yoiks wrote:Thanks guys. I agree that f stop is my primary concern which is why I was so interested in the 85mm lens. I'm gonna keep looking and hopefully pick something up before the holidays (so that I can get lots of goods shots of the kiddies).
It is not the best option compared to a 2.8 zoom in a 24-70+ range
but if shooting kids indoors is your primary goal, a wider angle fixed may fill in a gap at a relatively cheap cost. Since you are shooting full frame and already have a 50mm, you might consider something in the 28-35 range. Second, remember that when shooting indoors close to people, 1.4 may cause you issues as well. It will shoot with very low light that is true but the depth of field will certainly come into play. I am going to attach a link for you to have some fun with. Maybe you have already used a depth of field calculator. I have an app on my phone just so I can check things when I am shooting on occasion. http://www.dofmaster.com/dofjs.html
The reason lenses with such small fstops are so versatile is it allows you to get very creative with the backgrounds. IF you want things in focus, you can stop it down and go with a larger aperture. It allows for more focus in the background, etc. If you want a thin plane of focus, open that baby up. With 1.4 and at a close distance, you may be able to get the eyes of someone in focus and the tip of their nose starting to go out of focus. IN a living room from 13 feet, using 50mm you will get some numbers like this (I used the canon a 5d as the main setting since it is full frame)
Subject distance 13 ft
Depth of field
Near limit 12.2 ft
Far limit 13.9 ft
Total 1.73 ft
In front of subject 0.81 ft (47%)
Behind subject 0.92 ft (53%)
Now, reduce those numbers by about 30 percent and you will be very close to true focus. (most calculators are VERY optimistic on their ranges) Now, let's go practical. From 13 feet, using 50mm, if you focus on an eye, you will have reasonable focus 6" in front of focus and about 8" behind. That means if you shoot two rows of children, the back row will be out of focus. The picture may be bright, but you will not have focus for the whole group. In this scenario, you will need to get very close to f4 to get the back row and front row in acceptable focus. Now is the time to start working on the ISO capabilities of your camera. Bump it to 1000 or so, maybe 1600 and try that. Many of the older cameras could never produce decently in that range but even in my 7d I shoot 1000 or 1600 at the football games and get very decent pictures. By going to a wider lens, you will also gain some depth of field at comparable fstops.
Keep learning and have fun.