Which of the following would be least likely to cause a "fuzzy" image on oil power?

a. Using a plan-achromatic objective lens
b. Objective not screwed in tightly
c. Not enough oil on the slide when using oil immersion lens
d. Focus controls not adjusted

ANS: A
When an objective is not screwed in tightly, not enough oil has been used on a slide when using an oil immersion lens, or the focus controls are not adjusted, an image is likely to be fuzzy or unclear, making it difficult to visualize objects. Using a plan-achromatic lens gives a flat field so that the edges of the field are in focus at the same time as the center.