The Plane of Focus

[acp]

Technically speaking perhaps this is more than a beginners post, perhaps it is not. For the most part I understand apeture and depth of field, although I could probably know more, so considering the plane of focus, I know there is a certain amount in front of your primary subject and a certain amount behind the subject that is either in focus or out of focus. This ofcourse is highly dependent on your f-stop,creating blur or focus for infinity. However I thought that lets say given a large family shoot 8-10 adults, all arranged left to right and basically in the same plane, what I mean by that is no one person significantly forward or backward of each other, how do you get to a point where you can have a larger apeture f/5 or below, keep everyone in focus left to right and still create blurry background. As a rookie at portrait photography large families have been the bain of my existence. I guess I thought no matter your your f/stop at least all subjects on the same plane moving left to right should be in focus. What I find though is that even if I put my focus point the center subject those falling to the end of the left right spectrum don't seem to be focused like the center subject. Also does my physical proximity to the subjects affect this. Let's say if I were trying to shoot at f/4, does the amount in focus change (left to right) if I'm 8 feet away as opposed to 20 feet away?

I use a Canon 7D, my normal go to lense for portraiture is a 1.4 50mm, I recently rented a 24-70 f/2.8 for a large family shoot, by the way didn't care for the lens. Totally different question but can anyone recommend a good tack sharp wide angle lens.

 

[Helen B]

You may have a lens that does not have a particularly flat field (few lenses have a perfectly flat field). Do some tests with an imaginary group, framing the image so that you can see the ground where their feet would be. Look closely at the ground and see if you can see a curved zone of focus. Use that knowledge to arrange the actual group.

Curvature of field does vary with distance.

The other explanation, which can happen in conjunction with the first, is that your lens isn't especially sharp as you approach the corners of the image, especially when close to wide open.

 

[Derrel]

Yes, your camera-to-subject distance DOES affect the ability to throw the background decidedly out of focus. At longer camera-to-subject distances, it becomes more difficult to photograph a group of people AND also to make the background decidedly out of focus. The same goes for the size of the capture format; a larger capture format camera, like say a 120 rollfilm camera capturing 6x7 cm images will allow you to photograph a family group shot and also make the backdrop out of focus. With a small-format camera, like a 1.6x APS-C sensor size like that in your Canon 7D, the angle of view that will encompass a family group of 8 to 10 people will come along with a lens focal length that is quite small, and which has a MUCH, much closer hyperfocal distance. The practical upshot of shooting family groups of 8 to 10 people with a small-sensor digicam is that, in many "real-world" situations, your photos will have deeper depth of field and less background blurring than if you were to use a significantly larger-format camera, like the aforementioned 120 rollfilm camera shooting onto a big 6x7 negative.

An old technique used to involve literally tieing a string to the camera's tripod bolt, and using the string to align the group members the same distance from the center of the lens axis. As you probably know, at shorter distances, like those found indoors in many studios, objects positioned at the edges of the frame area are quite significantly FARTHER AWAY from the sensor than objects placed in the center of the lens's field of view. At longer shooting distances, the distance difference between edge and center field objects can easily be covered by the increased depth of field; at closer ranges, the disparity is enough to cause poor focus at the edges of the frame if the f/stop is wide or wide-ish. If you combine the difference in distance, plus the poorer edge performance of many lenses that Helen B mentioned, well, you've got some issues beginning to pile up. Some lens designs have HIGH AMOUNTS of curvature of field, so it's possible that you can encounter three separate factors that serve to mess up family group shots. Some lenses that have BAD curvature of field at wide f/stops and close ranges, like my old 1969 Nikkor 35mm f/1.4, have an interesting image characteristic, with a very sharp center area, and then oddly-soft edges and corners...looks super-cool for party photos shot in dimly-lighted areas with bounced flash...looks absolutely awful on group photos...

Tell you what...if you want to line up 8 to 10 people side-by-side, why not try the string method, and arrange the people in a very slight curved line....a slight arc...then use a string tied to the camera, and literally make sure that the string's end or a knot in it touches the chin of every person in the group...see what happens, okay?