Using Microscope Objectives For Beginners
Isn't it cool to use a microscope objective for macro photography? I mean, just by looking at it, it is cool. But when I first started using microscope objectives on my Canon 550D, I made quite a few mistakes. That was largely due to the fact that I did not understand what microscope objectives are and how they should be used to achieve the result I wanted.
There are many types of microscope objectives and it can be very confusing when you first start thinking about using them. So, a good understanding of microscope objectives can be very beneficial. Luckily, we are in internet age, a quick sear result this which I think is very comprehensive:
Daunting task right? Yeah, it is very confusing to read that article without actually getting hands on with it, just like everything else when you first start doing something. But before you go and buy an objective, let me share some of mistakes I made when I first started out.
Mistake #1 : Working Distance.
If you have read that article, you should remember the mentioning of the term Working Distance, often abbreviated as WD on the barrel of objective. From my experience, most reputable objectives manufacturer will have this marking. If WD marking is not shown on the objective, ask vendor or do a search if you know the model number of the objective. Working distance is usually the total distance from the front of objective to the subject when image is in focus. But some vendor has a different definition -- total distance from the front glass to the subject, but when the front glass is deeply recessed into the casing, then the effective working distance will be shorter. This usually happens to low magnification objective. So make sure to ask the vendor about this.
So why is working distance (or effective working distance) so important? Maneuverability when trying to focus, lighting and if you are doing focus stacking, total depth you can achieve. The first time I saw someone using a microscope objective on a DSLR, I was so excited and bought one with 10x magnification without paying attention to working distance. After setting it up, to my dismay, I had to move the objective so close to the subject, I bumped into the subject quite a few times, it turned out the working distance of that objective is ONLY 1.5mm.
Since the reason I bought that objective, beside cool factor, was to take series of images of flat surface to measure consistency of my rail, I decided to accept such short working distance, well, it was kind of "expensive" compared to what I thought it should be (wrong, very wrong). But then the second problem surfaced -- it was very hard to light because the gap between the objective and subject was so small, I had to practically put two small flashes very close to where the objective was, nearly touching it. You know, lighting is the most important part of photography and this shortcoming will definitely cause more headache in the future.
OK, since my purpose of using that objective was basically for measurement, I also accepted the lighting shortcoming. But once my rail prototype was built, I decided to do a quick focus stacking of a fly. Guess what? The total depth of field I can get is about 1.5mm, or the working distance. This is because I could not move the camera with that objective mounted on it by 1.5mm, the working distance. The fly's eye is about 3mm in depth, so I could not get a good focus stacked image. Duh!!!
Mistake #2 : Infinite vs Finite Objectives
Again, if you have read that article, you should know that an objective can be finite one or infinite one. A finite objective projects images onto a surface at designated focal distance. An infinite objectives projects images in parallel rays and requires a tube lens to project image onto a surface. The focal length of the tube lens can effectively change magnification.
I was so out of luck that the first objective I bought is a finite one beside short working distance. So I struggled to make sure the total distance between the sensor and objective is the required 160mm. I must say that without custom hardware, it is very difficult to do, I ended up buying an helicoid so that I can adjust the length between the objective and sensor plane.
Not only the difficulty of positioning it, but also I ended up with a fixed magnification of 10x. Though, it suited my initial purpose of using it as measurement tool, later on, when I tried to do focus stacking, I just had to do half of the eye of that fly.
For an infinite objective, things are very different. After that finite 10x objective fiasco, I decided to get an infinite 20x objective with 11mm working distance. Because an infinite objective needs a tube lens to convert parallel rays on to a focal plane, I decided to use my old 70-300 zoom lens for film. To my surprise, the objective worked very well with the zoom lens, not only easier lighting and deeper depth due to long working distance, I could change magnification!!! Though, it is not recommended to deviate too much from its designed magnification, it did have good image quality when stretched between 10x and 30x.
Mistake #3: Watch out for those infinite objectives that need special eyepiece
One drawback for the 20x infinite objective (Olympus MSPlan 20x) I got is that it needs a special correctional eyepiece to work well. This type objectives are earlier generation of infinite objectives and usually works fine without the special eyepiece but if you are trying to get the best out of it, you are out of luck. If you do not know, please ask vendor or if you know the model number, google it.
Mistake #4 : Achromatic vs Just Glass
This is probably an obvious one, probably not due to understanding of objectives, but due to the price alone. If you have read that article, it is known an achromatic objective corrects chromatic aberration, though not fully, it does produce better image quality. Achromatic objective is much more expensive than those simple glass one. Nowadays, only objectives with low magnification power are made of simple glasses, so it is easy to tell the difference of an achromatic one from the simple ones (costing about 3-4USD). My mistake was that I just purchased a cheapest 4x one to do focus stacking on that fly so that I can get the whole eye. The result was horrible. Only recently I am able to find an infinite 4x achromatic objective with reasonable price.
Since this blog is for beginners, these are just some considerations: Apochromatic objectives produce the best image quality, this type of objective often have APO marking on its barrel. The next in line is the Fluor type and it is often labelled as semi-APO and has FL or Fluor marking on the barrel.
So to conclude:
- get objectives with as long working distance as possible
- get infinite objectives as it is so much flexible to use
- watch out for those infinite objectives that need special correctional eyepiece.
- get at least achromatic objectives, if budget allows, get apochromatic or Fluor type.