Choosing Between 10x and 16x Eyepieces: The Cost of High Magnification
Total magnification equals objective magnification multiplied by eyepiece magnification. From this, many people draw an obvious upgrade idea: switch to a 16x or even 20x eyepiece and the magnification jumps up a step right away. You can do this, but an optical ceiling stands in the way, and it is called empty magnification.
The principle in one sentence: the eyepiece only enlarges the image formed by the objective lens for you to see; it cannot add any detail. The resolution limit is set by the objective's numerical aperture (NA). Once total magnification exceeds roughly 1000 times the objective's NA, the image only gets bigger, not clearer. That is empty magnification: blowing up a lot of empty blur.
An example: a 40x objective with NA 0.65 has a useful magnification ceiling of about 650x. Paired with a 10x eyepiece, that is exactly 400x, and the image still has headroom. With a 16x eyepiece it reaches 640x, close to the limit, and the image starts to look soft. With a 20x eyepiece it hits 800x, beyond the ceiling, and you are purely magnifying blur.
So when does a 16x eyepiece make sense? Two situations. First, paired with a high-NA objective, such as a 100x oil immersion objective with NA 1.25 and a ceiling of 1250x, a 16x eyepiece brings it to 1600x, still at the edge of the useful range, and detail still gains a little. Second, the observer's eyesight calls for it. For users with weaker vision, or those who want to examine detail comfortably, a slightly larger image eases the strain on the eyes. Even if resolution does not improve, the experience has value.
Conversely, the 10x eyepiece is a jack-of-all-trades for a reason. It lets the objective's resolution be fully magnified without waste, the field of view is relatively larger, the exit pupil diameter is reasonable, and viewing is most comfortable. When the budget is limited, rather than investing in a high-power eyepiece, invest in a better objective. Money spent on NA goes to the cutting edge.
There is another overlooked approach: a wide-field eyepiece (marked 10x/20mm or above) is worth the extra money more than a 16x. Raising the field number from 18 to 22 enlarges the diameter of the circular image you see by one fifth, and the efficiency of scanning slides goes up directly. This is seeing more, which is far more practical than seeing bigger.
In one sentence: resolution comes from the objective; the eyepiece is just the display. Want a clearer image? Change the objective. Want a bigger image? Change the eyepiece. Want both clear and big? Spend your money on the objective first.