def isolate_columns(logdf):
    """
    Isolates the subset of columns in a DataFrame log that are
    relevant to data reduction.

    Inputs:
        logdf: (pd.DataFrame) logsheet in Pandas DataFrame.

    Outputs:
        savedf: (pd.DataFrame) Pandas DataFrame to be used in reduction.
    """
    if "Method" not in logdf.columns:
        savedf = logdf[["Object", "ExpTime", "Filter", "Comments"]]
        savedf['Method'] = "saturated"  # default for now
    else:
        savedf = logdf[["Object", "ExpTime", "Filter", "Comments", "Method"]]

    return savedf