Lowering condenser to regulate aperture.

[rhamvossen]

Hi all,

I have an old Winkel-Zeiss microscope of wich the condenser diaphragma cannot be centered, something is misaligned. This is a problem often seen in older microscopes, the cause of which is not clear to me. Anyway, in such cases there is no centered illumination when the condenser iris is used to control the aperture. A solution to this is to open the condenser iris fully and to lower the condenser to the point were 2/3 of the objective back lens is illuminated. With a mirror the light of a fluorescent bulb then can nicely be centered.  While lowering the condenser is seen as bad practice I really see no objections at all. In cases were the condenser iris is misaligned this "bad practice" leads to better results. Have a look at the first picture were fig 1 is with the lowered condenser, fig 2 shows the image from the decentered iris. The specimen is P angulatum taken with the old Winkel-Zeiss objective 42/0.85.







The picture below shows the Winkel-Zeiss microscope, it has a simple 2-lens NA 1.2 Abbe condenser. Te light source is one of those new generation fluorescent bulbs you can buy at the supermarket, they give a very even illumation and are perfect for a microscope with a mirror.



The point is that lowering the condenser to control the aperture doesn't lead to inferior results in my eyes. I also have the impression that there is less glare when the condenser is lowered, improving contrast. Any thoughts on this or other comments?. Best regards,

Rolf

[rhamvossen]

Hi all,

I'm a bit surprised that no one has something to say about this rather fundamental issue. Were are the theoretical people out there that can explain why it should or should not matter if you lower the condenser in stead of using the diaphragma to regulate the aperture? Best regards,

Rolf

[JGR]

Hi Rolf,

For heavens sake what you doing with your lamp. 230V AC and no protective covering !! You are a gambler who plays with his life. Every electrican get heartbeats when seeing that. Children, please do not imitate.

regards
Juergen

[rhamvossen]

Juergen, is that all you can say? Protective covering for what? Do you really think that the stand is under current or what? It's just a lamp in a isolated socket, it's all there is.

Rolf

[JGR]

Sorry Rolf,

That it was somewhat OT but you can reach the bare socket of the bulb with your fingers or other conductive elements and that is the risk.
I'll hope that other will help you for your main problem.

sincerely,
Jürgen   

[treinisch]

Hi Rolf,

I did a little experiment to shed some light onto this problem.

I placed a little sheet of paper on the
object plate so that it is perpendicular to the plate showing the light-cone coming
from the condenser directed towards the objective lens.

Image 1 is showing the cone when the condenser is set up the way that it is usually done (at least to my knowledge,
which is admittedly quite limited)

Image 2 is showing the cone when the condenser is set up the way described by you

Both Images of course at app. the same magnification. And yes, the images show a 5x obejective,
but setup is done using a 10x objective and then switching to the 5x to give enough headroom for the little
sheet of paper.

Is it reasonable to assume, that variant 2 will yeald a considerable loss of brightness and a loss of
contrast due to light scattered by things beyond the area of observation on the objective plate?

Just some suggestions!

best regards

Timm

[rhamvossen]

Hi Timm,

Thanks for your experiment. I have done this experiment too in the past. When the condenser is lowered, there is indeed a larger specimen area illuminated, so you would expect an image with less contrast. But in practice the opposite is true. Something that might be a factor is that the light intensity drops when the condenser is lowered. Best regards,

Rolf