NSOpen Panel

Hello all. I am opening a panel to select an image and putting the resulting path in a field. No problem. But how can I determine which button opened the panel in the first place? My code is below.

Thank you,
                   Todd

-(IBAction)findImage:(id)sender;
{
    NSOpenPanel *op = [NSOpenPanel openPanel];
    [op beginSheetForDirectory:nil
                          file:nil
                         types:[NSImage imageFileTypes]
                modalForWindow:[NSApp mainWindow]
                 modalDelegate:self
                didEndSelector:@selector(openPanelDidEnd:returnCode:contextInfo:)
                   contextInfo:nil];
}

- (void)openPanelDidEnd:(NSOpenPanel *)op
             returnCode:(int)returnCode
            contextInfo:(void *)ci
{
    if (returnCode != NSOKButton)return;
    NSString *path = [op filename];
    [imagePathField setStringValue:path];
}