Enlarger falloff

Is it normal for there to be as much as a full top fall off at the

edged of a circle of illumination, no matter what size, no matter the

fact that two different heads were used? Both my Bessler condenser

and colorhead show immediate" falloff from center of at least .2

stops within 2 inches of the center of an illumination circle for an

8x10 print. In most cases this approaches a full stop within an inch

of the edge. All bulbs are good. Can my 150mm El-Nikkor (never used)

be to blame? It appears to look ok. And it doesn't matter how stopped

down the lens is. The enlarger is leveled at all stages but the

falloff seems a bit too much. Whats your experience? I'm new at this.

BTW, I'm using a Minolta meter with a flat incident lens to measure,

but you can see the problem visually.

The repeat question is for those that just peruse the LF forum.

Hi Wayne

 

There must be something very strange on your enlarger equipment.With my LPL 4551 I just got 1/10 fall off to the edges.

Maybe the lamps are not at the right place ( they need a very good adjustment) or with the lens is something not okay.

 

Good luck!

Ermmm.... are you referring to the circle of illumination with the lens shining down to the baseboard without a negative carrier in the light path or *with* a negative carrier in the enlarger?

 

If the former, pop a neg carrier in the light path and measure again - the illumination circle from the lens is much greater than that needed to cover the neg without significant falloff at the edges.

 

If you have 1 stop fall off WITH a negative & carrier in the light path then borrow another lens and try that.

 

Cheers,

Makesure the enlarger head is set for the correct size negative.

It's happened to me before and it's not nice.

There's always going to be some falloff of light from the centre to the corner with ANY enlarger and lens. It's due to the angle that light hits the edge of the printing paper, and comes from the edge of the negative to the lens. Theoretically, it follows the cosine of the projection angle, raised to its fourth power. This is the well-known Cos^4 law.<br>However, this can be modified quite drastically by the lens design. It might be made a little better, or even much worse than Cos^4.<br>The falloff will also change with the enlargement ratio.<br>Anyway, assuming that the enlarger lens is 'neutral', and doesn't modify this natural falloff, then you should expect the corners to be at least half-a-stop down from the centre.<p>BTW, I should ignore the reading you're getting from the incident meter. The only real way to assess how serious the falloff is, is from a print.<br>Most printers just get used to their particular 'lamp', and burn the corners in as a matter of course.

First you need to tilt your sensor toward your light source,

the enlarging lens. Second you cant beat the Cosine 4th Law.

You can reduce the effect by using the longest enlarging that

will allow you to make the size enlargement you need. The Cosine

4th law also works inside your camera and makes your negatives

thinner at the edges than at the center so the two often cancel

each other out. When using super-wide angle lenses on large

format a center spot ND filter is often used to correct the light

fall off. As odd as it may seem the Inverse Square Law of Light

Propagation is working in your camera and under your enlarger. <br>

<br>

For information on the Cosine 4th Law search <a

href="http://www.google.com/">http://www.google.com/</a> <br>

<br>

Hope this helps,

Well after a little fooling around, with a neg carrier in place I should note, I find that I have up to a 1/2 stop in the corners, with falloff of .1 to .2 stops occuring halfway to the edge of a 8x10 peice of paper. In certain directions the falloff is more than others, which would make me think that the lens stage is not level, but this isn't the case per say. I did level it with a bubble level on top of a lens board. My previous measurements were without a neg carrier. Can I expect to see falloff of a 1/3 stop with falloff approaching .4 to .5 stops in the corners? Seems a bit much; Enlarger is a MCRX.

 

It think the only way around it is to either burn the corners or make a neutral density mask using film. I did buy a D2 and will receive it this week, so I look forward to finding out how it compares to the Bessler.

The Cosine 4th Law is a fact of physics. You will find the same effect with an Omega D2 or any enlarger. If the lamp head is "perfect" there will be a loss of light to the edge due to the inverse square law. It not desirable to remove this effect as it happens in your camera. You have less exposure at the edges of your negative so the negative is thinner at the edge. Less light at the edges on your baseboard cancels this out.

 

The shorter your lens, taking or enlarging, the greater the effect the Cosine 4th Law has. In theory you should use an enlarging lens of the same focal length as your taking lens so the Cosine 4th effect is canceled exactly but in practice this is impractical.

 

If you succeed at the baseboard then you will need to do the same at the film plane. If however you have additional light loss at the edges due to an uneven enlarging light source that is best to correct this.

 

---

 

"The probe of the pm2M incorporates a cosine correction feature which is used whenever it is necessary to read a point on the projected image which is not directly underneath the enlarging lens." --Beseler pm2m Color Analyzer instruction manual.

 

I didn�t make this up!

BTW Beseler made a conical 4x5 diffuser the reduces light at the center. I put one in my Aristo D2 cold light head above the standard diffuser. The illumination is now extremely even and the Aristo head prints so fast I prefer a little less intensity. I also prefer to print 35mm with a 6x7 diffusion chamber in my Dicro 45 Color Head and I use the 4x5 chamber for all medium formats.

 

Hope this helps,