Contact versus In-camera Exposure Times
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- Subject: Contact versus In-camera Exposure Times
- From: Mike Ware <firstname.lastname@example.org>
- Date: Mon, 25 Sep 2006 14:26:28 +0100
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I think the time is overdue to set the record straight on this issue. If
you're uninterested in closely-reasoned technical matters, please don't
bother to read on...just look at the last paragraph.
Nine years ago I published a paper ('On Proto-photography and the Shroud of
Turin', History of Photography, 21:4, Winter 1997, pp261-9) with an explicit
order-of-magnitude calculation of the theoretical in-camera exposure time
needed for 'proto-photographic' processes (i.e. those having a quantum
efficiency ca. 1), such as photogenic drawing paper (salt prints),
cyanotype, gum dichromate, platinotype, palladiotype, etc. The theoretical
exposure (to a normal subject in bright sunlight) turned out to be in the
order of one hour at f/4 (or ten hours at f/12). The general correctness of
this is born out by all the empirical evidence of the photographic history
of these 'print-out' processes (- as opposed to the vastly faster
silver-gelatin development processes). Such a long exposure is virtually
inapplicable to any subjects except architecture and very still landscape,
and it is useless for most modern camera photography.
However, as every alternative photography practitioner knows, these same
processes can be contact-printed in bright sunlight with exposures of only a
minute or two. (Including 'new' cyanotype - traditional cyanotype is maybe
three stops slower.)
The unsurprising fact that 'in-camera' exposures are much lengthier than
'contact' exposures - to achieve the same maximum density on the same
sensitized material, given the same light source, of course - seems to have
presented some difficulty for T. King. On Internet Lists and View Camera
magazine, he has persistently denied this fact, claiming that he "has taken
light meter readings to prove that there is very little practical
difference" between in-camera and contact exposure times. Moreover, he goes
on to assert that, because it does not agree with him, the whole of
established photometric science must therefore be in error. Curiously, he
does not offer any evidence of his own actual exposures, by camera and
contact, to support his contention that the two should be about the same.
T. King's assertion obviously flies in the face of all common sense,
practical photographic experience, and accepted scientific theory. He has
described (to the PhotoHistory List!) his use of a light-meter, taking a
measurement through a camera lens - which is actually meaningless, and does
not measure what he thinks it measures. His erroneous reasoning betrays the
fact that he does not even understand the difference between Illumination
and Luminance. Misuse of a light-meter by an amateur is insufficient
evidence to demolish the theoretical foundations of the professional science
So let's examine the basic theory:
The necessary in-camera exposure time is proportional to the square of the
lens 'aperture' (f/stop number) - this will not surprise anyone who
understands the physical principles of photography.
The light intensity for in-camera exposure is given by the Standard Camera
Image Illumination Equation, also known as the Jones-Condit Equation, which
connects subject luminance with image forming illumination at the camera
focal plane. Details can be found in texts such as Dunn & Wakefield's
'Exposure Manual' p212, Jacobson's 'Manual of Photography' p97, James's
'Theory of the Photographic Process', and even in Appendix II of my paper
'On Proto-photography...' cited above.
All these texts are necessarily dismissed by T. King as "faulty". He has
even used them as an argument to "reject authorities" and to "take no man's
word for it" - his dreadful mistranslation of the motto of the Royal
Society: "Nullius in Verba". Of course scientists generally take each
other's word for it. That is professionalism. Science could not progress
(...apologies for the regal digression...back to the poisoned waterhole, for
An estimate of the ratio of exposure times, in-camera : contact exposure,
can be calculated by rearranging the Jones-Condit equation and entering
plausible numerical values for various camera and lens parameters, to get
the constant of proportionality. The desired ratio is then found, if I've
got the arithmetic right, to be given approximately by:-
5 times the square of the 'aperture' (or f/stop number)
To save us mental arithmetic, here is a table for this ratio:
f/stop (A) 2.8 4 5.6 8 11 16 etc
Ratio (5*A*A) 40 80 160 320 640 1280 etc
(This is for a lens focussed at infinity. If focussed for 1:1 reproduction,
multiply the ratios by 4. Remember this is only 'order-of-magnitude'
calculation - maybe good to a factor of two or three either way - it's not
This underlies my broad statement, persistently challenged and dismissed by
T. King, that in-camera exposure is about two to three orders of magnitude
(i.e. 100 to 1000 times) longer than contact exposure, all else being equal,
(and assuming that the law of reciprocity holds).
I have made 'in-camera' cyanotype negatives myself, and find that the
successful exposures fit these parameters. In practical terms, this means
that a process needing a direct contact exposure to sunlight of, say, one
minute (such as my new cyanotype process on a good day!) would require a
corresponding exposure of 160 minutes (ca. two and a half hours) in a camera
with the lens aperture set at f/5.6 - if the negative is to have the same
maximum density (for white objects) as the contact print.
T. King's "cyanotype rex" process should be about two to three stops faster
than my "new cyanotype", because he puts the ferricyanide in a "developer"
bath rather than in the sensitizer, thus avoiding the 'internal filter
effect' of the ferricyanide absorption band (peaking at 420 nm) which
attenuates the available actinic light. King has published an in-camera
"cyanotype rex" negative in the BJP - no exposure was stated, but I
understand that it was 45 minutes at f/4.5. This is entirely consistent with
the present theory, given the greater process speed, which is closer to my
theoretical ideal. But it still leaves the "cyanotype rex" many orders of
magnitude short of a truly viable "camera speed" process, as understood by
any reasonable, present-day photographer. King, however, claims a "camera
speed" which "revolutionises photography"! To publish his cyanotype camera
negative without disclosing the actual exposure was, in my view,
mischievous. (T. King, "Changing the blueprint", British Journal of
Photography, 151:7490, 21 July 2004, 31-33.)
Obviously, camera exposure times can be cut drastically by using larger lens
apertures, and by severely under-exposing the negative. This explains the
equally mischievous claim by M. Maunder on behalf of King's "cyanotype rex"
that "exposures in-camera of well under a minute become possible", (M.
Maunder, 'Ironing Wrinkles', Ag 40, Summer 2005, pp78-83). His subsequent
publication (M. Maunder, 'Herschel's Genius', Ag 41, Autumn 2005, pp54-63)
illustrates a cyanotype negative of 5 minutes exposure at f/2.8, made with
his cyanotype process which he presumptuously named "herschelotype". This
negative displays a pale blue sky - which is very pretty - and little else!
But skies in my cyanotype negatives were deep blue. Maunder's negative is
evidently underexposed by about four stops. Correcting for this false
"massaging" of the test, we arrive at an exposure of 40 minutes at f/2.8 -
or 160 minutes at f/5.6. That sounds familiar.
Contrary to the extravagant and misleading claims of T. King and M. Maunder
for their "revolutionary developments including camera speed iron processes
such as the cyanotype", I conclude that no cyanotype process yet devised
offers a viable addition to photographic practice for negative-making in the
camera (excepting, of course, Maunder's practice of direct photography of
the solar disc.) Nor are the resulting cyanotype negatives much good for
making prints by UV-sensitive processes: their lack of spectral density in
the blue and UV can only result in very poor tonal gradation in any
alternative process print, as I have personally confirmed in practice.
What a waste! To have spent all this time - just to arrive back at a
conclusion that nearly everyone knew already! But I hope that at least one
of the many muddied pools left in the alternative photographic landscape has
now been allowed to clear...
WWW : http://www.mikeware.co.uk