I seem to be writing another review. Mostly, I hold back from them because I don’t feel sufficiently expert to be able to express a strong opinion. However, in this case I think I can. Why? Well, I have been using microscopes for more than 50 years (God! Is it really that long!). I have had the good fortune to have used microscopes made by all of the major manufactures and had access to models that cost many £1000s and sometimes many 10s of £1000s – compound, stereo, and more specialist microscopes such as laser confocals. Thus, being retired and obsessed with the notion that ‘the closer you look, the more you see’, I wanted a compound microscope to use in my retirement but was worried that I had been spoiled by my professional experience of the highest quality instruments, and thus would find it hard to buy something of sufficient quality that was within my price range. So, what to choose?
There are a huge range of options for anyone wanting to buy a compound microscope. Their are some fantastic instruments to be had 2nd hand and at very affordable prices. Then of course there are new high-end microscopes made in Europe, Japan and the USA (or at least carrying the brands of those countries). However, mostly these instruments are far more costly than those made in China and elsewhere. For many people, myself included, the cost of new microscopes made by the likes of Zeiss or Nikon ruled them out. However, they remain options for 2nd hand purchases.
There is another choice to be made – actually lots (!), but lets begin with this one: tube length or infinity corrected optics? ‘Tube length’ or ‘finite’, microscopes are those where the objective is designed to form a real image behind it in a plane of focus for the eyepiece to pick it up and present it to the eye of the observer. The distance between the objective and the eyepiece in these microscopes is usually 160 or 170mm. Most compound microscopes employ the Deutsche Industrie Norm, or DIN, standard. The DIN standard has a 160mm distance from the objective flange to the eyepiece flange. The other available standard is the Japanese Industrial Standard, or JIS where the objective to eyepiece distance is 170mm. However, more recently, many microscopes have been made in which the rays of light emerging from objective travel in parallel and are focused by a lens in the body of the microscope (the tube lens – the tube lens is usually an achromatic lens with a focal length of 200mm ) and the eyepiece. Such designs are more complicated than finite conjugate microscopes but allow for other optics to be placed in the path between the objective and the tube lens and it is for reason of this flexibility that manufacturers have increasingly turned to making them. Quick historical note: the German manufacturer Reichert first made Infinity microscopes in the 1930s. They were followed later by Zeiss, Leica and others. However, the infinity optical system did not really take off until the 1980s.
The choice between finite and infinite is of some consequence to someone buying an instrument; there are lots of finite microscopes on the 2nd hand market and whether new or not, they are likely to be easier to find and cheaper, than their infinite counterparts. Further, if after buying a microscope you want to add more objectives etc. it will be easier and cheaper to do this with finite models. The fact that the 160mm microscope with RMS objectives is far more common than the JIS standard is also something to bear in mind. All of this is not to say that infinity models are not available on the 2nd hand market – they are, after-all they have been in manufacture since the 1980s. However, a quick scan of eBay confirms that they are far less common than their finite counterparts. Maybe this is because high-end infinity microscopes last a lifetime and most are still sequestered within science laboratories?
So what did I choose to do? Well, it may seem counter-intuitive but I decided to buy a Bresser Infinity microscope. Although Bresser is a company based in Germany, the microscopes that they sell come from China. Why did I decide on an ‘infinity’ model – a similar microscope can be had with ‘finite’ optics (Bresser TFM-201 and -301)? Two reasons. Firstly, I had it in mind to tinker with the design and the infinity space makes it easier to place polarizers, prisms, filters etc., in the optical pathway. Secondly, the greater cost of the infinity objectives versus the finite objectives from the same manufacturer, led me to believe they might be of better quality and likely of more modern design. However, I do not know if the latter is true – I do not have anything with which to directly compare the Bresser infinity objectives.
So, how did it go and was I pleased with my purchase or disappointed? To cut a longer story short, the microscope arrived extremely well-packed in double cardboard boxes and inner polystyrene packaging shaped to fit the component parts. It took only a few minutes to remove the transit screws and assemble it – the condenser, objectives and eyepieces were already in place and all that had to be done was to put the two halves of the microscope together. The impression of the frame and the quality of the mechanics is good – solid and precise. The image through the microscope is nothing short of truly excellent. The field of view is extremely wide and the plan achromatic lenses are remarkably flat across the field and there is no obvious colour fringing. I do not think that I could distinguish the quality of the images produced by the 4, 10 & 40X objectives from those of Zeiss, Leica etc. The 10X, 40X and 100X objectives are close to parfocal – only a quarter- turn of the fine focus is needed to bring objects focused using the 10X into focus with the 40X. Similarly from 40X to 100X. However, the 4X objective requires the use of the coarse knob to bring it to focus. The 100X objective is probably where the quality of the optics is not as good as that of more expensive microscopes. However, with some tuning of the condenser, the images are more than acceptable.
The microscope has an LED illuminator and Koehler illumination. Koehler provides optimum contrast and resolution by centring the light and spreading it evenly over the field of view. I like the fact that the LED lamp allows you to vary the light intensity over an extremely wide range. Checking out the design of the lamp housing that simply plugs into the frame, I can see it would be easy to change the LED for say a UV one to do some basic fluorescence imaging. Perhaps some more on that another time. The glass above the field iris provides somewhere that colour filters could be placed if one did not want to use the rather fiddly push-in filter-holder on the condenser – see below. The lighting is every bit as even as you would expect from a Koehler system.
Inspecting the rack and pinion for the condenser, the mechanics of the stage and some of the other mechanical parts, show that they are of course not of the same quality as found on premium brands. However, the focusing mechanism, mechanical stage and condenser rack all work very well and are more than adequate. One slight niggle was that I could not bring the condenser high enough to make my home-made Rheinberg and dark-field stops work properly and this no matter what size I made the central stops in the filters. I was concerned this was a manufacturing fault. However, after removing the condenser and more closely inspecting the mechanism, I discovered that there is a single Allan-headed screw that serves to stop the condenser being raised to the point where it could impact the slide. It was either poorly adjusted or had moved in transit – it is a rather crude mechanism that a simple lock-nut would fix. A couple of turns with an Alan key left the condenser hair’s breadth from the plane of the stage and dark-field began to work perfectly for the 10X and 40X lenses but I found I had to raise the slide to get good dark-field illumination with the 4X objective. One sign of the lower manufacturing standard of the Bresser is that the condenser iris moves rather too easily and its leaves are chromed rather than blacked. It has another slightly (no, very!) annoying feature in that the filter holder is a press fit rather than swing-out. Thus, the holder has to be pulled out every time you want to change the filter. However, because it drove me mad, I quickly set about fixing this by designing and 3D printing a swing-out holder that press fits into the base of the condenser (there will be a later article on this and filters in general).
The problems outlined above are really rather minor, and for the price (895 Euros via Amazon but for some obscure reason 1009 Euros direct from Bresser (!), it is an excellent buy. There is reasonable range of accessories for the microscope including dedicated dark-field and phase condensers and phase objectives. In conclusion, I would not have been disappointed to have had this microscope as a laboratory instrument – it cost less than many single objectives from Zeiss! I will be writing more about using this microscope in dark-field and other applications and also about how I attached a m4/3rds camera to it. Bresser – please talk to your manufacturer about supply this microscope with a swing-out filter holder!
A quick post-script: I bought a refurbished 20X objective from Bresser – superb quality and a very useful magnification to have.
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