Bulb to condenser distance?

[jmxphoto]

I did some forum searches and couldn't find much on this. I'm using a Lucky 90M enlarger and it has the ability to adjust the distance the bulb is from the condenser by about 4-6" or so. I was wondering what the purpose of this adjustment is. My Beseler 23c, which I'm not using due to space constraints, has no such adjustment. Is bulb to condenser distance something that you calculate, set and forget or are there aesthetic reasons for changing it?

 

[Marcust101]

JMX,

I have an old 4X5 enlarger with a similar degree of adjustment, I didn't pay much notice until I had to replace the bulb. I then discovered that the distance of the bulb from the condenser affected the evenness of the light projected, almost like it had be focused or something. I don't know when this works but it did look like the shape of the filament was faintly shown if the distance was changed significantly.

I'd be interested the actual reason too

regards

Marcus

 

[Ian Grant]

It's because at different size enlargements the distance sometimes need s to be varied. The condensers focus the light source to the lens which gives optimal illumination. But of course the position of the lens chages as you focus. Additionally not all bulbs are the same dimensions.

Yes you can calculate the correct difference it's the simple 1/f = 1/u + 1/v

Ian

 

[jmxphoto]

my bad, I just looked at the 23c and it does have a similar adjustment. It just moves the condenser, not the bulb and the calibration markings are for different film formats. Hmmm... interesting.

Ian, what do f, u, and v represent? I'm pretty new at this!

 

[Ian Grant]

Have a look here, go down the page to Image Properties, they use S1 & S2 instead of u & v but it's the same formula. It's far easier to look at a diagram.

It's very simple - f is the focal length.

Ian

 

[dancqu]

My Beseler 23c, which I'm not using due to space constraints,
has no such adjustment.

The greater the distance of the negative carrier from the
condenser the more narrowly focused is the light. The most
distant for only small formats. The condenser close to the
negative carrier can be used for all formats.

Small formats need greater enlargements so the provision
for higher light intensity; the more focused light. A provision
which is something of an extra and which some enlargers do
not have. Their condenser or condensers are set low so
cover all formats. Dan