Microscope 101: How to Focus a Stereo Microscope

I’ve talked about how to focus a stereo microscope over in the Microscope Basics section of MicroscopeGenius.com–especially the Stereo Microscopes section–however, I feel it’s worth reiterating to make sure the message is clear to all of my hobbyist readers.

I do get a few emails about folks having trouble getting an focused image out of their AmScope stereo microscope, or Omax stereo microscope, no matter what they have tried to do. Flailing around attempting to focus a stereo microscope can be irritating and frustrating, so knowing the basics really helps you figure out what you should be doing right, rather than using process of elimination to figure out what you’re doing wrong.

Microscope 101: How to Focus a Stereo Microscope

The key principle for focusing any optical instrument is that the relationship between working distance/field of view and magnification is an inverse relationship. This means that it’s like a teeter-totter–when one goes up, the other goes down, and the other way around as well. As we know, working distance is the term for how far away from the surface of the sample is from the end of the microscope’s objective lens. So, to focus a stereo microscope, we have to determine what working distance we need to be at for that specific magnification value.

Now, based on the inverse relationship, we know that to focus any lens, the higher the magnification it is, the closer the focal point is from the lens. By this principle, we know that when we increase magnification, we then need to move the sample closer to the lens, or the lens closer to the sample. Simple enough!

By the opposite, we also know that if we reduce magnification, to focus a stereo microscope, we have to move the lens or sample farther away (as we’ve increased our working distance and moved the focal point farther away).

Where people can get hung up on this is assuming that the working distance for a stereo microscope is fixed. Many manufacturers list their working distance as a firm number in their descriptions, but how can a working distance be fixed if the microscope has a range of magnification powers available? Every time one changes magnification, working distance is also changed. Compound microscopes use the same principle as well–the more magnified your lens, the closer you need to be to the sample to get it into focus. This is why your 4x objective is significantly shorter than your 100x objective.

The Basics

So, here are the steps I recommend taking to focus a stereo microscope with ease when you’re a new microscope user (when you get used to it, you’ll just have a feel for where it’s supposed to go).

1. Set the microscope to the lowest possible power on your microscope. This ensures that you will have the longest working distance that your microscope is capable of, which is usually provided by your manufacturer in the manual or on the listing.

2. Adjust the microscope so that it sits to about that distance, then use the eyepieces to fine tune the image to clarity. At this point, you should see an image with great detail. If not, the distance is not correct, or there is something wrong with the microscope. For example, AmScope stereo microscopes of the SM series are 7x – 45x out of the box. At 7x, we’re given it has a working distance of 4″. By setting it to 4″ away from the sample’s surface we want to focus on, I can achieve focus through the microscope.

3. Adjust the magnification higher, then adjust the working distance lower. Do this slowly to get a feel for the microscope’s focusing distance. Note that it’s fairly exponential how quickly the working distance drops off, so it’s normal to see that you have to move the stereo microscope head further down per magnification change as you go. I would try to adjust the magnification by .25x on the dial (estimate if your dial isn’t labelled to that significant figure).

4. Continue to adjust each part slowly until you reach the maximum magnification for your microscope and are focused.

5. Once max magnification/minimum working distance is achieved, try to do it again in reverse until you reach minimum magnification/maximum working distance. Remember to take your time to ensure you get each step right, and you’ll be able to focus a stereo microscope in no time.

6. If you’ve been successful, congratulations! You’ve gotten the basics down of how to focus a stereo microscope!

How to Focus a Stereo Microscope Using Barlow Lenses

Now, with that settled up, let’s get into the next step. Many stereo microscopes come with auxiliary lenses that amplify or reduce your magnification. These lenses are commonly called Barlow lenses. Typically, they screw onto the objective lens housing, which means that they change the magnification, and from what we learned earlier, also changes the working distance needed to focus a stereo microscope.

Let’s go back to our AmScope SM series microscope as an example. Earlier, we mentioned that the SM microscope head out of the box is 7x – 45x. Often times, they include either a 0.5x Barlow (SM05), a 2.0x Barlow (SM20), or both. Since total magnification is just the product of all magnifying elements, we can translate that as meaning they either come in 3.5x – 45x, 7x – 90x, 3.5x – 90x, or the original 7x – 45x, depending on what combination of lenses comes included.

If using a 0.5x Barlow lens, the magnification is cut in half, so down to 3.5x. At 3.5x, we’re given from AmScope that the working distance to focus a stereo microscope from the SM series will be 8″. Just like before, after installing the lens, move to the lowest magnification/highest working distance, and work your way down (steps 1-4). Once you get to the bottom at 22.5x, work your way back up.

If using a 2.0x Barlow lens, the magnification is doubled, so up to 14x – 90x. We aren’t given any working distances from AmScope at this point, which is where people get confused. We know that at 3.5x we get 8″ and at 7x we get 4″, so it’s safe to say at 14x we should expect around 2″. Start there, adjust as necessary, then repeat steps 1-4 as before, then finish with step 5 to work your way back up.

Note: Although you would not want to since these lenses will basically cancel each other out, you cannot stack Barlow lenses. It simply throws the image focus way far off from where it is expected, and really doesn’t do anything good for your image quality either. So the max range of the SM microscope without adding any higher power eyepieces for all SM stereo microscopes will be 3.5x – 90x.

How to Focus a Stereo Microscope When Adding a Camera to the Mix

It really isn’t too much different, but we do have to change one factor to focus a stereo microscope correctly when using a camera. Usually, when trying to focus a stereo microscope and a digital camera gets involved, we get into the realm of insecurity. We are no longer really sure of the total magnification being used, because the camera manufacturers rarely give it to us. That, and it’s all based on a digital sensor’s “natural” magnification it gets when it captures an image–something not easily quantifiable. So, more playing with it to focus the stereo microscope is necessary.

Most of the time (but not all), the magnification on a camera is going to be higher than your 10x eyepieces that came with your unit. Again, this is generalization, so check the numbers to be sure. If so, you will need to simply follow the rules above! The more magnification, the less distance. So, to focus the camera, don’t use the eyepieces at all–it’s pointless since they don’t focus at the same point. Use the screen on the camera, or the computer screen (depending on the kind of device you have).

Well folks, that’s about it! You’re now pros and able to focus a stereo microscope with ease (after some practice)!

If you’re looking to get a quality stereo microscope, we have quite a few reviews done on some great equipment at our Stereo Microscopes category section! Be sure to take a read and see if anything there will suit you and your needs! As always, please leave any comments, questions, or concerns in the comment box below!