Thursday, 25 August 2011

Focal-plane shutters and camera-phone slit scan

The principle of slitscans explained and done deliberately in the last post also applies to "ordinary cameras" under certain circumstances. In the "old days" of film photography, this meant focal plane shutters.

Focal plane shutters

Most 35mm film SLR cameras have focal plane shutters. A few compact 35mm cameras have them too (usually the ones like Leicas or Zorkis with interchangeable lenses.) Some medium format cameras have focal plane shutters, and a small number of large format cameras have them. Focal plane shutters have two moving blinds next to the film - one blind opens and the other closes. The advantage for the camera manufacturers is that the closing blind can start to close before the opening blind has finished opening. This makes the shutter look like a slit moving across the film. The slit is narrower the "faster" the shutter is. That's why the manufacturers like these shutters: they don't have to make anything actually move fast, just make the slit narrower. Of course moving slits play havoc with other aspects of photography: they are not good if you want to take pictures with flash, but they are cheaper.

The famous picture by Lartigue is an old one taken with a rather slow focal plane shutter set so the slit is rather narrow. In this picture he captures the car quite sharp (but not the wheels) but it is "slanted" as the shutter or slit is moving from top to bottom. In fact to make it more complicated, Lartigue also pans the camera in the picture, but not enough to capture the car vertically.

If you own a camera with a focal plane shutter you might experiment with it: set it to its fastest speed and take photographs of rapidliy moving objects. Helicopter blades or aeroplane propellers are good subjects. Try holding the camera the nomal way, on its side, or even upside down. Some focal plane shutters (the majority, probably) go from side to side, and some go up and down. For example my Olympus OM1 has a side-to-side movement, whereas my Praktica MTL has an up-and-down movement. (Because of the vertical shutter Prakticas of the 1970s are ideal for these experiments.) I must try this again, but unfortunately I haven't got any nice images from these cameras to show here right now.

Digital slitscans

But in the modern age, some digital cameras work as slitscans too. In this case there is no slit moving but the camera's processor is so slow and is programmed to grab the scene a line at at time, that this moving line as it is "grabbed" is similar to a slit. Try the camera attached to your phone. Or even better get a very old phone and try that. My phone is quite old, and I was pleased to get this image out of the window of a moving car.

Lance charge

With some panning you might get a nice artistic illusion of speed.

Moving bus

Try to work out which way your phone or digital camera scans. I find my images are much better if I hold my phone upside down!

Sunday, 21 August 2011

Slit scans on 35mm film

A while ago I wrote about multiple exposures and particularly about multiple exposure while winding film and panning or moving the camera. I promised I would explain the principle of the slitscan and how this evolves from the multiple exposure device. You may want to read the previous post at before going any further with this one.

To start with, take a camera which can take very long narrow images. The way I do this is I glue cardboard or metal directly onto the focal plane, just in front of the film, to stop light getting to any part of the film except the thin slit in question. A picture of a modified camera was on the previous post where it was pointed out that simple inexpensive point-and-click 120 film cameras are ideal.

With some practice, pointing the camera and winding on the film at coordinated rates, such a camera can produce pictures that are rather representational, because the panning and the winding on are both about the same amount.


A slitscan camera

If it wasn't for those nasty hard lines and rather clunky appearance this is almost a normal picture taken in a different way. Well how do we get rid on the lines? The idea is to arrange that the "slit" gets narrower and narrower until it is only a few pixels or so wide and the film is wound on smoothly past the slit by some mechanism. This is actually not too difficult to arrange. Here is a normal 35mm camera with a slit added

Petri TTL with focal plane slit 1

The process is likened to a flatbed computer scanner: this can only "see" a thin line at a time but with the line moving it can actually put together quite complex images. In fact flatbed scanners have been converted to slitscan cameras too, but not by me. Not yet at least.

My preferred material to build these slits is the metal from an aluminium drinks can. This can actually be cut quite accurately with a sharp knife and steel rule. The technique is to cut the metal to size and then cut it in half, and it's important to ensure that you don't swap or turn over either piece after it's cut - any imperfections in the cut on one side should match the other side. The width of the slit should be as small as possible and very even - don't allow the slit to be wider at one end that the other as this will give different exposures and sharpness. A with of below 1mm is ideal. I estimated mine were about 0.2mm.

The next things to arrange are the shutter, which must be held open, and some mechanism to wind the film past the slit. For my early attempts I wound the film by hand using a home-made rewind crank of very exaggerated size.

Modified Petri TTL

This works pretty well. The main disadvantage is the camera must be on a tripod. But there is something quite satisfying about winding and panning the camera with one smooth motion both about the same speed. Since most cameras have nasty wind-on locks and double exposure prevention devices, the technique I used was to shoot the whole film blank with a lens cap on the lens, then somehow arrange the shutter to be open (on B perhaps) with the rewind button pressed, and rewind and pan the camera simultaneously. Some photographs taken this way are almost normal.


As an alternative to panning the camera, you can arrange the camera is still and the subject moves. That's how these two cars were shot.

Two cars

The same principle is used commercially for cameras that take photographs of horses, greyhounds etc as they pass the winning post. Notice that as the camera is fixed the background is blurred in a characteristic "horizontal" way as the camera only sees a very thin slice of background repeated.

A combination of panning and taking pictures of moving objects can give surprising results. For example, one of the cars was moving here, making it appear much shorter than the car behind. In fact you can see both the front and rear of the car behind, with the car infront only covering the middle.


You'll find plenty of other very wacky photographs of this type on the web.

Conclusions and further developments

There are some problems with this method.

The exposure is governed by: the speed of the film as it goes past the slit and the width of the slit. It is quite easy to calculate that if the speed of the film is V mm-per-second and the width of the slit is W mm then the effective exposure is W/V seconds. For this to be something "sensible" like 1/100 the slit needs to be very narrow.

The slit should be narrow for another reason: the narrower it is the sharper the images will be. Sometimes the panning and the speed of film match exactly, and the images are unexpectedly sharp, but more often one gets slightly blurry images which may be good enough or not, depending on what you want.

I find it is difficult to cut any slit completely evenly, which accounts for the horizontal lines seen in many of my pictures. It is also difficult to wind film evenly, and unevenness in this accounts for vertical lines.

The slit does introduce the chance for the film to get scratched. In fact this is a minimal problem compared to the others, but it is a problem.

Finally, as I have described it, these cameras need a tripod.

On the other hand, these cameras have the advantage that the pictures they take are not limited to the width of a normal frame on 35mm film, but can be as long as you want. Panoramic photographs are very much possible.

I future posts I will give some other ideas that simplify the process and make the cameras easier to use. This involves perhaps using a motor to wind the film. There are also ways to simulate slitscans with a digital camera that can take video. You may also find that your camera's focal plane shutter already behaves as a slitscanner, or that your digital camera (especially phone camera) works so slowly it too behaves as a slit scan. Watch this space!

Wednesday, 17 August 2011

Gum Bichromate

If photography is art, the qualities of the media used in the final print are as much part of that art as the subject. In fact I would go further - the subject and whatever it is provided it is looked at freshly with an interesting eye is not important for "art". One does not have to take a photograph of a pretty girl or a cute puppy for it to be art: in such cases these cutsie subjects are more likely to be distracting from the purpose of the image than complementing it.

So other methods for making a photographic image on paper must be explored by any serious photographic artist. Or so my humble opinion goes. I am only at the beginning of this exploration, but I hope to share some of it here, and some details of the experiments I made.

The gum bichromate process appeals to me for a number of reasons: one uses real-art paper, and real-art paints (watercolours). The choice of colour is available in a bewildering range, and it is even possible to layer several colours on each other. The theory is that the dichromate chemical when exposed to ultraviolet light, hardens many natural and artificial gum or glue-like substances, and this can be used as the basis for making a print from a negative. An introduction to gum-bichromate is available at The Alternative Photography site and this site has much more than I can say here. However my personal experiences may be helpful.

The recipe

The recipes I read on the web were mostly of the sort "take a dollop of this and a small amount of that..." and were too imprecise for anything but experiment, not that there is anything wrong with that, but some precision is needed to help make the process more predictable.

I took some "half-pan" watercolours, student grade from an art shop, and chose one (for the first attempt, the black) and cut it into half. It makes a very thick gummy cube, rather difficult to spread without water, but I really don't want to add too much water as it is the reaction between the dichromate and the gum that is important. Ideally, the watercolour should go in a dedicated pestle-and-mortar. You might be able to improvise some alternative holder, or use paint from a tube which is easier to mix but less concentrated.

I dissolved 13g of potassium dichromate crystals in 100ml of de-ionised water. They don't dissolve very easily: what I did was put the crystals and water in a bottle, put the top on, then immersed the whole thing under the hot tap, shaking occasionally, until it had dissolved. See warnings on dichromate below.

I took 6ml of gum arabic solution (from a bottle supplied by Windsor and Newton, available in art shops) and mixed it and pounded it with the water colour. Gum arabic is from the acacia tree and is also available as powder - you could dissolve this in (very little very hot) water and use this instead. My notes say to use 30g in 90ml water, though I didn't test this.

In a darkened room - with definitely no daylight though some tungsten light should be OK - I mixed the watercolour and gum mixture with 6ml of the dichromate solution. This made a thick watercolour paint which just covered a sheet of A2 paper, using a good soft flat brush. The paper was stored in the dark to dry, and when dry cut into small pieces as required. I used medium quality watercolour paper. The textures in the paper will remain and add to the print, though very smooth paper will show more detail.

Printing out

Like most printing-out processes, the best source of light is the sun, but a UV light (from a fluorescent tube or mercury vapour lamp) is possible. The negative is place on top of and in contact with the paper, and the whole sandwich is held tightly under glass. There are special frames to hold all this together. I found that even on a rather dull overcast day, 2-5 minutes was sufficient exposure. In bright sun it will be less. This is much less exposure than is needed for printing out using silver-based emulsions.

Printing-out frames

Special printing-out frames are available to allow you to check the process as it happens. If you have them (I did) by all means use them, but in this process nothing appears to be happening and it is pointless checking anything. You just have to do a trial, develop it, and modify the time for the second attempt.

The UV light should have hardened the gum in certain areas of your paper, but as I said, at this stage nothing can be seen. To "develop" the picture you need to wash away the non-hardened gum revealing the white paper beneath. Run the paper gently under a stream of water from a tap for a minute of so. Then soak the paper image-side down in a tray of clean water for 5 minutes. Then change the water and repeat 2 times. I found that the paint needed some help to come away and I brushed the paper very lightly and evenly all over with the same soft flat brush I used for coating. When this works (and for me, it always worked, though some prints were better than others) it was wonderful to see how some paint comes away leaving the image.

After washing thoroughly (all of the orange dichromate colour should go) leave it to dry.

Study of yucca (gum bichromate)


The gum and modern pigments are safe to use but dichromate is toxic and probably carcinogenic. When handling crystals or powder wear rubber gloves, eye protection and a face mask so there is no possibility of breathing in dust. When handling solution or prepared paper use rubber gloves. If you use containers to handle chemicals don't use them later on for food. If you think you may have accidentally touched something with chemicals wash in plenty of running water immediately.

The paralysed cyclops

Just every so often a project - or even a single picture - is finished.

In most of the photography world, "finishing" a picture is (almost) just clicking the shutter button. The rest of the processing is a formality - whether it be film or digital. In the art world, finishing a picture can be a major event. In fact, many people say a picture is never finished, just "good enough". What is the difference?

Real art, I believe, is, as well as about looking and recording what is there, is about the processes of evaluating the marks placed on the paper, or canvas, and repeating them or re-doing them until they are right. This process of evaluation and development is what makes the difference between the fine qualities of marks or whatever seen in true art, and bad art or (sigh) most photography. One of my artistic heroes is Frank Auerbach who would paint over and over a painting perhaps a hundred times, most times each being only loosely based on what came before, but always looking for that elusive "finished" painting.

I have set up a companion blog for my "finished" work, The title comes from a statement by David Hockney, saying that photography cannot be art because it only has one eye (the lens). In saying this, I think he is wrong occasionally, but (sadly) this is right most of the time. I hope my finished photographs are the exception rather than the rule. Please look at it. All this talk of camera mods and hacks would be quite empty if there was not an objective, and it gives my best attempt at showing that objective.

Wednesday, 10 August 2011

Distressing pineapple

I have talked about distressing negatives using bleach (hypochlorite) and also with wire wool already in this blog. Here is an alternative method that promises better control than either: pineapple.

Actually papaya would probably work as well. Both these fruits contain an enzyme that rather slowly dissolves gelatine. Gelatine of course is that jelly-stuff made from boiling skins and bones of animals, and is in fact the stuff that the business-end of a piece of film - the emulsion, that carries the actual image - is made from.

Apologies to vegetarians: film photography is not vegetarian. Non-animal products were tried at some point but obviously were not good enough. In the distant days of photography 100 years ago or more, albumen from eggs was used as an alternative, but that isn't vegan. I am reminded of the story of some very precious waif-like model who turns up at a photo-shoot with her nose in the air, prattling on about how important it is to be kind to animals. When she hears the photographer is using film she complains about the cruelty involved in making film, and refuses to have her photo taken, storming off in a huff. She doesn't even stay long enough to take her coat off (which is very expensive, made from real fur).

Anyway, and back to topic. Pineapple on film. It should work equally well with colour or black and white, slide or print film. I never heard anyone else try this, so it's all very experimental. Try it your own way, but use my suggestions or experience here as a guide if it helps. Here is what I did today.

I got a small very ripe pineapple and took about a quarter. (I eat the rest!) I turned the flesh to pulp in a liquidiser. I taped some negatives emulsion side up to a sheet of glass (any flat surface will do) and got a very soft flat brush.

Liquidised pineapple

Now the idea is to brush the pineapple onto the film so that only a very small amount of the emulsion layer comes away, and you can put textures in the image. With the film I had, I discovered leaving pulped pineapple on the film for about 30 minutes removed a bit more than half of the emulsion. Some emulsions are thicker than others, so you may get a different effect. You know when something is really happening fast when the pineapple starts to go a greenish black from the silver in the film. (This is for black and white film. Colour film will behave differently.)

This was the best technique I found. I dolloped pineapple onto the negatives very thickly and spread it round, left it for 5 minutes to let it start working. Then I brushed the emulsion very gently to remove almost all the juice. At this stage the negatives dried quite quickly and the emulsion was soft. I brushed in alternate directions with a brush dipped into a small amount of pineapple, and left it to nearly dry each time. I did this for perhaps 5-10 minutes. Then I washed very carefully and thoroughly, without touching the emulsion.

This is a negative as scanned before the treatment

The Shire, Birmingham

and the same negative afterwards:

The Shire, Birmingham

The effect is only just noticeable on the negative looking by it by eye, but of course it is very obvious in the final picture. If you click the image you should be able to go to a larger version. It looks much better up close.

Here is another, first before

The Shire, Birmingham

and after

The Shire, Birmingham

To finish with some thoughts and suggestions for more experiments: the interesting thing is the texture that is now on the emulsion. You can try anything at all make a texture when the emulsion is soft. My brush was just a start. I also wonder if the resulting emulsion can be dyed, and some image made because the other different densities from the thickness of the emulsion and the dye. There are also some chemicals that can inhibit the effect of the enzyme, and if you are chemically minded you can look into that and see if you can get them to do anything interesting.

Colour film will behave quite differently as it is made from several layers corresponding to the various colours. This is definitely one to try some time.

Finally, some warnings. Pineapple juice is of course safe, and tasty, but black and white film contains silver which is poisonous. Colour film contains treatment agents to stop it being attacked by fungus, and formaldehyde was a common choice at least a few years ago. This is poisonous too, and film may also have residual chemicals in it. Keep food and food items away from photographic items. Also, I have no idea whether film treated this way will remain stable over a long period of time. It probably depends how carefully you wash it.