In discussion groups and photography groups you see questions about lenses from shoppers and the number one thing they want to know is if a lens is “sharp.” What many beginning photographers don’t understand is that lenses are not sharp at every aperture value and at every possible distance (especially with zoom lenses).
Therefore the responses one gets on these forums will likely be very subjective and based upon that photographer’s experience and how they use that lens. If a person reports a lens is "not sharp" they may not be giving you accurate information. Every lens has a “sweet spot” and sharpness is often confused with depth of field. This article will help clear up that confusion and help you figure out what your lens' "sweet spot" is and how to find it.
First, some background information, no lens is ever perfectly sharp. Lenses are designed for maximum performance based on what manufacturers believe the main function of that lens will be. For example, if you purchase a zoom 70-200mm lens, the manufacturer might assume that the main use of that lens will be at the 200mm end of the range and they will maximize its performance for that focal length. A lens that is a macro lens will be maximized for close up shooting distances. Never purchase a lens and expect perfect sharpness at every distance, aperture, and zoom level. Know what you will be using the lens for and buy a lens designed for maximum performance for that purpose.
Another common point of confusion for beginning photographers is understanding the difference between sharpness and focus (or depth of field). Sharpness is related to resolution and depth of field is related to how much of the image is in focus from foreground to background. When you are first learning photography you will probably hear the advice that if you want more of your image in focus to use a narrow aperture (larger f/# like f/11 or even f/16). While generally this is true, greater depth of field (or focus) does not always mean more overall sharpness.
A quick side note: Using smaller apertures will increase depth of field, but the depth does not increase equally in front of and behind the subject. Instead when you stop down your lens, the increase in the depth of field behind your subject will be almost twice as much as in front. In other words if the overall depth of field increases to 20 total feet, about 1/3 (~7 ft) of this will be in front of the subject and the remaining 2/3rds (~13 ft) behind the subject. (If you need a refresher or introduction to depth of field and hyperfocal distance check out our previous blog posts).
I took three photos of a flowering tree using f/4, f/5.6, and f/22. The photo at f/4 has the most narrow depth of field and the photo taken at f/22 has the widest depth of field, while the f/5.6 (not shown) would be somewhere in between. While there is more in focus (greater depth of field) in the photo taken at f/22, it is probably not as sharp compared to the in-focus parts of the same picture taken at f/5.6.
We can examine the sharpness by "pixel peeping" and looking at only the areas that were in the zone of focus. We should find that the image is sharper at f/5.6 and less sharp at f/4 and f/22. The image taken at f/5.6 will appear “sharper” even though more of the area of the scene (depth of field) is in focus at f/22.
Now let's actually take a look at those images below. The series of shots was taken from the same location and only the aperture was changed. These are parts of the same photos shown above that demonstrated depth of field taken at f/4 and f/22 (plus the additional shot at f/5.6). Note that the original images were already cropped and then I further cropped in on just one flower that was inside the depth of field (in the area of focus). At this "pixel peeping" level we can compare the sharpness. Remember that when not cropped to show detail, each of these photos "looked" sharp and in focus in this area of the image. The focus and focus point were not changed in between shots so any differences you see are not because of a focusing issue. We can see that none of the images are perfectly sharp, but the one in the middle is the best. The issue/difference between these images occurs because of the internal workings of the lens. All those glass elements have to align and at different apertures the quality differs.
Lenses are “softer” at both extremes of aperture – wide open and mostly closed. The details in an image won’t be as crisp or “sharp” when shooting at the maximum apertures like f/1.4 (or in the case above with my lens at f/4), or at minimum apertures like f/22. Both extremes are equally deficient when it comes to sharpness. As a general rule the maximum sharpness, or “sweet spot” of a lens is 2 to 3 stops from the maximum aperture of that lens. So for example, if the maximum aperture of a lens is f/2, then the “sweet spot” would be between f/4 and f/5.6.
The lens used for the photos above has a maximum aperture of f/4. Therefore the "sweet spot" should be between f/5.6 and f/8. The image at f/5.6 gave good results, but with further testing, I believe this lens would probably be sharpest around f/6.7 or f/7.1 which is about one and 1/2 stops above the maximum.
Once you know the range of where the “sweet spot” should be, it is easy to conduct a test to find it with more precision. To conduct the test you will need to use Aperture Priority Mode. In Aperture Priority mode we can set and test the aperture and the camera will set the ISO and shutter speed.
If you are unsure of how to switch to Aperture Priority and how to change the aperture settings in that mode, you may need to consult your camera manual (we have most manufacturers’ links here). If your camera allows you to use one-half or one-third stops, make sure that is selected in your settings.
To perform a test to find the “sweet” spot of your lens, first find the maximum aperture of the lens and add one half or one full stop to that number to use as your starting point. If your lens has a maximum aperture of f/3.5 (most kit lenses), then start at f/4.8 or f/5. If your lens has a maximum aperture of f/2, then start at f/2.4 or f/2.8.
Go outside in good sunlight. Use a tripod or something to stabilize the camera and to keep it in position between shots. You will need to take photos of something that is about 10 feet away (like my sample images of the flowering tree) and make sure your focus point is on that object. You will not recompose or refocus in between shots (after getting your focus point you can change over to manual focus to make sure your focus point stays set).
Start by taking a photo with the aperture at your starting point (from above). Then turn the aperture dial 2 or 3 clicks narrower (bigger f/#) and take another photo. Then turn the dial a few more clicks and take another photo. Take test shots from your starting point up to about f/11.
By taking a test shot every 2-3 clicks you will be sampling a range of f/stops that include some half and third stops. The “sweet spot” may not be at a full stop like f/5.6. If your camera only has full f/stops, then take sample test shots at each full stop.
Now it is time to "pixel peep." Upload your photos and zoom in on each one. Examine the area of the photo that contains the object you focused on (that was about 10 feet away) and examine that same area in every photo. You should be able to tell which aperture gave you the sharpest image.
You can then repeat the test using the range of apertures in 1/2 and third stops around that aperture to further narrow it down. In my test above f/5.6 was the sharpest, but to really narrow it down I would test again starting at f/4.8 using every increment up to about f/7.1 (I already had an image at f/8 that was not as sharp so I know I can stop before that point).
Once you know the "sweet spot" make a note of that aperture for that lens. Repeat for different lenses. Each one will have a different “sweet spot” but typically the maximum sharpness will be about 2 stops above the maximum aperture, somewhere in the range of f/4 – f/10.
If your lens is a zoom lens the sharpness and “sweet spot” may not be identical throughout the zoom. For example, I have a Tamron 18-400mm and the maximum sharpness at 18mm may not be the same aperture setting when shooting at 400mm.
We could not ever hope to detail all the possible parameters for every lens in the marketplace in a short article like this, but we can offer a little general advice. If you are ever out and about and need a sharp image and you are not sure what the “sweet spot” of a particular lens is, then use Aperture Priority Mode and take a series of photos from f/4 through f/11 and you will have all your bases covered!
Another often overlooked way to help improve image quality and sharpness is to use a tripod and a shutter remote. These reduce camera shake.
So in summary, shooting with a wide open apertures may result in reduced sharpness in your images. Additionally the narrow depth of field can be difficult to work with. At very low apertures this depth can be less than an inch and can cause one part of a face, like the eyes, to be in focus and the nose out of focus.
When examining your images to trouble shoot a “non-sharp” image, consider both the depth of field (focus) and sharpness (resolution) as separate factors even though both are related to aperture. Decide if the lack of “sharpness” you see is due to missing your focus point while using a very narrow depth of field, or if it is an overall resolution issue due shooting at the extremes of the lens’ aperture range.
Knowing that your lens has a “sweet spot” and how to find it can help improve your images overall, but much about photography comes down to compromise. Just because your lens has a “sweet spot” doesn’t mean you should always use that aperture. When out in the field you may have to decide which is more important; maximum resolution (sharpness) or depth of field. It may be that you want the wider depth of field for a landscape shot and to get that you may have to sacrifice some level of sharpness. Also, remember, that to see the difference in sharpness you may have to “pixel peep.” The actual difference in sharpness between two images in normal print sizes may not even be that noticeable to the naked eye (even though they are there). “Pixel peeping” can become a dangerous obsession. Don’t let the search for optimal sharpness interfere with your creativity and the fun and joy that photography can provide.