I am new to photo taking, so would like some advise on what considerations to take into when buying a prime lens.
I am currently using a D70 camera which comes with the usual Kit lens. I am considering one of the following prime lens as I would take some photos of people motion eg. dances, stage performances in dim light without the use of flashes (usually not allowed in performances).
I have asked around, and seems that there are some in the range:
1) f1.8 50mm (around S$200) (give or take S$20)
2) f1.8 85mm (around S$700)
3) f1.4 50mm (around S$700 - think so, can't remember)
4) f1.4 85mm (around S$1,800)
What are the factors I have to consider when choosing one of these lens?
I understand that there will be a price gap for different apertures, but why does the 50mm lens and the 85 mm lens differ by more than double the price for the same aperture?
What about speed of autofocus, whether the autofocus function can work on D70? Are these things I have to find out, or are they a given?
Are there any other factors affecting the quality of pics taken in these lens?
Thanks in advance.