canon 50mm f/1.4 vs. 85mm f/1.8

[MrJones]

Hi,
I feel that I need another lens for portraits.
most of my work is either photographing special events or senior portraits. right now I'm using the canon 70-200 f/4 and the 50mm f/1.8 (which I'm not that happy with).
I also own the 24-105 f/4 but I rarely use it when shooting seniors.

I was thinking of getting the 85mm f/1.8 or the 50mm f/1.4, any suggestions here?
I've read that that the 85mm has a better focus mechanism, is that true?

I'm shooting with a canon 40D but thinking on moving on to a full frame 5D soon (if that makes any difference)

thanks for the help

[inkista]

Full frame will only make a difference in terms of the field of view you get with either lens. Both are EF so they'll work on both a crop and a full-frame.

The USM in the 85/1.8 is the desirable "ring type", while in the 50/1.4, it's the "micromotor" type, but the 50/1.4 is a special case where the non-ring USM still allows full time manual focus, so you're liable not to notice too much difference.

What you really want to ask yourself is which is the right focal length for you? If your 50/1.8 is just right focal-length/framing wise, then get the 50/1.4. If it's too short, then get the 85/1.8 or 100/2. If it's too long, then get the 35/2, 28/1.8 or Sigma 30/1.4.

[MrJones]

thanks inkista,
since I'm gonna use it mostly for portraits and body shots, the focal length is important but I can adjust myself most of the times (specially when using a full frame body) and I'll have the time to do so.
the thing I care about the most is the sharpness.
I'm actually leaning toward the 85mm since I already own a 50mm.
is the difference between f/1.4 to f/1.8 makes a big difference?

[inkista]

It's two-thirds of a stop. But DoF wise, remember, that longer lenses, if framed identically to a 50mm lenses, will give a thinner DoF, so in that way, it's probably a wash between the two. In terms of sharpness wide open (50/1.4 on the left, 85/1.8 in the middle, 100/2 on the right):



And if you don't know how to read an MTF chart:

The vertical axis is light transfer.

The horizontal access is the distance from the center of the frame. So, left side is center performance, right side is corner performance. [Don't forget that a crop sensor's edge is at 13mm--this is what we mean by lenses looking better on a crop--you dodge corner performance issues]

"Ideal" performance would be a flat line at the top of the chart.

Blue lines are f/8 performance. Here, the two lenses are roughly equal.
Black lines are wide-open performance. f/1.4 is harder to make sharp than f/1.8. But obviously, here, the 85 is better.

The thick lines are at low resolution (10LP/mm) [contrast], the thin ones at high resolution (30LP/mm) [resolving]. Both are different aspects of sharpness.

The dashed vs. solid have the target lines running in different diagonal directions (meridonial vs. sagittal). The closer these two lines are together, the smoother the bokeh will be.

Both of these lenses perform pretty closely, but on sheer sharpness across the frame, especially wide open, the 85 is better, and the 100/2 is better yet. Longer/slower glass tends to perform better on these charts than wider/faster glass.

Now, this chart shows you nothing about distortion or C/A control. But it can be a useful way to compare lenses to each other, or to speculate on a lens's performance before it's released.

[MrJones]

wow!
thanks for the explanation inkista!
you really are the greatest
I think I'll get the 85mm but you certainly got me thinking about the 100mm too!
thanks again, you are the greatest!