What is a Diopter in Cinematography?

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?

5 Things You Didn’t Know About the Iris in Your Lens

Inside a lens, amongst the various glass elements, is an ingenious mechanism which we call the iris. Just like your biological iris, it controls the amount of light passing through the pupil to form an image. I’ve written about the iris’s use to control exposure before, and its well-known side effect of controlling depth of field. But here are five things that aren’t so commonly known about irises.

 

1. f-stops and the entrance pupil

This image shows the exit pupil because it’s seen through the rear element of the lens. A view through the front element would show the entrance pupil.

The f-number of a lens is the ratio of the focal length to the diameter of the aperture, but did you know that it isn’t the actual diameter of the aperture that’s used in this calculation? It’s the apparent diameter as viewed through the front of the lens. A lens might have a magnifying front element, causing the aperture to appear larger than its physical size, or a reducing one, causing it to appear smaller. Either way, it’s this apparent aperture – known as the entrance pupil – which is used to find the f-number.

 

2. No-parallax point

The no-parallax point of a lens is located at its entrance pupil. Sometimes called the nodal point, although that’s technically something different, this is the point around which the camera must pan and tilt if you want to eliminate all parallax. This is important for forced perspective work, for panoramas stitched together from multiple shots, and other types of VFX.

 

3. Focus

If you need to check your focal distance with a tape measure, many cameras have a handy Phi symbol on the side indicating where the sensor plane is located so that you can measure from that point. But technically you should be measuring to the entrance pupil. The sensor plane marker is just a convenient shortcut because the entrance pupil is in a different place for every lens and changes when the lens is refocused or zoomed. In most cases the depth of field is large enough for the shortcut to give perfectly acceptable results, however.

 

4. Bokeh shape

The bokeh of a 32mm Cooke S4 wide open at T2 (left) and stopped down to T2.8 (right). Note also the diffraction spikes visible in the righthand image.

The shape of the entrance pupil determines the shape of the image’s bokeh (out of focus areas), most noticeable in small highlights such as background fairy lights. The pupil’s shape is determined both by the number of iris blades and the shape of their edges. The edges are often curved to approximate a circle when the iris is wide open, but form more of a polygon when stopped down. For example, a Cooke S4 produces octagonal bokeh at most aperture settings, indicating eight iris blades. Incidentally, an anamorphic lens has a roughly circular aperture like any other lens, but the entrance pupil (and hence the bokeh) is typically oval because of the anamorphosing effect of the front elements.

 

5. Diffraction spikes

When the edge of an iris blade is straight or roughly straight, it spreads out the light in a perpendicular direction, creating a diffraction spike. The result is a star pattern around bright lights, typically most visible at high f-stops. Every blade produces a pair of spikes in opposite directions, so the number of points in the star is equal to twice the number of iris blades – as long as that number is odd. If the number of blades is even, diffraction spikes from opposite sides of the iris overlap, so the number of apparent spikes is the same as the number of blades, as in the eight-pointed Cooke diffraction pictured above right.

5 Things You Didn’t Know About the Iris in Your Lens

6 Things to Beware of with Vintage Lenses

Ever since digital cinematography became the norm, DPs have sought to counter the format’s perfection with characterful vintage lenses. Having just completed a feature film shoot, Hamlet, on Cooke Panchros and a Cooke 10:1 Varotal, I’m over the moon with the beautiful, creamy, organic look they brought to the production. However, I can’t deny that they have some disadvantages over modern glass which you should take into consideration before choosing the vintage approach.

 

1. Softness

Vintage lenses simply aren’t as sharp as their modern counterparts, particularly at the edges of frame and particularly when the iris is wide open. On Hamlet I deliberately shot with the Panchros wide open to soften the image, rather than adding a diffusion filter like I’ve often done in the past, but that look is not for everyone, and it does make things a little harder for your focus puller. Be sure to test the sharpness and view the results on a large screen before committing.

 

2. BreathING

Breathe is the phenomenon whereby a lens appears to zoom slightly in or out when the focus is pulled. The Cooke Varotal is especially prone to this. As a result, my focus puller Aristide Russo had to be very gentle with his pulls otherwise the breathing was distracting.

 

3. Veiling

Many DPs love lens flares, and beautiful, natural flares were one of the reasons I picked the vintage Cooke glass. But look out for veiling flare – a milkiness and lift in the shadows affecting the whole frame. I noticed this a lot when shooting under the practical fluorescents in Hamlet‘s stage set, especially with handheld shots where the veiling would appear and disappear depending on the camera’s angle to the lights. I decided to embrace it and make it part of the film’s look, but if maintaining high contrast at all times is important to you, lenses without modern coatings may not be the right choice.

 

4. Vignetting

Check for dark patches in the corners of your image. The Varotal I used vignetted at certain parts of the zoom range and not at others, so the dark corners would appear and disappear during a zoom. Although not ideal, it isn’t noticeable most of the time. Besides, I figured that most colourists add vignettes to most shots anyway, so I was simply saving them a little time!

 

5. Mechanics

Older lenses are, quite naturally, less reliable. Even if they have been rehoused, like our Cooke “Century” Panchros had been in 2000, you may find that the iris and/or focus sticks sometimes. Our 25mm started to play up halfway through our shoot, forcing Aris to use the rosettes to support the matte box, otherwise the motor wasn’t powerful enough to turn the focus ring. This possibility was flagged for me during testing when we had a similar issue with the 50mm. Even if all your lenses seem to be fine during prep, know that a vintage lens could start misbehaving at any time, and your rental house may not have another on the shelf to replace it with.

 

6. Uniformity

Don’t expect a set of vintage primes to all have the same maximum aperture or the same external configuration. The iris ring might be buried in the matte box, the matte box might not fit on at all, or it may be impossible to engage both iris and focus motors at the same time.

 

All this sounds quite negative, but the flares, softness, breathing and vignettes can be absolutely beautiful. Be aware of the downsides of using vintage glass, absolutely, but if they suit your story then embrace the flaws and get ready to be blown away by your dailies.

In case you missed them the first time, I’ll leave you with some highlights from my Hamlet lens tests.

6 Things to Beware of with Vintage Lenses

5 Facts About the Cinematography of “Dunkirk”

Some have hailed it as a masterpiece, others have complained it left them cold. Personally, seeing it on 70mm, I found Dunkirk a highly immersive and visceral film, cinematic in the truest sense of the word. The huge, sharp images free from any (apparent) CGI tampering, combined with the nerve-jangling gunshots and rumbling engines of the superlative soundtrack, gave me an experience unlike any other I can recall in recent movie-going history. I can imagine that it was less effective projected from a DCP onto a smaller screen, which may account for the underwhelmed reactions of some.

But however you feel about Dunkirk as a film, it’s hard not to admire its technical accomplishments. Here are five unique aspects of its cinematography.

 

1. It was shot on two huge formats.

Director Christopher Nolan has long been a champion of large-format celluloid capture, eschewing the digital imaging which has become the dominant medium in recent years. “I think IMAX is the best film format that was ever invented,” says Nolan in a DGA interview. “It’s the gold standard and what any other technology has to match up to, but none have, in my opinion.”

Imax is a process which uses 65mm film (printed on 70mm for exhibition, with the extra space used for the soundtrack) running horizontally through the gate, yielding an image over eight times larger than Academy 35mm. Following some test shots in The Prestige, Nolan captured whole sequences from The Dark KnightThe Dark Knight Rises and Interstellar in Imax.

For Dunkirk, Nolan and cinematographer Hoyte van Hoytema, ASC, FSF, NSC were determined to eliminate 35mm altogether, to maintain the highest possible resolution throughout the movie. Imax cameras are noisy, so they shot dialogue scenes on standard 65mm – running vertically through the gate – but Imax footage makes up over 70% of the finished film.

 

2. The movie was framed with three different aspect ratios in mind.

Those who watched Dunkirk in an Imax cinema got to see the native aspect ratio each sequence was captured in, i.e. 2:20:1 for the standard 65mm dialogue scenes but the much taller 1.43:1 for the Imax material, the bulk of the film. Those, like me, who attended a standard 70mm screening, saw it in 2:20:1 throughout. And those hapless individuals who watched it digitally apparently saw the standard Scope ratio of 2.39:1, at least in some cases.

This means that, when composing his shots, van Hoytema had to have two ratios in mind for the dialogue scenes and three for everything else. “Framing was primarily for the 2.40 [a.k.a. 2:39:1], then protecting what was outside of it,” 1st AC Bob Hall explains. This left close-ups, for example, with a large amount of headroom in 1.43:1, but the huge size of Imax screens made such framing desirable anyway.  “Imax is such an immersive experience that it’s not so much the composition that the cinematographer’s done as where your eyes are going on the screen that creates the composition.”

 

3. Parts of the camera rig were worn as a backpack.

Breaking with the accepted norms of large format cinematography, van Hoytema captured a significant proportion of the movie handheld. The 65mm camera package weighed over 40kg – about three times the weight of a typical Alexa rig – with the Imax camera only a little lighter. To avoid adding the weight of the batteries, video transmitter, Cinetape display and Preston (wireless follow focus) brain, these were placed in a special tethered backpack which was either worn by key grip Ryan Monro or, for water tank work, floated on a small raft.

Unfortunately, Hall quickly found that electromagnetic interference from the Imax camera rendered the Cinetape inoperable, so he ended up relying on his extensive experience to keep the images sharp. “I had to go back to the technology of the 1980s, where I basically guess how far famous people are from me,” he remarks drily in this enlightening podcast from Studio Daily.

 

4. A periscope lens was used to shoot spitfire cockpit interiors.

“I wanted to tell an intensively subjective version of this story,” says Nolan. To that end he requested over-the-shoulder views out of the windscreens of Spitfires in flight. Furthermore, he wanted to be able to pan and tilt to follow other aircraft passing by. Given the huge size of the Imax camera, there was no room to rotate it within the cockpit. Instead, custom periscope lenses were built which could snake over the pilot’s shoulder, and pan and tilt independently of the camera body, using prisms to maintain the correct image orientation to the film plane.

Other glass used on Dunkirk included an 80mm Imax lens belonging to Nolan himself, and converted stills lenses.

Note that the camera is mounted upside-down, to compensate for the flipped image generated by the prism in the periscope lens.

 

5. At one point the camera sunk to the bottom of the sea for an hour and a half.

A specific Spitfire POV required was from a damaged plane diving towards the sea and hitting the water. The practical effects department devised a catapult to launch an unmanned mock-up from a ship, the grips built a crash housing for the Imax camera which would be inside, and a plan was devised to recover it before the mock-up sank. But they weren’t quick enough, and the crew watched the plane and the camera disappear beneath the waves and plunge to the bottom of the English Channel, where it sat for 90 minutes until divers retrieved it. Incredibly, once dried out and developed, the film footage was found to be completely undamaged. “The shot was all there, in full colour and clarity,” says van Hoytema in the American Cinematographer article. “This material would have been lost if shot digitally.”

 

5 Facts About the Cinematography of “Dunkirk”