Depth of Field on a m4/3, the reality.


So, you have bought a m4/3 camera, perhaps, the Olympus OMD E-M5, or the Panasonic GX-1. You also bought a few primes (your secret love to shoot with). You got a Olympus12mm f/2, Panasonic/Leica 25mm f/1.4 Summilux, Olympus 45mm f/1.8 as your available light kit. You are used to shooting an APS-C DSLR, you love shooting at the largest f/stop (old school meaning the widest opening, NOT the largest f/number). And you love that shallow Depth-Of-Field (DOF hereafter).

I am going to back-up a bit before I reveal how to get shallow DOF using a m4/3 camera. And for clarity, the m4/3 sensor is about 1/3 smaller than a APC-S sensor. I am not a math major, so I said “About”. Because the sensor is smaller, several other things come into play with the lenses made for m4/3.  One of the main things that determines the perceived depth of field is the size of the point of focused light that forms the image on the sensor. As the sensor gets smaller, this point of light must get smaller also, to render detail equally as sharp as compared to larger sensors. This point of light is called “Circle-Of-Confusion”.

From Jo Plumridge:

Contrary to popular belief, depth of field in a photograph does not change abruptly from being pin sharp to soft. Instead, the softness occurs as a gradual transition. In actuality, objects both in front of and behind the focusing distance immediately begin to lose focus, even if this is not perceivable to the human eye. Because of this, a more exact term is used — “circle of confusion” — to describe how much a point needs to blurred before it is perceived as being soft and unsharp. When the circle of confusion becomes obvious to our eyes, that region is deemed to be outside the depth of field.

From: wjbaird/Warren  (Black words are inserted by me to make it a curent example)

What is the “Circle of Confusion”?

The circle of confusion allows you to mathematically define “appears in focus”. If you pick any single point in the real world, it’s image on the film will be a circle, and that circle will get larger and larger as the point gets out of focus. The ‘circle of confusion’ is the biggest that circle can become before you will no longer say that the image “appears in focus”. Since the sensor for a (m4/3) digital camera is smaller, the circle of confusion for a m4/3 digital camera must be smaller, (compared to an APS-C sensor).

Suggested Circle of Confusion for

  • Full Frame 35mm = .030mm
  • APS-C =………………. .020mm  (.030 / 1.5 = .020) The 1.5 = the Crop Factor to find the CoC for that size sensor)
  • m4/3 = ……………….. .015mm (.030 / 2 = .015) The 2 = the Crop Factor to find the CoC for that size sensor)

So, how does the CoC effect DOF? Well, as the CoC gets smaller because the lens covers a smaller area with the same angle of view for smaller sensors, the focal length must be shorter also to cover the same angle of view of larger sensors. OK, look, a 50mm lenses covers normal angle of view which is figured out by measuring the diagonal of  the sensor.  So, for a Full Frame 35mm camera [24×36], that will be around 50mm lens. For a APS-C sensor [16×22], that will be around 33mm, for a m4/3 sensor [14×18],that will be around 25mm lens.

So, these lenses [50mm (FF), 35mm (APS-C), 25mm (m4/3) all provide about the same angle of view for the sensor size they where made for. And all these lenses have a different size CoC to go with the sensor size they are to be used on.

Because the CoC is smaller with smaller sensors, the perceived  DOF is deeper, or larger. How much larger, would be the next logical question. That answer lies in the factor or difference in the size of the sensors compared to each other. The Full Frame 35mm being our standard as a starting point. So, if we look at this as ratios, then:

  • FF = 1:1
  • APS-C = 1:5 (sensor is about 1/2 the size of a FF) (1:5 because the normal lens is 33mm,  1.5 x 33 = 50mm (about)
  • m4/3 = 1:2 (sensor is about 1/4 the size of a FF) (1:2  because the normal lens is 25mm,  2 x 25mm = 50mm)

This way will help us figure it out with f/stops comparisons across sensor sizes. We use the same factor to figure DOF across sensors. Many people get very confused at this point. Here is why. They confuse light gathering ability with DOF changes, and interchange the two at will.

Simple Difference:

F/stop is the AMOUNT of light reaching the sensor, and does not change across sensor sizes. That is: f/2 on a FF camera, lets in the same amount light as f/2 for a m4/3 sensor. WHY? the opening is much smaller on a m4/3 camera. Because the f/stop IS A FRACTION of the focal length it is used in. Uhhh ???

f/2 on a FF lens 50mm lens is an opening of 25mm… Where is the fraction to find that?  Here:  f = focal length, divided by the denominator = the opening in mm’s.  OK, another way: 50 (focal length) / 2 = 25,  So, f/2 on a 25mm lens = 12.5 mm’s. The opening is smaller, but lets the same amount of light as a 50mm lens at f/2. Now, I know all about the micro math argument that will make it more precise.  And, that a larger sensor can gather light faster because of the larger photo-sites. FOR THIS article, I do not, nor want that level of explanation. As far as REAL WORLD usage goes, this works just fine for my article.

DOF is the perceived parts of the photograph that look sharp with any particular f/stop. A we use larger f/stop, the perceived parts become less. That is, there is less in focus. DOF has nothing to do with light gathering ability. JUST, what is perceived as in focus in the photograph.

I am not denying they don’t work together, THEY DO… BUT, please keep them as separate components. You will stay less confused in the long run.

Because the CoC affects the DOF, and with a factor you can know use to see how the same f/stop with  difference sensors have different DOF’s. Here, I will give 3 examples of how much the DOF changes on different sensors, all focused at 10′ from the camera with a normal lens for that sensor size.

  • FF: 50mm @ f/5.6  8.31 feet to 12.6 feet
  • APS-C: 33mm @ f/5.6  7.71 feet to 14.2 feet (equals f/8 on a FF  camera, or ONE STOP smaller, it  is not exact, but within a 1/4 inch each way)
  • m4/3: 25mm @ f/5.6  7.9 feet to 17 feet (equals f/11 on a FF camera, or TWO STOPS smaller, it is not exact, but within a 1/4 inch each way)

You can see as the sensor gets smaller, and we adjust the lens to be a normal lens for each senor size, the DOF gets wider, just slightly closer to the camera, but, behind the subject, it grows deeper, faster. Remember, the camera to subject distance remained the same for all 3 examples.

This is where we get 2 things from. The “Crop Factor” we have all heard.  m4/3 has a 2x crop factor, m4/3 = 2 stops smaller. APS-C has a 1.5x crop factor, APS-C = 1 stop smaller.. So, the crop factor also give us a clue on what f/stop to use with the adjusted focal length for the sensor, at the same Camera to Subject distance [as above shows] to achieve the same DOF.

BUT, two other factors control DOF, other than Camera to Subject distance.

  1. Focal Length
  2. f/stop

Here is a video that can explain it better.


OK, so how do I control the DOF with my m4/3?

1) Pick a longer focal length. The longer focal length, at a shorter camera to subject distance with a larger f/stop (wide iris opening) will do the trick in many cases. The Olympus 45mm f/1.8 is a good choice. But, if $400. is too much, but a used Canon FD, Nikon Ai, Minolta MD 50mm f/1.4, manual focus 50mm f/1.4 with an adapter for under $150.00 or so.. it will give you a larger iris opening, that, in turn you a thinner DOF. (the 50mm FF lens on a m4/3 will give you a field of view of a 100mm lens. The longer the focal length is, the thinner the DOF at any fixed distance, comparatively

Special thanks to “Harmonica” for allowing me to post this demonstration photos.

P/Leica 25mm f/1.4 at f2

————————————————————————————————————-

Olympus 45mm f/1.8 at f/2

——————————————————————————————————————-

Vivitar 55mm at f/2

————————————————————————————————————————

Pentax FA 85mm f/1.4 at f/2

I should add at this point: the longer the Focal Length, while maintaining the same framing of the subject will produce a larger Blur of OOF areas. SO, if what even blurrier OOF areas, use a longer lens, and frame as would with a shorter lens. The longer the lens, at the same framing of the subject, the larger the CoC, and the Blurrier the OOF areas. As the above samples show.

2) Move Closer to your subject, and use a wider f/stop. With this technique, you can use a shorter focal length, if need be. and, f/4 or so.  For instance, with a 14/42 kit lens, that is not very fast for shallow DOF. But, you can still achieve it with this technique, to a point.

3) Use a larger f/stop (wider iris opening). you can combine this technique with the other 2 techniques to achieve shallow depth of field also

These are the 3  ways to  control DOF with any camera / sensor size. Some, may not be convenient, but, you have other options you can combine to get what you want. If, you know your gear, and what lenses and f/stops to use to achieve what you want. the limits are easily overcome. I am talking about cameras with m4/3 sensors and larger.

I hope this was not too hard to understand. And that it has helped give you some ideas of how you can achieve the DOF you want.

9 thoughts on “Depth of Field on a m4/3, the reality.”

  1. I’m sorry, but are completely sure about these numbers?

    Suggested Circle of Confusion for
    Full Frame 35mm = .030mm
    APS-C = .025mm
    m4/3 = .015mm

    If full frame is .030mm and m4/3 (crop 2) is .015mm, then APS-C (crop 1.5) should be .030/1.5 = .020, and not .025, right?

    1. I too was not sure on the APS-C CoC suggested size. These were taken from the DOF Calculators website. But, .020 makes more sense. Since there are suggested guides, they should be relative to the crop factor between themselves. Thanks.

  2. Thanks for a very helpful explanation. I have a much better understanding of DOF. I’ve never seen the “blackboard” tool you used in your explanation. Is it commonly available?

  3. Very interesting post. I’m math repellent, so most of it is Greek to me, but I’ve shot FF, APS-C and m4/3 and the DOF control differences are real. How willing one is to go the extra mile to get that thinner DOF is a personal choice. Personally I’ll always consider using the larger sensor, but m4/3 certainly can deliver shallow DOF under the circumstances you’ve mentioned. Just more of an effort. APS-C mirrorless is a pretty nice compromise, but obviously m4/3 has the more lens options, so all in all it’s pretty much a draw…

    1. Because m4/3 does take a little more effort, we are seeing very fast lenses being made. 25/.95, 17.5/.95. Knowing your camera/lens combo is important also.. a small P/S with a long zoom, can also achieve shallow DOF.

  4. You know what? I actually think the distance from the lens to the sensor (or distance nodal point to sensor?) might also influence the amount of bokeh achieved. Because the relative difference between in and out of focus becomes bigger when the lens-sensor distance is smaller… you may get more out of focus bokeh rendering? Am I wrong? I just can’t totally figure it out…

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s