Getting Out of Your Depth

This article is motivated by all the wrong or misleading information about depth of field, especially in regard to Micro Four Thirds cameras, that gets spouted by YouTube photographers. It will reiterate and expand on some things I wrote about in a previous blog post called Focus is Overrated. It will also consolidate into a single article all the responses and reactions I have given to this misinformation when I see it on YouTube and other places online.

As far as I can ascertain, this nonsense about Micro Four Thirds was started by self-styled photography expert Tony Northrup, who seems to have something against the M4/3 format, on his popular YouTube channel. He actually accused the manufacturers of M4/3 cameras of lying about the specifications of their cameras and lenses, particularly in relation to apertures, focal lengths and depth of field, and essentially tried to blind the ignorant and uninformed with pseudo-science and faulty logic. His rationale was that because an M4/3 sensor had a crop factor of two, then the f/stop also had to be doubled along with the equivalent 35mm “full frame” focal length: i.e. an f/2.8 lens on M4/3 should be labelled as f/5.6. Unfortunately, a host of inexperienced and uninformed photographers believed his faulty reasoning and perpetuated the myth.

The exposure triangle (ISO/aperture/shutter speed) gets drummed into people, even though there seems to be a minority of photographers who shoot in manual mode for exposure, preferring to leave the camera to make some of the decisions. Having grown up using manual film cameras, I still prefer to use the manual setting for most of my digital photography, and very occasionally switch to aperture priority for situations that require quick reflexes in variable lighting. However, this current bokeh and shallow-depth-of-field obsessed generation don’t seem to be aware of the other triangle related to depth-of-field. Actually, I don’t even know if it is something that is generally talked about, or even exists. I certainly hadn’t heard of it before, but the exposure triangle. nomenclature was new to me too, for something that was quite fundamental. Now, the three sides of this imaginary depth-of-field triangle are focal length/aperture/distance. I will now attempt to explain how this works as simply and eruditely as I can, hopefully without the use of complex mathematical formulae or charts.

It turns out that length does matter, at least when it comes to lenses and where depth-of-field is concerned. And this is where a great deal of the confusion arises. For some reason the “full frame” sensor has become the standard by which focal lengths are equated. All camera and lens marketing material always gives a 35mm equivalent focal length for a lens, even with mobile phones. This seems like a pointless exercise because the majority of people taking photos today have never used a 35mm camera, or even know what the 35mm or “full frame” refers to. So, when they buy their shiny new smartphone with its 3/4/5 lenses they get given 35 mm equivalent focal lengths for the tiny little lenses, without having a clue what it really means. With DSLRs and MILCs, you get told a crop factor for your particular sensor, whether it’s 2x, 1.5/1.6x or 1x for M4/3, APS-C or 35mm “full frame” respectively. For someone like me, who has been shooting with 35mm cameras for forty-plus years, it is a handy quick reference when buying a lens because I am familiar with and have used a whole range of lenses over those decades. This use of equivalents is where the confusion comes in, especially regarding depth-of-field.

Firstly, it’s important to understand that both focal length and aperture are physical measurements of the lens, with focal length being calculated from the front element to the film/sensor plane. However, through the use of different types of lens elements the distance doesn’t have to be physically the same as the focal length, e.g. a 50 mm lens doesn’t have to be exactly 50 mm long. As with the focal length, the aperture is also a physical measurement relating to the diameter of the hole made by the lens diaphragm to control the amount of light passing through it. The f-number calculation is based on that diameter and the lens’s focal length. Due to the smaller size of the Micro Four Thirds sensor, the diameter of a lens specifically made for it can be smaller than one for a larger format as it has to only cover a smaller image area.

I’ll use M4/3 as my base example, to keep things easier to understand, and because of it being the apparent reason for the confusion. On M4/3, the 25 mm f/1.8 could be considered a standard prime, with its equivalent on 35 mm being the popular nifty fifty. ‘Equivalent’ meaning that both lenses have the same angle of view (AoV) on their respective sensors, in this case, both have a diagonal angle of view of approx. 47°. The 50 mm f/1.8 will obviously be a bigger lens because its imaging circle has to cover the larger 35 mm sensor, and the physical size of the aperture will be proportionally bigger, but it will let through the same amount of light as the f/1.8 aperture on the M4/3 25 mm lens. The idea that the aperture on the M4/3 lens should therefore be the equivalent of or labelled as f/2.8 is total nonsense. The confusion comes from equating the same field of view with different depths-of-field, while ignoring the actual focal length.

For example, with a 25mm lens at f/2.8 on an M4/3 camera it will have the aforementioned 47° AoV. If it is focussed on a subject for a head and shoulders photo from 1 metre away, it will give a depth of field (DoF) of 125 mm. Using a 50mm lens, also at f/2.8 on a “full frame” camera for the same AoV, i.e. image framing (although bearing in mind the different (3:4 vs 2:3) ratios of the sensors) standing 1 metre away, we get a DoF of 62 mm. Although the framing (AoV) of the portrait is approximately the same, the longer 50mm lens will give you a much shallower depth of field – almost half, which most people seem to understand. Stopping down to f/5.6 will give the same depth of field as the 25 mm at f/2.8, but that doesn’t mean the lens is mislabelled, because at it is letting in two stops less light to achieve the same exposure, which is the primary function of the aperture, as previously mentioned.

Now, let’s see what happens if you put a 25 mm lens on a 35mm camera – although 24mm is a more common focal length for a prime, for the sake of argument and simplicity, we’ll stick with 25 mm. Now, that lens would have a diagonal AoV of approx. 82°. To make the same head and shoulders portrait we would need to stand only 50 cm (0.5 m) away and, apart from making an unflattering portrait due to a certain amount of distortion, it would give a depth of field of 62 mm (the same as the 50mm on the 35mm sensor). However, if you stood at the 1 m mark, the DoF would be 257 mm, double what it was on the M4/3 camera, but you would be taking in area around the head and shoulders, and would have to crop the image to get the same framing, which is why sensors smaller than 35mm are called “crop” sensors. However, it would have a much greater depth of field due, to the distance from the subject and the shorter focal length.

Now, if we put the 50mm lens on an M4/3 camera (and if we were using a “full-frame” DSLR in this experiment we could use the same lens, with an adapter) it would give an AoV of 24.4°, the same as a 100 mm lens. To get the same portrait shot, we would need to stand 2 metres away (double the distance of when using the 25 mm lens) and at f/2.8 you would get a DoF of 125 mm, which is the same as using the 25 mm on the M4/3 camera from 1 m away for the same image frame.

What all those figures show is that shorter focal lengths inherently have more depth of field than longer ones, and although aperture changes the depth-of-field, at the same f-stop, it is the distance from the subject that also affects it, when trying to create the same image using different size sensors. Essentially it comes down to: the shorter the focal length, the greater the depth-of-field; the smaller the aperture, the greater the depth of field; and the further from the subject, the greater the depth of field. And vice versa, obviously. Longer focal length, shallower DoF; wider aperture, less DoF: and closer to the subject, less DoF.

I’m not sure if this has made it any clearer, or dispelled any misinformation about depth-of-field and the perceived disadvantage of Micro Four Thirds sensors. Once you can figure out the DoF triangle, you will see that it is possible to achieve the same effects with any sensor, using the right mix of focal length, distance and aperture, with bigger sensors having the advantage for shallow depth-of-field, and smaller sensors for greater DoF.

Where M4/3 does excel is in macro photography, where extremely close focus distances mean razor-thin depth-of-field. Because the sensor is so much smaller than “full frame”, it’s much easier to fill the frame with the 1:1 image (and not, as some people claim, giving a 2x magnification compared to 35 mm), which means shorter lenses and therefore a bit more depth-of-field. Apart from the 28mm for Canon’s numbered-days EF-M mount, the Zuiko 30mm macro for M4/3 is the shortest focal length true macro lens (at least 1:1 reproduction), which actually does 1:1.25. Canon also do a 35mm EF-S macro too. The advantage of the Zuiko is you don’t need to get as close to the subject as the Canon lenses. But when you couple the 30mm’s DoF with focus stacking or focus bracketing, which is built-in to many M4/3 cameras, you don’t need as many shots to composite together to get deep focus on the subject, and still keep the background distractions out of focus.

Going deeper

In the early 20th century, there was a group of photographers, which included greats such as Ansel Adams and Edward Weston, called Group f/64. They were so named because f/64 was the aperture they needed to use to ensure they got as much of the image in focus as they could when using their large format cameras. As the great Henri Cartier-Bresson said, “Blurred backgrounds in colour photographs are distinctly displeasing”, and he was shooting with a 35 mm Leica and 50 mm lens, and primarily in monochrome. They had a completely different aesthetic to today’s fashion for extreme shallow depth-of-field.

Never Mind the Bokeh

Apart from all the misinformation about depth-of-field, which I hope I have managed to clarify without further confusing the matter, one of my long-running and seemingly never-ending bête noires is the obsession with shallow depth-of-field and bokeh, especially amongst YouTubers. It is this, and the aforementioned misinformation, that has kept lots of photographers from considering Micro Four Thirds as a viable camera system. Even amongst video shooters it seems to be losing favour. The irony is, not only do they want razor-thin DoF, therefore super-wide apertures, they also want their images to be as sharp as possible, except the backgrounds, which they want blurred but also “creamy” or “buttery smooth”. And with video, because they can’t shoot at high shutter speeds, they have to add ND filters, further reducing image sharpness (depending on the optical quality of said ND filter). And they also want their equipment smaller, and not too expensive. It’s a bit like the old work conundrum where you can offer a service, but with only two out of three options of cheap, fast or good. When they talk about a lens, they say it has great depth-of-field, when what they actually mean is, it can achieve very shallow depth-of-field.

One of the things I love about M4/3 is the fact that it has more depth of field because of its smaller lenses. Shooting live music, where everything has to be shot wide open with the fastest lenses as possible, and where the subject is nearly always moving, a bit of extra DoF is always welcome. For portraits, showing more than a slither of the person’s face in focus is always nice, as is a bit of their environment. And I’ve already addressed the advantage for macro.

And what is it with bokeh? It’s become a feature of the lens that is now a major selling point in lens marketing and advertising. People obsess about the quality of it in photos, or, worse still, make it the subject or (ironically) the main focus of the photo. This definition of bokeh is from The Grammarist website, “The term bokeh is borrowed from Japanese, where the word is rendered as bo-ke. In Japanese, bo-ke means a haze or a blur, and is also used to mean the brain fog of senility or old age”. Kind of says it all. I’m sure the fad will eventually pass, but the brain fog of old age might have caught up with me by then.

Nerd to know

Calculating focal length

Calculating f/stop measurement

Calculating depth-of-field

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