Benefits of Wide Field of View Microscope Objectives
Some of you might understand what field of view is, but what does it mean in microscopy? How can we calculate the field of view of an image blown up hundreds of times and passed through several lenses?
This article goes into detail on what a microscope’s field of view is, what limits the field of view, how magnification affects the field of view and even looks at emerging boundary-pushing technologies.
What Is Field of View?
A microscope’s field of view is simply the amount of physical space that a microscope is looking at during any given moment. When measuring this, we tend to make some qualifications, such as whether the image produced within this field is free of aberrations or other distortions that compromise image quality.
These aberrations are caused by redundant beams of light filtering in through the microscope as they bounce through the light channel, so most microscope lenses are carefully tuned to minimize these; we’ll get more specific about this later on in the article.
The field of view is a rounded area measured by its diameter, typically in millimeters or micrometers. Importantly, as our microscope objectives gain higher magnification and look closer into a sample, their overall field of view shrinks because the precise area they are looking at is further defined.
This, of course, means that using a microscope objective of a lesser magnification also presents the benefit of a larger field-of-view, which is an improvement in certain scenarios.
How Do I Calculate Field of View?
According to the New York Microscope Company, the field of view on a microscope is calculated by taking the microscope’s field number, or its individual field of view, and then dividing that by the total objective magnification.
Important to note, some models of the microscope may use a secondary optical lens, so if yours does, then the magnification of the auxiliary lens should be multiplied by the magnification of the objective lens before dividing.
As an example, say you have an eye-piece that has 10x magnification and a field of view of 22 and are using an objective lens with 40x magnification. We would take our eyepiece and objective magnifications and multiply them to get 400, and then divide our field of view (22) by that 400 for a total of 0.055; these results are also measured in millimeters but are converted to micrometers easily.
Through this math, we directly see how our microscope’s total magnification diminishes the total field of view but also provides deeper clarity.
Is High Field of View Bad?
There are benefits and detractions to the field of view that might seem somewhat apparent but are actually more complicated than meets the eye.
As stated, a higher field of view typically means lower magnification, but as lens technology advances, this is not always necessarily the case. According to MicroscopyU, there are wide field of view microscopes in production that offer larger viewing images, but these need microscopes corrected to take advantage of this extra space properly.
By utilizing highly correct eyepieces, these devices can achieve field numbers of 26 millimeters and greater. If the eye-piece in our example above had a field number of 22 instead of 26, then our field of view would’ve increased from 0.055 to 0.065.
Another important factor in understanding is how magnification and field of view correlate to the depth of field. As light becomes more narrowly channeled into the objective lens, the field of view becomes shallower or closer to the lens itself.
While this is not a problem in certain applications, in others, it causes complications. For instance, many life science microscopy samples require them to be submerged in water, meaning that they must be fully sealed and observed through another layer of glass, as well as the liquid. If a lens has too narrow a field of view and depth of field, then it is unable to properly observe the sample without image refraction and aberrations. To this end, microscope lenses and objectives are corrected for certain mediums and thus only work within those mediums.
As MicroscopyU also explains, the optical lens and its field diaphragm determine the field of view, which is the component that helps filter out the redundant light. This field diaphragm takes place either before or after the field lens, which is either presented in a convex or concave configuration, depending on its placement. The field diaphragm helps to present a clearer image, but this does come at the cost of some field of view and a smaller viewing angle.
With all that context, we can then think of the field of view as a trade-off, a larger viewed area in exchange for less detail and image depth. To that end, we need to think of the field of view situationally as something to be considered when viewing specific specimens with a microscope.
Do we need to see an entire sample simultaneously? What is the working distance for this sample? Could I utilize image stitching here? With the answers to these questions, you can consider what might be the right solution for you and determine whether your field of view is a benefit or a detriment to your application.
Will Microscopes Ever Reach Higher FoVs?
Currently, some microscopes are pushing the literal boundaries of what is possible with imaging. By attaining true 4K resolutions, these revolutionary microscopes are able to offer the same level of image quality and detail across a broader surface. This is possible because the extra pixels on the sensor absorb more data, meaning that optical zoom is not the only factor at play in data collection. Instead, these microscopes do not need the same intense zoom factor as they can better utilize their space.
Additionally, a group of Chinese researchers recently experimented with what they called metasurfaces or nano-fabricated surfaces that could both manipulate and retain light even at microscopic scales. They published their work in the November 2020 edition of SPIE’s Advanced Photonics, where they were able to improve the efficiency, field of view, and polarization, all with an ultralight and ultrathin architecture.
To recreate the capabilities of a digital microscope, they directly integrated their metalens with a CMOS image sensor, creating a MIID, or Metalens-Integrated Imaging Device. They were able to configure these surfaces with an array and utilize multiple CMOS sensors for wide-field imaging while also emulating image stitching.
Image stitching is the simple technique of taking multiple images and stitching them together through a sophisticated algorithm that detects overlap and properly aligns them. For the MIID, they were able to integrate image stitching into their image capturing, allowing for the image to be corrected as it was taken by multiple sensors simultaneously. They called this polarization-multiplexed dual-phase, a complex technique requiring software and hardware to communicate with one another properly.
This is a disruptive technology in its infancy that carries a lot of potential applications. In theory, it is able to best the microscope in capability across several verticals. However, there is a lot that needs to be understood about it for the technology to be implemented on a large scale or standardized. With more standard microscopes in higher resolutions, these, too, can achieve a higher field of view without the need for software image stitching that potentially alters collected data.
With respect to standard microscopes and objectives, it is possible that other intersecting technologies offer new solutions for expanding current microscopes. In addition, we said earlier that there are some high-FoV microscope solutions. However, these are niche and more expensive than their counterparts.
As with all technologies, engineers continue to work for new and innovative methods to improve not just products but research tools and methodologies.
Conclusion
The field of view in microscopy is a more complicated subject than that in standard photography. It encompasses the levels of magnification from not one but two lenses, as well as the depth of field, immersions, and even emerging metalens technologies.
We now understand that a higher field of view sometimes means a lower magnification level, but that that isn’t always bad. Sometimes our samples require larger depths of field or simply become larger in ways that ask for a higher field of view.
In terms of sheer benefits, we understand that while a higher field of view correlates to a lower magnification level, we know this isn’t inherently bad. Lower magnification means a clearer image, and of course, results in a larger specimen size viewed, which for certain applications is immensely valuable. The important thing is that you understand your individualized application and what best benefits the data you are collecting.
For more discussions about Field of View, Image Stitching, and Immersions, visit Navitar.
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