The Blog

Everyday updates and longer photostories

Mar 25 2018

Sharpness testing

By Mark Beresford | Comments 6

I wanted to look at lens sharpness across the standard nine f/stops. I used my Canon 135mm L lens at the closest focusing distance it allows. I didn’t sharpen the images at all, just cropped them and converted them to JPGs without resizing.

I used a studio strobe to light the compass to allow me to keep the shutter speed at 1/160th. The continuous LED lights I use for video are not bright enough and I would have had to use longer shutter times for the smaller apertures, introducing camera shake. Instead, I adjusted the strobe brightness for each photo.

f/32

f/22

f/16

f/11

f/8

f/5.6

f/4

f/2.8

f/2

It’s standard wisdom that f/8 is often the sharpest aperture, and this is true for this lens. For product photography, you usually want the sharpest possible photo, so f/8 would be the default. As a caveat, some product photography and food photography does use narrow depth of field to create more emotion around the image. But for technical product photography you want to maximize sharpness.

With a mid-telephoto lens like this one, f/8 gives you a short depth of field. I focused on the number 32. Two numbers away, and the focus is already falling off.

To increase depth of field, you can stop down to a smaller aperture. At f/16, sharpness on the number 32 is falling off, and by f/32 it is significantly degraded. This is caused by diffraction, where some of the light is deflected as it passes close to the narrowed blades of the lens iris. The deflected light rays overlay the nicely parallel light rays that pass straight onto the sensor, and has a muddying effect. The numbers at the back of the compass are almost as sharp as the number 32, so there is a lot more depth of field. So you trade loss of sharpness caused by diffusion with improved depth of field.

If I’d focused about two-thirds of the way back I could have better overlapped the depth that’s in focus with the compass to improve the result slightly.

Of course, at f/2, the lens is wide open and there is hardly any depth of field at all. The lens is also soft at this aperture. So, wider apertures are just bad for product photography. Good for portrait photography where you want to isolate the subject and too much sharpness shows up too many skin flaws. But bad for product photography.

So what if you want a product photography to be sharp and have a wide depth of field? You can address this partially or completely in a few ways:

  • Increase the distance to the subject. The down-side is that you’ll have to crop the image more, losing pixels which may be important.
  • Use a wider angle lens that naturally has a deeper depth of field. The downside is that as you get closer to the subject with a wide angle lens, you introduce perspective distortion. You can correct this in Photoshop to some extent. A closer distance can also put the camera inside the lights so it casts a shadow.
  • Keep the aperture at f/8 for maximum sharpness, then take a series of about 15 photos, with each one focused further into the subject. Then, you have to blend them together using software. This is called focus stacking, and you can do it in Photoshop or specialized software such as Helicon focus. I’ve tried this and have had hit-and-miss results — mostly miss. It’s a challenging job for software. First the software has to align each image, as the process of changing focus changes the magnification slightly. That’s called focus breating. Then the software has to figure out where each image is sharp and use different parts of each image to create the result. Here’s an example of the artifacts you see with focus stacking. The sharpness all around the dial is pretty good, but look at the large E, and other parts of the image. There are obvious seams.

  • Use a tilt-shift lens. These lenses angle the plane of focus. In my compass image, I would angle the lens so the plane of focus falls forward so it’s in between the horizontal plane of the compass surface and the sensor plane of the camera. I would lose some vertical depth of field, but that isn’t critical because I’m most interested in the numbers on the dial. The downside of this method is the high cost of the lens and the extra setup time.

Depth of field isn’t so much of a problem with large objects because you’re shooting from a distance or using wider angle lenses that are appropriate to the situation. It is a problem with close-up photography of smaller objects. It’s a huge issue for macro photography, but it’s also an issue for anything with dimensions smaller than a couple of feet.

Everything in photography is a compromise. Being aware the trade-offs helps you make informed decisions that lead to better results.

Finally, here’s an image taken at f/8 with my Canon 100mm macro L lens, along with a cropped detail. With this lens I can get in much closer so I don’t have to crop so much. I also cleaned up the compass first, spent some time getting nice lighting, and sharpened this image in Lightroom. Being able to get in closer makes a big difference. I could have got in much closer for a detail shot had I wanted to take more time. These images are not perfect, but they are pretty nice shots of a beautiful compass.

For a complete list of blog entries, see Archive