ALTDEV DURING VISUAL APCH WHILE TRYING TO MAINTAIN A VISUAL LOOKOUT FOR OTHER TFC.

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FLT WAS FROM DEN TO TUS. TUS APCH CTL HAD CLRED US TO 8000 FT. WE HAD PASSED A LIFEGUARD TWIN CESSNA ON OUR R SHORTLY BEFORE CLRNC TO 8000 FT. VECTORS PUT US ON A HIGH L BASE LEG TO RWY 11L OUTSIDE OF RIVAL FAF. THE PREVIOUS WK ON AN IDENTICAL FLT; APCH CTL HAD CLRED US FOR A VISUAL APCH SHORTLY AFTER CLRNC TO 8000 FT. WHILE OBSERVING THE CESSNA ACFT AND PLANNING FOR A VISUAL APCH TO RWY 11L. I (PF) INADVERTENTLY DSNDED BELOW 8000 FT (7600 FT). I IMMEDIATELY CLBED BACK TO 8000 FT. NO ACFT CONFLICT OCCURRED AND WE WERE CLRED FOR A VISUAL APCH SHORTLY AFTER. PROB AROSE WHILE MAINTAINING VISUAL WITH CESSNA ACFT. ALSO; APCH CTL KEPT US VERY HIGH FOR A L BASE VISUAL TO RWY 11L. THERE WERE NO OTHER ACFT IN OUR IMMEDIATE AREA AND WE WERE NOT TOLD THE REASON FOR THE HIGH BASE LEG ALT. ANTICIPATING A VISUAL APCH CONTRIBUTED TO THE ALTDEV. ALL SYS AND WARNING DEVICES OPERATED NORMALLY WHICH KEPT THE DEV TO A MINIMUM. SUPPLEMENTAL INFO FROM ACN 299278: I WAS WATCHING THE CESSNA WHEN THE ACFT WENT TO 7600 FT. AT THE ALT ALERT; THE CAPT CLBED BACK TO 8000 FT. THE PROB OCCURRED DUE TO THE CESSNA ACFT BEING OUR CONCERN AND ANTICIPATING A VISUAL APCH.