Through a glass, darkly.

So I was listening, as one does, to some of those poddy talk-show things. A couple of them mentioned the use of filters in black and white film photography. The discussion came around to what effect filters have and what to use them for. I started by shouting at the radio (I was listening in the car while commuting). Then it migrated to ‘as any ful kno’. Then it changed to my thinking how best to describe what filters do. This is that description. You may prefer that I stayed the silent fool.

Let’s get colour out of the way first. If you avoid the use of those awful Cokin filters of yore (you thought a tobacco graduate was a university-leaver making a poor career choice?), you are left with two basic filters: neutral density and polarising.

Tobacco graduate - what was I thinking?

ND filters will let you either use long exposures for those creamy moving water shots or, in graduated form, hold back a bright sky to give the foreground a chance.

Polarisers can take reflections off glass and water, darken some areas of the sky and increase contrast. Here endeth the colour filters.

Black and white is capable of much more manipulation. The basic idea is that a coloured filter lightens its own tone on the negative and darkens the complimentary (meaning opposite) colour. “Fine” you may think “but what’s the opposite of green?”. Strictly it’s magenta, but you can vary between blue and red to get the exact effect you want.

There is a very good online resource here that describes how colours are grouped and work together and has an interactive colour-picker to let you experiment. As an aside, if you find a colour you like, it gives you the RGB values for it, which you can use in Photoshop to tone an image. Perhaps a subject for a future post.

Traditionally, anyone shooting landscapes on mono film used a yellow filter. Yellow darkens the appearance of blue, so makes the skies darker and the clouds stand out more. The haze you see over distant objects is also blue, so a yellow filter cuts through this a bit and makes the scene look a bit clearer.

This works for all colours, but there is a little complication that stops the story ending right here – colour sensitivity.

The makers of black and white film had to work hard over many years to make it sensitive to all of the visible colours of light. The first films were really only sensitive to blue and were known as Orthochromatic. Since they can’t really ‘see’ red, they can be developed under a safelight. Most photographic enlarging paper is orthochromatic. Being only sensitive to blue doesn’t matter, as the subject has already been converted to black and white on the negative. There is a real benefit in being able to see what you are doing in the darkroom (or the ‘red room’ as watchers of Stranger Things are calling it).

Gradually the manufacturers added sensitivity to green and red, giving us modern Panchromatic film that can see the full visible spectrum.

You can see the difference in portraits taken on old and new film. The old Ortho film darkened skin and lips and struggled to render skies as anything but white. And note – there is no point in putting a red filter on a camera using Ortho film – it would only have the effect of a fairly strong neutral density filter. On the other hand, if you want to recreate the old Ortho look in a scene or portrait, use a blue filter on a modern Panchromatic film.

The fun comes in that, in adding sensitivity to red light, some films strayed over the line (bad films!) into the infrared. Using a strong red filter on these films can give the effect of using specialised infrared film. This renders blue skies as black, cuts through any atmospheric haze and can render leaves and grass as white. This is because the chlorophyll reflects infrared quite strongly. So a red filter, that you might have expected to make green go very dark, can actually cause it to go white. It depends on the film though, and on the amount and direction of sunlight.

So where does this take us with filters? Filters lighten their own colour and darken the opposite colour, within the constraints of what the film is capable of seeing. A red filter will darken grass and blue skies, lighten brickwork and make pale skin paler. A red or orange filter can subdue the appearance of freckles on pale skin. A green filter will darken blue skies but also lighten grass and leaves and darken brickwork.

You can see how filters treat colours differently if you have a copy of Photoshop or Elements that can do layers. Open a colour picture in the application. Add a new layer above the image – make it a hue/saturation adjustment layer and make the blending mode Colour (or Color). Name it Filter.

Make a second hue/saturation adjustment layer on top of this one and name it Film. Double-click the layer thumbnail and drag the saturation slider to -100. This will turn the image to black and white. The layers should look like the picture below.


Go back to the Filter layer and double-click the layer thumbnail and drag the Hue slider from side to side. What you are doing is seeing how a black and white film renders a colourful scene through various colours of filter.


The picture above shows what you can do. The original image is top left. The other images are various twekas of the Filter settings. The weird image bottom right is what the colours look like in my favourite settings, with the mono-conversion layer switched off. You may just prefer this anyway.

It’s worth a play, as you may find a setting that gives you a really sharp tonal separation in what would have been a blur of blending greys.

Actually, there is one more thing you can do with colour film or digital and filters and it involves flash. Put a blue filter over the lens and a yellow filter over the flash. Shoot portraits close enough to be flash-lit. The background will go a deep evening blue. This is how to shoot day-for-night.


Try it, and try the mono filters thing too.

Catting logs

One of the benefits of digital picture files is that you can embed information in them or use tools to make them searchable. Looking for that picture of a smiling seal you took – who knows when? All the software tools exist to find it, and all the other pictures that are similar. Simples.

What do you do with physical objects though, like negatives and slides? Fine if you scan and label them all, but over years of shooting you could be looking at pterodactyls of storage and months of boring filing.

When I was young and had more time than money, I started a card filing system. I would make a contact print of my negatives and then add the film reference number to each relevant subject card. I could go to my Seals card and find the references, then pull the matching negative sleeves from their ring binders and eyeball them to find the one I wanted. This rigour and discipline lasted maybe four years. What killed it was when time got shorter (university) and the rate of taking pictures exceeded the time I was willing to put down my books (ho ho – beer, more like) to do filing.

That lasted a fair bit longer than four years, as both time and money reduced to zero.

Then, one day, I bought a scanner.

That started a craze of poring through negative sheets and slide boxes and scanning things that took my fancy. Which, obviously, all ended up in random folders without any identifying information. And I still hadn’t the time or inclination to scan every frame and log them.

What I did eventually do is take myself in hand (insert a Kenneth Williams ‘ooh!’ here).

This was mainly because I bought some proper negative filing sheets to let me move stuff from envelopes to proper A4 sheets. This made them visible. So my next trick was to drop a complete sleeve of negatives on my lightbox and take a digital picture. Same with slides: arrange into a rectangle and shoot.

So the random and invisible pictures became a set of named folders, each containing the digital equivalent of a contact sheet. And still no way of finding the smiling seal.

The final step was to add a plain text file in each folder, with the same name as the contact sheet and the folder itself, containing a description of the pictures. Basically, a simple list of what is on the negatives or slides, whether it’s good or bad, film type, development notes, location – all that jazz.

And the big question on everyone’s lips is “fine, but how do you find that picture of a smiling seal?”. Every set of pictures gets its own folder on the hard drive. The folder is named for the date of shooting in YYYY MM DD format so that they sort in date order. The folder will also probably have the main location or activity added after the date. This makes it quick to find the main home of a set of pictures, or if I have been to the same place or shot the same thing several times, I can find the occasion I am looking for. If I need to do a better search than that, I fire-up a search utility (Agent Ransack, but other products are available). This will zip through the text files in each folder and find things like ‘smile NEAR seal’.

It’s a lazy-arse and simple method, but it works for me.


Ah, screw kit!

So I’ve built myself my ultimate screw-fit kit. You may wonder why. I wonder why. As if our continued existence in this vale of tears wasn’t hard enough. Let me explain.

It all started when I realised that what I thought was a broken lens (that still seemed to work well) was actually a fine example of a rare and exotic optic. I did think of selling it, driven mostly by fear: I was walking around with a lens that on its own cost more than most of my gear combined. But the second thoughts said that the value of a lens is primarily in what it can do. So I kept it.

I had an old Pentax SV to drive it. It’s an fine old thing with a pretty good focusing screen (WAY better than a Zenit). No meter, but we’re all Real Photographers, right? We can gauge the light just by looking. Luckily I had a handheld meter too. There was no particular reason to own the SV – it was a cheap and useful fitting for the screw-mount lenses I had lying around.

Then a funny thing happened. I wandered into a charity shop and there was a Spotmatic with Pentax’s 80mm f1.8 lens on it. £40. Again – who needs a Spotmatic? But the lens was interesting. We went for a cup of coffee while I thought about it. After a very quick coffee I was back in the shop and running through the shutter speeds and apertures. And so it came to pass that it too became mine.

So now I have a brace of Pentaxes (Pentii?), a wide 50mm and a wide 80mm. Toss into the bag a Pentax 35mm f3.5 Lens that has been around for years. Add an old Weston meter and we’re done.


Ooh er. I’ve just assembled a screw-fit system. When I said it was my ultimate kit, I meant my only. It was more by accident than design. No, it was entirely accidental.

So how does this ancient ensemble work in a world of computers? Slowly. All those people who say they shoot film to slow them down could be right if they use screw kit.

The main thing is changing lenses. I can do the bayonet-twist as quickly as any psychopath. But a screw lens doesn’t tell you when it’s free – you just keep turning till eventually it comes off, while trying not to drop it when it does. Then you offer-up the new lens, get it square, try not to cross the thread, find the point where it engages and screw gently in case you made a mistake. And the base cap on the removed lens doesn’t just click and twist – you have to screw it fully home with the same care.

This is probably why old-time photographers were pictured with two or more cameras round their neck: it took too long to change lenses.

Apart from that, they are just cameras. Having a separate meter can be a bind, but you get over it. Just keep checking the reading each time the conditions change and remember what to set the camera at.

There really is nothing to get in the way. The camera will never make a mistake or get confused, because it can’t. The lenses are simple and silent. And there is a certain Zen pleasure in using the simplest equipment.

Would I use it for something important or where automation could help (like sports)? No. I use it because lets me use two lenses that I like a lot. If I had those same lenses in bayonet mount the screw-kit would never have been created. Indeed, I do use them with a bayonet mount adapter but with the loss of the auto-diaphragm.

So why the fuss and what’s the point? The cameras were cheap and work, but the main thing is that they let me use easily a bunch of old lenses I had in drawers and boxes. There is also a load of old and interesting lenses made in M42 that are either not available in the ‘proper’ mount of your choice or would be too expensive. It does make me really appreciate the invention of the bayonet mount though. For the curious, this is the line-up:

  • Yashinon 55mm f1.2
  • Pentax 80mm f1.8
  • Pentax 35mm f3.5
  • Industar 50mm f3.5
  • Helios 58mm f2
  • Soligor 35mm f3.5 (a repurposed paperweight)
  • Hanimex 200mm f3.5

I honestly didn’t start out to build a screw-fit kit: this stuff just accumulated from historic purchases of cheap stuff when I found it, plus a couple of lucky finds. I guess the creation of a system is just post-rationalisation for hoarding.

So, do I use this stuff or does it sit on a shelf and preen? A sub-set of the kit was out this weekend for a walk in some shady woods where I was going to need wide apertures (what idiot loads Pan F for anything other than clear desert skies?). Can you hand-hold an 80mm lens at 1/30th? Yes if you press the camera against a tree, safe in the knowledge that it doesn’t matter if you scratch it. Or the camera.

Plus, come the apocalypse or nuclear EMP, these things will still be working. Not that it will matter when we are fighting over the last tin of dog food. Which is a cheery thought to end on.

Awkward focus

How do you focus a camera that doesn’t tell you when it’s in focus? That’s really awkward.

If you can learn how to do this trick, there are loads of interesting old cameras that you could use. They can be reasonably cheap too, as people do prefer things you can focus. And yet, you may have heard the pundits talking about magical solutions like zone focussing or hyperfocal distances. What’s a poor boy to do when you thought zones were something to do with exposure and hyperfocals were what old people went to opticians to get?

Focusing a lens means moving it nearer or closer to the film or sensor, so that the light from your chosen subject is brought to the least fuzzy point. There was some stuff about it here. Many lenses have a built-in screw thread so that turning them moves them in and out, without letting light leak past. Other arrangements are possible, but they mostly all do the in and out thing.

Now, our eyes are not perfect and there is a lower limit to the size of things we can see. As you get older, that might be grandchildren. Grant me though, that we can’t see atoms, or even molecules. So when an image is projected onto the film or sensor by a lens, there will be a range of distances where everything appears sharp. If you could look closer, perhaps with a microscope, you could see that that amount that was truly sharp was less than it appeared to the eye. This is why small prints or pictures often look sharper than they are when you make them bigger.

But for practical purposes, there is a range of distances in front of the camera between which stuff looks sharp. This is the depth of field. Where this zone falls and how deep it is depends on several things. Let’s assume for now though that these things are outside your fine control: you can make a basic choice like the camera you are using, but you can’t change the lens on it. Let’s also assume that your camera might have one of the two forms of focusing: controlled and guess, where guess includes fixed and not adjustable. If your camera provides accurate and adjustable focusing and that is what you want, then move along – there is nothing to see here. But there can be good reasons why you might want to use your adjustable camera as though it was not. The main one is often speed of use. Focusing takes time.

So, how do you make best use of either what you are stuck with or what you choose to adopt? According to type is the answer. Guessed focusing comes in three forms: fixed, zone and scale.

Fixed is where there is no adjustment possible. It’s not autofocus – it means the focus of the lens is fixed and you do your best to put the subject in the sharp zone. If you are lucky you may be able to find out from the manual or t’interweb where the focus distance is, or what the depth of field is. I have a fixed-focus camera, and the manual lists the range of sharp(ish) distances for each aperture setting. Without this information you may have to find out, or just live with it. It’s a fair assumption that a fixed-focus camera will be set to somewhere around the distance where you can get an adult in the frame, around mid-length. My own fixed-focus camera is set for about 8 feet. You could leave it at that and just work with it, or use a bit of film in testing. What you need is a long fence or railings that you can shot at an oblique angle so that your picture shows it from close to far. Before you shoot, pace out some distances and mark them with chalk or a stone. Then examine the developed image to see where it is sharp and how far away that is. Then get someone to stand that far away and look at them using the camera, to get an idea of what that distance looks like. Or make a simple version of the card rangefinder. Then shoot everything at the sharp distance.


Next up is zone focusing. This is where the lens offers a set of symbols for where it will focus. These are usually head and shoulders, group, mountains. Again, you can work with it or do the fence test to get an idea of what each setting does.

Zone focus

In the case of my camera, head and shoulders works out around 1.5 metres or a bit less than 5 feet. Groups fall at around 5 metres/ 15 feet.

Cameras like this can be very quick to use – pick the type of picture you are making, set the symbol for focus and go. Providing the aperture is around f8, you are likely to have enough depth of field to not have to worry.

Scale focusing is like using the symbols, but without the symbols. This is where the lens is marked with real distances, but you have to guess or measure the distance of your subject and adjust the lens accordingly. The lens on the camera above has both a distance scale and symbols. It sounds dreadful – how will you ever be able to estimate the distance acccurately? Use some basic rules:

  • A head and shoulders is around 5 feet, or a bit less.
  • An adult, shot vertical on 35mm with a 50mm lens, just about fills the frame at 10 feet (3m).
  • A group will be around 15 feet, or 5m.

Then use a reasonably small aperture like f8 and it will mostly work. If you are picky or nervous, make yourself a card rangefinder. It will easily fit in the camera case or your wallet.

You can even use a ‘proper’ camera with scale focusing. The street photographers do this for speed. You need to have a lens that has depth of field marking on it.


If I set this lens to f8, then everything between 2.5 and about 5m will be sharp. If that’s the most likely distance for stuff I want to take pictures of, I can set the lens and aperture and use the camera like a point and shoot. It would let me do slightly wide head and shoulders shots through to slightly tight groups without having to adjust a thing. This is what news photographers used to do, to give them the reaction time they might need to get the decisive moment (as legend would have it).

Then we come to the secret weapon of landscape photographers: the hyperfocal distance. Given a particular aperture, the hyperfocal distance is the point you focus the lens at that gives a depth of field spanning from half that distance out to infinity. It sounds like magic, and the actual point you need to focus on varies with the film or sensor size, the lens and the aperture. You don’t have enough fingers and toes to do the maths. So you either use an online resource or an app to calculate it for you, or use the depth of field markings that the lens maker gave you.

Say I’m using the lens in the above photo and I want both a group of people and the mountains in the background to be sharp. So I want a depth of field from say 4m out to infinity. I twist the lens to find a pair of aperture markings that put infinity on one side and my closest distance at the other. Then set the aperture to match the marks – the point of focus is already set correctly. Job done.


In this case I need f11 and my closest sharp distance is perhaps 3.5m. The actual point of focus of the lens is 6m, but I don’t care.

This also works well if you are taking pictures of things that occur a bit further away, but variable. Some sports or activities, for example. Set the depth of field to cover the area of the action and concentrate on taking pictures.

So there you are – sharpness made simples, and a way to make use of the cheap end of the camera market.

In perspective

Want to start a fight? Ask a few photographers what effect the choice of lens has on perspective. Perhaps not a fight, but you will get a lot of hearsay rules and theories. But, as we know, a proper theory is one that can be falsified and makes testable predictions.

So a camera is basically a pinhole sitting at some distance from a sensor or film. The angle of view is set by the size of the sensor and how far away it is from the hole. A big sensor further away from the hole can have the same field of view as a smaller sensor closer to the hole.

Angle of view

Since the distance between sensor and hole is basically the focal length, this explains why different sizes of film or sensor need different focal length lenses to get the same angle of view. For example, a 50mm lens would be a telephoto (narrow angle of view) on an APS-C sensor, a normal lens on 35mm and a wide angle on medium format. Or, to put it the other way round, a standard lens on a small sensor would have a shorter focal length than one for a larger sensor – see A and B in the diagram above.

Let’s ignore the sensor size for now and just look at the angle of view of the lens.

Angle 1

The lens we have fitted has a wide enough angle of view to take in both the person in the foreground and the building in the background. If I stay in the same position and fit a lens with a narrower angle of view, it gets just the person’s face and a small section of the background.

Angle 2

What you will notice though, is that the relative sizes of the person and the building do not change, you just get a narrower slice of the wider version. You can test this by taking the same picture from the same position on both a wide and a narrow angle lens. Or use a zoom. Enlarge the wide-angle shot so that the central portion matches the narrow-angle picture and you will find that they match perfectly. This is the falsifiable test. This means that perspective, in the sense of the relative sizes of objects in the frame, does not change with your choice of lens if you stay in the same position. All that changes is how much stuff you get in the frame.

So what does a wide-angle lens change? If you get close to the subject, it changes the relative sizes of the nearby subject and the distant background.

Angle 3

In this diagram I shoot a head and shoulders with a narrow-angle lens (the dotted lines). It can see a narrow section of the background, so the background looks quite large in relation to the subject. If I get close to the subject with a wide-angle lens, so that I still get a head and shoulders (the solid lines in the diagram), the subject is the same size but I see more of the background and it looks smaller in relation to the subject. What changes is called the diminuition – the rate at which objects get smaller as they get further away.

This is why we usually avoid shooting portraits with wide-angle lenses. If we get close enough to fill the frame, the relative size of things near and further from the camera changes. These could be the nose or an arm or leg. But we do use wide angle lenses for landscapes, where we want to give prominence to a foreground object (like a Joe Cornish rock).

So there you have it – the angle of view of the lens only controls how much you can fit into the picture, providing you stay in the same place. You can use the angle of view of the lens to control how big the background is in relation to the subject, but only if you move nearer or closer to the subject.

What does this mean? That perspective is controlled by position, not the field of view of the lens.

Packing it all in

I’m away on my hollybobs. Special ones, too. But I might be taking more cameras than clothes. Let me explain.

I passed one of those milestone birthdays this year. The sort of birthday that your younger self couldn’t even imagine. I remember at school working out how old I would be in the year 2000, which seemed an impossible distance away. Anyway, to mark the event I’m doing something special – I am going diving in clear, warm water.

I have dived in clear water before, but never as an experienced diver. All my experience has been in British waters so I have never really seen what good visibility is like, or been able to dive without a duvet under my drysuit. But now I am off to parts abroad (after October, to be known as ‘parts foreign’). Clear water, warm, small or no tidal range, little or no currents. Filled with smiling fish and marvellous things. Huzzah! But what camera do I take?

I need to be able to switch between macro and normal, as I want to be able to show the environment as well as the residents. No-brainer then: take the digital plus housing. The camera itself is an old model of Canon. Perfectly adequate for the job but old enough that I got a second one off eBay for a fiver. That way, if I do get a flood, it’s a camera swap rather than a crisis. So that’s one housing in the main suitcase and a couple of small digital cameras in the carry-on. Plus their batteries, as airlines get nervous about Li-on cells where they can’t see them.

What about the Nikonos? If I take it, I will need to take the bracket and two flashguns. So that’s two suspicious-looking bits of electronics and a chunk of metal in the big bag and a heavy X-ray opaque camera and the flash batteries in the carry-on. Plus film.

What about shooting video? If I use the digital camera for that, I’ll need to take a video light. I have one and it can double-up with a grip as a dive torch. Sorted. Except that’s another battery for the carry-on.

Then, what do I use for my touristy shots on land? The spare Canon digital could do the job, plus the Nikonos is amphibious. Do I need anything else? But I would really like to take another ‘proper’ camera with settings so that I can do the arty-farty bit. But then I would need lenses. What about a point and shoot? My mobile phone has a very capable camera plus editing software, so I could just use that. What if I could find the 80mm lens for the Nikonos? No, that last one is daft. My sensible head tells me to use the spare Canon, as I am carrying it anyway.

So: Nikonos plus two flashes and bracket; housing plus two Canon digitals; video light. Rational choices, but the film freak in me still wants to take another film camera. And the Nikonos is heavy to carry around. So is it the tiny Olympus XA, accepting that it has the same 35mm lens as the Nikonos, or is it an old Pentax point and shoot that has a zoom lens but could die on me? But if it does die I just wind the film back and ditch the camera. And the built-in flash is far more capable than the one for the XA. OK, so add a Pentax to the pile, feeding from the same film stock as the Nikonos. And add a film retriever in case I do have to rewind a part-used film. That won’t look at all odd on the luggage x-ray. Not a bit.

Then add to the pile a bunch of rechargeable AA cells for the flashes. Plus chargers for them, the Canons and the video light. And for my phone. And some film – all in original boxes so the airport guards don’t get nervous. Sorted.

Unless I change my mind again.

Plus seven pairs of pants and tee shirts. I’ll let you know how I got on.

Too many socks?


And of course, I changed my mind again. The little demon of perversity sat on my shoulder during the drive to work and whispered in my ear. The Pentax point and shoot is as big as a housebrick, so why not take something smaller? The Balda folds up really small, and a couple of rolls of 120 film will take up no space at all. Plus it will fulfil my desire for something manual and awkward to fiddle with. And it delivers my real desire, which is to shoot black and white. So it’s Pentax out and Balda in.

Unless I change my mind again.


Guess what? I realised that the Nikonos was going to be hooked-up to its flashgun, and that I didn’t want to be undoing and re-doing the connection every day: too much risk of introducing a leak. Plus I weighed the bag and it was well under the limit. So I sneaked the Olympus XA in. So that’s one Nikon, two Canons, one Olympus and a Balda. Totally ridiculous, if it wasn’t for the need to take pictures underwater and on the surface, plus the desire to have enough resilience against breakage or problems.

Making a card rangefinder

I have referred to this a few times, with links out to pages on t’interweb where these things are spoken of. The drawback is that the web resources seem to use maths, when what we want is simplicity. So here’s my version.

What you need is a bit of card (I know, the title was a spoiler). Anything from credit card to an index card will do (ask your parents what a card index was). The main thing is that the card has one good right-angled corner. You also need a tape measure and a second scrap of card or paper.

Find a nice vertical line – a door frame or the edge of a wall. Measure a distance of say six feet from it and stand facing the target line with your toes at the distance mark. I say feet, but you might also measure the distance in meters. Use whatever units your camera lens is marked in.

Hold the card out in one hand at arm’s length. Choose the hand you will be using when you do this for real with a camera. If you are likely to be holding the camera in your right hand and using the rangefinder in your left, then hold it out in your left hand. Let’s assume you will be using your left hand. Close your left eye and line-up the edge of the card with the vertical target. Without moving the card, close your right eye and open the the left. Hopefully the vertical target will appear to move along the card, away from the edge that was originally lined-up. If it moves the wrong way, swap eyes.

That second scrap of paper is used to mark where the vertical edge appears to move to. Pinch it against the main rangefinder card and slide it sideways until your distant mark appears to consistently move to the same place. Keeping the rangefinder card and the scrap of paper pinched together, mark on the rangefinder card a line for that distance. In this example, six feet. Change the distance and repeat.

Card rangefinder
Left picture is what you see with your right eye open. Right picture is with left eye open – mark the measured distance where the yellow card joins the white one.

You can now throw away the scrap of paper and keep the rangefinder. You have built a rangefinder that works for you and your eyes and will measure close distances with enough accuracy to focus lenses using their marked distance scale.

Here’s one I made earlier. Marked in meters and made to be held in the right hand (which is why the distances come in from the left).

Go ahead and make copies of the card so that you can keep one with each camera. Laminate them. Print the scale on the back of your business cards. Get the lines tattooed on your finger or mark them with a pen before you go out.

The only time you will need to change the card is if the length of your arm changes, or the distance between your eyes. Or you decide to become a pirate.