECEMBER I GOT THE TAMRON 17-28MM LENS AND A FELLOW POSTER SUGGESTED TO DO THIS TEST FROM LONELY SPECK ASTRO WEBSITE TO SEE IF LENS WAS CONSISTENT.  I AM GETTING THE TAMRON 28-200 LENS LATER THIS WEEK AND LEAVING SATURDAY FOR A WILDLIFE TRIP.  A FEW PEOPLE ON THE FORUM HAVE  GOTTEN A POOR QUALITY  EDITION FOR FOCUSING AND HAD TO RETURN FOR ANOTHER.  WONDERING IF THIS TEST WOULD SHOW QUICKLY IF I HAVE A PROBELM AS HAVE A SHORT WINDOW TO FIGURE THIS OUT.  ADVICE WELCOME. BELOW IS LINK AND INFO ABOUT THE TEST.

https://www.lonelyspeck.com/a-practical-guide-to-lens-aberrations-and-the-lonely-speck-aberration-test/

A Practical Test Method

For astrophotography, aberrations can be a distracting element of the photo. I usually gauge the quality of a lens for use with astrophotography by its aberration performance at low f/numbers.

I’ve been working on an easy method to test all of my future lenses for their aberration performance. I wanted to make the test reasonably easy to understand and provide a good visual representation and success criteria for judging lens aberration performance. The goal of the test is to allow photographers to understand how one lens performs versus another and to help define the threshold of aperture setting (f/number) beyond which the lens performs acceptably.

Now I like to test my lenses by using them for real-world photography, so I originally thought I could just make my tests by shooting the stars with each lens and then showing the results. But this presents a lot of variability, dealing with light pollution, the moon cycle and the changes of the night sky throughout the calendar year. For my aberration test, I instead decided to make a simple test chart for my computer screen (.zip). It’s simply a black image with a square grid of 360 white dots. It can be used on LCD screens with a resolution of at least 1440px by 900px or higher.

By photographing this image on my computer screen with a lens at various apertures, it’s possible to see and measure the aberration characteristics of the lens at those aperture settings. Every lens is tested by making a photo perpendicular to the screen such that the screen occupies the upper right quadrant of the image. As long as we’re reasonably consistent with framing, the image size of the dots should be roughly the same size relative to the sensor size, regardless of the lens or camera system used, which gives us a good way to compare the aberration shapes of one lens to another and across multiple systems. As an example, I have samples made with a Canon EF 50mm f/1.8 STM lens which can be used on either a full-frame camera or an APS-C camera. Two sets of data are collected: one at each aperture framed with a full frame sensor and another set framed on an APS-C sensor (or via APS-C crop mode). If testing on another sensor size like Micro 4/3″ or 1″ sensors, the test would be performed identically.

Example Full Frame Aberration Test – Canon EF 50mm f/1.8 STM lensExample APS-C Aberration Test – Canon EF 50mm f/1.8 STM lens

Key of Common Aberrations

One of the cool things about this test is that it allows us to see, very obviously, the kind of aberration the lens is producing. I’ve made a key with our most common aberrations to help us define the aberration. In real life, we’ll often encounter slightly different shapes (or a combination of them), but this key is fairly representative of the pure aberration shapes, just as I described earlier in this article.

The Lonely Speck Aberration Test

I wanted a way to quantitatively compare lens performance so I thought of making a simple scale for measuring the aberration: measuring the width of the aberration — Adobe Photoshop has a simple measurement tool — at its longest point (in pixels) and comparing that dimension as a percentage of the height of the image (4000 pixels in the case of the Sony a7II).

So for example, if an aberration measures at 62 pixels wide, the resulting aberration level is: (62/4000)*100% which is approximately 1.5%.

Just from my testing of how I like to use my lenses, an aberration level of 0.4% or less is considered “excellent,” less than 1% is “acceptable,” while aberration levels of 1% or higher is considered “poor” performance. It’s kind of a coarse and arbitrary scale, but it gives us a way to compare lenses on pretty much any camera system. APS-C corners, which tend to show lower aberration levels anyway, are always measured relative to the height of the APS-C frame. If the test were performed on even smaller camera systems, like Micro 4/3 or 1″, they would be measured relative to the height of those respective images. For some visual reference, the dimensions of each square crop in the test below is exactly 2mm by 2mm as measured on a full frame camera’s sensor and 1.33mm as measured on an APS-C sensor.

In this example using the Canon EF 50mm f/1.8 STM (Amazon / B&H), the lens shows very poor performance at f/1.8 in the corner of the full frame image with aberration levels of 1.5% but reaches a very good level of 0.5% by f/2.8, which is consistent with how I’ve tended to use the lens for astrophotography. We can also see by the shape of the aberration that the lens has a complex mix of coma at the APS-C edge that transitions into distinct sagittal astigmatism by the full frame edge.

Usually results like this tend to correspond well to the sharpness of a lens. In the case of the 50mm f/1.8 STM, it does tend to produce lower sharpness images at f/1.8 and gets a lot sharper by f/2.8 — consistent with our test.

Canon EF 50mm f/1.8 STM Aberration Test @ f/1.8Canon EF 50mm f/1.8 STM Aberration Test @ f/2Canon EF 50mm f/1.8 STM Aberration Test @ f/2.8

Now some technical notes: this test isn’t perfect.

Some lenses, particularly those with floating lens elements (usually zooms and lenses with internal focusing mechanisms), won’t always render aberrations exactly the same at different focus positions. Since we’re photographing a computer screen close to the camera, rather than stars at infinity, it’s possible that some lenses might show different real-world performance when compared directly to my test. That said, in my experience, the aberration shapes tend to stay fairly representative of what you should expect from a lens, even in my tests with lenses that use floating lens elements. I’ll make it a note to always compare the test results with real world results to make sure that there are no pronounced differences.

Here’s what the crops from the Canon EF 50mm f/1.8 STM look like when shooting actual stars at full frame. The aberration shapes look very similar to what the test above showed.

Stars with the Canon EF 50mm f/1.8 STM @ f/1.8100% Crop of Stars with the Canon EF 50mm f/1.8 STM @ f/1.8Final Thoughts

The presence of aberrations can be particularly distracting in astrophotos. My test makes for a simple standard way to compare the aberration characteristics of lenses, hopefully without being too technical. I think it’s a good standard test that should prove useful for finding lenses with particularly good performance for astrophotography.

Now this test method is only one metric for the quality of a lens (there are other things like actual sharpness measurements, build quality, etc.). While I think it’s an excellent test for gauging the performance of a lens for use with astrophotography, definitely keep in mind that even lenses that “fail” an aberration test at certain apertures can still be excellent otherwise. While I wouldn’t use the Canon 50mm f/1.8 STM at f/1.8 for astrophotography, it’s great for other applications like portraits at that setting. The test lets me know that if I want the best compromise between light gathering and aberration performance for astrophotography, I should stop the lens down to at least f/2.8. For more information on how I like to choose lenses for astrophotography, check out my article on how to pick a lens for Milky Way photography.

I plan to run all of my future lens reviews through the Lonely Speck Aberration Test to hopefully make for a great way to compare choice lenses for astrophotography use. If you have a lens you would like to test with this method in order to compare with my results, please feel free to download the test chart (.zip) and try the same test on your equipment. If you’d like, contact me with your results and I’ll add them to a running spreadsheet of lens tests.

I hope this article gave you a practical and hopefully not-too-technical set of knowledge about lens aberrations, how they affect your photos, and a simple way to test your lens’s performance. There’s a lot more that can be learned about optics out there. For some extended reading on lens design and aberrations, check out some of my resources: Wikipedia: Optical Aberration, Roger Cicala’s Seven Deadly Aberrations article on LensRentals and finally, The Astronomical Files for Black Oak Observatory. This post would not be possible without their sharing of knowledge on optical aberrations.

I hope you enjoyed this article. If you’re interested in learning more about photographing the Milky Way, check out the rest of the Astrophotography 101 lessons or head straight on over to How to Photograph the Milky Way.

 WhistlerNorth's gear list:WhistlerNorth's gear list
Sony a6500 Sony a7 IV Sony FE 28-70mm F3.5-5.6 OSS Samyang 12mm F2.0 NCS CS Sony FE 70-300mm F4.5-5.6 G OSS +5 more
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