Finding your Sweet Spot - Kev Lewis

A quick way to improve the pictures you get from your digital camera.

Every camera lens, with an adjustable aperture, has one aperture setting that will give you the maximum sharpness and detail. This is referred to as the lens' "Sweet Spot", and it is very easy to determine which aperture it is.

Please note that image sharpness has nothing to do with Depth of Field (DoF) other than the aperture selected will give you a specific DoF.

As you change the Aperture setting from large numbers (f22) to small (f2.8) more light is allowed through the lens, how long it will strike the sensor or film for is defined by the shutter speed. I know it's the opposite of what you would think. The lower the number the bigger the Aperture and consequently more light is allowed through the lens. More light requires a faster shutter speed to avoid overexposure. That's why lenses with bigger openings are referred to as "Fast Lenses".

Here is how to find your lens' sweet spot:

If you are a geek, like me, you would set your camera up on a tripod and shoot a static subject, which is placed normal to the optical axis of the lens. Using mirror lockup if possible to minimise vibration, take images at the full range of apertures available on the lens, analyse each frame to look for the image with the most detail i.e sharpness and this is your lens' "Sweet Spot".

If, on the other hand, you just want a quick way to check it:

Pick a bright sunny day and choose a test subject with lots of detail and color. I would use a something with a lot of texture in it, like a brick wall. It will need to be in full sun or use flash as you need to be able to take a series of images using all the apertures available on the lens WITHOUT allowing variables, like camera shake, to affect the results. Use a tripod if possible, mirror lockup and cable release or self timer to avoid this problem.

Using Av or manual mode set your lens to the largest aperture (smallest number) and take a picture. Change the aperture (and shutter speed) and repeat the photograph until you have images taken at each aperture.

Download the image files to your computer and compare each of them at 100% magnification. The pictures will be similar but they will have subtle differences.

What you are looking for is the image with the most detail, NOT the most Depth of Field because obviously that will change from aperture to aperture. Look at the edges of things in the image this will be the area where the difference is most apparent.

One image will be sharper than the rest and typically, it will be 2-3 f-stops from wide open i.e. if the lens is an f4 lens then the sweet spot will be somewhere around f8-f11. Unusually, doing this test on a Canon 100mm f2.8 macro lens I found the sweet spot was about f11 i.e. 4 stops down from wide open. This is probably down to the optical design favouring a smaller aperture for macro work.

So now you know the Sweet Spot for one of your lens, each lens will differ so it is worth checking them all.

Does this mean that you MUST shoot at this best Aperture? Nope it doesn't, before you take a photograph you need to decide what is important and what is appropriate for each photograph. Some images will require a big DoF so you would stop the lens down more but not too much or the image will suffer from diffraction softening (another kettle of fish to be aware of lol), or the ambient light may mean shooting with the lens wide open to get a reasonable shutter speed.

You just need to be aware that the further away from the sweet spot you go the less sharpness will be in the image, so it is always a compromise between DoF and sharpness.