Ilford Multigrade with dial in filtration?

[Mike O'Pray]

Rob. I was probably not fully explaining what I do with my meter as my main issue was equal times but in fact I meter without filtration on a highlight in the neg with just discernible features and this is the exposure time that gets stored in the meter. Of course the meter has to be calibrated first with a test strip.Then I meter on a shadow detail with just discernible detail and by pressing another button the meter performs a function that works out a suggested grade. This grade is then dialled in on the dichroic head. The probe is placed on the same highlight spot as before and reads the exposure time with filtration in place. For quite a long time I gave things no further thought about exposure times. The prints always looked reasonable and even when some negs on the same roll needed a half grade more or less or occasionally a full grade more/less, it worked quite well. Then I began to think about the raison d'etre for dual filtration, namely constant exposure times for different grades and thought I'd try the experiment of leaving the probe on the chosen highlight and dialling in all the dual filtrations given by Ilford, expecting a constant time and finding that while times were within a small range between say grade 2 and 3.5dae there wasn't a constant time.

By then I had joined a certain analogue users website and saw a link to the Paul Butzi article on establishing the curves for Y and M to ensure that times were constant. I never tried to establish my own curves. I didn't have a Stouffer step wedge for a start.

At no point do I remember the article mentioning what the constant time represented( mid-grey/zone VI?) and in my naivety assumed that that it was a constant highlight time as this was what my meter was designed to measure for correct print exposure.

So I then assumed that I hadn't got constant times because I hadn't done the curves for my machine, my paper etc but I just got on with things.

Then I saw the Ralph Lambrecht site called Darkroom Magic where he very kindly makes public a lot of info and lo and behold discovered that he had produced dual filtration for both Ilford MG and Agfa papers and on a Durst enlarger with the same number of units as mine, namely 130.

Maybe this was the answer and I didn't need to do my own curves anyway. I noted that his dual filtration for both Ilford paper and Agfa paper differed from Ilford's and Agfa's own dual filtrations. So maybe these differences were what produced the elusive constant times i.e his filtrations were spot on and Ilford's and Agfa's were slightly out.

I tried the same experiment with my probe, using RL's filtrations and as you know from my previous posts this didn't get me constant times either.

Then just to complicate things or maybe expand on things to give a fuller picture, RL seemed to be saying in another section and table he produced that even with his dual filtrations, some amendment to times were necessary.

If I have understood his table correctly then when you change from one grade to another, you add or subtract fractions(1/12ths I think) of an fstop to get to the elusive constant times.

I then speculated that maybe my probe was in fact reflecting RL's table of fstop alterations when it came up with different times for different filtrations. However as I don't work in fstops this was going to be difficult to work out. I have yet to try it but I'd need to know the formula for working out how to arrive at 1/12ths of a fstop of whatever the probe said the exposure time was. RL does give a fairly comprehensive table of times but unless my probe time is exactly the same then there aren't of much value.

So can you or anyone else explain how you work out fractions of an fstop for any exposure time and I can use this to see if my probe times in fact correspond to or more nearly correspond to waht RL's table says the corrected times need to be for grade changes.

Finally can anyone offer an explanation why RL's table only allows for grade changes within a restricted range? That is to say it covers say a two grade change but no more. It doesn't give changes from say grade 1 to grade 4.

If my reference to RL's table isn't clear, it will hopefully be clear if you have a look at it on the Darkroom Magic site.

So that's my story. It's sad but true. This last sentence refers to the DS site. That's DS as in Del Shannon. He was refering to "Runaround Sue" in 1961 who was nearly as frustrating as runaround dual fitration but frustation with Sue vanished with teenage angst, whereas darkroom frustration gets worse with age not better.

Mike

[Argentum]

The way you have described how you use your meter means the following (I think).

The first set of readings with no filtration determine contrast and time. The second set of readings with the filtration dialed in allows the meter to calculate the filter density change on that highlight value and therefore caculate an appropriate time change to allow that highlight value to stay constant.
i.e. it automatically makes that highlight value the speed point for that exposure, thereby overiding the speed point of the combined Y+M values.
That means you don't have to worry about the natural speed point of the combined durst Y+M figures except that they should ideally give even spacing between grades which they do on ilford papers. For that reason, I would suggest you don't need to use dual fltration with your meter as you can use just Y or M as the meter works out the speed point for you.



When using Ralphs figures, they will also be overidden.
If you want to find out if his fugures are good, then you must print without using the meter and using a step wedge and then chart the results to see if they all cross at the same point.

[Argentum]

the time factor for any fraction of an fstop is:

2^(fraction) so a twelth is 1 divided by 12 = 0.08333333

2^0.08333333 = 1.059463094

so if you have a time of 12 seconds, then if you multiply that by 1.059463094 twelve times, you will get 24 (near as damn it).

A quarter stop time factor would be 2^(1/4) = 2^0.25 = 1.189207115
A third stop time factor would be 2^(1/3) = 2^0.33333 = 1.259920759
A half stop time factor would be 2^(1/2) = 2^0.5 = 1.414213562
etc

Recognise that 1/2 stop factor. You should do, its the square root of 2. The 1/3 stop factor is the cubed root of 2 and the 1/n the nth root of 2.

So as quick test, meter a negative as normal before putting in any filtration and it gives you a time. Then multiply that time by the half stop factor and then close down your aperture by 1/2 stop and re-meter and you should get that time.

What about opening up aperture or reducing filtration. Simple, you use a negative exponent so a 1/3 stop would be 2^-0.333 = .793718864
You can work out the rest.
Have fun...

[Mike O'Pray]

Originally Posted by percepts

the time factor for any fraction of an fstop is:

2^(fraction) so a twelth is 1 divided by 12 = 0.08333333

2^0.08333333 = 1.059463094

so if you have a time of 12 seconds, then if you multiply that by 1.059463094 twelve times, you will get 24 (near as damn it).

A quarter stop time factor would be 2^(1/4) = 2^0.25 = 1.189207115
A third stop time factor would be 2^(1/3) = 2^0.33333 = 1.259920759
A half stop time factor would be 2^(1/2) = 2^0.5 = 1.414213562
etc

Recognise that 1/2 stop factor. You should do, its the square root of 2. The 1/3 stop factor is the cubed root of 2 and the 1/n the nth root of 2.

So as quick test, meter a negative as normal before putting in any filtration and it gives you a time. Then multiply that time by the half stop factor and then close down your aperture by 1/2 stop and re-meter and you should get that time.

What about opening up aperture or reducing filtration. Simple, you use a negative exponent so a 1/3 stop would be 2^-0.333 = .793718864
You can work out the rest.
Have fun...

Rob It has been many years since I have had to do any but very basic maths so you'll need to bear with me a little. I have understood how you arrived at 0.083333 as the decimal equivalent of 1/12th but have failed to follow what 2^ means which is needed to get to 1.05946.

I have then understood why I need to multiply my exposure time by this factor to arrive at the time for 12 secs plus 1/12th stop and I think that I then multiply this resultant figure by 1.05946 to get to 2 x 1/12th stops etc

Maybe I'll understand the "negative exponent " example as well once I grasp the 2^.

Thanks

Mike

[Argentum]

The ^ character is used on the web to mean power or exponent. So 2^2 means 2 raised to the power of 2. Or 3^1.259920759 means 3 raised to the power of 1.259920759.
You will need a calculator which has a button for raising numbers to a power to work those out.

Yes it is the result which is multiplied each time.

You can divide the time by the factor instead of using the negative exponent if you like. So (12 / (3^0.333)) is the same as (12 * (3^-0.333 ))

[Argentum]

If it makes it any easier you can convert those time factors to percentages.
1/12 stop is 5.95% or rounded to 6% which an easily remembered number. So if you want to make a 1/12 stop adjustment using time. Then its just time plus 6% of time. So 12 seconds plus 6% = 12.72 seconds and that is simple enough to do in your head without resorting to a calculator or lookup table.

[Mike O'Pray]

Thanks. My basic calculator is therefore useless for such things. I had tried everything I could think of on my calculator short of tapping out "Scotland The Brave" on it but to no avail.

Just as an addendum I had another look at RL's site and it appears he is claiming that with the exposure factor table you can use highlight readings( he gives factors for both zones VII and VIII to arrive at exact times for constant exposure.

It appears to involve running test strips at each dual filtration at 1/12th fstops to arrive at times which give zones VII and VIIIs and inputting these into an excel sheet which he provides but I am not sure I completely grasp what he is saying as his explanations assume a certain level of knowledge and do not start from first principles.

It also seems that within a very restricted range of grades, movement half a grade either way call for no change to time and one grade change depending on what the old and new grade can be as little as 1/6th of a stop which might hardly be noticeable.

This is a speculatiive conclusion on my part but if you were to do contrasty negs such that the vast majority were right for prints at grade 2 then you might produce pretty good prints from those negs calling for grade 3 as well without having to work out a compensating time. If the neg variation was any more than this however then constant times produce prints which are noticeable less than optimum

So 3 MF backs or 3 35mm SLRs to do N, N-1 and N+1 might enable you to forget about the problems we've discussed.

Mike

[Mike O'Pray]

Rob Thanks again for the easy method. Our last two posts crossed and my useless calculator comment pertains to factors not percentages. I might even manage a quick and dirty 6% of a time in my head thanks to 1950s and early 60s schooling.

Remember needing to work out how much change the boy was due when he bought 16 apples at 3.5d per apple and had handed the greengrocer two half crowns?

You don't see such thing in maths books nowadays. I think they banned such questions under the Geneva Convention or was it Human Rights legislation?

Mike

[Argentum]

see earlier post:

http://www.film-and-darkroom-user.or...3&postcount=36

[Argentum]

As a follow up on this, I re-tested MGIV FB. First I used some of what I think is the new formula but I have had it for several years. I was somewhat shocked by the result which you can see in the first chart. Only ISO(R) 93 for grade 5 which is really bad. At first I thought the paper was creamy colour but it dawned on me that this paper had gone off (badly) so I bought some new. And you can see the result in the second chart where I get ISO(R) 46 for Grade 5. This is damn close to Ilfords claimed 40 and only goes to show that dichroic filters are perfectly capable of achieving high contrast.
Even better is that the G2 Y+M curve almost matches the zero filtration curve exactly. The moral of the story is to only use very fresh paper for doing this kind of test otherwise it proves nothing. N.B. Dmax was 2.24 which is more than 7 stops range.
I will also post charts for MG Filters on fresh paper shortly.