How and Why to Shim Your Cine Lenses

Cinema lenses are an entirely different world with various mounts and customizations. You can truly refine a lens to be absolutely perfect for your system – if you are willing to put in the effort. Swapping a mount out or simply getting the accessories for smooth rack focus and aperture are straightforward. Less simple is starting to take apart your lenses to shim them and adjust back focus.

If you are looking to get your cine lenses just right, the guys at CVP have put together a great explainer on how and why you should shim your cine lenses. And if you invested in good glass you should absolutely make sure you are getting the most out of it.

The explosion of more affordable cinema lenses means that this glass is making its way into more users hands. It also means there might be a bit more variance between the lens and the camera mount that makes shimming the lens an important step. A lens that isn’t properly set will have a focus scale that is off, an inability to focus to infinity, and parfocal zooms not actually being parfocal.

Shims allow you to correct any back focus distance to exactly what you need it to be. You will also find there is no one answer on how much you need to correct. The distance can also change if you use the lenses on more than one camera body – even if they are the same exact model. You will need to verify using specialized tools or just a lot of practice and double checking.

Just note that cinema lenses are precise, delicate tools. Working in a clean, dust-free area is a must. You’ll also want some bits and pieces to keep things like screws organized as you will likely have to remove parts of the lens. While CVP has advanced tools like a collimator you can still do this yourself at home.

Image Credit: Laowa

Make sure you have the exact adapter/mount, a solid tripod, a focus chart, a tape measure, and tools you need for your specific lens. Some tweezers can help. Pen and paper for notes is also going to be helpful.

Set up your camera facing the focus chart and mount the lens. Open up the iris to wide open. Distance may depend on your space or, ideally, the one provided by the lens manufacturer’s instructions. Make sure that any measurements you perform are done from the sensor mark on your camera.

Now you can focus the lens on the center of the chart. An external monitor to make sure you are dialed in is a nice idea if you have one. Punch in, use peaking, do anything you want to make sure you are at the ideal setting. Note what your lens’ focus scale is reading and the actual distance to the focus chart.

If the marked distance is less that the measured distance you will need to add shims. If the marked distance is greater than the measured distance you will need to remove shims.

Be extra careful handling shims. The extra thin ones can be bent and this can impact functionality. This is where the tweezers come in. 

Image Credit: Atlas Lens

Time to open up your lens and please, please follow the instructions. This is not the part you want to mess up. Usually, if the lens is designed right and has shims as an option you will have a relatively easy time. There are also sometimes some specific instructions for each brand, such as how many shims you should have at most in a lens.

Repeat the focus chart setup again and again until your focus scale provides a mark that lines up with the measured distance.

Parfocal zoom lenses are generally similar. You can use the same focus chart setup. Zoom in all the way and open up the iris and focus the lens. Check your distance – this is S1. Now zoom out all the way and refocus the lens. This distance is S2. 

If S2 is less than S1 you need to increase the shim amount. If S2 is greater than S1 you will need to decrease the shim amount. This should maintain the parfocal nature of the lens.

You may need to adjust the focus scale instead or in addition to shimming, though some lenses don’t have this as an option. 

It can be daunting but shimming your cine lenses is the way to get the best performance out of them.

[source: CVP]

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