Depth of field riddle

[BermyLady]

This is my first posting in this forum, just joined. Was looking for information about a weird phenomenon I am experiencing. I am a Canon user, 5d Mk2, and am currently focussing on using just my 17-40mm lens to cover events. I love people photography and am studying documentary photography with another photographer. Now, the other day, we had a parade. At several moments I decided to open up my aperture to 4 (the maximum on my current lens) so that the background would blur. We have a very colourful cultural dance group and I wanted to try and capture one of the dancers, but with the background blurred. It did not work! I am stumped, as I have always learned that a big aperture (low value) will give me the blurring. It does with my other lenses, but it is much more unpredictable with the wide-angle, or so it seems.

The photo shown is not my best as my flash did not fire here, although I wanted it to. But it does demonstrate my point. Do any of you have any pointers here as to why the blurring did not happen?

I was prob at 2 meters from my subject...

All input is very much appreciated!

[FrankMi]

Hi BermyLady, as you have discovered, the wider the lens, the greater the Depth of Field. You would have more success with DoF by backing up a bit and shooting with the normal lens. The DoF is even less on a telephoto lens, but to get the same size subject, you would need to back up even further. This is why I love a wide lens for landscapes, darn near everything is in sharp focus!

[BermyLady]

Hi Frank,
That IS still weird as I have shot with the same lens on aperture 8 and all of a sudden there WAS blurring.... I do not like to shoot events with a tele lens because I want to be right in the middle of things. It's just that it is almost impossible to see all of the background elements in the viewfinder. I am trying to train my eyes to do so as much as possible but this sure is a learning curve!

[Markvetnz]

The basic problem is the use of a wide angle lens. I checked your exif data and it looks as if your focal point was 100m away.! This seems illogical because the subject appears much closer. Assuming your subject was 30 ft away, on a 35mm camera at f4.0 on a 40mm lens the depth of field is just under 80 feet, and at 17mm it is infinite! So quite logical that your background is not out of focus. There is also significant wide angle distortion in your photo. This can be corrected using lens correction software. The solution to your problem is to use a longer lens.

[FrankMi]

It looks like my second post yesterday didn't take. First, I apologize for not giving you a warm welcome as this is your first post to CiC. We would like to encourage you to post a bit about yourself on the "Meet the Members" thread and if possible, update your profile with at least your first name.

Mark's analysis would account for the difference you are seeing between the F/4 and the F/8 images. Wide angle lenses are more difficult to focus due to their wide Depth of Field so unless focusing is on Auto, it is often easier to manually set the focusing ring using the distance markings on the lens.

[BermyLady]

Frank, and Mark...
Thanks for your comments and yes, Frank, will update my personal data soon. My name is Nicky...
My focal length in this picture was 23 and my subject was less than 4 m away, so I don't know where Mark got his details?
The distortion is acceptable to me, by the way. I actually like that as a by-product of the wide-angle, although I will change to a different one soon that is said to give less. We will see.... Will confer with my National Geographic colleague to see what he thinks....

[Vandenberg]

this always helps me

http://www.dofmaster.com/dofjs.html

[BermyLady]

Great! Thanks Rotterdam!

[rpcrowe]

Frank, and Mark...
Thanks for your comments and yes, Frank, will update my personal data soon. My name is Nicky...
My focal length in this picture was 23 and my subject was less than 4 m away, so I don't know where Mark got his details?
The distortion is acceptable to me, by the way. I actually like that as a by-product of the wide-angle, although I will change to a different one soon that is said to give less. We will see.... Will confer with my National Geographic colleague to see what he thinks....

This just goes to show you that different photographers have different opinions regarding virtually any image. No one is right and no one is wrong and it doesn't mater what anyone else says. If you like the effect, it is right for you. However, when you post an image with questions, you open yourself up to answers; some of which you won't like and many of which you won't accept.

I will sometimes use a UWA lens to shoot people but, when I do, I tend to get a lot closer to the subject and let the perspective distortion separate the subject from the background. I normally detest the distortion in perspective which the upper torso of a human appears significantly larger while showing the lower portion as small and spindly. However, that is my view of the subject and certainly doesn't mean that this is the way images should be shot.

If this is the effect that you desire to show, you have done a good job. If you like it, it is a good capture! However, if you would like to use the same approximate focal length at the same distance and not portray the individual as having a massive upper torso and spindly legs; shooting from a lower position will change the perspective. Shooting with the camera at mid-chest height of your subject will provide less distortion than shooting at the subjects shoulder or head height.

And, if you want to separate the subject from the background by using selective focus, you might want to stay away from UWA focal length lenses and opt to use a longer focal length to compress the DOF...