Updated April 17, 2018 By Ellen Murphy
When you change from low power to high power on a microscope, the high-power objective lens moves directly over the specimen, and the low-power objective lens rotates away from the specimen. This change alters the magnification of a specimen, the light intensity, area of the field of view, depth of field, working distance and resolution. The image should remain in focus if the lenses are of high quality.
Changing from low power to high power increases the magnification of a specimen. The amount an image is magnified is equal to the magnification of the ocular lens, or eyepiece, multiplied by the magnification of the objective lens. Usually, the ocular lens has a magnification of 10x. A typical lab-quality standard optical microscope will usually have four objective lenses, running from a low power of 4x to a high power of 100x. With an ocular power of 10x, that gives the standard optical microscope a range of overall magnification from 40x to 1000x.
The light intensity decreases as magnification increases. There is a fixed amount of light per area, and when you increase the magnification of an area, you look at a smaller area. So you see less light, and the image appears dimmer. Image brightness is inversely proportional to the magnification squared. Given a fourfold increase in magnification, the image will be 16 times dimmer.
Going to high power on a microscope decreases the area of the field of view. The field of view is inversely proportional to the magnification of the objective lens. For example, if the diameter of your field of view is 1.78 millimeters under 10x magnification, a 40x objective will be one-fourth as wide, or about 0.45 millimeters. The specimen appears larger with a higher magnification because a smaller area of the object is spread out to cover the field of view of your eye.
The depth of field is a measure of the thickness of a plane of focus. As the magnification increases, the depth of field decreases. At low magnification you might be able to see the entire volume of a paramecium, for example, but when you increase the magnification you may only be able to see one surface of the protozoan.
The working distance is the distance between the specimen and objective lens. The working distance decreases as you increase magnification. The high power objective lens has to be much closer to the specimen than the low-power objective lens in order to focus. Working distance is inversely proportional to magnification.
Microscopes magnify an object's appearance by bending light. Higher magnification means the light is bent more. At a certain point, the light is bent so much that it can't make it through the objective lens. At that point – usually around 100x for standard lab microscopes – you'll need to put a drop of oil between your specimen and the objective lens. The oil "unbends" the light to stretch out the working distance and make it possible to image at high magnifications.
Do you need some examples of images at different magnifications under a microscope? The different images below were taken with two different types of microscopes. The images of Paulownia wood, hair, and frog's blood were captured with a high power compound microscope using a Nikon camera adapter. The compound microscope typically has three or four magnifications - 40x, 100x, 400x, and sometimes 1000x.
The images taken of the sunflower with the moth pupa were taken with a low power or stereo microscope. A stereo microscope is a good instrument for viewing insects, coins, leaves, or anything you might hold in the palm of your hand, but need to see more detail on the item. The images of the moth were taken using an OIympus camera using a digital SLR camera adapter. Paulownia Wood Under a Compound Microscope
These images are provided courtesy of Robert Lavigne. Paulownia are deciduous trees native to much of China. The Paulownia fortunei tree is a fast-growing tree that is often grown commercially for production of hardwood timber. More information on Paulownia Wood can be found here. Human Hair Under a Compound Microscope |
Human Hair under a Compound Microscope 100x Magnification | |
| Human Hair under a Compound Microscope 400x Magnification |
Human Hair under a Compound Microscope 400x Magnification |
Frog's Blood Under a Compound Microscope
Frog's Blood under a Compound Microscope Image captured using the Lumenera Infinity 1-3 Camera 400x Magnification | |
Frog's Blood under a Compound Microscope Image captured with a Nikon Coolipix 3mp Camera 1000x Magnification |
Moth Pupa Under a Stereo Microscope
Sunflower with Moth Pupa in the Stem under Stereo Microscope All images captured using an Olympus SLR camera 10x Magnification | |
Moth Pupa under Stereo Microscope 30x Magnification | |
Heath of Moth Pupa under Stereo Microscope 60x Magnification |
You may find these similar articles interesting and helpful as well:
Types of Microscopes
What is a Compound Microscope?
What is a Stereo Microscope?
An important point to understand when working with compound microscopes and their objective lenses is that the field of view changes as the magnification changes. Typically a lower magnification objective lens will have a larger field of view, and a higher magnification objective lens will have a smaller field of view. For more information on how magnification affects the field of view read this article on microscope magnification and field of view.
Is More Microscope Magnification Better?
It is easy to think that more microscope magnification will result in seeing more detail and a better image. However, keep in mind that more magnification means you will see a much smaller area on your sample. Additionally if you get too much magnification (anything over 1000x), you will end up with empty magnification and poor resolution. So no, more microscope magnification is not necessarily better.
Which Microscope Objective Should I Start With?
Start Low!
Since the 4x objective lens has the least magnification, but a larger field of view, it allows for more of the specimen to be seen, as well as locating the part of the sample you wish to view. This in turn makes it easier to focus on the sample. Most microscope objective lenses are parfocal, meaning once you have one objective lens in focus, all other objectives should be in focus as well as you move from 4x to 10x, etc. Occasionally, when switching to the highest magnification lens you may need to make a slight adjustment to the fine focus, but this is not required when you start out with the 4x lens. So starting your microscopy viewing at the lowest objective is usually the most simple way to start. You can certainly use another magnification to begin, but it will likely take more time and can be a bit discouraging if you just can't seem to locate the area of your specimen you wish to view, especially if you are a microscopy novice.
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