Do I need a 100x oil immersion objective?

Here I explain why I am against 100x oil immersion objectives for microscopes in schools and for beginners.

What are oil immersion objectives

Many microscopes these days come with a 100x oil immersion objective. When using it with a 10x eyepiece, you will get the magnification of 1000x, what is the about the maximum that microscopes are able to resolve.

An oil immersion objective requires you to place a drop of immersion oil on the slide and then to rotate the 100x oil objective into the oil. The bottom part of the objective is then covered in oil and will require cleaning after the observation session. This way there is no air between the slide and the objective and this increases both brightness and resolution.

The disadvantages of using Oil Immersion

A very common problem is, that also the non-oil objectives might become contaminated with immersion oil, wen not carefully used. People forget about the immersion oil and then switch magnification back to the 40x “dry” objective. Due to the small distance to the cover glass, this objective is also dipped in oil and needs to be cleaned. It is also possible for immersion oil to enter the objective itself and cause problems there. Trust me, it’s gonna happen. I have done it myself. As a matter of fact, I even managed to dip my 40x objective into mounting medium for making permanent slides. This turns hard and must definitely be cleaned. Maybe even worse than immersion oil. This video documents my carelessness:

For this reason we have banned oil immersion objectives, immersion oil and mounting media from the school lab. Students ended up destroying the 40x objectives, which became useless after a short time.

Oil immersion can not be used with permanent slides. The immersion oil will make the slide dirty. You will not be able to clean the slide completely from the oil smear on the slide will also disturb the image quality when looking at the slide with lower magnifications. For this reason a 100x oil immersion objective is not recommended when using commercially made permanent slides (which you want to keep clean and oil free).

Oil immersion objectives also have a low depth of field. Moving microorganisms will drift in and out of focus. For this reason oil immersion is best used only for static specimens.

If it has limited use and so many disadvantages, why do they include it?

I am sarcastically inclined to say: “So that customers dip their 40x objective into oil and destroy it this way so that they have to buy a new replacement objective. It’s good for business.” This rather admittedly cynical argument does not hold, however, becasue many microscope companies do not even sell replacement objectives. I think that the reason is rather more subtle.

It is for marketing reasons. The company includes a cheap 25x eyepiece as well and can then advertise the microscope with a magnification of 2500x, even though a magnification like this is totally meaningless because of lack of resolution. Most buyers of microscopes don’t know that and will use the maximum magnification as a criterion for comparing microscope quality. Many people think that the more you magnify the image the more you will see. Only those that know a bit about microscopy understand that a higher magnification will cause you to see less because:

  • The image is darker
  • The depth of field is smaller:
  • The field of view is smaller: Oil immersion is mostly useful for non-moving objects becasue otherwise they will quickly move out of the field of view.

Considering the relatively high cost of oil immersion objectives and low range of applications, I would rather save the money or invest it in a more stable microscope body. The problem is, that many companies do not give you an option to exchange objectives (exception: the high end manufacturers). Alternatively you might consider to buy a 20x objective to have in intermediate magnification between 10x and 40x, or a 60x objective. The oil immersion objective is useful if you already know that you will be observing mostly heat-fixed bacteria, or sub-cellular structures, such as the chromosomes of dividing cells. For general-purpose microscopy, like the observation of water samples etc. it is generally not useful, becasue the magnification is too high anyway to see the whole organism at once.