Time calculations when switching print sizes

Dave Miller · Jul 10, 2006

RH Designs said:
Just done a rather more scientific check and you're absolutely right - if you substitute lens-paper distance for print length in my previous post then it will work accurately.

Good, I'm glad that's settled.

Dave Miller · Jul 10, 2006

Could I expand on the above a little by suggesting that it is the image size, and the lens node to paper surface that are the important dimensions. Paper size is not important.

Chan Tran · Jul 10, 2006

One can mark the negative to paper distance on the enlarger column but can not do so for the lens to paper distance, but lens to paper distance is what we really need. Now can we calculate the lens to paper distance if we know negative to paper distance as well as the lens focal length?

Dave Miller · Jul 10, 2006

Chan Tran said:
One can mark the negative to paper distance on the enlarger column but can not do so for the lens to paper distance, but lens to paper distance is what we really need. Now can we calculate the lens to paper distance if we know negative to paper distance as well as the lens focal length?

Chan, why do you want to calculate the lens/paper distance when it is quicker to measure it?

Chan Tran · Jul 10, 2006

Because I want to install an encoder in the enlarger head lift and display the height directly. If I can calculate the lens distance then I can make the gizzo display the exposure time automatically. Just for fun really AS we all know it's much easier to measure the light intensity.

dancqu · Jul 11, 2006

Chan Tran said:
AS we all know it's much easier to measure
the light intensity.

At a basic level that's where the EM-10 comes in.
RH or some other party should introduce an upgrade.
The upgrade would have linear response and digital
readout. Upgrade Plus would have some memory.

Half a mind to put one together myself. I think it
likely a rather straight forward off the shelf type of
project. Basic, $74.50, Plus $94.50. Dan

Papa Tango · Jul 11, 2006

For simple changes (5x7, 8x10, 11x14) I use my handy dandy Beseler PM2M. Once I have determined the exposure for a particular size, I find an area near or approaching middle grey. Null out, and change the enlargement ratio. Then, open the aperature a stop or so until the meter nulls again. Perfect every time!

If I find a spot with a densitometer beforehand that is middle grey, I can use the pre-programmed modules to make changes across a variety of papers or contrast grades. I have modified my analyzer for completely BW, meaning that all four metering positions are balanced for white light. This simply involves replacing the color gels on the sensor tube rotator with white diffusion gel and bench aligning. One module can then provide me with four different contrast grades for each paper.

This method of course does not change the time factor, which can be an issue with some dense negs that require opening the lens past optimal.

RalphLambrecht · Jul 14, 2006

Chan Tran said:
Now can we calculate the lens to paper distance if we know negative to paper distance as well as the lens focal length?

Sure you can. You do it by using the magnification factor.

lens-to-paper distance: d1 = f*(1+m)
neg-to-lens distance: d2 = f*(1+1/m)
neg-to-paper distance: d = d1+d2

Chan Tran · Jul 14, 2006

thanks Ralph! Your equations requires that we know the magnification factor, which we can find by comparing the size of the projected image / the negative size. However, I think we can figure out d1 and d2 with only the focal length f and the sum d1+d2 are known.

Chan Tran · Jul 14, 2006

Assuming the magnification is greater than 1, I can use the following formulas to calculate d1 and d2 from the known value of d and f:
d1= -d - sqrt(d^2 - 4*f*d)/ -2
d2= -d + sqrt(d^2 - 4*f*d)/ -2

If the magnifcation is less then 1 then d1 and d2 are reversed.
what do you think?

RalphLambrecht · Jul 14, 2006

1. Measure the distance to the nodal point of the lens.
2. Solve the first equation for m.
3. Solve the second equation for d2.
4. Solve the third equation to get the neg-to-paper distance.

That is what you wanted, isn't it. A way to get from the lens-to-paper distance to the neg-to-paper distance.

Chan Tran · Jul 14, 2006

No Ralph! I want a way to get from the neg to paper distance to the lens to paper distance, not the other way around. The reason is that the lens to paper distance is what we need to calculate exposure time but the neg to paper distance is what we can derive from the scale on the enlarger column. However, like in my previous post I have figured it out.

pnance · Jul 14, 2006

If you measure the height from the bottom of the lens to the paper with a metric tape you can use an easy equation, (ot/oh^2)*nh^2=nt, (old time divided by square of old height) times square of new height equals new time. If you put this in a programable calculator it makes minor changes in the height of the enlarger easy to adjust for. I have it programmed in a TI-83. Works like a charm. Having a calculator handy also helps when you change paper types.

RalphLambrecht · Jul 15, 2006

I see. Well done!

RalphLambrecht · Jul 15, 2006

I think you need to change these to:

d1= (-d - sqrt(d^2 - 4*f*d))/ -2
d2= (-d + sqrt(d^2 - 4*f*d))/ -2

Since your equation doesn't include the magnification, I see no need to reverse d1 and d2 in case of m<1.

Chan Tran · Jul 16, 2006

Thanks Ralph! I wrote the equations by hand and use the long horizontal line for divide so I didn't need the bracket. I forgot to put them in when translate the equation to computerized form.
The 2 equations are solutions to the quadratic equation -d1^2 +d*d1 -f*d =0 and thus the top equation always yields the larger distance. When the magnification is <1 then d1 < d2, but using the 2 equations I posted the d1 is always the larger one.
I derived the above quadratic equation by using the equation 1/d1 + 1/d2 =1/f and substitute d2=d-d1.

Chan Tran · Jul 17, 2006

I think some of my posts as well as Ralph's are deleted from this thread.

RalphLambrecht · Jul 17, 2006

I think so too. It happened yesterday or the day before. Nevertheless, I think we had this one nailed anyway.

Time calculations when switching print sizes

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