I’ve spent lots of money on what I understood was a great lens, but I am not satisfied with the results I am getting. Do you think I have a bad copy, or am I doing something wrong? To give you a little more information, I use a Canon 80D with a Canon 16-35mm f/4 L IS lens. I mostly shoot landscapes with maximum depth of field for extra sharpness.

The problem is that the extra sharpness just isn’t there. Any ideas?

Hannah (UK)

Marcus says:- This is an easy one to answer, but takes a little explaining. The lens you have is probably the sharpest wide angle zoom on the market. While it is possible you have a bad version of this lens, it is very unlikely.

The issue probably lies in the fact that you say you are shooting to achieve maximum depth of field, which suggests you are using apertures of around f/16 to f/22. When using such small apertures, although you theoretically increase the depth of field, you lose sharpness across the entire image due to something called diffraction. This is where light which comes into contact with the aperture blades gets scattered as it passes through the lens, causing a loss of image sharpness onto the sensor. Think of when you turn your kitchen tap on and watch the water flow out of it into the sink below. The water flows in a neat and organised manner until it comes into contact with a hard object. If you placed a knife (or any object for that matter) onto the edge of the flow of water, that edge would become chaotic after contact with the knife, as the water particles are scattered rather than flowing in a neat orderly fashion. The same happens with light as is comes into contact with the aperture blades. If you use a large aperture (say f/4) then only a small percentage of light passing through the large opening comes into contact with the aperture blades, so most of the light passes through in an orderly manner to create a sharp image. As the aperture opening gets smaller though, the percentage of light coming into contact with the blades increases. Once you get down to f/16 and f/22 on DSLR lenses, this will be enough to start noticeably affecting the sharpness of an image. This is compounded on your camera to some degree because you have a crop sensor which means that you then need to enlarge the image to a greater degree to get it to fit your computer screen. The more you enlarge an image which is suffering from diffraction, the more noticeable the diffraction will be.

Another problem with shooting at small apertures is that it results in slower exposure times. Unless you are using a very solid tripod (and we mean a very solid tripod - not some cheap nonsense) then this will also have an impact on the sharpness of the image. Diffraction may make 10% difference and then enlarging the image by an extra 50% over that of a full frame sensor turns 10% into 15%. Then you may be adding another 5% of motion blur from use of a slow exposure time, and you can see that you have suddenly lost 20% of the sharpness of your lens.

You don’t mention what focal length (if there was a particular one) which you use, but if you use the extremes of the range (ie 16mm or 35mm) along with a small aperture, then you are using the lens at its very weakest points. Try shooting at f/8 between 20mm - 30mm and I bet you’ll see a massive improvement in your results. The lens you have is the best you can get, so you are right to have high expectations. Good luck and let us know how you get on.