Oil Immersion

Can't Focus at 100x? Check These Four.

A quick reference for a slide that is sharp at 40x and foggy at 100x.

Tim Pellerin · Sky Optical Services

Sharp at 40x but nothing but fog at 100x? Almost every case comes down to the slide, the coverslip, the oil, or the order you focused in. These four checks take less time than cleaning the objective, so do them first.

Do this

Slide coverslip side up. Upside down, the 100x cannot reach focus.
Use a coverslip. 100x oil objectives are made for coverslipped slides.
Add one drop of immersion oil. Without oil, 100x looks like fog. One drop is enough.
Focus at 40x first. Then swing to 100x and use only the fine focus.

Avoid this

Swinging the 40x through the oil. Oil on a dry objective causes hazy images. If it happens, clean the 40x right away.
Coarse focus at 100x. The working distance is tiny. Coarse focus can drive the lens into the slide.
Too much oil. Extra oil runs onto the stage and can seep into the objective.
Still blurry after all four. The 100x front lens may be dirty or damaged. That is a service call.

The habit, every time

STEP 1

Slide face up

STEP 2

Coverslip on

STEP 3

One drop of oil

STEP 4

40x first, fine focus

New to oil immersion, or not sure how much oil to use? See Protect the Scope. One Drop Does It.

Try it in 3D or VR

See the gap between the objective and the slide shrink from 4x to 100x, and what a crash does. Open Focus Without Crashing on a phone or computer, or in the Meta Quest browser for VR.

Watch it in a minute

Microscope Minute from Sky Optical Services

Still blurry after all four? The 100x front lens may be dirty or damaged. That is a service call.

Request Service Printable Guide (PDF) Watch the Video

Need hands-on help? See our microscope repair and maintenance service in Houston.

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