Aug 222011

In this second part of ‘Understanding Depth of Field’ I will look at the problems associated with using very wide and very small apertures. The main problems that I will look at are:

  • Why can’t I see the depth of field in the viewfinder?
  • Problems with using a wide aperture (focusing and lens softness)
  • Problems with using a small aperture (diffraction and shutter speed)

Why can’t I see the depth of field in the viewfinder?

This is a question that I am asked a lot!

Let’s imagine that you are using aperture priority mode and you change the aperture from f4 to f11. If you look through the viewfinder as you change the aperture you see absolutely no change in depth of field (DOF). However, when you take a photograph there is a big difference in DOF between the f4 photo and the f11 photo.

So why, when the camera is set to f11 do you see the DOF for f4 in the viewfinder and not the DOF for f11?

The reason is that when you mount a lens on to the camera, the camera automatically sets the lens to the widest aperture. This is to give you the brightest possible image in the viewfinder for composition and focusing. It is only when you press the shutter release that the camera sets the lens to your chosen aperture.

If you do want to see the depth of field before you take a photograph then many digital SLR’s have a ‘depth of field preview’ button. Pressing the button sets the selected aperture and you can see the depth of field, the viewfinder image also goes very dark!

Problems with using a wide aperture

 

This photograph of my daughter Erin was taken with a 50mm f1.4 lens at f1.4. Only her left eye is really sharp. Using a smaller aperture such as f4 would allow me to get her face in focus but maintain the blurry background.

From last week’s blog you will remember that a very wide aperture will produce a shallow depth of field. If the depth of field is too shallow then your focusing needs to be very accurate. This can be very difficult to achieve if your subject is moving!

Another problem is that not all of your subject will be in focus. E.g. If your subject is not looking directly at you, then one eye may be in focus but the other eye is soft.

Another problem with using a wide aperture is that many lenses are only sharp in the centre of the frame when they are used at their widest apertures.

If you have plenty of light to work with, then trying using a smaller aperture to overcome focusing problems and lens softness. E.g. if you are taking a portrait with a 50mm f1.8 lens, then try the shot at f2.8 or f4. You will still see a blurry background but your subject will be sharper and you will have fewer problems focusing.

If you don’t have plenty of light to work with then try one of these solutions:

1. Increase your ISO and use a smaller aperture.

2. Move further back.

3. Prefocus on a point that you expect your subject to pass and take multiple frames as the subject passes that point.

Problems with using a small aperture

Probably the biggest problem with using a small aperture is the slow shutter speed you will get when taking a photograph in low light conditions. The easiest solution is to mount the camera on a tripod and use a remote shutter release. If using a tripod is not possible then increase the ISO but beware of noise!

The second major problem with using a small aperture is diffraction. When rays of light pass through a small aperture they diverge and interfere with another and create a diffraction pattern. In simple terms; if the diameter of the diffraction pattern is larger than your camera’s pixel size (pixel pitch) then the resolution of your photograph will be reduced.

When is diffraction really a problem? To be honest you will only notice diffraction when you are producing large prints.

There are four ways to solve diffraction problems:

  1. If you are using a small aperture to get a slow shutter speed then use a neutral density filter instead of small aperture. E.g. use f8 and a 3 stop ND filter instead of f22.
  2. If you are using a small aperture to get a large depth of field then focus at different distances and use either Photoshop or Helicon Focus to combine the different photographs and increase depth of field.
  3. Alternatively, if you are using a small aperture to get a large depth of field then use a tilt-shift lens and tilt the lens to move the plane of sharpest focus. I will explain in a separate blog post in the very near future!
  4. Perhaps the easiest alternative, if you are using a small aperture to get a large depth of field is to use a wider angle lens.

Below are two photographs which show diffraction. These are cropped photographs from a Nikon D3x with a 105mm macro lens. The lens was manually focused.

In the first photograph, the Nikon D3x sensor has resolved a fine hair in the centre of the photograph, in the second photograph there is greater depth of field but the fine hair has almost disappeared because the diffraction pattern at f32 is much bigger than the D3x’s pixel pitch and therefore the sensor can no longer resolve the detail.

Macro photo of a paint brush at f11. Notice the fine hair near the centre.

Macro photo of a paint brush at f32. Compare the fine hair near the centre with the previous photograph.

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