|Technical Level: Intermediate-Advanced|
DOF and Hyperfocal Distance
The Hyperfocal Distance (HD) is an optically calculated constant based on the combination of lens focal length and f stop. It is used to determine what the best focal distance would be to gain maximum DOF. If your camera focuses at the HD, any object beyond the HD will be in acceptably sharp focus. With the camera focus set at HD, most items in front of the HD will be in acceptably sharp focus. You should run a DOF calculation to see how far the Near Focus Limit extends in front of the focal distance. You can still focus where every you want to focus at any focal distance from the camera. If you focus at a distance less than the HD, everything behind the HD will not be in acceptably sharp focus.
For example Diagram 01 shows the Hyperfocal Distance and Near Focus Limits for 18mm and 50 mm lens at various f stops. The shaded areas on the diagram also shows the DOF for each f stop when focused at the HD. Click on the diagram to see a larger version.
An evaluation of Diagram 01 shows:
- Longer focal length lenses have a HD that is farther from the camera. For example the HD for a 50mm lens at f/8 is about 34.3 feet while the HD for a 18mm lens at f/8 is about 4.4 feet.
- The Near Focus Limits are closer to the camera for shorter focal length lens when compared at the same f stops. For example the Near Focus Limit for the 18mm lens at f/8 is about 2.3 ft while the Near Focus Limit of the 50mm lens at f/ 8 is about 17.8 feet. Thus a lens with a wider angle of view and a shorter focal length would be able to include objects that are very close to the camera in the portion of the image that is in acceptably sharp focus.
- The total DOF is greater on shorter focal length lens compared to longer focal length lenses. The total DOF for the 18mm lens at f/8 extends from about 2.3 feet to infinity while the total DOF for the 50mm lens at f/8 extends from about 17.8 feet to infinity.
If you choose to focus your camera at a distance other than the HD, your Near and Far Focus Limits will change accordingly. Focusing closer than the HD will bring your Near Focus Limit closer but you may lose some sharpness in the background elements depending on there distance. The inverse is also true in that focusing farther away than the HD will move your Near Focus Limit away from the camera and will most likely assure that your distant photo elements are in acceptably sharp focus.
It is highly recommended that you obtain a Depth of Field Calculator, either on a smart cell phone, on your computer, on a paper chart etc. to provide some guidance as you work with and master this subject. DOF calculators are great “What if” tools to learn how DOF and HD work.
How do I apply this information in the real world? How do I estimate distances? We do have segments on estimating distances in lesson DP-112C Depth of Field and DP-112E Depth of Field Exercises to help with estimating distances and lessons DP-111C Focus and DP-111E Focus Exercises to get information on locking focus. Some lenses have distance scales on the lens body which can be approximately used to set your focus at the HD.
From a practical standpoint it is better to focus just a little beyond the Hyperfocal distance to assure that the far focus limit is at infinity.
Do I have to be super accurate? No. Focusing at a distance of 18. 63 feet is difficult and impractical. Get the focus in the ball park and your shot will most likely get as much DOF as you need.
What if I don’t have a DOF calculator with me? The rule of thumb used by many landscape photographers is to focus about 1/3 of the distance into the scene if you don’t have a chart or smart phone with you to determine what the Hyperfocal distance should be for a given lens focal length and f stop. By focusing 1/3 of the way into the scene, some very close foreground elements may not be in the zone of acceptably sharp focus. That could be remedied by moving back about 10 feet (if possible).
The basis for this is as follows: Let’s assume we are using a 28mm lens at f/8. The HD for a 28m lens at f/8 is about 10.7 feet. We will round this to 11 feet. If I have a grand vista in front of me consisting of a mountain, lakes etc. and the mountain is 1 mile away or 5280 feet, the 1/3 rule of thumb would have me focus about 1700 feet into the scene (.33 x 5280 = 1740). So what is the difference between focusing 11 feet away or 1700 feet away? In both situations the Far Focus Limit is still infinity so everything beyond the focus distance is in the zone of acceptably sharp focus. The only thing that will change will be the Near Focus Limit. The Near Focus Limit for a focal point 1700 feet away is 10.6 feet compared to a Near Focus Limit of 5.6 feet if I focus at a distance of 11 feet. The net effect is that I have lost about 6 feet of Near Focus Limit. If I am not including anything in my image that is closer than 11 feet away I have not lost a thing. This only becomes a factor in this situation if I want to include a foreground object in the frame that is less than 11 feet away from the camera.
One thing you will notice from this last HD example about focusing 1/3 of the way into the scene (@ 1700 ft) instead of 11 feet into the scene, is that Depth of Field and Near Focus Limits are not linear. That’s why you need a DOF calculator to learn how DOF works.
So, get hyper focused on your Hyperfocal Distance.
Go to the next blog in the series The impact of Lens Focal Length on DOF.