Fresnel Lens for Bowens S mount
From time-to-time I’ve thought it would be nice to have a fresnel lens spot available when shooting with strobes. Profoto has one for its system. Its a 6″ glass lens in a housing that sells for $1,558.00 at Adorama. Even if it could be modified for the strobes I use its way too much of a financial investment for what I wanted to try. I wondered how hard it would be to buy an LED fresnel spot and modify it so I could mount a speedlite or larger strobe.
Then, when doing some research on Amazon I came across this product: foto4easy Studio Light Focus Mount . It comes with 4 filters and focuses from 40 to 10 degrees of spread. Its fresnel lens is about 4 1/2 inches across. Best of all, it mounts to the strobe via a Bowens S mount.
A quick test with a CL-360 mounted on an S bracket illustrates that its an effective light modifier. The images below show the fresnel lens mounted on the S bracket with the CL-360. An image of Manny shot with it focused to 10º spread. Finally, the pull pack shot showing relationship of light to Manny. The EXIF data shows the camera settings. The light was set to 1/4 power and it fired in HSS mode and was controlled by an X1Ts on a Sony A7RII. The second image — Manny against the ski — was shot Av at -2EC. Since I was using a lens with a maximum 5.6 aperture the shutter was fairly slow. 2 EV wider aperture would have been well within the max shutter speed and had power to spare on the light. Outside with bright sun I don’t mind sharper shadow transitions. Its a by product of the fresnel lens. Watching television or motion pictures it doesn’t take long to realize that a lot of the shadow transitions are relatively sharp. Probably because fresnel spots are being used for lighting. I first used fresnel spots doing theatrical lighting in collage. They have been in the theatre and on motion picture sound stages since the beginning.
One thing I discovered is that a 10º spread is hard to aim. Will have to do a bit of thinking about ways to aid that process. Modeling light won’t be effective outside in sun. Probably would work in studio or at night.
Comparing Fresnel lens to grids
Another approach to narrowing the light beam from a strobe is adding a grid to a reflector. Both narrow the beam spread but the light on the subject may be different. I have a set of grids for a standard 7″ reflector so a comparison seems relevant.
One question is what impact the grid has on exposure compared to the fresnel lens adapter. The following series of images answer that question as well as illustrating the difference in light character. The easiest way to determine the difference in exposure was to get an exposure with the fresnel adapter at its 10° setting, then shoot all the variations at the same power setting on the strobe. The AD360 was set to 1/64th power.
This first set of images are the fresnel adapter set to 10°. These are the baseline images for exposure. ISO 100, 1/60 shutter, F5.6 aperture, AD360II at 1/64 power. Each set has two images, one at the camera settings for exposure, the other with exposure adjusted in Lightroom to get the same reading on Manny’s cheek as the baseline image — 77%.
The next set of images is with the 7 inch reflector with 10° grid. All camera and strobe settings are the same. The first image in the set is at the camera settings. The second is with the exposure increased 2.20 EV to get the 77% reading on Manny’s cheek. While the light fall off is similar there is a significant difference in power required to get the same exposure.
The next set is the fresnel adapter at 40°. The wider beam spread required a 1.7EV exposure adjustment in Lightroom to get the 77% reading. This confirms that the fresnel adapter does focus the light to get the tighter beam. This is similar to the zoom on a speedlite increasing the guide number as the focal length gets longer which equates to a narrower light beam.
The following set is with a 40° grid inserted into the 7″ reflector. It requires a 1.9EV increase to get the 77% reading. The grid and fresnel are about equal in power required from the strobe to properly light a subject at 40° beam spread.
This testing process could be more scientific but I doubt the results would be substantially different.
The fresnel lens adapter narrows the light spread from a strobe by focusing the output into a narrow beam. The grid narrows the light spread by blocking light that is reflecting outside the beam angle.
Because the fresnel is a focusing lens, the position of the flash tube behind the lens impacts the nature of the light beam. The advantage of the Godox S bracket and the AD360 is the ability to position the flash tube in the adapter. The farther back the better. With a strobe like the AD600 with its flash tube extending well into the adapter it loses the ability to focus the light into a narrow beam. More expensive fresnel lens adapters are available and probably are more precise. For my purposes this is an adequate fresnel adapter, especially at the price.