The Concept:
Staying along the theme of Macro, in Today’s Photo Minute, I wanted to discuss depth of field a little more because it is a critical factor when doing close-up photography of any kind. We know that Aperture controls depth of field primarily. The higher the aperture, the more depth of field, the lower the aperture the less depth of field. However, when we start doing close-up photography, two other factors come into play.
Distance to the Subject and depth of field – The distance to the subject also has a major effect on the amount of depth of field (or more accurately, the lack of). In essence the closer the camera get to the subject, the less depth of field you will get even at the same aperture. In the illustration below, you can see how much more out of focus the 2nd flower is as I moved from 4 feet away (the photo was cropped in to match the tighter image) to 1 foot away. Based on this, sometimes the way to get more depth of field is to stay further away from the subject and crop in later (if you have enough resolution to do so).
Focal length and depth of field – The other variable when it comes to depth of field beyond aperture and distance to subject is the focal length. While often you will want to use whatever focal length (wide angle, telephoto, etc.) that fits your situation, it is good to remember that the lower the focal length, the more depth of field and the higher the focal length the less depth of field. You can see this illustrated in the example below where the back flower is much sharper at 50mm (image cropped in to match the tighter shot) than it is at 300mm. If you have been following some of my wildlife images, you might have noticed that even on a songbird at F6.3, because I am using a 600mm focal length, the back of the bird may even be outside of the depth of field zone and therefore, a little out of focus.
The Assignment:
The assignment for today is to see these principles at work for yourself. Choose a subject with some depth to it and in aperture priority mode, land on the same aperture for all of the shots. You might try F8 or F11 as a starting place. Photograph the subject from a few feet away and then if you are doing macro, photograph the same subject from much closer (maybe even as close as you can get). See how the depth of field shifts.
Then with the same or a different subject, stay in aperture priority mode and at the same aperture and take a photo with your zoom lens at a lower focal length (18mm maybe on an 18-55 lens) and then at a higher focal length both at the same distance from the subject and note the differences. This does not have to be a macro exercise necessarily if you want to do a scenic photo but getting very close to the foreground (let’s say a fence and then your back yard in the distance, etc.). You could even get two people to stand a distance apart (at least 6 feet!) one being closer to the camera and one being further way and then do shots at different distances to them as well as different focal lengths. Whatever you do, this is a great time to really gain understanding about depth of field and the factors that control it.