From my personal experience, in my early photography days, i used to click all my photos at the maximum and minimum aperture using the Canon 50mm f/1.8 lens. I thought that clicking at f/1.8 will guarantee me the best result possible and that clicking at f/22 would give me the sharpest result from this lens. Later, i was disappointed to learn that be it any lens, it is not the sharpest when it is used at the highest aperture value (F/22 in the case of Canon 50mm f/1.8). Every lens gives the sharpest result when photos are clicked using the aperture value which is the ‘sweet spot’ of the particular lens.
What is sweet spot?
The sweet spot of a lens is the aperture value of a given lens that produces the best sharpness in the photo, from the center to the edges. While clicking at low aperture value (such as f/1.8 or f/2.8), you might get great level of sharpness at the center, but to achieve edge-to-edge sharpness you will have to find out the sweet spot of your lens.
How to find out the sweet spot?
The thumb rule of calculating the sweet spot of a particular lens is to find out the middle range aperture values. E.g. the aperture values of Canon 50mm f/1.8 ranges between f/1.8 and f/22, therefore, that sweet spot of the Canon 50mm f/1.8 lens is somewhere between f/5.6 to f/8.
But, there is a better and practical method to figure out the exact sweet spot of your lens. Here is the list of things you would require to get started with the sweet spot test:
- DSLR mounted with the lens you want to test
- A steady tripod
- A sheet of newspaper
- A strobe/studio light/bulb to ensure the lighting does not change throughout the test shots
Mount the DSLR on a steady tripod and make sure to switch on 2-second timer to avoid shake. Set your camera on aperture priority mode.
Take a sheet of newspaper and paste it on a flat vertical surface, make sure the paper is flat for accurate results.
Light-up the newspaper using an artificial light source (strobe/studio light or bulb) so that the lighting does not change throughout the shoot.
Position your tripod, with the DSLR mounted on it in such a position that it is parallel to the newspaper. Focus on the newspaper and turn-off the autofocus mode on the lens to avoid the lens from focusing every time you click a photo.
Set the minimum aperture value on your camera, for example f/1.8 while using the Canon 50mm f/1.8 lens. As mentioned in the Step 1, make sure your camera is on aperture priority mode.
After taking a shot at f/1.8, take another shot by increase the aperture value by one stop, i.e. f/2.8. Then click the third shot at f/4, and then f/5.6 and so on till the maximum aperture value of your lens.
Sharing the aperture stop values for your reference:
f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/44
Here’s a look at the results i achieved using the Canon 50mm f/1.8 lens (images are at 100% crop cropped from the bottom left corner):
Concluding from the image above, the photo is the sharpest in the bottom left corner when clicked using f/8. Which means that the sweet spot of Canon 50mm f/1.8 lens is f/8.
It doesn’t mean that while using this lens i should always click on f/8, to get overall sharpness anything between f/5.6 – f/11 looks good from center of the image to the corners. But there might be situations when you need to open up the aperture to f/2.8 or f/1.8 to achieve shallow depth of field, which is completely fine.
Now it’s time to put your lens to test and check the sweet spot at which the lens will give the optimal shaprness.