Output Pin Working Without Being Set

Hi, why does digitalWrite(53, HIGH) turn the LED on (however it is very dim) even when I don't have pinMode(53, OUTPUT); at the top?

And why is it dim without defining it as output, and then normal brightness when I do define it as output?

Thanks in advance, David

Because writing a HIGH to a pin configured as an input turns on the internal pullup resistor. Tha value of this resistor is about 20k.