Question Re Mixing Chambers

[Richard K.]

OK, I've done Physics in a past life and I used to understand this stuff but, tell me, what exactly is the advantage of using a 6x7 mixing chamber in a 4x5 enlarger over a 4x5 chamber if enlarging 6x7?

Am I right about the following?

If you use a 4x5 chamber but block the light to emerge into a 6x7 area over the neg, there is little difference in emerging brightness (say per suare cm) compared to using the whole 4x5 area.

OTOH if you make the chamber physically smaller, it will be *brighter inside (for the same wattage light source!) and hence brighter at the emergent 6x7 area.

Why, exactly, is the smaller chamber brighter? Isn't the absorption, reflection of light a constant dependent on the chamber's physical characteristics?

Is there a good scientific discussion of this anywhere on line? Thank you!

[Richard K.]

Oh BTW, anyone have a spare 6x7 mixing chamber for a LPL Saunders 4x5 enlarger? :rolleyes:

[cyrus]

I have often wondered about the same thing. The chambers on an Omega are not smaller, only the hole which goes over the negative is smaller. In practice the use of a smaller format chamber for a smaller format negative means that there's less light spilling out of the sides of this hole. That's about the only thing I can think of.

[Richard K.]

So a pin-prick hole should be REALLY bright?!?!?

[ic-racer]

It can be considerably brighter. The mixing chamber is not a mask. It is lined with reflective material. A lot of technology in the Durst two-part 8x10 mixing boxes to narrow the light field, and keep it even and make it brighter. The 35mm mixing box gains over 3 stops. Thats a lot when doing a mural print of 35mm frame. For example, going from a 2 minutes exposure with the 10x10 mixing box to 15 seconds with the 35mm mixing box.

I think an easy way to understand it is to imagine a mirror on the bottom of your 4x5 mixing box with a 35mm hole cut in it. The mirror reflects a lot of light back into the white styrofoam of the mixing chamber ceiling and makes it physically brighter. So brighter light comes out of the 35mm hole.

[ic-racer]

So a pin-prick hole should be REALLY bright?!?!?

Not 'should' but COULD. You'd have to engineer the system right. The ultimate would be to get all the light waves in synch as they come out the pin hole (Laser).

[cyrus]

But apart from thr laser, the brightness should be the same over an area. Whether you have a 1sq inch or 10 sq inch hole it should be the same per sq after it leaves the mixing chamber. {Lasers do the opposite of a mixing chamber - they concentrate light instead of diffusing or "mixing" it} but of course this doesn't take into account that that the larger hole for a 10x10 means there's less surface area for internal reflection and presumably therefore the 10x10 box would be less bright per sq in than the 35mm box which has a greater area of internal reflection as ic-racer points out

Personally I am quite confused by the Durst two-part mixing system. I am trying to set it up to be my only enlarger capable of handing 35mm through 8x10. I have the lower and upper boxes for the 4x5 and 8x10 and have bought the upper boxes for the 6x9 and 35mm but I really don't know if they were all that necessary since they are not significantly diffrerent size wise, or if there are matching lower boxes in existence (nevermind what crazy name Durst would have assigned them)

[Brian Ellis]

On the Beseler MXT use of the mixing chamber cuts the exposure time in half according to Beseler. I thought that sounded good because I used the Beseler/Minolta 45A head which was very slow with black and white printing. So I ordered the mixer, paid about $100 - $150 for it IIRC. Only after I bought it did it dawn on me that I could have accomplished the same thing by opening up the aperture one stop, which would still have left me within the usual optimum for enlarger lenses (one to two stops from wide open).

[ic-racer]

6or if there are matching lower boxes in existence (nevermind what crazy name Durst would have assigned them)

I have the 6x9 upper and lower. So you are saying the 35mm and 6x9 upper are nearly the same?

[gary mulder]

The diffusers in the mixing boxes from the durst are not all flat they are convex. For some of the boxes there where 2 different diffusors two compensate for light falloff of the taking lens.

[Greg Blank]

Not sure LPL supplies a 6x7 per say. I have not seen one in 6 years, and Omega Brandess is the distributer. The 35mm Mix chamber OTOH, has a lens built into the bottom that consolidates the light into a much more narrow output.