Why isn't this shot sharp?

codrus

So I took my new Sigma 50-150 f/2.8 lens to the San Francisco Zoo and shot a bunch of pictures with it on my 50D. Results were generally good, but there are some shots that didn't work out as well as I'd hoped, and I'm trying to figure out why.

Compare these two shots:

http://codrus.smugmug.com/gallery/7063880_s89sd#452580845_JaPeY

http://codrus.smugmug.com/gallery/7063880_s89sd#452578501_KEiok

Both are at f/2.8, 150mm, and a fast-enough shutter speed that camera shake shouldn't have been a concern, yet the polar bear is sharp and the eagle isn't. Any thoughts on why?

One difference is that the eagle was shot at a much closer range, maybe 6 feet vs 30+ for the polar bear. Depth of field is proportional to the focus range, right? I know the lens is front focusing to a degree (I've got the MF adjustment maxed out and it didn't quite deal with it), so maybe the eagle's head has fallen out of the DoF while the polar bear's hasn't? There's a bit of green fringing at the back of the eagle's head.

Does that make sense? Any other thoughts on why I might be getting sharp shots on one subject but not on the other?

thanks,
--Ian

manbou

Looks like the polar bear was in better light than the eagle. My guess is that the low light on the eagle created more noise and less sharpness.

Cygnus Studios

It looks more like light issues vs focusing to me. Look at the eyes of the eagle. Killer sharp. I am sure Ziggy can offer the correct fix for this.

digismile

If your distances are correct, your issue is definitely depth of field. At f2.8 and 150mm, your DOF at 6 feet would be .05 feet, about half an inch. At 30 feet, your DOF will be 1.29 feet.

Take a look here for the calculations. Even at f8, your DOF would only be .14 feet, so when your in REALLY close, you need a higher f stop.

hth,

LiquidAir

codrus wrote:

Both are at f/2.8, 150mm, and a fast-enough shutter speed that camera shake shouldn't have been a concern, yet the polar bear is sharp and the eagle isn't. Any thoughts on why?

One difference is that the eagle was shot at a much closer range, maybe 6 feet vs 30+ for the polar bear. Depth of field is proportional to the focus range, right? I know the lens is front focusing to a degree (I've got the MF adjustment maxed out and it didn't quite deal with it), so maybe the eagle's head has fallen out of the DoF while the polar bear's hasn't? There's a bit of green fringing at the back of the eagle's head.

Does that make sense? Any other thoughts on why I might be getting sharp shots on one subject but not on the other?
--Ian

DoF and a bit of front focus are probably part of the problem on the eagle, but I don't think that is the whole story. The eagle is also back lit so there is some fringing and flare. I am also wondering how well your lens performs near its close focus limit (lens sharpness does depend on focus distance for close subjects). Finally the eagle was shot at 1/250s (compared to 1/6400 for the bear); 1/250s is slow enough that motion blur could be a factor. My feeling is that you are seeing a combination of factors rather than one single problem on the eagle shot.

pathfinder

Why the choice of f2.8 for so many of the shots?

That lens ( any lens ) will be significantly sharper two stops down, at f5.6, than it will be wide open. And you will have more depth of field to help keep your subject in focus also.

When shooting with a telephoto at a near distance, you can shoot at f4 or even f5.6 and still have an adequately shallow depth of field.

I usually do not choose to shoot my lenses at their maximum aperture, unless I know precisely why I am shooting wide open. Modern cameras are good enough that I would rather raise my ISO one stop than shoot wide open if I have a choice. Bear in mind I say this even with great glass, like my Canon 300 f2.8 IS L.

Maybe I am just old fashioned.

Tee Why

If this is a consistent issue, then it may be the lens or the camera. If not, then I suspect something unique to the shot. Perhaps the bird moved a bit? Raptors are fast so who knows.

ziggy53

Cygnus Studios wrote:

It looks more like light issues vs focusing to me. Look at the eyes of the eagle. Killer sharp. I am sure Ziggy can offer the correct fix for this.

Everybody is giving great answers and my guess is the same as was covered.

jeffmeyers

Yeah, it's definitely a depth of field issue. Also you lens will probably not be as sharp at f2.8 as it will be at, say, f8 or f11.

codrus

Thanks for the help, folks! A couple responses to various people:

I hadn't realized the DoF was quite that shallow at 150mm f/2.8!

As for why I'm using f/2.8 -- this is really the first time I've used a telephoto lens as fast as 2.8 for anything other than a few test shots (at least, the first time with one that actually worked). I've been using my 17-55 f/2.8 a lot, and with that lens I really like using it wide open. It doesn't seem to hurt the sharpness much on the 17-55 (of course, that's a significantly more expensive lens than the Sigma, so maybe that's why). I started out the day at the zoo using 2.8 for most of the shots, then started trying stopping down later. The polar bear is a morning shot, most of the tiger and lion shots are later.

On the eagle shot, I'm not sure I could have gotten away with stopping down much beyond 2.8, though. It's already at 1/250 (on the edge of being not handholdable at 150mm on a crop body), and I had the ISO up to 400 already (I find the noise at 800 annoying and 1600 unacceptable). The bird wasn't moving around much, so I don't think target motion is to blame for it.

On the distance front, 6 feet might be underestimating it on the bird, but it wasn't more than 10. Thanks for the pointers on making better use of a faster telephoto zoom!

thanks,
--Ian