Finding Your Lens’s “Sweet Spot”?

You may have heard about a lenses sweet spot. Knowing this and acting on it, will improve your images. It is something that even some old pros have either forgotten or don’t think about. The Sweet Spot on your lens, what the heck is it in the first place?

There is a lot of science happening with your lenses. There is the expensive quest for the “fastest” lens, which is one with the lowest f-stop. I just saw a 50ml lens with a reported .75 aperture. Wow. Your lens is a mix of curved glass lenses, heavily coated with a design to spread the light it sees across the flat surface on the sensor.

Each f-stop does not arrive at the sensor with the same amount of focus and clarity. Yes, it is well known that the smaller the aperture (higher the f-stop number) the greater the depth of field. Or rather the greater is the area of perceived focus.  While the wider the aperture is the narrower is the focus plane. (Smaller f-stop number)

Sharpness is determined by how distinct small lines in the image are reproduced. The Sweet Spot of a lens is where this distinction is uniform across the sensor and thus across the resulting image.

To experiment, put up a newspaper flat on the wall. Take a shot after carefully focusing at the two extreme aperture settings of your lens.  Closely inspect the results in your imaging program with an enlarged view. It should reveal the sweet spot pretty quick.

The rule of thumb for all lenses is that the Sweet Spot is 2.5 to 3 stops above the lowest f-stop the camera has. This also means on some zoom lenses it is a floating spot since the lowest f-stop of the lens changes with the zoom.  In real numbers a lens with a 2.8 aperture will have the Sweet Spot between f5.6 and f8, on my Canon 2.8 lens that comes in at 7.1.

This puts my focus plane evenly and clearly sharp across the whole plane. It is gorgeous.

The better the quality of the lens means you will not see a huge difference but it will be there, the image just feels richer. This is the result of what is called “spherical aberration” where the lens cannot hold the focus evenly out from the focused point. It’s most visible at the edges of the image.

Often at the edges you will see color fringing along with the blur.

Rather than testing all your lenses you could try searching the Internet with this string, your lens model with f-stop then add sweet spot.  Eg: canon 50mm 1.2 sweet spot.

If you are really technical based you can find online all sorts of products for finding the best focus point of your lens. You can use these same targets for your sweet spot testing.

This is not to say the other apertures, especially the lowest, are not usable, just that is not where the lens best performs. Some of the older cameras, like Holga camera uses the lack of a sweet spot for its charming retro effect, while LensBabies go to extremes to destroy the need to find a sweet spot.

As you select your shooting aperture for each situation you have to balance the depth of field you want with the sharpness you want. It’s just good to know what you are balancing.

 

Mark Laurie map, spa, FellowPPOC
Mark is a photographer, instructor and writer. His 35 yr old Calgary based studio specializes in photographing women. Mark has had his own TV show, written 3 books plus 4 limited edition books. His Revealing Venus experience workshops run out of Italy.
http://www.marklaurie.com
http://www.innerspiritphoto.com