Some Thoughts on Chromatic Aberration - 02 Apr 2020

Hooded Merganser, SOOC, 1:1 crop, Sony a7RIV, Zeiss 35/2.8, Digidapter™, Swarovski STX85@25X

I had the opportunity to digiscope a pair of Hooded Mergansers this past Monday in the ponds next to our house. With the early morning Sun shining on the pair as they swam in the dark waters I knew that exposure would be a challenge, especially with a gorgeous drake displaying a bright white hood bordered in black feathers. I made sure to expose for the white, and for the most part, did well. However, I noticed chromatic aberration (CA) creeping into some of the photos, and decided it was worth a discussion. I made sure to be selective in culling the images so that I could demonstrate some of the challenges that we digiscopers will run into when trying to capture high-contrast situations.

I was shooting with a Sony a7III and Zeiss 35mm f/2.8 pancake lens attached to a Swarovski STX85 and 25-60X Zoom eyepiece set to 25X using a Digidapter™. I had just mounted the camera to the Digidapter™that morning and thought it was properly aligned (trying to get the sharpest vignette circle at the edges of the frame and even illumination throughout the field-of-view, FOV). However, as I reviewed images in Lightroom CC I noticed some very slight CA even the center of my view, where I should not see any. The following is a series of images where the drake swam across my FOV with the first image of the duck in the center of the camera:

There is a very slight amount of CA along the edges of the white border on the hood and in the shoulder. Its not bad, but noticeable, and is easily removed during post-processing.

Now, look at the drake when its toward the edge of the frame. You'll notice more pronounced CA along the white edges with purple-fringing on one border and blue-green fringing on the opposite. Again, it can be removed in post, but it does highlight the fact that CA becomes progressively worse in all scope eyepieces as you move from center to edge. 

Top of the line scope manufacturers have glass and coatings that minimize CA, but cannot remove it entirely. That's why proper alignment of the camera in front of the scope eyepiece is critical to optimizing image quality and reducing CA. One of the great strengths of the Digidapter™is the ability to achieve proper alignment by being able to adjust camera position in front of the scope eyepiece. 

Luckily for me, it turned out that the locking screw on the Digidapter™ had prevented me from pushing the adapter completely in position. So, when I removed the adapter and replaced it on the scope the system operated as expected and the CA issue was greatly reduced. 

How does focusing influence CA? I found that Alignment, not Focus,  is the culprit behind the CA issue I was experiencing. I typically roll focus through the subject when I digiscope; that is, I shoot a high-speed burst while adjusting focus in front, on, and behind the subject so that I improve my chances of getting one frame as sharp as possible. I find that CA does not improve no matter how sharp the image is. That said, I also find that out of focus subjects remain CA-free when proper alignment is achieved. The images below illustrate this last point:

To expand on this, also note that zooming a scope's eyepiece will alter the critical point of focus and can magnify CA if it is a problem. There may be more detail, but typically it is at the sacrifice of image sharpness. Therefore, its a general rule of thumb that cropping an image for magnification is better than zooming an eyepiece for magnification. That said, I still tend to try my luck at zooming an eyepiece to improve magnification BUT ONLY AFTER I'VE EXHAUSTED LOW-MAG CAPTURES. 

The takeaways from this discussion?

Proper alignment of the camera in front of the scope eyepiece is key to reducing/eliminating chromatic aberration.

CA gets progressively worse toward the edges of the scope's field of view, so make sure to keep your subject in the CENTER OF YOUR FOV and CROP FOR COMPOSITION (don't frame for composition).

Digiscoping will expose any flaws that are inherent in a spotting scope or digital camera. If you are in the market for a spotting scope the first thing I recommend to potential buyers is to point the scope at a subject of high contrast, like white siding in bright sunlight to evaluate the amount of CA that you'll likely encounter. Most scopes do great in normal lighting and show no CA, so this is a great way to evaluate the quality of the optics. The same goes for testing eyepieces, whether they are zoom or fixed. And just know that adding another camera and lens in front of the scope eyepiece with magnify any flaws.

Finally,  did you notice that the first image in this post was taken with a Sony aRIV but I talked about the a7III? That's my next post.

Keep shooting, and stay well. And Stay Home!