Sony a7S Full-Frame Mirrorless Camera, Part 1: Lens Options

Before Minolta brought auto focus SLRs to the masses with the Minolta Maxxum 7000 in 1985, many zoom lenses were designed to hold manual focus from telephoto to wide. This was an important and desirable lens feature that assisted photographers to attain critical focus by allowing them to zoom in, focus, and then zoom out and reframe -- all while maintaining the subject in focus. Once autofocus SLRs replaced manual focus cameras, professionals no longer required parfocal lenses and varifocal lenses became the norm.

A parfocal lens is still important in video, even with lens and body systems that support autofocus, and especially for lens and body pairings that don’t support autofocus. The reason is that videographers need the ability to reframe a shot while recording by changing the focal length (zoom) without taking the shot out of focus. This is less important for handheld work as it is for tripod work but for me, the parfocal nature of a lens is the first thing I test and if a lens is not parfocal, I don’t even consider it.

Having a parfocal lens and long zoom range are the most obvious lens features that videographers gave-up when many moved to DSLRs and many event producers have adapted to life with 3x zoom lenses when 12-20x was the norm on their camcorders or they film entirely with prime lenses and zoom with their feet -- but some lines of work still require the ability to change focal length while filming, like speeches, dance recitals, and most anything live or filmed with a single camera.

Autofocus

There are two different autofocus technologies to consider. Contrast Detection Autofocus (CDAF) works to maximize the contrast in an image, which occurs when a subject is in focus. In order to determine when it is in focus it needs to also go slightly out of focus so it is common to see a little focus jog before a camera settles into focus. CDAF works a bit slower than PDAF and is common on camcorders.

Phase Detection Autofocus requires two images, one through the lens to the sensor, and the other through the lens to a secondary TTL sensor. Both images are compared and the autofocus mechanism calculates if the lens is front of back focused and moves the focal point precisely and quickly.

Photographers use autofocus way more than professional videographers do. Mostly this is because autofocus behaviour can be too unpredictable or fast for video work. In my own use with the Sony FS700 and LA-EA2 adapter paired with α-mount lenses, which allows me to switch from manual focus to phase detection autofocus mode, I found that the autofocus system frequently tried to focus away from my subject and the results we jarring. Photographers value fast autofocus but for times when the camera decides to focus on something different in the frame, a fast autofocus magnifies the mistake too much, especially for shallow depth of field work. E-mount lenses on the Sony α7S focus using CDAF autofocus and Sony claims they have designed many of their E-mount lenses with “movie-friendly” smooth focusing.

Focus Peaking set to highlight in-focus edges in red.

Focus peaking has made manual focussing much easier for videographers and photographers who chose to focus manually. When engaged, the sharpest edges on an image display colored edges. The Sony α7S has focus peaking in red and yellow and sensitivity adjustments of high, medium, and low. I often use an external monitor and select focus peaking (and/or zebras) in one of my two displays (my external monitor also supports focus peaking) and I leave the other monitor with a clean feed.

Manual or Supported Electronic Iris

Being able to adjust the iris on a lens is very important but this can be an issue if the lens you want to use can’t communicate with the camera body electronically or if it doesn’t have a manual iris ring on the lens barrel. Sony E-mount lenses talk directly to Sony camera bodies, including iris control, and Sony α-mount lenses require a supported smart adapter like the Sony LA-EA2/3/4 to attain linear iris adjustments. Some lenses, like the Nikon G series of lenses, lack an iris ring on the barrel but this limitation can be overcome with a simple adapter that has an integrated manual iris ring. Unfortunately the same is not true for Canon lenses.

Shawn Lam’s collection of lens mount adapters for the E-mount.

The Canon 5DMKII was the first DSLR that could be used for professional video and Canon video DSLR users far outnumber Panasonic, Sony, and Nikon DSLR and mirrorless camera users for video combined. The problem with Canon lenses is that when you try to use them on Sony E-mount bodies with an adapter is that Canon lenses don’t have iris rings on the barrel as the iris is electronically controlled by the camera body. As a result, early Sony E-mount video camera users couldn’t pair their Canon lenses with their Sony bodies unless they filmed everything with an iris that was fully wide open. Eventually the demand for a smart adapter for Canon lenses to the E-mount that offered iris control led a company called Metabones to develop such an adapter. The Metabones Canon EF lens to Sony NEX Smart Adapter (Mark III) is now a third generation product and also adds EXIF data and image stabilization features.

Lens Gears

Cine lenses often feature stepped rings for use with a follow focus control. I own a Rokinon 24mm Cine T/1.5 lens with stepped gear rings for both the iris and focus but I can still use it without a rod, rail, and follow-focus system if I want. One of the other advantage of a cine lens is that it has a click-free iris so you can smoothly adjust the iris without being restricted to hard F-stop increments.

Rokinon 24mm T1.5 Cine ED AS IF UMC Full Frame E-mount Lens on Sony α7S with geared focus and aperture rings.

By the way, a lens that is marked with T-stops measures the actual light transmission of the lens while an F-Stop is a calculation of the focal length divided by the diameter of the entrance pupil and varies between lens manufacturers and lens designs. If your lens doesn’t sell in a cine version you may also be able to have it professionally converted with a cine-mod to at least de-click the iris.

Zoom Ring/Power Zoom

I look for evenly dampened zoom rings on a lens that lets me change focal length smoothly while filming. I don’t always need to zoom while filming but for when I do, I don’t want a lens that has jerky movements. Like many of the above desirable characteristics of a great video lens, a smooth zoom ring isn’t terribly important to a photographer and as a result, many older lenses outperform newer lenses for video use.

Sony 28-135mm f/4 Power Zoom Full Frame E-mount lens prototype as shown at NAB 2014

In 1991, Minolta developed the Maxxum xi Power Zoom still photo lens. A power zoom for a still camera doesn’t make too much sense but it only seems fitting that Sony, who purchased the Konica-Minolta company that was a merger between Konica and Minolta, is now launching power zoom lenses for their E-mount cameras. A power zoom brings servo zoom control that is common in camcorders but is missing from large sensor video cameras. Current Sony power zoom models are designed for APS-C sensors but at NAB 2014 Sony was displaying a 28-135mm F/4 power zoom FE mount lens designed primarily for video use that they plan to release later in 2014. This one lens could make lens buying decisions covering this impressive 4.8x focal range an easy decision. Keep in mind that most zoom lenses that are suitable for video work are limited to a 3x zoom range and this lens has the ability to best the Canon 24-105mm F/4 that works reasonably well as a video lens.