My partner is the usability expert when it comes to flashlights. Just ask Nightcrawler / Metalhed. He'll tell you.
The TK40 is NOT perfect and the SUI is probably the least perfect aspect. For some, access to the head and a way to add front-end accessories is another gripe.
We've not finished the evaluation of the TK40 yet ( I just shoot the pics) so the review is not up but you should see it here in a couple weeks (delayed due to illness in his family).
http://www.imagometrics.com/FLReviews/FLR.home.htm
Here's a summary that I'm authorized to share:
Activation Switch:
SUI (Solid User Interface)
Functionality
From the OFF state, the user depresses the rear-mounted tail cap switch to click the light on. The light remembers the last setting, which is excellent for repeatable ON/OFF cycles. The light has the following setting from a single click in consecutive sequence : Low, Medium, High & Turbo
This means that each IMMEDIATE depression of the switch button, results in the next higher setting and from Turbo the light reverts down to Low and not backwards to High, Medium etc.
One Long click turns the light off.
To actuate strobe, SOS and flashing modes, entails a rapid double click from the static levels.
Comment: This SUI functionality is learnable and though not intuitive, is logical and linear.
We've never loved dual function switches. The Fenix TK40 is actually a triple function switch since one button controls : ON/OFF, Levels & Mode. As a result, practice combined with muscle training are required for a fluid and fail-safe usage. It is well within the capability of the average operator, but mistakes may occur under duress.
Ergonomics
Rearmounted tailcap switches are not the most comfortable to use, as few operators end up holding such lights with the same posture for searching as they must for actuation. As a result, a lot of hand repositioning occurs in situations that require frequent State, Level or Mode switching alternating with hand holding in overhand or underhand gestures..
On the other hand, tailcap mounted switches are easier to locate with little or no error when compared with side mounted switches ( which require indexing for fail-safe location). Further, rear mounted switches typically accommodate weapons mounted or vehicle mounted use more easily than side mounted switches.
Comments:
Engineering constraints frequently result in a compromise between switch type, location on the light and range of features due to PCB, waterproofing (depth rating) and other space constraints.
The TK40 provides adequate haptic feedback (soft click) to inform the operator that an action has occurred in response to actuation. Missing from the TK40 is a momentary 'ON' soft activation. As it is not marketed as a 'tactical' light per se, this seems to me a minor constraint for a general purpose light, particularly, when all the signaling modes are taken into consideration.
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Does anyone have any complaints on operating this light with the rear switch?
It seems as if it would not be user friendly and in many cases possibly difficult to hold and operate the switch with only one hand. Mind you this is an assumption since I have not had the opportunity to either hold or operate one, but compared to a maglite it seems it would be unnatural.
Holding a maglite at the rear feels unbalanced, but choked up on the head it feels secure and stable. I have a T20C2 that feels natural, but it is short and fits comfortably in my hand. Would love to hear some thoughts and opinions especially those with LE experiences/stories?
Just out of curiosity, has anyone experimented with running this light using a different battery configuration? I know a lot of people didn't like the AA's when it came out.