A lot of the value of the Letus 35 comes from what it does automatically that you have to be concerned about with other 35mm adapters and with do-it-yourself adapters. One item that might be confusing to some is the use of the terms “Front Focus” and “Back Focus”. In a camera without an adapter, the “Front Focus” refers to the way the camera lens focuses on the focal plane of the object being recorded, and the term “Back Focus” refers to the way the image is focused from the camera lens onto the camera backplane (film or sensors). However, 35mm adapter users have re-purposed these terms. So “Front Focus” means focusing using the 35mm lens from the focal plane onto the screen, and “Back Focus” means focusing the native camera lens on the image on the screen so that it can be recorded by the camera’s sensors.
In the diagram above, notice the items labeled a, b, c, and d. The distance between the 35mm lens and the screen, “a” in the diagram, determines whether the 35mm lens can focus properly at infinity. In some adapters, the screen can be moved backward or forward and the user must “set infinity” by adjusting the screen position. In the Letus 35 Extreme, the screen is preset at the correct distance for infinity focus and the lens mounts for each brand of lens changes the positioning of the 35mm lens to maintain the correct distance. In the Letus 35 Extreme, the image is inverted, “b”, so that it is viewed and recorded upright. In some adapters the image is inverted and the user must adapt to working with upside-down imagery during production and post. The distance “c” between the camera and the screen, and the magnifying element “d” that enables the camera to focus on the screen are critical to image quality. In some adapters, the magnifier is replaceable with different quality optics for DV and HD work, and can be positioned at different distances to the screen, making the user responsible for correctly setting distance “c” and for ensuring that the camera properly aligns with the magnification element and the screen. All of these settings are handled by the Letus 35 extreme. The unit screws onto the front of the camera in the filter mount, ensuring correct distance and alignment.
Setting the Back Focus
With the vibrator off, and without a 35mm lens attached, I used the auto focus on the HVX to focus on the grain pattern on the screen. Following some advice I read online, I set the zoom around 77. Then I turned the focus to “manual” to freeze it. I normally use a small bit of gaffer’s tape to secure the camera lens when I am on a shoot. It’s too easy to accidentally bump the camera’s focus ring and not notice the slightly out-of-focus image.
After that, I attached a prime 50mm Nikon f2.8 lens, turned on the vibrator button, and started shooting.
The testing that I performed was not rigorous mathematical measuring or shooting pattern charts. I didn’t hook up studio monitors and take light measurements. There are a number of other people who have done rigorous tests of the Letus 35 Extreme and measured it in various ways, and there are some really good articles available online (notably on DVXuser.com) if you are interested in that kind of technical detail.
As always, I was looking at it more practically: Can I use it to make movies?
And, as always, I am trying to answer these five questions:
What does it buy me in terms of frame control?
What does it buy me in terms of blur control?
What does it buy me in terms of motion control?
What does it buy me in terms of image control?
And finally, does that additional capability come at a price that is worth the money, time, and the effort required?
Testing the Letus 35 Extreme
Take a look at this! Now that’s 35mm shallow focus!
