The high power objective on a microscope is the 40x lens, and its job is to magnify a specimen enough to see fine cellular details that lower-powered lenses can’t resolve. When paired with a standard 10x eyepiece, it produces a total magnification of 400x, making it the strongest “dry” lens on most microscopes (meaning it doesn’t require oil). It’s the go-to lens for examining individual cells, bacteria preparations, and tissue structures in biology and medical labs.
How It Magnifies and What You Can See
The 40x objective sits between the medium-power 10x lens and the 100x oil immersion lens on a typical microscope’s rotating nosepiece. You can identify it by its light blue color ring, following an international color-coding standard where red marks 4x, yellow marks 10x, green marks 20x, and light blue marks 40x.
At 400x total magnification, you can distinguish structures inside cells: nuclei, chloroplasts in plant cells, and the shape of individual bacteria. This level of detail comes not just from magnification but from the lens’s numerical aperture, a measure of how much light the lens can gather. A standard 40x objective has a numerical aperture around 0.65, while higher-quality fluorite and apochromat versions reach 0.75 and 0.95 respectively. The higher that number, the finer the details the lens can separate.
Why the View Gets Smaller and Darker
Switching to the high power objective dramatically shrinks your field of view. With a standard 10x/22 eyepiece, the visible area through a 40x objective is only about 0.55 mm across, with a surface area of roughly 0.24 square millimeters. For perspective, that’s smaller than the period at the end of a printed sentence. At lower magnification, you’re seeing a much wider slice of your specimen, which is why you always locate your target on low power first and then switch up.
The image also appears dimmer. A higher-magnification lens spreads the available light across a larger image, so less light reaches your eye per unit area. You’ll typically need to open the condenser diaphragm wider or increase your light source to compensate. This is a normal part of working at 40x, not a sign that something is wrong.
Depth of Field Drops Sharply
One of the most noticeable changes at high power is how thin the in-focus “slice” of your specimen becomes. According to calculated values from Nikon’s MicroscopyU, a 40x objective with a 0.65 numerical aperture has a depth of field of only about 1.0 micrometer. Compare that to 55.5 micrometers at 4x or 8.5 micrometers at 10x. That’s a massive difference.
In practical terms, this means only an extremely thin layer of your specimen is sharp at any moment. If your sample has any thickness to it, you’ll see different structures come in and out of focus as you adjust the focus knob. This is actually useful: you can “optically section” through a specimen, examining one layer at a time. But it also means focusing requires a gentle touch.
Why You Only Use Fine Focus
This is one of the most important rules for using the high power objective: never touch the coarse focus knob once it’s in place. The coarse adjustment moves the stage (or lens) in large increments, and at 40x, the gap between the lens and the slide is very small. A single turn of the coarse knob can slam the lens into the slide, cracking the glass or scratching the lens surface.
Most microscopes are parfocal, meaning that once you focus on a specimen at low power, switching to a higher objective keeps the image roughly in focus. After clicking the 40x lens into position, you should only need minor adjustments with the fine focus knob to sharpen the image. The fine knob moves the stage in tiny, controlled increments, keeping both your slide and your lens safe.
How It Differs From the Oil Immersion Lens
The 40x high power objective is a “dry” lens. It works with an air gap between the lens tip and the slide. The 100x oil immersion lens, by contrast, requires a drop of special oil placed directly on the slide. This oil has the same refractive index as glass, which prevents light from bending and scattering as it passes from the slide into the lens. That scattering is what limits resolution at very high magnifications.
Because of this design, the 100x oil immersion lens achieves numerical apertures of 1.25 to 1.40, compared to the dry 40x lens’s typical 0.65. That translates to significantly finer resolution. But the oil immersion lens comes with extra steps and cleanup, so the 40x remains the practical workhorse for most observations. If you can see what you need at 400x, there’s no reason to go higher.
One critical caution: never get immersion oil on the 40x lens. If you swing the nosepiece past the 100x while oil is on the slide, the 40x objective can pick up oil residue, which degrades image quality and requires careful cleaning with lens paper.
Getting the Best Image at 40x
Start by finding and centering your specimen at low power (4x or 10x). Move the area of interest to the exact center of your field of view, because the much smaller field at 40x will only show what’s in the middle. Then rotate the nosepiece to click the high power objective into place.
Adjust only the fine focus knob until the image sharpens. Open the condenser diaphragm slightly if the image looks too dark, but don’t open it all the way, as too much light can wash out contrast and actually reduce visible detail. For stained specimens, a moderate light level with the diaphragm partially closed often gives the crispest image.
If your specimen is thick or three-dimensional, slowly roll the fine focus up and down to scan through different focal planes. Because the depth of field at 40x is only about 1 micrometer, each tiny adjustment reveals a different layer, letting you build a mental picture of the specimen’s full structure.