Achieving Sharp Focus



Getting sharp images is a key element to most kinds of photography. Achieving sharp focus is largely a function of aperture and shutter speed but not the same as getting a maximum depth of field. The focal length and quality of your lens are also factors.

The first step in getting sharp focus is to make sure your viewfinder is calibrated to your eye. This is done with a viewfinder diopter usually located on the exterior of the viewfinder. Simply look through the viewfinder and auto-focus on something. Then adjust the diopter dial until the image looks its sharpest to your eye.


Diopter adjustment dial

Reciprocal Rule
Slow shutter speeds lead to soft images (See camera shake in the Exposure section). The reciprocal rule is a guideline for determining the slowest, safe shutter speed for handheld shooting.
The steadier your camera the sharper the image will be. Use a tripod when available or brace your camera against something solid.

The Rule: The slowest shutter speed should be no slower than the reciprocal value of the focal length you are using. So think of your focal length as a fraction and use the reciprocal fraction as your slowest shutter speed.
50mm lens (50/1) = 1/50th of a second
200mm lens (200/1) = 1/200th of a second

Lens Quality
The higher quality the lens, the sharper the images. High quality optics and better build quality will always give better results. Some lenses will also have image stabilization features. Image stabilization will allow sharp images to be taken up to 4 steps slower than the reciprocal rule states. Unfortunately high quality lenses are considerably more expensive than the average consumer level lens.

Lens "Sweet Spot"
All lenses will have an fStop value at which the lens will produce its sharpest image. All lenses are different but generally the sharpest performance for a lens is about 2 stops down from the lenss widest fStop (the smallest number).

To test your lens follow these steps:
What you need: - The camera and lens you wish to test
- A tripod
- A magazine or some kind of page with a lot of sharp text
- A light source if you are not in a brightly lit area

1. Set your camera to aperture priority. This will simplify the process as you will not have to adjust shutter speed for each shot.
2. Set the ISO setting to 100 or 200. This will provide clean, noise free images.
3. Set the camera to the shortest self timer setting. This will prevent camera shake.
4. Tape a page of the magazine with a lot of text to a wall. And point your light source at the page if you need more light.
5. Put your camera on the tripod and set it up at a distance that will allow a full frame shot of the magazine. Make sure the distance is great enough that your lens can achieve focus.
6. Set the aperture at its widest setting (smallest fStop number)
7. Focus on the text of the magazine. Auto focus should work fine.
8. Take a series of shots, each time adjusting the aperture down by one stop. (f2.8, f4.0, f5.6, f8.0, f11, f16, f22). Do not move the camera at all, if the camera gets moved, start over.
9. Download the photos into your computer and do a comparison of each photo. View the photos at at least 100% magnification. The image with the sharpest appearance will be the lens sweet spot. Use the photos metadata to see which fStop value is the sharpest.

In this case this lens appears to be sharpest at around f10.

If you enjoyed this tutorial, be sure to check out the other tutorials that are available in the Discovery Center. If you have any ideas for tutorials or projects that you would like to see in the future, please leave us some Feedback.


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