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Exposure Compensation Chart (pdf file)
Subject: Exposure Compensation Chart
From: mccoll@gte.net
Date: 1998/05/19
Newsgroups: rec.photo.equipment.large-format
This is the long explanation to a short chart.
Peter and Labrat rightly point out the differences between my excessive
striving for perfection and the realities of photography in the field.
Nonetheless, I often work toward reliability and accuracy despite the weak
links in my procedural chain of aperture ring, shutter speed, voltage
fluctuations, etc. In this way I know I'm doing everything I can to bring
the art of photography into the predictability of the science of
photography. I am also compensated by the assurance that I can at least
partially predict how new and novel photo problems can be resolved without
reinventing the wheel each time. That is, I can eliminate certain variables
because I have already determined their parameters and effects (as much as
my math, equipment and procedures will allow).
The goal, of course, is to convert ritual into formula, thus allowing "art"
to somersault
gracefully into the clean air of creativity, diving off the reliable
springboard of accurate
calculation.
So, what kind of chart or formula would be of most use to field
photographers who find themselves working somewhere in the magnification
range between 1:1 and infinity? Measuring film-to-lens distances is
probably easiest (as opposed to measuring subject length at mid-plane of
focus, or lens-to-subject distance). Perhaps a cross-platform basic chart
would be useful because many of us also shoot small and medium format?
So let's try this approach, we'll calculate 1/3 and 1/4 stop bellows
factors for symmetrical and non-symmetrical lenses in 3 film formats
regardless of lens focal lengths. This is a neat trick so stay tuned.
1.
First we need to know where to measure from each time. The front of the
lens? The middle? Let's find a nodal point.
Most accurate:
Set up your camera for a 1:1 magnification.
Measure the distance between the subject and the film plane. Divide the
distance exactly in half. Mark that halfway point on the side of the
shutter.
Less accurate:
Focus the camera at infinity or some other faraway object (like the moon, or
winning the lottery). Measuring from the film plane, mark a point on the
lens or shutter
which is exactly your lens' stated focal length away from the film plane
(150mm or whatever).
Now that your measuring points are clearly marked. Use them each and every
time for consistency.
2.
We now need to do some basic math to determine factors for 1/3 and 1/4 stop
adjustments. I have found that working over a two stop range, from f 1 to f
2 to be sufficient for accurately calculating all other f-stop ratios.
A one f-stop adjustment either doubles or cuts in half the amount of light
being transmitted. Thus, the aperture sequence is based on the square
(and square root) of 2.
1/3 and 1/10 stops do not fit into such a sequence. How are they calculated
numerically?
The way I do it is to calculate a single stop into 32 segments (2 to the 4th
power). The actual sequence looks like this:
1.00000000
1.01088928
1.02189715
1.03302488
1.04427378
1.05564518
1.0671404
1.0787608
1.09050773
1.10238258
1.11438674
1.12652162
1.13878863
1.15118923
1.16372486
1.17639699
1.18920712
1.20215673
1.21524736
1.22848054
1.24185781
1.25538076
1.26905096
1.28287002
1.29683955
1.31096121
1.32523664
1.33966752
1.35425555
1.36900242
1.38390988
1.39897967
1.41421356
Keep in mind that this sequence is a RATIO. Each number in the series has
the same relationship to the next larger and next smaller number. We cannot
simply divide the difference between f 8 and f 11 into 3 equal steps. Such
a division would be innaccurate because the ratio between each division
would be different.
But by sequencing the range between stops into 32 steps, based on the square
root of 2, we can closely estimate where a 1/3 stop might express itself
numerically.
1
1.120 (1/3 stop)
1.126 (2/3 stop)
1.414
Quarter stops are relatively simple:
1
1.091 (1/4 stop)
1.189 (1/2 stop)
1.297 (3/4 stop)
1.414
These numbers will be used to calculate magnifications for a base chart.
I'll quit here for now and post some more in a few days.
Jerry
-------------------------
Exposure Calibration Chart, Part 2
Previously I shamelessly bragged about how I was going to construct a bellows
extension chart that would not use lens focal length as part of the chart.
In that same discussion we looked at f-stop sequences and how to calculate
them out to nauseating but useful extremes. The resulting sequences we will
now use are in ¼ stop and 1/3 stop increments. These sequences are:
(1/3 stop) (1/4 stop)
1 1
1.120 1.091
1.269 1.297
1.414 1.414
1.580 1.542
1.780 1.682
1.834
2 2
These numbers are related to magnification in a direct way. The formula is
(e.f./f)-1 = M. That is, effective f-number divided by the f-number =
magnification.
Or, to put it another way, subtract 1 from an f-number and you get the
magnification at which you need to make that amount of correction (using
f 1
as the starting point). For example, subtract 1 from f1 and you get zero. At
zero magnification you need to make zero correction.
Subtract 1 from f1.414 and you get .414. At .414 magnification you need to
make one stop correction (the difference between 1.414 and 1).
Subtract 1 from f 2 you get 1. At a magnification of 1 (lifesize) you
need to
make a 2 stop correction.
This is the basic relationship between magnification and f-numbers, and it
assumes we are using a SYMMETRICAL lens, with a PUPILLARY RATIO of 1, which
often we are not.
So a simple base chart for 4x5 might look something like this:
Mag. Subject size Image size f-stop adjustment
(long dimension in inches)
.091 55 5 ¼
.120 41.6 5 1/3
.189 26.5 5 ½
.269 18.5 5 2/3
.297 16.8 5 ¾
.414 12 5 1
.542 9.2 5 1 ¼
.580 8.6 5 1 1/3
.682 7.3 5 1 ½
.780 6.4 5 1 2/3
.834 6 5 1 ¾
1 5 5 2
But this chart is accurate only for a lens with a pupillary ratio of 1. Ok,
Jerry, I hear you ask. How can I tell if my lens has a pupillary ratio of
1?
Well, lets pull out the damn thing and measure it, shall we? Usually,
process lenses of a symmetrical lens design designed for close-up flat field
photography have pupillary ratios close to 1. Wide angle and telephoto lenses
have pupillary ratios greater than or less than 1. These pupillary ratios
affect the above chart.
To measure your lenses pupillary ratios do the following:
1. Take the lens off the camera. 2. Open the shutter (if necessary) and open
the diaphragm fully. 3. Hold the lens away from you toward a well lit surface
or window and look thru the FRONT of the lens. 4. With a ruler, measure the
diameter of the entrance pupil (use metric, its more civilized). Write the
number down. 5. Now flip the lens over and repeat the procedure for the exit
pupil. Write that number down. 6. The pupillary ratio is simply the exit
pupil diameter divided by the entrance pupil diameter.
Telephoto lenses usually have pupillary ratios between 0.3 and 1. Wide angle
lenses have pupillary ratios from 1 to 2.0 or greater. It is these differing
pupillary ratios which make generic bellows extension charts unreliable. For
example, my 90 mm Fujinon has a pupillary ratio of 2. At magnifications near
lifesize, my exposure compensation is almost 1 full stop different than the
above generic chart.
I will conclude this segment here. The next installment will be a chart of
exposure compensation in f-stops plotting pupillary ratios to magnifications.
This will be a base chart applicable to ANY film format and ANY focal length
lens.
--------------------------------
Exposure Calibration Chart, Part 3
In Part 1 we looked at f-stop sequences and saw how they are based on the
square and square root of 2.
In Part 2 we discussed the direct relationship between f-stop and
magnification and got our first look at pupillary ratios and how to
measure them.
Hopefully by now you are getting a glimpse of how f-stop adjustments
actually work. Effective aperture is based on MAGNIFICATION and
PUPILLARY RATIO.
The focal length of the lens is irrelevant!
The focal length of the lens is irrelevant!
The focal length of the lens is irrelevant!
The only reasons lens focal length enters into any calculation is when
we wish to know DISTANCES - from lens to film, lens to subject, subject
to film, etc. But we dont need to know the actual focal length in order
to make an extremely accurate bellows extension chart for any
particular lens! All we need to know is the magnification and the
pupillary ratio!
The following link is a chart in PDF format. Save it and print it
out for your reference file. Ive never seen another chart like it
(though Ive never really looked).
http://home1.gte.net/mccoll/980522.pdf
This Universal Chart is an accurate base chart for all film formats and
all lenses. To use it you must know a few things about your own system:
1. The pupillary ratio of each lens in your arsenal. Measure your
lenses as I instructed in Part 2, and record the information in your
records or on the chart itself.
2. How to calculate magnifications. Briefly, its Image Size divided
by Object Size. I normally use the long dimension of my camera format:
4x5 = 5 or whatever size is easy to measure on the groundglass.
35mm = 36mm (the actual long dimension of a 35mm frame)
So heres where the Universal Chart comes into play. Since this is a
large format newsgroup, well stick with 4x5 format.
Put the lens (for which you have already determined the pupillary ratio)
on your camera, and mark a 4 inch line on the ground glass.
4 inches times 0.1 magnification equals 40 inches. So adjust the camera
and lens so that an object exactly 40 inches long is reduced to the 4 inch
line marked on your ground glass. Be sure its in focus.
Now go to the chart and find the f-stop adjustment for your lens
pupillary ratio at the .1 setting.
At this point you can mark the rail, mark a ruler or string dedicated
to that lens, measure the film-to-subject distance, or use whatever
method seems easy and convenient for referencing that distance in the
field or studio. Personally, I use film-to-subject distance, but any
method will work as long as its reliable.
Note that the pupillary ratio has a BIG effect on the f-stop adjustment.
For example, at 1:1 Magnification (life size) the f-stop adjustment for
a wide angle lens is close to 1 stop, while the adjustment for a telephoto
lens can be as much as 3 or 4 stops. Thats quite a difference!
Conclusion:
In writing this paper I threw together a spreadsheet that calculates
out all aspects of focal length distances, f-stops, magnifications, etc.
It would be nice to see a web site where someone could input data and get
an instant calculation of all necessary information for their camera system.
I hope this information has been of use.
Jerry McCollum
Subject: Re: Exposure compensation chart for bellows extension
From: LabRat stickley@crosslink.net
Date: 1998/05/14
Newsgroups: rec.photo.equipment.large-format
There are a few too many variables among lenses to have a simple chart
for bellows extension.
Jerry's right but...
I focus each lense on infinity (eg, the moon) and measure and record the
distance between standards along my rail (This is not focal length but
gives a reference) Then, when doing close-up, I measure again and
calculate the ratio. Ratio squared is APPROXIMATELY the added light
needed to illuminate the film the same as it would have been at
infinity. Not precise but film latitude saves my bacon .... Labrat
Subject: Re: Exposure compensation chart for bellows extension
From: Peter De Smidt pdesmidt@fdldotnet.com
Date: 1998/05/13
Newsgroups: rec.photo.equipment.large-format
Jerry McCollum wrote: > > There are a few too many variables among lenses to have a simple chart for > bellows extension. Any chart generic enough to cover all lens types is > invariably inaccurate. Pupillary ratio differences between wide angle and > telephoto lenses alone will render generic charts useless and throw > exposures WAY off. > It's true that using a chart won't work for telephoto lenses, since we often don't know what their optical center is. But most lenses are not telephoto, and so the chart should be good enough. > I have calculated bellows extension factors for various lenses in my > reproduction studio to 1/10 stop. Why this excessive accuracy? My > electronic flash units and digital spot meter are this precise, so why not? > The manual diaphragm on the lenses is the least precise factor in the > system, so I use an optimum aperture and leave it at that setting. I adjust > my lighting to compensate for bellows extension, accurate to 1/10 stop. > That's all well and good in a studio, even though I doubt your shutter is accurate to 1/10th stop, although with strobes the shutter accuracy would not matter. I also doubt that your spot meter is that accurate. What about flare in the meter, differences between meter's sensitivity and the sensitivity of the film, voltage fluctuations in the strobes... Mind you, I'm not saying that in your circumstances striving for as much accuracy is not ok, but only that I doubt you actually achieve 1/10 stop accuracy. In the field, such accuracy would not be needed nor practically possible, especially, if one uses older lenses. > If anyone is interested I can post some methods and calculations for > constructing your own own charts for your lenses and equipment. It's easy > once you understand some basic relationships between magnification, focal > length and pupillary ratios. > Please do. > FYI, most people don't even know the ACTUAL focal length of their lenses. > They assume a "150mm" lens is what it says it is. Usually not so. > > How do I measure the actual focal length of a lens? Briefly, obtain a 1:1 > magnification, divide the film to object distance by 4. > Your probably right, but I doubt that the difference in the field would be all that important. One could easily test to see if it was important. Most film speed/development testing is done with the lens focused at infinity. OK. Now run the test at 1:1. Do the negative densities match what is predicted by the formulas? If so, great, if not does the variation matter? If so, adjust your chart. Regards, Peter De Smidt
Subject: Re: Exposure compensation chart for bellows extension
From: "Jerry McCollum" mccoll@gte.net
Date: 1998/05/13
Newsgroups: rec.photo.equipment.large-format
There are a few too many variables among lenses to have a simple chart for
bellows extension. Any chart generic enough to cover all lens types is
invariably inaccurate. Pupillary ratio differences between wide angle and
telephoto lenses alone will render generic charts useless and throw
exposures WAY off.
I have calculated bellows extension factors for various lenses in my
reproduction studio to 1/10 stop. Why this excessive accuracy? My
electronic flash units and digital spot meter are this precise, so why not?
The manual diaphragm on the lenses is the least precise factor in the
system, so I use an optimum aperture and leave it at that setting. I adjust
my lighting to compensate for bellows extension, accurate to 1/10 stop.
If anyone is interested I can post some methods and calculations for
constructing your own own charts for your lenses and equipment. It's easy
once you understand some basic relationships between magnification, focal
length and pupillary ratios.
FYI, most people don't even know the ACTUAL focal length of their lenses.
They assume a "150mm" lens is what it says it is. Usually not so.
How do I measure the actual focal length of a lens? Briefly, obtain a 1:1
magnification, divide the film to object distance by 4.
Jerry McCollum
P.H.Groepper pgroeppe2@estec.esa.nl wrote in article > > Can anyone tell me is there such a thing as a simple exposure compensation chart for bellows extension on a 4X5 camera? > > here it is: > > Lens-to-film dist. exposure value (EV) compensation > ------------------- ---------------------------------- > Focal length x 1 -> 0 (i.e. don't change at infinity) > " x 1.41 -> 1 (i.e. open 1 f-stop) > " x 1.73 -> 2 (i.e. open 2 f-stops) > " x 2 -> 3 > Focal length x Factor -> Factor x Factor - 1 > > Note, that exposure compensation is never negative, minimum compensation > is zero at infinity setting. Also, it's not specific to any camera or > film size. > > You can refine the above table yourself with the formula. It's practical > to make the table directly fit for a given focal length. You then need > one table for each focal length. > > cheers, Peter
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