Light falloff w/ 90mm on 6x7 negative

[lifein2x3]

How many kinds of condenser lenses were made for your enlarger? Usually, multiformat enlargers have several sets of condensers that need to be switched when switching the lens' focal length.

I'm given to understand just the one; the 45M model is from the 1950s.

 

[Paul Sorensen]

How many kinds of condenser lenses were made for your enlarger? Usually, multiformat enlargers have several sets of condensers that need to be switched when switching the lens' focal length.

The Beseler 45M series enlargers have an upper bellows that allows you to adjust for the negative size by moving the condenser lens relative to the negative.

 

[Ryuji]

Paul,

Thanks, come to think of it I remember that but I've always been a Durst user so I was confused.

 

[dancqu]

Usually, multiformat enlargers have several sets of
condensers that need to be switched when switching
the lens' focal length.

That is half true. With the condensers set for greatest coverage
no changes are Needed. But for small area negatives the light level
will be low. A low light level because greater diameter enlargements
are being made, at least for same size prints.

Going from a small area coverage to a greater area coverage
will need rearrangement. Dan

 

[dancqu]

I did adjust through the entire range of avaliable distance
between the light source and the negative plane, ...

A little more understanding of the workings of condensers may
help. The workings of condensers is very simple. They gather light
from a broad source. The flat surface of the condenser sees the
bulb across it's entire surface. The light is gathered and focused,
condensed. The two lenses together have a very short focal
length. Some Beselers lift the lamp house there by projecting
an even more narrow beam of light upon the negative stage.
Adding an auxiliary condenser also narrows the beam.

So why narrow the beam? After all complete coverage of any
designed negative size is assured with the beam least condensed.
The reason for the more narrow beam is a needed increase in the
intensity of light falling on small negatives. They usually receive
a greater magnification.

Condensers are not needed. Most know that. Many years ago
there were a lot of enlargers built which had only a large diffusion
chamber with a contained light bulb. Another design, and the first
of the cold light types, used a bank of fluorescent tubes just
above the negative stage. I had one of those in the late
1950s; a 4x5 Omega and it was war surplus. Condensers
are something extra, an additional feature. Dan

 

[dancqu]

So why narrow the beam? After all complete coverage of
any designed negative size is assured with the beam least
condensed. The reason for the more narrow beam is the
needed increase in the intensity of light falling on small
negatives. They usually receive greater magnification.

I should mention enlarging lens focal length. I started
years ago with 4x5 Omegas, the one I mentioned in my
previous post and two more in the service. Another was
a Beseler. None of the four had any instructions. Never
added or removed auxiliary condensers or adjusted the
Beseler when changing format. Must be the light
levels were always sufficient.

Lifting the Beseler lamp house or adding auxiliary
condensers shortens the effective focal length of the
condenser set. That is, the length of the cone of light
from the negative stage to it's focal point is shortened.

The lens is adjusted up and down for focus. So to
say that the focal point of the condensers must fall at
the lens' optical center is incorrect. I think it correct
to say that any point at which the enlarging lens is
located must fall within the effective focal length
of the condensers.

There is no trick to measuring the effective focal length
of the condensers. Using an empty negative carrier or none
at all and a piece of card stock measure the distance twixt
the negative stage and the focus of the condensers.

The maximum lens bellows extension I believe is then
known. That is, I've not tested but think the lens should
not be extended beyond the focal point of the condensers.
That be the case with a very long bellows even though the
negative is evenly illuminated. Dan

 

[lifein2x3]

Well, I just tried a Nikon 80mm f5.6 enlarger lens, and I had the same problem only worse. So, I have to conclude it's the light source. Something isn't right with the condenser, or I need to switch to a diffusion setup.

Any suggestions for a DIY diffuser? Or something relatively cheap?

 

[Ryuji]

That is half true. With the condensers set for greatest coverage
no changes are Needed. But for small area negatives the light level
will be low. A low light level because greater diameter enlargements
are being made, at least for same size prints.

Going from a small area coverage to a greater area coverage
will need rearrangement. Dan

That's not the proper use of condenser system. In order for the condenser system to function properly, you have to have the light source focused by the condenser lens at the entrance pupil of the enlarging lens. If you don't satisfy this condition, even if you get even illumination of the negative area, you are basically using your light source more diffused than collimated, and you might want to use a diffusion enlarger instead.

 

[kb244]

Hrm odd considering I use my Graflex 90mm f/6.3 optar on my beseler and print 6x7 without a problem, and just this past weekend I just did a few prints off 6x7 with a Nikkor-EL 75mm f/4 lens. I just make sure the condenser set appropriately.

Edit by the way, I apparently did not read thru the entire thread.

 

[dancqu]

In order for the condenser system to function properly,
you have to have the light source focused by the condenser
lens at the entrance pupil of the enlarging lens.

As I've already explained that is not possible save for some
of the Beselers which have infinitely variable condenser focusing.
In order to focus the image the enlarging lens is moved up and
down, so the entrance pupil also moves up and down. Dan

 

[jmdavis]

Well, I just tried a Nikon 80mm f5.6 enlarger lens, and I had the same problem only worse. So, I have to conclude it's the light source. Something isn't right with the condenser, or I need to switch to a diffusion setup.

Any suggestions for a DIY diffuser? Or something relatively cheap?

Tom, see if you can find a copy of the 45M manual. I'm wondering if the enlarger was disassmbled and the condensor lenses put back incorrectly. I had no problems with the 80mm Rodenstock covering 6x7 on my 23C.

 

[lifein2x3]

Tom, see if you can find a copy of the 45M manual. I'm wondering if the enlarger was disassmbled and the condensor lenses put back incorrectly. I had no problems with the 80mm Rodenstock covering 6x7 on my 23C.

I'll have to do that; I can't think of anything else that'd be causing the problem. Although I can't see how the condenser lenses could go together any other way, since they're both identical and they're both pointing inward (convex side in).

 

[michiel fokkema]

Hi,
The condenser is important also.
I can't print 6x6 with 80mm on my 4x5 enlarger with the 6x9 condensor. It only works with the 100mm.

Cheers,

Michiel Fokkema