Pete Askew
Admin
Yes, you can certainly achieve quite high magnification by reversing a lens, especially of you combine it with some extension tubes or bellows. This works well with certain biological specimens (insects and flowers) but flatness of field can be an issue with some subjects. The biggest problem is lighting though, as the closer the lens gets to the specimen the harder it becomes to get enough light onto it without causing flare and reducing contrast. Obviously with a transmitted light microscope (and transparent specimens under whatever macro setup you use) this is less of a problem although controlling stray light is still an issue (which is partly why microscopes have complicated condensers below the specimen stage). With opaque specimens it becomes much more difficult as the subject to objective (lens) distance becomes very small at higher magnification and so it is difficult to shine light through the gap onto the 'face' of the specimen. So, the solution is to shine the light down through the imaging lens and this is what is being used on the microscope used for this image.
Here is a picture showing the light source on the left with the incident lighting system in the middle and the objective (lens) sitting on top of it - note that this is an inverted microscope and so the lenses sit below the specimen. I have drawn the light path onto the image and you can see that the light enters from the left (yellow) and then strikes a half-silvered prism in the block with the circle on it. It is then diverted through 90º vertically through the outer part of the lens (see the images of an 8X incident - epi - objective below. The first is an image through the rear of the lens showing both the outer, illuminating 'ring' and the inner, imaging element and the second showing that only the inner, imaging lens element is accessible by the reflected light) so that is strikes the subject. Incident light from the subject (blue) then enters the central part of the objective and passes through the half silvered prism and forms the image that one can look at with the eyepieces or project onto a camera focal plane (film or sensor). I have removed the binocular eyepiece holder so you can see the incident component better but where the blue line ends is another mirror that diverts the light forward to the point at which it will form an image (in this microscope, 250 mm away from the subject) and it is this that the binocular eyepieces etc are used to look at / magnify further.
Sony RX100 V. PP in LR / Nik
Nikon D3 + Zeiss 100 mm Makro Planar ZF f1:2, ISO 320, 1/80s at f1:8.0. PP in LR / Nik
Nikon D3 + Zeiss 100 mm Makro Planar ZF f1:2, ISO 320, 1/80s at f1:8.0. PP in LR / Nik
The lighting unit of this (and many other) microscopes produces Köhler illumination when set correctly. Basically what this achieves is perfectly defocussed light at the subject plane. This is important as it results in very uniform light with no parts of the light source itself being in focus (e.g., the filaments of the bulb) and few colour artefacts. The point at which it enters the microscope (with the light haze around it - the source contains a 12V, 100W projector bulb, parabolic mirror and concentrator lens) is a polarising filter which can be rotated by known angles. To the right of that is a iris diaphragm that lets one control the size of the light patch illuminating the specimen and thus minimising flare etc. The other levers are for dark field etc and are switched out in this configuration. Above the first prism sits the interference contrast unit which splits the polarised light into two out of phase sources. Light reflected by the subject (and therefore containing phase differences) is then recombined and during this process the constructive and destructive interference occurs producing the characteristic in-relief look of DIC - the lever on it allows one to control the colour 'shift'. The light then passes through a second polarising filter (the analyser) set at 90º to the initial one (you'd end up with a double image if you didn't) - you can just make out the knurled adjuster on the right below the incident unit / main housing - to become the image you can visualise through the eyepieces, camera etc.
Here is a picture showing the light source on the left with the incident lighting system in the middle and the objective (lens) sitting on top of it - note that this is an inverted microscope and so the lenses sit below the specimen. I have drawn the light path onto the image and you can see that the light enters from the left (yellow) and then strikes a half-silvered prism in the block with the circle on it. It is then diverted through 90º vertically through the outer part of the lens (see the images of an 8X incident - epi - objective below. The first is an image through the rear of the lens showing both the outer, illuminating 'ring' and the inner, imaging element and the second showing that only the inner, imaging lens element is accessible by the reflected light) so that is strikes the subject. Incident light from the subject (blue) then enters the central part of the objective and passes through the half silvered prism and forms the image that one can look at with the eyepieces or project onto a camera focal plane (film or sensor). I have removed the binocular eyepiece holder so you can see the incident component better but where the blue line ends is another mirror that diverts the light forward to the point at which it will form an image (in this microscope, 250 mm away from the subject) and it is this that the binocular eyepieces etc are used to look at / magnify further.
Sony RX100 V. PP in LR / Nik
Nikon D3 + Zeiss 100 mm Makro Planar ZF f1:2, ISO 320, 1/80s at f1:8.0. PP in LR / Nik
Nikon D3 + Zeiss 100 mm Makro Planar ZF f1:2, ISO 320, 1/80s at f1:8.0. PP in LR / Nik
The lighting unit of this (and many other) microscopes produces Köhler illumination when set correctly. Basically what this achieves is perfectly defocussed light at the subject plane. This is important as it results in very uniform light with no parts of the light source itself being in focus (e.g., the filaments of the bulb) and few colour artefacts. The point at which it enters the microscope (with the light haze around it - the source contains a 12V, 100W projector bulb, parabolic mirror and concentrator lens) is a polarising filter which can be rotated by known angles. To the right of that is a iris diaphragm that lets one control the size of the light patch illuminating the specimen and thus minimising flare etc. The other levers are for dark field etc and are switched out in this configuration. Above the first prism sits the interference contrast unit which splits the polarised light into two out of phase sources. Light reflected by the subject (and therefore containing phase differences) is then recombined and during this process the constructive and destructive interference occurs producing the characteristic in-relief look of DIC - the lever on it allows one to control the colour 'shift'. The light then passes through a second polarising filter (the analyser) set at 90º to the initial one (you'd end up with a double image if you didn't) - you can just make out the knurled adjuster on the right below the incident unit / main housing - to become the image you can visualise through the eyepieces, camera etc.
