In the dark

Terrence

I have a Canon 50mm f/1.8 that I am using for indoor shots. I am primarily shooting my son in ambient room light (lamps, maybe some sun through windows) and using a flash when needed to raise shutter speed. One thing I am wondering, is what is the advantage of a fast lens for people pictures in low light, since the DOF is so shallow wide open? The DOF is so shallow with this lens wide open that I can't get all of someone's face in focus. To get more in focus I have to stop down, defeating the advantage of a fast aperture. At that point, I'm thinking I can use my Sigma 17-70 f/2.8-4.5, and maybe ditch the 50mm altogether.

I suspect I am missing something important but simple, as is usual with me.

For pics of small (fast moving) children in ambient room light with flash, what's your favorite lens or aperture/shutter combos?

Terrence

Never mind. Should have searched first. http://www.dgrin.com/showthread.php?t=53853 explains what I am asking.

pathfinder

That is an excellent thread regarding the value of faster lenses.

I think of three very good reasons for using faster lenses, and couple to avoid them.

Faster lenses collect more light, which allows faster shutter speeds or shooting in very low light. Large apertures have shallower Depth of Field - this can be a good or bad thing. It certainly can lead to lovely images with shallow DOF. The DOF is also a strong function of format size - DOF is very large for very small negatives/sensors, while DOF gets shallower for larger formats at a given aperture. That is why 8x10 view cameras have lenses with f64 apertures....

Fast Lenses ( F2.0 or larger )also allow the shooter to see better through the viewfinder, even if not used wide open.

Generally, faster lenses cost significantly more, but they are usually some of the the best optics a manufacturer makes. It is hard to think of an f1.4 or f2.0 lens from Nikon or Canon that is a poor lens.

Fast lenses are larger to collect more light, that means they are larger and way more. They will need larger diameter filters if a filter is needed.

wxwax

And as you've pointed out many times in the past, faster autofocus as well.

pathfinder

Thanks, Sid, I should have mentioned that as well.

Although, how fast AF is, is a complex series of variables. For example, the older 85 f1.2 L is not known to focus fast at all..... The newer vesion is faster, but I have not seen how it compares to its slower brethren, the 85f1.8 which focuses rather quickly.

In that regard, is the 50mm f1.2 L faster at AF than the 50mm f1.4 or f1.8?

Terrence

Thanks for the extra and reinforcing info. You two have probably forgotten more than I will ever learn about this topic.

LiquidAir

pathfinder wrote:

Thanks, Sid, I should have mentioned that as well.

Although, how fast AF is, is a complex series of variables. For example, the older 85 f1.2 L is not known to focus fast at all..... The newer vesion is faster, but I have not seen how it compares to its slower brethren, the 85f1.8 which focuses rather quickly.

There are two parts AF speed: how fast the camera makes the focus distance decision and how fast the motor moves the elements to the right place. On faster lenses the camera can make the decision more quickly. In particular, Canon bodies will use a better AF sensor when the aperture is f/2.8 or wider. However if that fast lens has a slower motor, the total time to AF can still be slow. By the time a lens is as fast as f/2 I don't think wider apetures make much of a difference in focus decision speed. As for focus speed and the 85/1.2, I think the issue there is that the motor is moving a big hunk o' glass so it can't accelerate as quickly as the motor in the 85/1.8.

pathfinder

I had typed out some discussion of AF aquisition, versus time to move lens elements with my earlier post, and decided to leave it out. Thanks for clarifying that anyway, LiquidAIr.