Michael Rammell

Micro Four Thirds Depth of Field Explained

MicroFourThirds, Tutorials & Tips, AudioMichael Rammell8 Comments

Hold on to your hats folks, because this weeks' post is a long and technical post! But In recent weeks I've seen quite a lot of misinformation going around when it comes to Aperture and Depth of Field with Micro Four Thirds Cameras. So today I want to put a stake in the ground and set out in the simplest of terms exactly what impact a Micro Four Thirds Sensor (and an APS-C crop sensor for that matter) has on Aperture, Depth of Field and Effective Focal Range.

If you're in a position to listen then go ahead and hit play on the audio player below, if not, scroll down a little further and you can read through today's post in your own time. Either way, the audio is the same as the text to make sure you're not missing out. After hitting play give it about 5 - 10 seconds to start:

I spoke briefly about this in a post I wrote called 'Why Olympus' in which I looked to demystify many of the myths about the MFT format. But the subjects of Depth of Field (or, an apparent lack of according to some) and the focal lengths of lenses are debates I have seen come up again and again.

I expect many people who subscribe to my blog to already be familiar with the concepts of crop factors and the impact they have on a lens and a resulting image, but for those of you who are very new to photography; my aim is to make this as clear as I possibly can and to start with the very basics. Here I'll explain aperture and focal lengths in as basic a way I can.

If at any point you don't understand please do get in touch with me via email or message me on Facebook - I'm always happy to chat directly.

Before I start I want to make a few things clear:

  • A mirrorless camera is not a worse or lesser camera than a DSLR
  • The fact that a camera has no internal mirror in front of the sensor doesn't mean it is 'lacking' something. This is by design and gives the camera a series of benefits over a DSLR. It arguably also represents some challenges too, but these are mostly things of the past now.
  • The inclusion or exclusion of a mirror does not affect Depth of Field.
  • Depth of Field is affected by the lens and the sensor

Sensor Sizes

In recent years (in the digital age at least) the three most common sensor types found inside cameras that the masses of photographers use, are:

  1. Full Frame Sensors. This includes such cameras as:
  2. APS-C Sensors. Cameras include:
  3. Micro Four Thirds Sensors. Cameras include:

In order of size from largest to smallest in the list above, it is Full Frame, APS-C, MFT.

There are a series of other sensor sizes too, such as Medium Format etc, but if you're reading this post you're most likely interested in one of the three above. For the sake of this post, I will only be looking at these.

Full Frame vs APS-C vs MFT - Affects on lenses

Quite simply, a sensor size will affect all of the properties of a lens; it's aperture and it's focal range. 

When you buy a lens it will have both a focal range and an aperture. These will be written on the box and also on the lens itself (most of the time). For example, let's look at the Canon 24-70mm f/2.8: with this lens, the '24-70mm' part denotes its' focal range or, if you like, its' zoom range: the smaller the number the wider the field of view. The larger the number - the longer it is able to reach. The Aperture is then reflected as 'f/2.8'. The lower aperture number is; the shallower the depth of field (again in simple terms lets say that a lower number will give you a blurrier background. For example the Canon 50mm f/1.8 will give you a really blurry background...)

Our example lens; the 24-70mm f/2.8 will fit and work on both Full Frame Canon camera bodies and also their Crop sensor bodies. BUT, these cameras have different sized sensors and so the lens will behave differently on each system. This is due to what is called the Crop Factor. IF these lenses could somehow fit an MFT camera (they won't, unless you use an adaptor), the lenses would again take on different characteristics. The different effects that these sensors cause is due to their 'Crop Factor'

Again, trying to use the simplest terms possible, the crop factor is, if you like, effectively the magnification rating. (this isn't technically entirely accurate, but, if you're new to this concept then this will help to explain).

A Full Frame has a crop factor of x1, an APS-C has a crop factor of x1.6* and a Micro Four Thirds Sensor has a crop factor of x2.

*The APS-C sensors from Canon and Nikon are actually slightly different sizes and therefore have slightly different crop factors. See the table below for more details. 

How Crop Affects Focal Range

What you may already be aware of is that the differing sensor sizes and as such crop factors actually change the effective focal range of a lens. This is worked out by very simply multiplying the lens' focal range by its crop factor. For example, on a Full Frame Body, a 100mm lens is multiplied by a crop factor of 1. Therefore, the focal range remains 100mm

On an APS-C sensor, that same 100mm lens will be multiplied by 1.6. 100 x 1.6 = 160mm

It's even easier with a Micro Four Thirds sensor as the crop factor is x2. So, without much thought at all, you could work out that a 100mm lens then takes on an effective field of view of 200mm. 

This table below sets out a series of common focal ranges with their respective, inherent effective focal ranges when used on each of the different sized sensors

X2 = Micro Four Thirds
x1.6 = Canon APS-C
X1.5 = Nikon APS-C and Fuji X Series
x1 = Full Frame Cameras

Your initial thought could simply be that surely a smaller sensor is a good thing right? After all, you get more zoom from a lens! To many this certainly is an advantage of APS-C over Full Frame as it can negate the need for you to buy a longer lens. However, it isn't always the case that this extra reach is wanted; those ultra wide lenses you can buy then become slightly less ultra wide.

Secondly, the effective aperture is also affected too:

How Crop Affects Aperture

In continuation of keeping this very basic, I won't explain in full exactly what aperture is, but rather I'll explain the things it affects. Put simply it affects the blur of the background in an image (Depth of Field) AND it determines how much light is let through the lens to hit the sensor. The second part is important because the more light that comes through to the sensor will mean that an image can potentially be less noisy and grainy.

Depth of Field 

Again, sticking with the idea that I am explaining this in it's simplest form; Depth of Field is basically this: From the point of sharp focus on your subject how much immediately after does the scene or subject begin to blur and how blurry is that blur. Experienced photographers may be reading this cringing with this description, but remember I'm trying to keep this is as simple as possible. You can look up a better definition of Aperture and Depth of Field on Google. I'm not here to explain that, I'm trying to explain how a lens affects this property: So, remember how the focal range was multiplied by the crop factor to give a new focal range? Well, we do the exact same thing with the aperture to give us our new values: f/1.8 will become f/3.6 on a Micro Four Thirds Sensor (1.8 x 2 = 3.6) . F/2.8 will become f/5.6 on a Micro Four Thirds Sensor (2.8 x 2 becomes 5.6) and f/4 will become f/8 (4 x 2 = 8) and so on. You can work out the APS-C Aperture difference by multiplying those same details by 1.6 too. So let's again say that f/1.8 x 1.6 = 2.88 (or 2.9 if we round up).

Now, if you know anything about aperture you will know that the lower the f number, the blurrier the background and the shallower the depth of field (the amount of your subject that is in focus). So, if you buy an f/1.8 lens you need to know that it will less shallow on an ASP-C than it would, were that same lens on a Full Frame camera.

The Martin Bailey Podcast Companion App, with DoF calculator

If you want to learn more about Depth of Field I strongly advise you look at Martin Bailey's eBook 'Sharp Shooter', available through Craft & Vision. In this book, Martin goes into great depth about aperture and it's effect on an image. Martin runs a superbly educational podcast and is an excellent photographer to boot! If you're interested in listening that podcast you can subscribe via iTunes or download Martin's Podcast Companion App from the App Store. Handily, this app also includes a Depth of Field Calculator, so when you're learning about Depth of Field you will be able to use this to help.


The relationship of Aperture & Speed

Often, when we talk about lenses with low f numbers, such as f/1.8, f/1.4 or f/1.2 you may hear photographers referring to them as 'Fast' lenses. The reason for this is that these lenses let a lot of light through them, which will afford you the use of a lower ISO or a higher shutter speed. A lower ISO will invariably give you a cleaner, less noisy image and a faster shutter speed will aid you in freezing fast moving subjects and potentially sharper images. Now you may assume at this stage, give that the focal range and depth of field properties have been impacted by the sensor size, that the amount of light a lens can gather is also affected.  This is not true. 

In terms of depth of field yes - an f/1.8 lens on a Full Frame sensor will give a shallower DoF than if it were mounted to a Micro Four Thirds sensor. That I think we have established now. But, the lens' aperture rating in terms of its ability to let through light remains the same regardless of the sensor it is paired with. The 'speed' remains the same.

So, now that we know that your camera (or rather its' sensor) will have an effect on the lenses you're using, this should aid you in choosing the right lenses for the right job...

DSLR Lenses - One Lens - Multiple Sensor Sizes

When it comes to DSLR's from Canon and Nikon, the best lenses are typically designed to work best with their Full Frame models. Even now you know that an f/2.8 lens is not the same on an APS-C as it is on a Full Frame sensor, they are still advertised and sold as an f/2.8. (they don't tell you that a 100mm will become a 150mm or a 160mm for example). You're left to work this out for yourself. But, this is to be expected; these companies develop lenses that can be mounted to two different sensor formats.

Fuji & MFT - Lenses designed to take crop factor into account

When it comes to Fuji and Micro Four Thirds lenses are developed to take the crop factor into account. Each lens for a Fuji Camera, for example, will only fit (natively) onto a Fuji X-Series camera, meaning at the point of designing the lens they know it will be affected by a 1.5 crop factor.

The same is true of Micro Four Thirds, hence the reason Panasonic lenses fit Olympus bodies (and vice-versa). These lenses are optimised for one size of sensor. The crop factor is embraced!

You may be more familiar with the more traditional and mature lens stables from Canon or Nikon. If that is the case then some of the focal ranges on offer from Fuji, Panasonic and Olympus may seem odd at first, and the number of f/1.2 & f/1.4 lenses on offer may seem quite 'exotic' too, but these odd focal ranges and exotic apertures (a 23mm or a 27mm for example) are intentional.

Going back to our crop factor multiplications from earlier, let's look at some of the lenses on offer from Fuji, Panasonic and Olympus to see why these lenses have literally odd numbers...

Olympus Lenses

We'll start with Olympus purely because these are the lenses I am most familiar with.

In the process of moving from Canon to Olympus, I sought out similar focal ranges in the system in an effort to more seamlessly replace my Canon System. My favourite lenses were my Canon 70-200mm f/2.8 and my 85mm f/1.8. Without being familiar with the topics in this post, it may at a glance appear that Olympus don't have their own equivalent lenses. But they do.

So, remembering that the crop factor is x2 on an Olympus camera, such as the OM-D E-M1 we know that the equivalent lens for Canon's 70-200mm f/2.8 is arguably the Olympus M.Zuiko 40-150mm f/2.8 (80-300mm equivalent field of view, remember?). (You can read my review of the 40-150mm f/2.8 right here by the way)

Here are a couple of images shot with my new favourite lens, the Olympus M.Zuiko 40-150mm f/2.8 PRO

What about my favoured Canon 85mm portrait lens? Well, Olympus offer us the M.Zuiko 45mm f/1.8 (45 x 2 = 90mm of course).

The 90mm focal range is very versatile; from close up details, to head & shoulder portraits to wider portraits too. Great focal range coupled with shallow Depth of Field.

You may at this stage be thinking that the apertures simply aren't the same. If we apply that same x2 crop factor to the 40-150mm f/2.8 we effectively have an 80-300mm f/5.6 after all). Well, this is where you now have to weigh up whether that extra reach and compact system size are of more benefit to you than the difference in aperture. Remember, though, the DoF may be equivalent to f/5.6, but it is still an f/2.8 in terms of its speed, as per our lesson earlier in this post.

Fuji Lenses

When I first dipped my toe into Mirrorless this time round it was Fuji that I looked at. Initially,  found the focal ranges quite odd at a glance, but again, if we apply the 1.5 crop factor that the Fuji X-Trans APS-C offers, the focal ranges actually equate to more 'traditional' ranges that we're likely to find with a DSLR. For example:

Fuji has the 23mm f/1.4 XF lens. Whoever heard of a 23mm lens? Why not 25mm? Well, if you do the same math as before, the 23mm f/1.4 works out to be a 34.5mm f/2.1 when mounted to a Fuji X-Series camera. It's near enough a 35mm f/2 equivalent.

Another example could be the 56mm f/1.2 XF Fujinon lens. Multiply that and you now have a fast and shallow portrait lens: 56 x 1.5 = 84mm. 1.2 x 1.5 = 1.8. So, let's agree that it is, in effect, an 85mm f/1.8 lens that would suitably replace the Canon 85mm I mentioned earlier.

The difference between 84mm and 85mm is so negligible that it hardly matters really.

Additional Reading and Factors affecting Depth of Field

I mentioned a few times at the start of this post: this post is written in an attempt to keep things as simple as possible. However, one thing that can't be left out is the factor of camera-to-subject-distance and the effect that has on Depth Field and Background separation. But, rather than me explaining this I would really encourage you to go and pick up Martin Bailey's book: 'Sharp Shooter'. In this, Martin discusses focal distances and how to calculate a depth of field. His explanation and illustrations are far better than I can explain!

Has this helped? Anything to Contribute? Let me know what you think

I hope that this post has helped to explain in some detail exactly how Aperture and Depth of Field are affected by sensor size. If you were unfamiliar with the lenses on offer from any of the camera manufacturers heavily invested in the mirrorless market then I also hope that this has helped to clarify the use of their focal ranges too. If this post has been helpful in any way at all please do remember to like, share and tell your friends. Oh, and please do leave a comment below. Feedback and contributions are always welcome!

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