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What is a Diopter in Cinematography?

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If you’re a fan of early Steven Spielberg films, you’ve probably heard of split diopter shots (also spelt “dioptre” in the UK). Brian de Palma used them a lot too, and Robert Wise’s Star Trek: The Motion Picture is absolutely riddled with them.

A split diopter allows half the image to be focused at one distance and half at another. Shallow depth of field is sought after by many filmmakers today, but it wasn’t always so popular. Sometimes a director or DP wanted both characters in a deep two-shot simultaneously in focus, and slow film stock and/or lenses meant that it wasn’t possible to just stop down the iris.

So what is a split diopter (a.k.a. split-field diopter)? It’s a convex lens of a semi-circular shape that can be slotted into your matte box. Light passing through this lens is converged so that it focuses closer, while light passing through the empty half of course goes unaltered to the main lens.

Bog-standard diopters are available too, with no missing half, enabling the entire image to be focused closer than normal. (In stills photography they are sometimes called close-up lenses or macro filters.) These are especially useful with anamorphic and zoom lenses, which tend to have greater close-focus distances than spherical primes. There’s no stop loss, so you don’t have to compensate with more light.

I carried a set of diopters with me on Hamlet and we used them for a couple of shots with the vintage Cooke Cinetal 25-250mm zoom (CF: 5’6″) when I was shooting quite tight and quite close to the talent.

So that’s what the physical object is. But a diopter is also a unit of measurement. A typical set of physical diopters (full or split) contains ½, 1, 2 and 3 strengths. What do those numbers mean?

A diopter is defined as a reciprocal metre, or 1 over the focal length. It’s the same unit used to define prescriptions for glasses. The important thing in cinematography is what effect a diopter of a given strength has on your minimum and maximum focus distances. Apps like pCAM Pro will work these out for you, but let’s do the maths ourselves because it’s my blog and I said so.

The formula for minimum focal distance is

where

  • x is the normal minimum focus of the lens (in metres),
  • x’ is the new minimum focus,
  • and d is the strength of the diopter.

Let’s take my Cooke Cinetal as an example. The 5’6″ close focus in metres is 1.68, so with a number 1 diopter…

… or with a number 3 diopter…

Diopters can be stacked; simply add the strengths together and then drop that number into the formula, so in the above case we’d have a total of 4 diopters (diopters the units, not diopters the objects!) which would produce a close focus of 0.22m. Keep the strongest diopter closest to the camera when stacking.

You have to be careful that you don’t reduce your maximum focal distance too much and find you can’t hold focus on a character as they move away. Your new maximum focal distance y’ (when the main lens is set to infinity) is

So that would be 1m with a number 1 diopter! Pretty restrictive, huh? A number 3 diopter gives you a maximum focal distance of 0.33m, even worse! I remember having to cheat Ian McKellen (CLANG!!!) a little closer to camera when we did a diopter shot on Hamlet, so that we could pull focus to him from a foreground actor’s hands.

If you bear that caveat in mind, however, a set of diopters is a very useful thing to have with you.

What is a Diopter in Cinematography?

Time Up for Tungsten?

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Poppy Drayton, in “The Little Mermaid”, lit by a tungsten 1K bounced off poly

Last October, rental house VMI retired all of its tungsten lighting units as part of its mission to be a Net Zero company by 2030. I know this mainly because I am currently writing an article for British Cinematographer about sustainability in the film and TV industry, and VMI’s managing director Barry Bassett was one of the first people I interviewed.

Barry is very passionate about helping the environment and this is reflected in numerous initiatives he’s pioneered at VMI and elsewhere, but in this post I just want to discuss the tungsten issue.

I love tungsten lighting. There’s no better way to light a human face, in my opinion, than to bounce a tungsten light off a poly-board. (Poly-board is also terrible for the planet, I’ve just learnt, but that’s another story.) The continuous spectrum of light that tungsten gives out is matched only by daylight.

Dana Hajaj lit by another tungsten 1K bounced off poly

Tungsten has other advantages too: it’s cheap to hire, and it’s simple technology that’s reliable and easy to repair if it does go wrong.

But there’s no denying it’s horribly inefficient. “Tungsten lighting fixtures ought to be called lighting heaters, since 96% of the energy used is output as heat, leaving only 4% to produce light,” Barry observed in a British Cinematographer news piece. When you put it that way, it seems like a ridiculous waste of energy.

Without meaning to, I have drifted a little away from tungsten in recent years. When I shot Hamlet last year, I went into it telling gaffer Ben Millar that it should be a tungsten heavy show, but we ended up using a mix of real tungsten and tungsten-balanced LED. It’s so much easier to set up a LiteMat 2L on a battery than it is to run mains for a 2K, set up a bounce and flag off all the spill.

Shirley MacLaine lit by a tungsten book-light in “The Little Mermaid”

I admire what VMI have done, and I’ve no doubt that other companies will follow suit. The day is coming – maybe quite soon – when using tungsten is impossible, either because no rental companies stock it any more, or no-one’s making the bulbs, or producers ban it to make their productions sustainable.

Am I ready to give up tungsten completely? Honestly, no, not yet. But it is something I need to start thinking seriously about.

Time Up for Tungsten?

What’s in a DP’s Set Bag?

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I used to own a whole bunch of equipment – camera, lenses, lights – but for reasons I’ve detailed elsewhere I got rid of all that back in 2017. These days I travel pretty light (no pun intended) to set, but there are a few items I wouldn’t like to be without.

Here’s what’s in my set bag, roughly in descending order of importance.

 

1. Phone

Alright, this isn’t technically in my set bag, but it is the most used thing on a typical day on set. I use Chemical Wedding‘s Artemis Pro app all the time to find frames and select lenses, the same company’s Helios Pro to look at sun paths, and occasionally other specialist apps like Arri Photometrics (to work out if a particular light is powerful enough at a particular distance) and Flicker Finder (to check if a light will flicker on camera). I’ve also got Lux Calc installed but so far I’ve never used it.

Other common uses of my phone are looking at call sheets and other production documents if hardcopies aren’t supplied, checking my Google Sheets breakdown to remind myself of my creative intentions for the scene, and taking photos of lighting set-ups in case I need to recreate them for pick-ups.

To enable Artemis Pro to simulate wider lenses with my iPhone 7’s relatively tight built-in lens I also carry a clip-on 0.67x wide angle adaptor.

 

2. Light Meter

I’ve written before about why light meters are still important. My Sekonic L-758D gets heavy use on set, mostly in incident mode but sometimes the spot reflectance mode too; see my post on judging exposure to learn about what these modes do.

I make sure to carry spare batteries for it too.

 

3. Gaffer’s Glass

On The Little Mermaid the crew took pity on me using a broken ND filter wrapped in ND gel as a gaffer’s glass and bought me a proper one. This is like a monocle with an ND 3.6 filter in it for looking into fresnels and other directional fixtures to see if the spot of light is aimed exactly where it should be. I mostly use mine to look at the clouds and see when the sun is going to go in and come out, but you shouldn’t use one to look at the naked sun because even with all the ND it can still damage your eyes.

 

4. Power bank

With the heavy use my phone gets on set the charge doesn’t always last the whole day, so a power bank is essential to keep it running, as of course is the mains charger just in case.

 

5. Travel mug/flask

Most productions are environmentally conscious enough now to dissuade people from using disposable coffee cups and water bottes (though there are still a million half-finished water bottles on set at the end of the day). I always bring my own travel mug and metal water bottle. Keeping the mug clean(ish), especially when switching between tea and coffee consumption, is a daily struggle.

 

6. Croc clips

I always keep a couple of croc clips on my belt when shooting. Although I rarely gel lights myself on larger productions, I find them useful for adjusting curtains to admit just the right amount of daylight, or attaching a rain cover or light-blocking cloth to the camera, or clipping my jacket to something as a last-minute lighting flag.

 

7. Multi-tool

On some productions I’ve worn a multi-tool on my belt every day and only used it once or twice (usually to open wrap beers), so now it stays in my bag unless it’s specifically needed. As a head of department I theoretically shouldn’t be doing any tasks that would require a multi-tool, but it’s annoying to need one and not have one.

 

8. Tape Measure

I think my mum gave me this tiny tape measure which I keep in my set bag because it’s so small and light there’s no reason not to. I’ve used it exactly once so far: to work out if an Alexa Classic with a Cooke 10:1 zoom on would fit into certain tight locations on Hamlet.

 

9. Gel swatches

I picked up a set of Rosco filter swatches at either the BSC Expo or the Media Production Show. I don’t think I’ve ever used it.

 

10. Compass

Occasionally Helios Pro isn’t playing ball and I need to work out roughly where the sun is going to be, so out comes the traditional compass.

 

One final thing. Until very recently I carried a pair of gardening gloves for handling hot lights, but again I shouldn’t really be doing this myself and incandescent lamps aren’t too common on sets any more anyway, so when my gloves became worn out enough to need replacing I decided not to bother.

What’s in a DP’s Set Bag?