I've seen many articles on f-stop printing and would like to give it a try. I do not, however, want to buy an f-stop timer. I am unemployed and cannot afford one. Does anyone know of the formula to calculate the f-stop intervals?

-- Mark Callf (mark@callf.fsworld.co.uk), August 12, 2001

Answers

I can't offhand provide formulas, but to increase exposure time by 1/3 stop, multiply your time by 1.273 (divide by 1.273 to reduce it). And to increase by 2/3 stop, multiply the result of the first multiplication by 1.273 again, and so forth. To get a 1/4 stop change in time, use the factor 1.2. I derived these factors by using actual times provided by my f-stop timer, which is set for 1/12-stop increments: specifically, increasing exposure 4/12 stop from 15 seconds, gave 19.1 seconds, and 3/12 gave 18. Somewhere I figured the formula on paper several years ago but have lost track of it. These increments should do you in practical applications.

-- Keith Nichols (knichols1@mindspring.com), August 12, 2001.

How does this work in practice to make a test strip?

My method is to hold a piece of cardboard over the paper, uncovering a bit at a time from left to right so from the right side I have sections with 5 seconds, 10 seconds, 15 seconds, etc. How would you make a test stip based on stops instead of time? I guess I'm just trying to wrap my head around the math.

-- David Parmet (david@parmet.net), August 12, 2001.

Check out unblinking eye, in it there is a table with the F stops etc. The URL is http://unblinkingeye.com/index.html

-- Jorge Gasteazoro (jorgegm@worldnet.att.net), August 12, 2001.

The Unblinking Eye piece is useful in providing the math with which to calculate exposures. To make a test strip, you have to calculate each of the exposures you want to see and expose the successive segments for the increment required to get there from the preceding segment. E.i., to make a five-segment, 1/4-stop test strip, starting with 10 seconds you would want to see strips receiving 10, 11.9, 14.16, 16.85, and 20.06 seconds. So your exposures would be 10, 1.9, 2.27, 2.69, and 3.21 seconds. I'd run up tables based on the exposure times I usually start with and thereby not have to calculate each time. My f-stop timer automatically makes seven-segment strips, with the suggested exposure in the center and three segments under and three over on either side of it.

-- Keith Nichols (knichols1@mindspring.com), August 12, 2001.

I have been teaching my students to burn and doge passed on fstops for years, long before i found out that there was a timer as well as a method that was popular. Could you share the referrence your talking about. It correlates to camera len use. 1 f stop more is twice as much light, etc. 1 f stop less will be half the light, or time so to speak. Referrence is made to middle gray and what tone you wish the highlights or shadows to be within that relationship. I believe Tim Redman's book has a printed table in the appendix section.

-- Ann Clancy (clancya@mediaone.net), August 12, 2001.

Ms. Clancy: The reference you want is the URL in Jorge Gasteazoro's message above. It is an item by Ed Buffaloe in which he gives the math for figuring exposure times as functions of f-stops and fractions thereof.

-- Keith Nichols (knichols1@mindspring.com), August 12, 2001.

Some thoughts on this: I used the f-stop process of exposure some years ago after reading about it in Camera & Darkroom. After a trial period I found the system to be time consuming and unnecessarily complicated. There is no advantage in the f-stop system to a time- percentage method. For example, from a test strip I know the proper exposure for some notional print will be somewhere between 10 and 13 seconds. I might see that the exposure will be closer to the 10 than the 13. So, guess what the exposure will be???? I don't need to calculate some portion of an f-stop to figure out the correct exposure. This is not rocket science. Things to remember: 1. The most valuable commodity in photography is your time. Want to spend it working a calculator? 2. Photography writers exist by writing articles. Photography magazines exist by publishing new articles. It doesn't really matter to either that the content of what they publish is meaningful. Sure, the f-stop method works, but it is overkill. R.

-- ricardo (ricardospanks1@yahoo.com), August 13, 2001.

If my print exposures are between 7 and 25 seconds I typically just make my test strips at 3 second intervals, but if I need to make tests at longer times I use precise 1/4 stop intervals. Adding 3 seconds to a 40 second expsure doesn't do much--the 1/4-stop increment is at about 48 seconds--so understanding how to calculate accurate 1/4-stop intervals can sometimes speed things up for you. But everyone has a different way of working, and if you aren't having a problem why change things?

-- Ed Buffaloe (edb@unblinkingeye.com), August 13, 2001.

Well, for the formula, a 1/3 stop is the cube root of 2 (from memory, about 1.25). I try to follow this when printing. I made my own timer from an old Radio Shack Model 100 computer, so I wrote the program to do the initial exposure for a test strip at 2,4,8,16 seconds. I then pick the two best exposures and run a second strip at 1/3 stops in between. Most of my printing is in the 8 to 16 second range (8, 10, 12, 16). To make a test strip with these times, you follow this sequence: 4, 2, 2, 8 seconds. The 2-16 sequence is 8, 4, 2, 2. A real F-Stop timer would be nice, but the Model 100 was free and I can make it do just about any time sequence I want.

-- Dave Mueller (dmueller@bellatlantic.net), August 14, 2001.

WOW, higher math! My second hand $15 analite meter by-passes all this!

-- Gene Crumpler (nikonguy@att.net), August 14, 2001.

I have know worked out the formula from your responses. I Have now written an f-stop printing calculator in Visual Basic 5 for the PC. It should be available on my web site once it is uploaded. (www.callf.fsworld.co.uk)

-- Mark Callf (mark@callf.fsworld.co.uk), August 14, 2001.

Iīm not sure the question I have relates to f stops, but here it is: I have a Leika Focomat V35, and canīt figure the need of changing the aperture on the diafragm on the 40mm lens. When closing it, I just end up with a longer expotion time needed, but I cant fing any improvment in the image. When will it be desireble to play with the difragm aperture?

Thanks

-- Francisco Cacho (pacocacho@yahoo.com), August 14, 2001.

OK guys and gals, now my humble $0.02. I seems to me overly time consuming and nit-picky to calculate 1/4 or 1/3 stop intervals for print exposures using square and cube roots, calculators, tables and the like when the same thing can be accomplished much easier. After all, the goal is to make a fine print as easily and repeatably as possible and not to engage in unnecessary calculations. I simply use percentages. Adding 50% to an exposure is the same as a 1/2-stop increase and so forth. For test strips in matched intervals just use an approximate 20 or 30 percent exposure increase between strips (e.g. 5, 6, 8, 10, 13, 17, 22, 29... seconds is close enough to 30% to not matter in practice). All my dodging and burning is done in percentages as well (dodge 10%, burn left edge 30% etc.) so that when scaling up or down or changing paper speeds all one has to do is find the basic exposure and figure the percenages (a lot easier than getting out the calculator to do cube roots) in order to arrive at the proper, and repeatable for any size, dodging and burning scheme. One soon acquires a feel for what a certain percentage of exposure increase or decrease looks like (I think I'll subtract 15% and try this print again) which gives consistent change regardless of basic exposure time, be it 10 seconds or 2 minutes. If you really need to think in f-stops, 30% is close enough to 1/3-stop, 50% to 1/2-stop and 70% to 2/3-stop to not make a practical difference when printing. You can even make these changes by adjusting the aperture if you like (although a 10% aperture change is next to impossible to estimate!), but most of us try to standardize on the sharpest one or two apertures on our enlarging lenses and vary the exposure time accordingly. Regards, ;^D)

-- Doremus Scudder (ScudderLandreth@compuserve.com), August 24, 2001.

This works for me : I compared it with an F-Stop Table and it is sufficiently accurate and absurdly easy. Method : Enter your printing time in seconds into a calculator ie 10 (secs) For + 1/4 F time,press "+",19,then the % key :result 11.9 (sec)...+1/3 F, enter 26....+1/2 F :41...+2/3 F : 58..+3/4F :68 For Less exp ,press "-",then % ,and the figure to enter,as above,is, for 1/4 F less : 16...20...29...37..and 40 for minus 3/4 F.

-- Ian Gordon Bilson (igbilson@earthlight.co.nz), August 25, 2001.