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DIOPTERS, OR CLOSE-UPS

For a proper result when working with anamorphic adapters, the constant use of close-up filters is required, thus many anamorphics need some modification to their front part, since they don’t have standard (or any) filter threads. These are the front clamps, which tend to be very specific to each anamorphic model and usually have 58mm or 72mm filter threads.

Buying step rings on eBay and shipping them straight from China or Hong Kong is a very simple and cheap way to use different filter sizes if you need them. While online step rings go for something between US$1 and US$5, it’s very hard, in Brazil, to find them for anything less than US$20. Redstan front clamp for Kowa Bell & Howell, 72mm filter thread. For the vast majority of lenses, minimum focus is around 2m. In some cases, 1.5m, which is still a very long distance to achieve a standard close up shot (shoulders and head) or anything tighter than that. Besides, when using any aperture wider than f/4, focusing can become a challenge on its own. Diopters – or close-ups – are actually auxiliary lenses with filter threads that consist of one single optical element or two (achromatic doublets or achromatic diopters). They’re grouped according to their focal range, measured in DIOPTRIAS (to make it more understandable, DIOPTRIAS will be called “strength” from now on). There are lenses with strengths ranging from +0.25 up to +10 (the plus signal means they’re converging lenses). A diopter is always placed in front of a complete lens/optical system and its effect is to “limit” the distance represented by the infinity focus mark. What we shall call the “new infinity” is much much closer and stands for the closes focusable distance which can be determined according to the strength of the auxiliary lens. The math is quite simple, but we first need to define two variables. All measurements will be done in meters. MaxF = maximum focusable distance, in meters. S = diopter’s strength $\text{MaxF}=\frac{1}{\text{S}}$ Taking +0.5, +1.25 and +2 strength diopters as examples, we have, respectively: $\text{MaxF}=\frac{1}{0.5}=\frac{1}{\frac{1}{2}}=1\times\frac{2}{1}=2\text{m}$ $\text{MaxF}=\frac{1}{1.25}=\frac{1}{\frac{5}{4}}=1\times\frac{4}{5}=0.8\text{m}$ $\text{MaxF}=\frac{1}{2}=0.5\text{m}$ As the strength increases, maximum focus distance shortens. But, in a practical way, how do these lenses improve anamorphics performance? Simply because their image quality is considerably superior when focused closer to infinity, losing quality as the subject comes closer to the minimum focus mark, between 1.5m and 2m. If infinity is now limited to a couple meters or centimeters ahead through the auxiliary lens, excellent results are now achievable in this area that used to be minimum focus. There are two obvious consequences to using them, though. The first one is that it becomes impossible to focus anything further than the predefined distance. The second is that every single focus mark on the lens barrel is now wrong. It would be necessary to recalculate every single one of them by the new rules, but this process is very much useless. It’s faster and easier to eyeball focus when using diopters. As for minimum focus, for diopters up to +1 strength it’s simpler to consider half the maximum focus as a safe distance to get good quality images. Anything closer than that starts to dive into that “close focus” zone we’re trying so hard to avoid and you should just swap the current diopter for a stronger one. When considering stronger diopters (anything higher than +1), 3/4 of the maximum focus distance is the closest I’d go if I want to keep a high image quality. Taking a +2 diopter as example, maximum focus sits at 0.5m. Anything closer than 0.35-0.4m already has a low image quality. Let’s see some examples. Pay close attention to the defocused objects in the background. On the first image, almost everything is in focus, but in the last one, the icons on the monitor are barely recognizable. 50mm taking lens + Iscorama 54 @ minimum focus: 2m. 50mm taking lens + Iscorama 54 + +0.5 diopter @ minimum focus: 1m. 50mm taking lens + Iscorama 54 + +1 diopter @ minimum focus: 0.5m. 50mm taking lens + Iscorama 54 + +2 diopter @ minimum focus: 0.25m. There is an operation to determine precisely the minimum focus of a lens when attached to any diopter. This operation can also be used backwards in order to determine the strength of an auxiliary lens if you know the lens minimum focus with and without the diopter – particularly handy for poorly written eBay listings or buying weird zoom lenses diopters that don’t have their strength explicitly written on the side. X = lens minimum focus without the diopter, in meters X’ = lens minimum focus with the diopter, in meters S = diopter strength $\text{X'}=\frac{\text{X}}{(\text{SX}+1)}$ eBay is a great source for diopters of all sizes. It’s just a matter of looking for the ones most appropriate to the lens you’re using. A standard for the smaller lenses (like Century, Kowa, Optex, Sankor, Iscorama 36…) is the 72mm thread. Bigger lenses such as the Iscorama 54 or most projection lenses require bigger diopters and, consequently, more expensive and harder to find. The idea here is not get attached to any particular brands. Most of them have some sort of “magical” numbers to portray the diopter’s thread and strength but usually the sellers already have that information and put them directly onto the listing information. Otherwise, a google search is a great way to getting extra information. I’ve identified some weird patterns along the way and in order to make things easier, I’m putting them right below. Fujinon: The first number is the maximum focus distance, the second number is the filter thread. For example Fujinon 16086 has its focus limit at 160cm (which equals a +0.6 strength) and an 86mm thread. 190101 stands for 190cm maximum focus and 101mm threads. Fujinon has some weird and good diopters that aren’t usually picked up by the common buyer, so keep and eye for them. Canon: The newest series (250D and 500D) have their measurements in milimeters. 250mm stands for +4 and 500mm stands for +2. The “D” comes from “double element” which means they are achromatic doublets. The older series use similar measuring, like the 1300H, which equals 1300mm, 130cm or +0.75, or even the 900H (quite rare), with 105mm threads and +1.1 strength. A good option in order to cover a wider range of thread sizes is the use of several step rings to achieve the proper fits. The problem is that these rings can easily get stuck or lost. A different solution is to look for threadless diopters. The ideal ones are called Series 9 and have a diameter of 83mm. With the glass at hand, you just need the right adapter. Taking the Series 9 as example, adapters were usually made by Tiffen and Kodak in sizes that range between 67mm and 86mm. This means you can use the very same diopter with lenses that have widely different thread sizes (67mm, 72mm, 77mm, 82mm and 86mm), covering almost every anamorphic currently available. Both adapters and diopters can be hard to find, but they come up every once in a while and for a fairly low price. The adapters consist of two aluminum rings that screw on top of each other, holding the glass in between them. Then, you just screw the whole thing in front of the lens, as a regular filter. Adapters and rings for Series 9 filters. And what is so special about these achromatic doublets, by the way? Their prices are way higher than regular diopters and they’re much harder to find too. Fortunately, they exist. These were first used in situations that require very short distances between lens and subject and extraordinary image quality (the kind of thing you expect from a microscope, for example). The kind of result you simply can’t get when using single-element diopters, which will always create strong chromatic aberration. The two optical elements (a positive and a negative one) are combined in a way they cancel out their weaknesses and enhance the resulting image quality, specially around the edges where the chromatic aberration is easier to spot. The doublets holy grail is Tokina’s 72mm +0.4, which goes for around US$350.

While looking for more information and ended up coming across a list of achromatic doublets made by various brands including detailed information such as thread size, strength, rarity and even a price range. From my own experience following the selling and buying online I’ve also came up with a short list with the most common and desired close-ups along with their price range over 2013. Might have a changed since then.

• Tokina +0.5 72mm – $150 • Kenko +0.5 72mm –$90
• Tokina +0.4 72mm Achromatic – $350 • Kenko +0.3 105mm –$350
• Canon 500D +2 72mm Achromatic – $100 • Sigma +1.6 62mm Achromatic –$20
• Angenieux +0.25 82mm – $330 • Kinoptik +1 82mm Achromatic –$530
• Foton-A +1 or +1.25 – $900 • Tiffen +0.5 to +2 138mm –$50 or more
• Tiffen +0.5 to +5 Series 9 – $1-50 • Tiffen/Kodak Series 9 Adapters –$20-40
• Tiffen +0.5 to +2 4.5″ – \$10-50 (rare)

TO CHAPTER IIIG – ANAMORPHIC LENS-YCLOPEDIA >>

• December 27, 2015 at 8:30 am

[…] up the prices. If you want to know more about diopters in advance, before my video on the subject, check this link, it’s quite […]