Discussion Photo Focussing Options

[Mox]

I was wondering if anyone could give me some info on the settings that affect the way that, for example, the background appears, such as a coin/bar in focus and the background blurred?

And any other things that I can do to change the way parts of the picture appear?

[Silverlocks]

@Charliemouse ^^^

[Charliemouse]

1 hour ago, Mox said:

I was wondering if anyone could give me some info on the settings that affect the way that, for example, the background appears, such as a coin/bar in focus and the background blurred?

The part of the image that appears in focus is determined by the 'depth of field' (DoF).  This is a fancy way of saying the range of things that are in focus.  This topic can get really technical and mathematical, but I will try to keep it simple.  That will mean some things may be simplifications, or not perfectly accurate, for the sake of brevity and comprehension.

Four things determine the depth of field:

1. The size of the camera sensor.
2. The focal length of the lens (or where the lens is set, if it is a zoom).
3. How far the subject is from the camera (the focal distance).
4. The aperture setting.

Here is a real world example.  I will take a typical 'every day' photograph, and vary each factor one at a time, to see what effect it has.

A full frame camera, 50mm lens, f/8.  For a subject 2 meters away, anything 32cm in front and 46cm behind, will be in focus.  So that would mean it would be safe to photograph someone standing up and have them completely in focus from the front to back (the tip of their nose to their ears could easily all be in focus).  We would say the depth of field was 78cm.  It's always bigger behind the subject than in front of it, because physics.

Now say we have a cropped sensor camera (APS-C for example).  DoF is now smaller at 50cm, 22 in front and 28 behind.

Go back to a full frame camera, but let's use a 100mm lens.  The DoF shrinks to just 18cm, 8 in front and 10 behind.

Go back to 50mm, but focus on something that is 5 meters away.  The DoF is now a much bigger 6 meters, 1.5m in front and 4.5m behind.

Finally, back to a subject 2 meters away, but change the aperture to f/16.  Now the DoF has grown to 1.7m, 50cm in front and 120cm behind.

So summarising the effects on depth of field:

1. The depth of field is smaller when you use a smaller sensor.
2. The depth of field is smaller when you use a longer lens.
3. The depth of field is bigger when your subject is further away.
4. The depth of field is bigger when you use a smaller aperture (larger f number).

To be continued...

Edited August 4, 2023 by Charliemouse

[Charliemouse]

Let's do another example, but this time use a typical coin photography set-up.

Base configuration: Full frame camera, 150mm macro lens, f/8, subject at minimum distance of 380mm.  Depth of field is 0.48mm - yeah, half a millimetre.

Using a smaller APS-C camera, the DoF would be 0.32mm

Using a 50mm lens, but moving closer so at the new (closer) minimum distance, the DoF would still be 0.48mm - interesting, huh?

Changing to f/16, DoF would be 0.96mm, i.e. twice as deep.

So summarising again:

• Depth of field at macro distances is really tight.  Only a thin band, half of a millimetre thick, is in focus.
• Changing the focal length of the lens does not change DoF, if you move the camera so that the subject is still the same size in the frame.  This makes sense because you are changing both variables 2 and 3 in the previous example, one increasing DoF and one decreasing DoF.
• Doubling the aperture number (quartering the amount of light) will double the depth of field.

To be continued...

Edited August 4, 2023 by Charliemouse

[Charliemouse]

So let's break this down into two common problems.  They are the exact opposites, but you may have one or the other.  The most common one is probably:

I can't get the whole coin in focus

So here are some solutions.  You can try one or several:

• Move further away.  Firstly, if none of the coin is in focus, you may be within the minimum distance of your lens.  In which case you need to move further away anyway.  But if you find that only a thin band of the coin is in focus, then try moving back.  Yes, you will sacrifice detail, but that's just life.
• Align the camera with the coin.  If the coin is straight on, you may be able to fit the depth of the coin within the depth of field, so everything is in focus.  However, this can be tricky, especially without a tripod, and may mean compromising on lighting.
• Use a smaller aperture.  Try f/11 or f/16.  I do not recommend going further than this, unless you are desperate, because you will begin to lose overall sharpness in the image.  Again, because physics.  Narrowing the aperture will reduce the amount of light getting through, so that means slower shutter speeds or higher ISO (and noise).  So you will almost certainly need a tripod.
• Focus stacking.  You can cheat physics.  If all else fails, and you want a nice angled photograph of a coin, with a perfectly sharp surface but blurred out background, then focus stacking is your answer.  It involves taking several (often many) photographs of the same scene, only varying the focus point by a tiny amount, 0.2mm or so, each time.  Then magically combine all the individual images into one super sharp image.

The background is in focus, and it is distracting

This is the very opposite problem to the above.  Apart from reversing some of the above advice, you could also try using a darker background that shows less detail, or using Photoshop to blur out the background.

In general

Tune the aperture so that just the right things are in focus and out of focus.  The easiest way is to do just do it.  I would strongly recommend using 'aperture priority' mode on your camera.  This allows you to set the aperture, which directly affects the appearance of your photographs, and the camera will balance the other settings to make the photograph work.

Edited August 4, 2023 by Charliemouse

[Charliemouse]

This is a bit of an aside, and doesn't really have anything directly to do with depth of field.

The area of a photograph that is out of focus is called the bokeh.  (Super nerdy aside: It's a Japanese word that is close to rhyming with OK.  In IPA it is: /ˈbəʊ.keɪ/ )

Generally you want bokeh to be smooth, with no detail of its own, so that it doesn't distract from the subject.

Different lenses will produce different bokeh, some smoother than others.  Dedicated macro and portrait lenses will often have more diaphragm elements (the petals that make up the lens's aperture), which will generate smoother and more pleasing bokeh.

It can make a difference to some photographs.

Edited August 5, 2023 by Charliemouse