Thanks for your questions about the Feisol leveling bases.
Basically, a leveling base provides a quick, easy way to provide a level shooting platform. This is especially handy when shooting multi-image panoramic images (Quicktime VR or stitched panoramas). While you can level your camera using a tripod head, that's not sufficient for these types of multi-image panoramas. For these types of images, the platform the camera rotates on (top surface of the tripod) must be level or you will end up with a tilted horizon.
When setting up a tripod on uneven ground, you can create a level platform by individually adjusting the length of the legs until the top surface of the tripod is level. However, this takes time and often requires multiple iterations of adjusting leg lengths until you get it just right. With a leveling base, it only takes a couple seconds to create a level platform. So, it's a nice convenience and it can be a real time saver for photographers who shoot a lot of multi-image panoramas.
To answer your questions specific to the Feisol leveling bases:
Does it go into a center column mount?
Yes, this is correct. It installs in place of the standard stock flat mounting plate, or the optional center column. It shares the same mounting mechanism as these other options. So, you can't install both a center column and a leveling base at the same time. It's either one, or the other (or the flat mounting plate).
The Feisol leveling bases are specific to certain tripod models, and not all Feisol tripods support this feature. Here's a list of which leveling bases work with which tripods:
CT-3301, CT-3401, CT-3402 Standard Class Tripods:
Leveling Base: Not Supported
CT-3441S Traveler Tripod
Leveling Base: Not Supported
CT-3342, CT-3442 Tournament Class Tripods:
Leveling Base: LBL-7567
CT-3371, CT-3471 Tripods:
Leveling Base: LBL-75100
CT-3372, CT-3472 Tripods:
Leveling Base: LBL-7572
Does it have some sort of release and lock mechanism?
Yes, it's a ball-and-socket arrangement, sort of like half a ballhead.
Is this something you put under the tripod head that allows you to level below the head?
No, you still mount your camera on top of the tripod. You can use the leveling base with, or without a tripod head. If you are shooting multi-image panoramas, you will need some way to rotate (pan) the camera. This can be accomplished with a tripod head with a built-in panning function or a panning base (such as the Feisol PB-70 or PB-90).
All I can see is that it has a knurled shaft and a level on the side other than that, it's a mystery to me.
The knurled shaft is actually the handle. You twist the handle to lock/unlock the leveling base. Once unlocked, you use the handle to level the platform using the built-in level as an indicator. Then, you twist the handle to lock it in place.
Here's a photo of the LB-75100 that may help you better visualize what I am describing:
If you need further clarification, or have any additional questions, just let me know.
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