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Old 14th January 2010, 12:04 PM
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Default Durst M307 BW - Uneven exposure

Hi all,

I've recently acquired a Durst M307 for B&W from the local Sixth-Form college (they've upgraded to colour enlargers throughout), but I'm having problems with uneven exposure. Does anyone here have any experience with these enlargers?

I've adjusted the bulb position as well as I can, but there is still about half a stop's fall-off towards one end of the frame (using 50mm lens and 35mm neg carrier).

I've cleaned and checked the alignment of the condenser, mirror, lens-board etc.... but these made no difference. I've got a spare bulb on order, and I can work around the problem by burning-in that end of each print, but it's bugging me! Can anyone suggest anything else to try?

Thanks!

Andrew
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Old 14th January 2010, 01:26 PM
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Andrew, at risk of stating the obvious, as I don't know this particular model, does it have the correct condenser(s) fitted? I remember years ago printing some 35mm negatives on a Durst enlarger which had condensers fitted for 6x6cm negatives and I wondered why the edges had such full-off
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Old 14th January 2010, 02:57 PM
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Hi Trevor, thanks for your reply!

I wondered, but am not sure quite how to tell - I'll take it apart again. I do know that it only has one condenser, but I'm pretty sure it's only meant to have one (can anyone confirm?). Now I think about it though, it didn't have a lens when it was pulled out of the store for me, and the lecturer found me a spare 50mm lens and lens-panel for me, but maybe it had originally been set up for 120 film. I'll check with him again next week.

Thanks again,

Andrew
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Old 14th January 2010, 03:17 PM
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Mind you, wouldn't one expect the fall-off to be at both "ends" of a 35mm frame if it was the wrong condenser? It seems to be more at one end than the other...

Andrew
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Old 14th January 2010, 03:58 PM
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Quote:
Originally Posted by Trevor Crone View Post
I remember years ago printing some 35mm negatives on a Durst enlarger which had condensers fitted for 6x6cm negatives and I wondered why the edges had such full-off
I've never used a condenser enlarger and don't know how the optics work in detail, but this sounds counter-intuitive to me - surely if the light covers 6x6 wouldn't it be perfectly even over the smaller 35mm area?
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Old 14th January 2010, 04:15 PM
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The bulb should be moveable up and down and left to right by tilting it. Also the mirror that reflects light down should be square and the condensor should be sitting flush on its mountings. Assuming all of that is correct then you may have a problem with the neg and lens stage.

The neg stage should be 100% parallel with the lensboard stage and the lens must mount perpendicular to the lens board. That is a bit tricky to test but the quick and dirty way is to score a fully exposed neg (film leader) from corner to corner with a pin and place in enlarger. You can then check with focus finder that the scored line is sharp at each corner. If not then adjust enlarger alignment.

It does sound as though you have a light or mirror positioning problem though.

Obviously enlarger column must be perpendicular to basebaord/easel too.

I assume negs have even lighting across their surface.
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Old 14th January 2010, 04:16 PM
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Quote:
Originally Posted by RH Designs View Post
I've never used a condenser enlarger and don't know how the optics work in detail, but this sounds counter-intuitive to me - surely if the light covers 6x6 wouldn't it be perfectly even over the smaller 35mm area?
You have different condensors for each film format.
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Old 14th January 2010, 04:59 PM
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Quote:
Originally Posted by RH Designs View Post
I've never used a condenser enlarger and don't know how the optics work in detail, but this sounds counter-intuitive to me - surely if the light covers 6x6 wouldn't it be perfectly even over the smaller 35mm area?
Indeed, it does sound counter-intuitive, but there was clearly observed fall-off, probably compounded by using a 50mm lens and not an 80mm?
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Old 14th January 2010, 05:10 PM
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Thanks everyone for thinking about my problem!

Quote:
Originally Posted by RH Designs View Post
I've never used a condenser enlarger and don't know how the optics work in detail, but this sounds counter-intuitive to me - surely if the light covers 6x6 wouldn't it be perfectly even over the smaller 35mm area?
That is what I'd expect too - and if you use the condenser for a larger format than your negative I'd just expect the projected image to be dimmer. However, I'm quite happy to believe that my understanding of the optics is deficient - I've never been quite sure of how a condenser is supposed to provide even illumination!


Quote:
Originally Posted by percepts View Post
The bulb should be moveable up and down and left to right by tilting it.
It moves up and down, and can rotate on its axis, but doesn't seem to tilt.

I'll check the alignments of mirror/condenser/neg stage again.

Quote:
Originally Posted by percepts View Post
That is a bit tricky to test but the quick and dirty way is to score a fully exposed neg (film leader) from corner to corner with a pin and place in enlarger. You can then check with focus finder that the scored line is sharp at each corner. If not then adjust enlarger alignment.
This is brilliant! Thank you for the suggestion.

Quote:
Originally Posted by percepts View Post
I assume negs have even lighting across their surface.
Do you mean that the negs are evenly exposed? Yes, a scan of a neg comes out much more even than an enlargement.

Thanks again, everyone.

Andrew

Last edited by arcr1; 14th January 2010 at 05:11 PM. Reason: For style...
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Old 14th January 2010, 05:25 PM
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opal bulbs used to be tested to make sure the element was centred in the bulb. But these days the manufacturers produce garbage. So you need to be able to centre the element. You can do this by rotating the bulb which would save needing it to be able to tilt. If you have a spot meter you can check brightness across baseboard as you rotate / shift bulb positions.

The trick with scoring a neg and checking sharpness only works to a point. It will give you the best alignment for the current state of film plane and lens alignement. It won't correct for film being out of parallism with condensor base or lens being out of perpendicular with neg stage. You need special equipment for perfect setup using either mirrors or laser. But for most purposes the scored neg will get you close.

I have a durst modular 70 with a condensor head. The condensor for 35mm is only one lens and not two. You probably use the exact same condensor as mine as a lot of durst parts for the smaller enlargers are interchangeable.
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