August 02, 2011

Van Dyke Prints: An Overview


For a little background to these overviews, see here.
History: Introduced in 1889 by Arndt and Troos, the Van Dyke print is part of a group of iron based processes which draw on Sir John Herschel's work on the Argentotype silver-iron process developed in 1842.

Negatives:
A density range of about 1.5 is suitable.

Sensitizer: Part A: 9gms ferric ammonium citrate (green) + 35ml water
Part B: 1.5gms of tartaric acid + 35ml water
Part C: 12gms silver nitrate + 35ml water
In moderate light mix A, B and C (in that order) and age for a few days before use. The solution keeps well for about a year if kept in a cool, dark place. At times, owing to trace chemicals in various supplies of ferric ammonium citrate, the sensitizer may develop a muddy precipitate after about 2/3 of solution C is mixed. This can be allowed to settle and the clear sensitizer on top used, or adding about 2.5 gms more of tartaric acid can also dissolve the precipitate. The additional tartaric acid results in a fainter or no print-out image, but the developed image looks fine.

Contrast control: Contrast control can be achieved by (a) using ferric citrate in varying proportions in Part A of the sensitizer, (b) by adding about 10 drops of a 10% solution of potassium or ammonium dichromate to 500ml of the second bath during washing. (c) printing in diffused light (with perhaps a burst of direct light at the end) also subtly affects contrast. Refer to Christopher James' book for details.

Coating the paper: Use a glass rod or hake brush to coat the paper. Once the sensitizer sets, further coats can show up as streaks, so quick coating is helpful.

Printing: Print until the image appears about a stop darker than you want the final result.

Processing: (1) Wash in distilled water or tap water with a pinch of citric acid added for 5 mins. (2)Wash in running water for two mins. (Can be toned after this point). (3) Fix in 3% sodium thiosulfate solution with .2% sodium carbonate (washing soda) for about a minute, (4) followed by a final wash in running water for 30 minutes. Hang to dry. Ironing the print with a hot iron can increase the contrast slightly and change the color.

June 24, 2011

Cyanotype: An Overview



As I explore a photographic process, I will post brief summaries of its essential technical elements. These summaries are not meant to be comprehensive or to substitute for books that deal in-depth with these processes. They are more field notes for myself and might be useful for a quick lookup while working with these processes. Remember that many of these alternative processes have been around for a century and a half and more and they have evolved considerably over that time. Remember, too, that these processes were not originally meant to be used with silver or even digitally printed negatives as most modern practitioners of alternative photo processes do. There are endless variations of formulas and techniques rather than one simple "right" method as my quick overview might imply to the superficial observer. But hopefully these summaries will serve as a quick reference or encourage you to read and explore further.

History: The Cyanotype was first described by Sir John Herschel in 1842. Numerous variations on the original formula are available.

Negatives: Generally a density range of about 1.35-1.4 is okay. That is, many negatives meant for ordinary darkroom printing with diffusion enlargers can be used. I find slightly contrastier negatives with a range of about 1.5- to be more suitable when using vinegar instead of water as the developer.

Sensitizer: Solution A: 20% ferric ammonium citrate (green) solution
Solution B: 8% potassium ferricyanide solution
Store both solutions in separate bottles and mix 1:1 just before use. Solutions will keep indefinitely in sealed dark bottles. Bacteria that might grow on solution A can be filtered off.

Coating the Paper: Any non-buffered paper can be used with gelatin sizing if necessary. Alkaline environments will degrade Cyanotypes. Coat evenly with brush or glass rod under low tungsten light. Can be air dried or dried with mild heat.

Printing: Printing times can be fairly long. Check the highlights by opening the split back to see if they are a shade or two darker than you want in the print. They will lighten considerably in the wash. The shadows are not good indicators as they might begin to reverse during the printing out process.

Processing: Cyanotypes can be developed using water. But white vinegar gives slightly more midtone contrast, about a stop and a half more dynamic range and a sharper print. But vinegar tends to tint the highlights light blue instead of pure white. I have found a 1:1 mixture of white vinegar and distilled water to give the best balance between retaining highlights and a longer tonal range. Develop by agitating in a tray for 45 secs to one minute and then put in a running water wash for 5 minutes. Hang to dry. A 0.3% bath of hydrogen peroxide (1+10 sol of commonly available 3% hydrogen peroxide solution and water) just before the final wash will oxidize the print to a deep blue. This is not necessary as the print will slowly oxidize in the air as it dries, but the peroxide bath lets you see the final color at once.

Toning: Tea or coffee or tannic acid can be used to tone cyanotypes. Bleaching with a solution of one tablespoon of sodium carbonate (washing soda) per liter of water solution before toning is said to reduce staining effects but I have not yet found a satisfactory method to produce repeatable results while toning cyanotypes.

June 06, 2011

The Keepers of Light: Book Review


This book takes its subtitle - "A History and Working Guide to Early Photographic Processes" - quite seriously. It is as much a history of early photography as it is a practical guide to early processes. So, while most handbooks for these processes have a bit of history included for 'background' as a matter of course, Crawford dedicates the major chunk of the book to a detailed, sustained and quite insightful history of early photography. The practical guide to these processes is quite competent but it is almost an afterthought after the exhilarating tour-de-force of the first section of the book on the development of photography from its earliest days to well into the age of the silver gelatin print in the first half of the twentieth century.

I have always found most histories of photography to be quite tedious. They usually read like a long list of dates and developments and brief backgrounds of the persons associated with them. This approach is akin to the older sort of history as a narrative of the rise and fall of kingdoms and rulers - a history based around personalities and a more or less linear notion of progress. But Crawford takes a leaf out of newer historians who write social history not as a linear narrative but as a 'genealogy' of inter-related strands perpetually interacting with each other - 'history from below.' Of course, he isn't writing with the grand developments within academic historiography in mind, but his approach makes this a very apt comparison.

The Keepers of Light lays out the evolution of what Crawford calls the 'syntax' of photography. The first chapter uses the examples of traditional printmaking and painting very effectively to lay out what exactly constitutes a 'syntax' for a visual medium. The parallels to modern theories of linguistics and semiotics are unavoidable and Crawford uses them very effectively to illustrate his point that every visual medium has a basic 'vocabulary' and 'ordering pattern' which combine to produce a coherent 'meaning' for the observer. This can be constituted of the most basic elements like the direction and thickness of strokes in an etching or the overall formal and technical limits of a medium. And the syntax of a medium can emphasize particular elements like form, texture, tone etc at the cost of others. Crawford argues that even though photography is sometimes seen to be a medium without a syntax - a process that merely reflects the world and is therefore in a direct and 'natural' correspondence with it - in fact each particular photographic process is strictly governed by a syntax. Every detail of the process - the tone, the detail, the reproducibility, even the duration of exposure required or things like generic conventions contribute towards the syntax. Together they define the limits and possibilities of the medium, how it produces the photographic artifact and how we, as the audience, make sense of that artifact. If anything, the photographic process is so intimately tied up with technology that the limits and possibilities of the syntax must be confronted every step of the way.

Thus Crawford sets out to write a history of early photography not as a history of linear events, but as one of the evolution of the photographic syntax. Part I of the book, divided into 12 chapters, traces this evolution in terms of changing conventions, demands and technical challenges that forced nineteenth century photography to constantly develop and evolve. The result is a breathtaking history which makes the twentieth century or even the fast pace of recent change with the emergence of digital seem quite tame by comparison. In doing so, it returns the reader to the sense of awestruck wonder that the practitioners of photography or its first audiences and consumers might have felt. To capture a veritable image of the world on a sheet of paper - as if not made by man but by Nature herself - this is something the magical, awesome quality of which we have lost sight of. We with our oh-so-sophisticated cameras, our 51 point autofocus and our MTF charts - even those of us who think of ourselves as 'traditionalists', use older film cameras and obsess over the zone system and sharpness. All of this is mere trifle compared to the wonder - the very sorcery - of the crude image of Paris rooftops on Niepce's bitumen covered glass. And isn't that why so many try alternative processes? To discover a hint of that magic, the discovery of the new world, the first step on the moon? It is akin to the jaded business traveler ticking off his air miles discovering the struggle that went into man's conquest of flight - from the myth of Icarus to the Wright brothers.

And the syntax is not merely a thing-in-itself - a mere description of the technical boundaries of a process. It lives in and interacts with the world in a way an unilinear perception of 'progress' cannot grasp. Thus the Daguerrotype reigned supreme since its introduction in 1839 for a couple of decades. Technically it was capable of producing results that would satisfy the zone system or sharpness junkie of more than a century later. But there were both socio-cultural and technical demands that its syntax couldn't satisfy. Crawford traces how the more painterly rendering of the Calotype and other processes proved more pleasing to an age for which photography had not been established as a completely unique and independent medium and the reproducibility of negative images proved too strong a technical demand to resist. So the syntax moved on, it evolved, it responded to demands.

It is in conceiving of the history of photography as the history of this dialog between form and content, between demand and response that Crawford's approach is so refreshing and unique. It is full of insightful detail accompanied by vignettes and bits of humor that are all the more charming because they do not quite fit with Crawford's somber academic style. Consider for example, the following revelation: "Subjects in early daguerrotypes frequently sit with one hand supporting the chin. They look like deep thinkers: They were actually concentrating on not moving their heads" (9). Similarly delightful are his observations on the limitations imposed by technique and form on content: for example, the depopulated cityscapes in early long-exposure Calotypes, or how the exposure limitations of certain processes dictated the look of many early street photos. Amusing and astonishing, too, are vignettes about the crisis in egg supply when factories were consuming 60,000 or more eggs per day to coat albumen paper or the fact that there were studios in the 1860s churning out over 1,000 copies from a negative per day! All of this leads to a history of photography down to the period of dominance of the silver gelatin print that is truly novel in approach and eye-opening in detail.

I have spent most of this 'review' on Part I of the book. This isn't unintentional because that is the part I found most compelling. The other two parts constitute a practical guide to some of the major processes and techniques of preservation. They are chock-full of detail and great for anyone wanting to try it out, but there are relatively few illustrations and some of the materials and sources from this 1979 book have obviously fallen out of date. For a pure practical guide, perhaps more recent books like Barnier's or James' might be better, or even one of the many internet sources for quick summaries of processes. But Crawford's basic introduction to sensitometry (while a little advanced in that it tends to assume familiarity with conventional darkroom sensitometry) and technique is very helpful. In other words, buy another book if you want merely a DIY guide, but definitely read this book if you want a perspective that will change the way you look at photography - not only its past, but even its present.

Crawford, William. The Keepers of Light: A History and Working Guide to Early Photographic Processes. New York: Morgan & Morgan, 1979.

May 30, 2011

Alternative Photographic Processes: A Bibliography

Here is a general bibliography on alternative photographic processes. I will keep adding to it as I come upon more resources and, over time, will put up more detailed reviews of titles I get to know well enough. If you have more suggestions to add to the list, please let me know.
Web Resources:

April 20, 2011

Hacking St. Ansel: A Homemade Densitometer

If, like me, you are still holding fast and shooting some film amidst the digital deluge, chances are you have delusional visions about Saint Ansel going forth and spreading the the light of the pure craft of photography (the light, of course, is divided into zones neatly marked I-X). But pure or not, the craft of analogue photography requires quite a bit of exactness and repeatability to give optimum results and for this, testing film and developer combinations often becomes necessary. But the one thing that hindered me most from really testing my film and having sleepless nights over geeky things like N-1 development was the lack of - or rather the cost of - a densitometer. I know those things cost a lot less than they used to - a few hundred instead of a few thousand several years back - but they are still expensive and bulky beasts. But not to be discouraged from my vision of photographic nirvana, I managed to put together a kit that cost me all of $30 - and can conceivably be done for less. NASA wouldn't trust their pictures of space aliens to its vagaries, nor perhaps would Adams be satisfied with the texture of the moon over Hernandez had he used this contraption, but at the moment it works for me.

For this densitometer I use the cheap photodiodes easily available at Radioshack or any other electronics store. It cost me about $2 for a handful of them. Next, I cut a small square piece of cardboard about 3x3 inches to form the base of my densitometer. Take a diode and bend its legs outward at right angles near the middle so as to form a small inverted 'T' with the light sensor at the top and the legs sticking out on each side. Two pieces of tape should be enough the stick the legs of the diode to the cardboard. Black electricians tape works well for me. Once you have done this you should have the inverted T standing on the board with the sensor sticking out. Now, we need something resembling an empty cylinder with a small hole on top to cover this so that there is only one inlet for light reaching the diode. This is easily made by cutting a small circle in the base of an opaque film canister - I made mine using a plastic reloadable film cassette with the center spool removed. This has the advantage of having a ready made hole in it. After you are done, you should have a structure that allows light to reach the photodiode only from a hole on the top and the two arms of the diode should be protruding from the sides.

Now onto the more interesting bit. As the intensity of light falling on the diode changes, so does the resistance that can be measured by sticking the sensors of a cheap multimeter (I prefer the digital ones) to the ends of that diode. How does this translate into film densities? Well, if you think of the formula for film densities its the difference of logarithms and not tied to any units. So put your 'densitometer' under a steady and relatively bright source of light. An enlarger works fine but I just use a table lamp for this. With full light falling on the sensor take a resistance reading - say the reading is 6. Now hold a piece of unexposed but fully developed film over the hole. The intensity of the light reaching the sensor decreases and the resistance correspondingly increases to 9, say. So now you have your filmbase + fog density which is log 9 - log 6. It's that simple, really. Now, hold a negative that you have exposed at zone 1 over the hole and say the reading changes to 12 - you can now easily calculate the density as log 12 - log 6 and then subtract your Fb+F density from it to get your zone 1 density.

But that is not the end of the story. The chief drawback of this setup is that even though it's quite accurate for the lower densities around zone one, the resistance curve of the cheap photodiode is not linear. That is to say, as the light reaching it decreases the increase in resistance is not quite proportional. As a consequence you will get lower readings than expected for higher densities like zone 8. A simple way around this problem is to 'calibrate' your densitometer. You will need either one of the calibrated step wedges that Stouffer, Kodak and perhaps others make. Or you can get by if someone with a densitometer just agrees to read a strip of film exposed by you at various densities. If you have a set of known densities, it is then easy to read those in our meter and find out by how much the reading is 'off'. So if you know that a particular strip has a density of 1.1 above Fb+F and your meter is coming up with 1 then you know that at that density the reading needs to be 'corrected' by 10 percent. I do all of this in a quick spreadsheet so it's easy to keep track of. Once you have read a set of densities and noted their correction factor, your densitometer is good to go on it's own. It's definitely accurate enough for the purposes of film-developer tests and produces repeatable results. Finally, I might mention that this setup works great if you are exposing a test roll of 35mm film - be sure to hold the negative over the light hole for a few seconds for the multi-meter reading to stabilize.

DIY Quick Release Plates, or, How to Put a $5 Camera on a $500 Tripod!

If you have a lot of plastic and toy cameras and like me want to use some of them as pinholes or use them on bulb mode for long exposures, then you've probably tried to device ways of putting them on a tripod. If the tripod quick-release system you are using is something like the Arca-Swiss then the cost of plates at about $50 or more a pop can become a major concern - especially on cameras which usually cost under $5!

I used Bogen's clamp system for a while and now use the Arca-Swiss system. They both share the same basic design although the sizes vary widely. The only pieces of equipment I wont put on a tripod without solid metal plates are my 'real' cameras - the Chamonix 4x5, and ยต4/3 Panasonic. But for most of my other cameras (I never seem to shoot rangefinders from a tripod, somehow) I have devised a simple way of making my own plates - modeling clay!

It's available from most art supply stores. I recommend the kind that hardens on baking. Cut out a block that has approximately the cross-sectional dimensions of your quick release plates but is slightly larger. For Arca style plates this should be about 38mm wide at the bottom, with 45 degree angles and at least 3mm high. That's it - that's the basic Arca-Swiss design. Beyond that you can put in whatever improvisations, anti-twist lips and other cool Really Right Stuff tricks. Now to do the fine shaping, open your clamp as wide as possible, slide the slice of clay in and tighten it gently. The clamp will act as a mold to shape the clay. I suggest spraying the clamp with a bit of talcum powder before you put the clay in so that it doesn't stick or get distorted when you take it out. Now put it in the oven and bake it as per the instructions. And voila! You have a quick release plate to fit the Arca-Swiss clamp that's perfectly suited for cheap and light cameras. If you have a sander bit for a dremel tool, you can also sand out any imperfections.

Most of these cameras don't have tripod mounts, so I just stick them to the camera with epoxy glue and allow it to set - works great. But it's easy to go one better and install a tripod mount. Just buy a 1/4"-20 hexagonal nut and embed it in the middle of the plate while soft. You can put a longer 1/4"-20 screw through this to mount to a tripod socket, if your camera has one. Bake the whole contraption together.

Remember, do not use these plates for any heavy and/or valuable equipment! Scrounge on EBay until you find the proper Kirk or RRS plates. But for having fun with cheap cameras, this method works for me!

October 11, 2009

Metering by Eye

Light is the basic element of photography, just as sound is that of music. A good photographer should be as familiar with light as the good musician is with notes and scales. Photography is, at its core, based on a very simple principle. An image of the world is captured by allowing a certain amount of light to fall on a piece of photosensitive material. Whether the photosensitive material in question is a silicon chip, silver film, glass plate or salted paper, this elegant little concept holds. Whether or not the light is shaped by the latest cutting edge in glass and coating technology, an old brass lens or indeed a humble pinhole, the same principles apply. The crucial questions of how much light reaches the photosensitive material and in what ways that amount may be controlled go to the very essence of photographic technique – the determination of exposure.

“Reading the light” or “metering by eye” can be easily mastered with a little practice, yet most photographers leave this central photographic decision almost entirely to their camera. Of course, camera meters work, but it helps to know precisely how a camera sets exposure and what a particular process' limitations are – in other words it is useful to know when to disagree with the camera and to take control of exposure decisions. But many new photographers think metering is some unfathomable mystical art best left to eccentric geniuses and pieces of silicon. Camera companies have done their bit towards obfuscating the issue by bragging about the wonders of “35 area evaluated metering,” “3D matrix metering,” “40 segment multi pattern metering” and such. These modes can indeed be powerful and useful tools for the right job but they don't yet supplant the intelligent photographer's brain. So let's try to lay out a practical approach to metering by eye, or being able to set the correct exposure without the help of a light meter. I shall assume that the reader is familiar with the basics of photographic exposure and can use a light meter efficiently (although I'll deal with those basic issues in a later article).

LV and EV: Ways to Think About Light
For the purposes of figuring out incident light levels we will try to think in something called LV or “light values.” LVs are a sequence of numbers representing intensity of light. Each number is double the previous one and half the next one – i.e. they are on a logarithmic scale. Thus LV 11 denotes twice the amount of light than LV 10 and only half the amount of light compared to LV 12. This might remind you of the f-stop scale and that is exactly what we are going to think of the LV scale as. Henceforth we will think of LV values in terms of “stops of light” - thus we could say LV 10 is two stops below LV 12 and so forth.

To avoid confusion, we might briefly mention EV here (although you won't need it for our purposes of metering by eye). EV, or exposure value, which you will see on many light meters, refers to particular exposure levels which can be achieved by certain aperture shutter combinations. For example EV 15 can be 1/100 at f16 which is the equivalent of 1/200 at f11 or 1/400 at f8 and so forth. All of these shutter-aperture combinations result in the same level of exposure (although the depth of field will differ, of course) and are designated by EV 15. If you notice we have not yet taken account of the sensitivity of the film or sensor – i.e. the ISO value which is the other major factor that influences exposure. So naturally as ISO varies, the same EV shutter aperture combination will produce different levels of final exposure. So while for bright sunlight EV 15 will be the correct exposure for ISO 100, the same exposure will be produced by EV 16 for ISO 200 or EV 13 for ISO 25 etc. Key point: EV numbers change with changing ISO.

LV numbers, on the other hand refer simply to the level of light and don't take into account the specifics of photographic exposure. Thus, they provide a better and more uniform way to think about light levels. If you familiarize yourself with the LV scale you can see a certain amount of light and immediately think of its LV value whether or not you have ISO 1600 or ISO 100 set on your camera. This separation of light levels and specific exposure factors like ISO-aperture-shutter has its advantages.

How do LV and EV numbers relate (or, another technicality you might want to ignore)? Well, they are exactly the same for ISO 100. But if the ISO changes to anything else, EV numbers naturally change while LV remains constant. If you figure the ISO in terms of an offset from 100 in number of stops then you can get EV by adding that number to the LV value: EV=LV+offset. A couple of examples will make this clear. Let's say you have ISO 400 film – that's two stops more sensitive than 100. So the EV value would be LV+2. If it's bright sun, the LV value is 15 and the EV value is 17. For ISO 50, the offset will be -1. So the corresponding EV value would be 15+(-1)=14. If this seems too complicated, ignore it – in fact, totally ignore EV for the moment and think of light only in terms of LV.

Recognizing LV values
Bright sunlight – that is, LV 15 - and that is our first cornerstone. Silly though it might seem, familiarize yourself with bright sunlit conditions - how intense is it, what kind of shadows does it cast? We will learn to think of other light levels in terms of how they differ in intensity from bright sun. And once you have mastered LV 15, think of everything else in terms of how many stops of light darker (in very few cases brighter) it is compared to bright sun. The best way to master this is to walk around for a couple of days with a light meter or even a camera with a meter. Whenever you see a new light level – take a guess how many stops below bright sun it would be and cross-check against your meter. Cloudy? That's maybe 2 stops below (LV 13). You will be surprised how fast this can be picked up.

Once you know your bright sun, there is another light level that I recommend familiarizing yourself with thoroughly – that of a moderately well-lit indoor setting. This is definitely harder to define in any concrete manner, so a light meter will definitely help. Look for LV 5 – or 10 stops below bright sun. It should be a about the light level of a moderate sized room lit with a 100 watt bulb. Once you find it, familiarize yourself with it fully, because it is much easier to think of night exposures as offsets from this instead of from bright sunlight. A brightly lit hall? A stop above – so EV 6. A lamplit street? Two stops below – so EV 3.

Converting LV Values to Exposure Settings
You may know the “sunny 16 rule” which says that in bright sun the correct exposure is 1/ISO value. So if you have ISO 250 film you'd set f16 at 1/250. What happens when your light is, say, four stops below bright sun – or EV 11. You can simply take the f16 at 1/250 combination and “open up” four stops. This can mean opening up the aperture or slowing down the shutter or a combination of both. So open up to f4 and keep 1/250; or alternatively keep f16 and slow your shutter to 1/15; or open aperture two stops to f8 and shutter two stops to 1/60. The combinations don't matter – as long as you “open up” the requisite number of stops. So, always start with the ISO value of your film/sensor for your shutter speed and f16 and then calculate how much you want to open up each. This implies of course that you are totally familiar with the aperture and shutter speed scales. That f2.8 is five stops from f16 or that 1/8 is 5 stops from 1/250 should be second nature – as much a reflex as scales under a pianist's finger. Fortunately, you'll find mastering this set of apertures and shutter speeds - if you don't already know them well - is much easier than learning to play the piano! Once this is accomplished, setting exposures for any LV will not be a problem. Let's say you're shooting in a relatively well lit restaurant that you think is LV 6. You have ISO 500 set on your digital camera and a fast f1.4 lens. However, you would like as much DOF as possible but don't want to handhold below 1/30. You need a total of 9 stops below bright sun. So set shutter to 1/30. That's 4 stops. The other 5 stops you'll need to get from aperture – so set it to f2.8 and presto!

In the age of advanced camera meters this might seem tedious and way too complicated at first. But just like synth-loops haven't put good drummers out of a job yet, the good photographer will know exactly when the meter can handle a scene and when to take control. I'll end with an ordinary example. This shot was made very hastily indeed. I saw these cool looking guys walking unconsciously in formation lost in the latest football news and had only a few quick seconds to frame, focus, set exposure and shoot. This is exactly the kind of scene where you want the quick automation of a modern intelligent camera meter, right? Wrong! Not because the meter isn't competent – but because it simply can't read my mind. There's more than one way to shoot this scene, surely. What if I wanted to have the street perfectly exposed for bright sun and have the guys looking like cool silhouettes? Most old camera meters would be “fooled” by the bright background and do just that. But it's not a question of getting fooled – it's about knowing the intention of the photographer. No matter what the camera companies tell us – there is not just one "correct exposure" of a scene. The exposure depends on the photographer's intended interpretation – just like this scene would work with at least three or four different exposure levels and produce four different interpretations. So unless they make a camera that plugs into a USB jack behind my ear to pick up what my brain is thinking, the camera meter simply doesn't know what effect I am going for. So even if you have a fancy meter, knowing the ins and outs of exposure will let you tell it just what to do (by setting the amount of compensation you want, for example). For me in this scene it involved a couple of quick and simple steps. I wanted to expose for the guys' faces but not totally blow out the background. I know that in bright sun, full shade is almost four stops less bright. So I decided to go for 3 stops below LV 15 (underexpose their faces by a stop at most but hold as much detail as possible in the background). So with ISO 250 film, I set my aperture to f8 and shutter to 1/125 – three stops – click!