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Depth of Field (the Illusion)


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Depth of Field (the Illusion)

25 Sep 2021 4:02PM   Views : 461 Unique : 345

It's something we often talk about but does it exist? Is it Scotch mist or the pot of gold at the end of the rainbow?

A video I recently came across on YouTube set me thinking. It basically said that depth of field is an illusion. And in fact illusion is the most apt description for the idea once you look at what's involved.

A lens can only focus on one plane at a time. Anything else, by definition (no pun intended, of course) is out of focus. You see this through the viewfinder a you're viewing an image created by the lens at full aperture. It may not be so obvious when looking at a general scene, but as you focus on closer subjects it becomes apparent.


The narrow zone of focus at f/2.8

The idea that more of a subject comes into focus when using smaller apertures is a useful one, and both landscape photographers and macro workers use the principle to their advantage. So in that sense it's something that clearly (again, no pun intended) works in practice. How is it an 'illusion'?

There are mathematical formulae for working out the depth of field or any focal length and aperture. That won't concern us here, a my brain (and possibly yours too) goes into blue screen mode at the very thought of them. The basic idea revolves around a 'circle of confusion' which is the size at which a point source is rendered as larger than a point on the sensor. Imagine out of focus street lights in a night-time portrait. That's crucial, as defining that size affects the value of depth of field calculated. I hope you're still with me.


More in focus at f/8

The amount of depth of field will vary depending on format too, as it depends on those parameters mentioned above. It's true that smaller formats have an apparent greater depth of field than larger formats, but the laws of physics and optics come into play. Back in olden days you could buy tables of depth of field for large and medium format lenses as well.as the upstart miniature format of 24x36 mm. It's worth noting that depth of field doesn't start and stop abruptly. You can tweak those parameters to 'give' you as much depth of field as you want. There's a gradual change from sharply focused object to out of focus area.

Films of the time generally were of lower resolution and larger grain than those later in the 20th century. Larger negatives need less enlargement for a given print size than smaller negatives. Which raises another point about resulting image size. A small print will look sharper and have good depth of field because all the details are small. Think of a 10x15 cm print. Go up to A3 and you may well see some areas aren;t as sharp as you thought. But if you're viewing the small print at arm's length and the A3 from a metre or so away, then you'll likely see no difference because the apparent size of the image would be similar.


The zone of sharpness is extended at f/16

That last point is also crucial, and easy to appreciate for prints. But these days it's s easy to zoom into an image at high magnification and get disheartened with an apparent lack of sharpness. But you're not really going to view all your images like that. With the huge resolutions of the latest cameras it's easy to find 'fault' It's not a fault. Such high resolution will allow you to see slight softness in parts of the subject (compared to where you actually focussed) that film grain in the past and lower resolution digital cameras wouldn't allow. You wouldn't, ordinarily, admire a large print in an exhibition from a few centimetres away. An image on an advertising billboard is designed for viewing from a distance (go up close and you'll see the limitations). Nor would you go right up close to your 4k monitor with the image at 100%.


Viewing close reveals how narrow the sharp area is at f/2.8

That's all very well as theory so I tried an experiment. I took a series of images using a 100 mm macro lens at full stops from f/2.8 through to f/32. Looking at the results, it's easy to see that there is a greater amount of detail visible when using smaller apertures. However, looking closely, and the 100% crops show this, that the point of focus is still the sharpest region. Areas that are within the 'accepted' sharp area covered by depth of field are still ;'soft'. This is extreme perhaps, but you need to explore the limits as such o understand what's going on. Viewed 'normally' there isn't an issue. One useful outcome of using f/32 was that found my sensor was clean, because such small apertures show up dust particles very clearly.


Even at f/11 the edges of the figures aren't fully sharp when viewed close

I discovered that there is more overall softness at f/22 and f/32 due to diffraction. Because of high resolution sensors and the ability to enlarge so much, the diffraction effects become evident. It's worth knowing the performance of your lenses just as it is, for example, knowing he noise performance of your sensor. You know what to expect and, importantly, what level is acceptable to you. That's another story.

That it 'works' is sufficient. Depth of field is, when it comes down to it, really a measure of how much unsharpness, or out-of-focusness, are you willing to accept?

If you're wondering, the pot of gold does exist but HMG got their first (Boris needs the cash).

Scotch mist certainly exists, it rolls in off the Clyde.

Depth of field is an illusion, albeit a clever one.


Scotch mist

All text and images Keith Rowley 2021


mistere Plus
9 16 7 England
25 Sep 2021 5:36PM
Depth of field is an 'optical' illusion. Specifically when talking about photography and art. What you see or how sharp or soft it is depends on how good your eyesight is (or your glasses are).
What the camera shows you is a very different interpretation of the scene that your eyes see.
Claude Monet had cataracts, Van Gogh had a condition called xanthopsia, a vision deficiency that causes the sufferer to see more yellow.
Rembrandt and Leonardo da Vinci are both thought to have had a squint. The eye problem actually enables sufferers to better represent faces and objects in three dimensions, as well as depth in landscapes. Good eyesight, or lack of it, plays a major part in everything that we do and, more importantly, perceive.
chase Plus
17 2.4k 638 England
25 Sep 2021 5:46PM
An excellent demonstration of Depth of Field Keith, very well put together and illustrated.

Quote:Nor would you go right up close to your 4k monitor with the image at 100%.

Oh, that's where I am going wrong....note to self...wear your glasses when looking at the monitor Wink
dudler Plus
19 1.9k 1950 England
25 Sep 2021 9:25PM
You're dead right about modern kit, Keith - the lack of sharpness away from a single thin plane with 42mp and f/8 is distressing, and makes me wonder a lot about focussing!

On the other hand, as you suggest, rather poor lenses give apparently greater depth of field.

I feel that I'm quite lucky to be shortsighted: it means that I don't have problems missing the wood for the trees, and certainly individual leaves don't usually distract me from the tree...

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