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  • Tathagata Mukherjee

Sensor Uncensored

Are megapixel images are going to eat your computer for lunch?


One question often pops up in our mind does sensor size matter ? If yes, then how to achieve a great depth-of-field and a bokehlicious effect. Well, in that case we need to understand the factors that create the effect.


Let’s clear up a few things before we dive in.


“Bokeh” is an English word that is a translation of the Japanese word “暈け” or “ボケ” that means: blur—specifically out-of-focus blur. So, why not just use the already established word “blur?” Because the simple English word “blur” can be applied to motion blur as well. Also, the word bokeh also encompasses the Japanese word “ボケ味“ meaning “blur quality.” So, bokeh is more than the blur, it is a word used to describe the aesthetic quality of blur.

The word is a noun. It is only capitalized when beginning a sentence. “Bokeh” is not a proper noun.

How do you pronounce it? Most people say that it is pronounced “bow” (like a bow tie) and “keh” (like the “ke” in Kelvin) with equal emphasis on each syllable.


The other thing feeding the bokeh obsession is the fact that the human eye, due to its excellent depth of field, does a poor job creating the kind of bokeh many viewers like to see in photographs or motion pictures.


How does it work ?


In general the bigger the sensor the more light it captures, more (fine) detail and less noise. So you can say that MF should be the best and M4/3 the worst.

One of the great advantages of using a full frame DSLR or even larger formats such such as 645 or even 4×5 view cameras is that there is a shallow depth of field and extreme background blur component that can’t really be matched on smaller formats such as APS-C or Micro 4/3.


The only real option on Micro 4/3 is the Voigtlander 17mm f/0.95.  This gives a field of view and depth of field equivalent to 35mm f/1.9 on full frame.  For APS-C, you’re stuck using full frame 24mm f/1.4 lenses, which yield equivalent FOV and depth of field similar to a 35mm f/2.1.  Close, but not quite what these larger formats can do.

FULL FRAME EQUIVALENCE

With smaller than 35mm sized sensors, you will often hear talk of a camera’s or lens’ “full frame equivalent” focal length or aperture. This can often be a source of great confusion among new shooters, and it can also be a point of disturbingly odd derision for other people, especially with regards to ‘aperture equivalence’.


What does “Full Frame Equivalent” mean?

First off, what does it mean when someone talks about a sensor or lens in terms of 35mm or ‘full frame’ equivalence? Well, quite simply, it is a way to compare angle of view, and more recently, the look you’ll get with respect to depth of field, between a full frame sensor and a ‘crop’ sensor. A few important terms to know:


  • Full Frame: A full frame camera has a sensor that is the same physical size as that of a frame of 35mm film. That is, 36mm wide x 24mm high.

  • Crop Sensor: A crop sensor camera, like on many DSLRs (which use the APS-C size) or the Four Thirds sensor (used in Four Thirds DSLRs and Micro 4/3 mirrorless cameras), is simply a sensor that is smaller in physical size than a full frame sensor.

  • APS-C: APS-C stands for “Advanced Photo System – Classic” (a reference to APS film), and means a sensor (or camera with that sensor) with a physical size between 22.2mm x 14.8mm and 23.6mm x 15.7mm.

  • Four-Thirds or 4/3: The sensor size used in Olympus and Panasonic DSLRs and mirrorless cameras (those of the 4/3 or Micro 4/3 system). The standard sensor size is in a 4:3 aspect ratio and is 17.3mm x 13mm.

  • Field of View or Angle of View: The angle of view that the sensor will record for a specific camera and lens combination. For instance, a 24mm lens on Full Frame has a diagonal angle of view of 84°.

One term that has been around since DSLRs made their entrance is ‘crop factor,’ which details the difference in focal length required for the same field of view between a smaller sensor camera and a full frame camera. This term was coined to help visualize that when you use lenses designed for full frame cameras on a crop sensor camera, the field of view is narrower…like cropping the photo in camera.


The first DSLRs used APS-C sized sensors because making viable full frame digital sensors at the time was cost prohibitive and very difficult. As the industry progressed, the APS-C sized sensors became somewhat of a sweet spot for high image quality with lower production cost.


The Crop Factor for an APS-C sensor is either 1.6x (Canon) or 1.5x (most others). What this means is that if you use a 50mm lens on an APS-C camera, it will have the same Field of View as a 75mm lens (50 x 1.5) on a full frame camera. The crop factor on 4/3 sensors is 2x, so a 25mm lens on a 4/3 or Micro 4/3 camera will have the same field of view as a 50mm lens will on a full frame sensor.


You will often hear people say “well, you have a crop sensor camera, so your 50mm lens becomes a 75mm lens on that camera.” This is WRONG. Focal length is a property of the LENS and the LENS ONLY, and it does not change in any way regardless of what camera you mount it on. What is true is that on an APS-C DSLR or CSC, a 50mm lens will have the same FIELD OF VIEW as a 75mm lens on a full frame camera.


The key point to remember about crop factor is that it is ONLY a reference point. That is, if you are used to shooting 35mm film or a full frame DSLR, using the crop factor will help you select a lens focal length that will give you the same look that you expect from your experience with a full frame lens. If you know what a 50mm lens looks like through your camera, you now know you need about a 33mm lens on APS-C or a 25mm lens on 4/3 to get the same field of view. That’s IT. It serves no other purpose.


Aperture ‘equivalence’:


This is a new one. In the past few years, people have also been using the crop factor to relate so called ‘aperture equivalence.’ That is, they’ll multiply the F-stop of a lens by the crop factor to get the ‘equivalent aperture’ of a lens. This has some basis in reality, but it is a pretty major fallacy, in my opinion, and it really skews people’s perceptions. I’ve gotten angry and rude comments on this blog about how I’m delusional about Micro 4/3 because of aperture equivalence. Interestingly enough, this term really only started to be thrown around when Micro 4/3 started getting popular.


A few things regarding aperture:

  1. The maximum aperture of a lens is the size of the light opening of a lens when the blades of the aperture diaphram (the blades that open and close to let more or less light in) are wide open. More specifically, it’s the effective size of the opening that determines the cone angle of the light rays entering the lens. Aperture size is generally given as a ratio of the effective aperture size to a lens focal length. This is called the f-stop. If a 50mm lens has a maximum aperture of 25mm, it would be an f/2 lens. (Focal length/aperture = 50/25 = 2). The reason there is a division sign is because f/2 means the aperture size is the Focal Length / 2. (in this case, 25mm.)

  2. The f-stop is one of the key components in exposure. The intensity of light hitting the film or sensor will be the same for the same f-stop, regardless of the focal length or actual physical aperture size. If you have a 200mm lens at f/2.8 and an 18mm lens at f/2.8, they both will have the exact same intensity of light hitting the sensor…the same number of photons per unit area. This, combined with ISO and shutter speed, helps determine how bright or dark your picture is. A larger number in f-stop means a smaller aperture (remember, it’s division: f/8 means the aperture is 1/8 the size of the focal length), which means less light hits the sensor.

  3. A ‘full stop’ means the exposure is doubled or halved. With aperture and f-stops, a difference of the square root of 2 is one full stop. (1.4 is a good approximation to use). So, f/1.4 to f/2 is one full stop, as is f/5.6 to f/8 (5.6 x 1.4 = 7.9, or approx. 8).

  4. Depth of Field: Depth of Field (DOF) is the depth of an image that appears to be in focus. Depth of field depends on three things: Focal Length, F-stop and Focus Distance (distance to your subject). These are all directly related. In fact, it is such that if you FRAME your subject the same way, all lenses will have the same depth of field for the same f-stop on the same format. To visualize this, consider a portrait where your subject is framed with the tops of the shoulders at the bottom of the frame, and the top of the head right at the top. If you frame your subject with a 50mm lens at f/2, then move BACK twice the distance you originally stood and use a 100mm lens at f/2, the depth of field, or amount that subject is in focus, will be the same.

  5. Background Blur: The amount that the area behind your subject is blurred. Using really wide-aperture enses, like f/1.4, can yield very blurry backgrounds, while your subject remains sharp. While background blur is related to depth of field, they are NOT the same thing. Background blur is dependent on the same three things as depth of field, but in a different way. While depth of field relations depend on focus distance, f-stop and focal length, background blur can be simplified from that: It is wholly dependent of focus distance and physical aperture size. Let’s look at that example from the previous bullet point. While a 50mm shot at f/2 and a 100mm shot at f/2 that’s taken from double the distance as the 50mm shot will have the same depth of field, the 100mm shot will have a BLURRIER background.

So, after all that drivel, what’s this aperture equivalence speak? Well, it refers entirely to the comparison of depth of field for a given sensor/lens combination. That is…you can multiply the f-stop by the crop factor to determine the aperture on a full frame camera that will give you the same depth of field.

So, if I shoot a portrait with a Micro 4/3 camera and a 25mm f/1.4 lens, and I shoot it at f/1.4. The field of view, depth of field and amount of background blur will be the same as if I’d shot the image at the same spot with a full frame camera, and a 50mm lens at f/2.8. Since Micro 4/3 has a crop factor of 2: 25mm x 2 = 50mm lens for the same field of view, and f/(1.4 x 2)= f/2.8 for the same depth of field. Similarly, 25mm/1.4 = 17.9mm aperture size and 50mm/2.8 = 17.9mm aperture size (so same amount of background blur at the same focus distance).

This means that, all things being equal, a smaller format will generally have DEEPER depth of field and less background blur than a larger format. This makes sense because smaller format cameras use shorter focal lengths for the same field of view, and therefore similar f-stops mean a smaller physical aperture size: less blur.


Full Frame is not a Magical Reference Format

Why Full Frame Equivalency Doesn’t Matter

The biggest issue I have with all the equivalency talk is that it treats the 35mm size as some sort of magical reference format where lenses act their ‘true’ self. This is flat out not the case, and it comes mainly from the introduction of the ‘crop factor’ to help out 35mm photographers who transitioned to digital when APS-C was pretty much all there was. Before 35mm became popular, most photographers shot with a larger format. 35mm was the ‘mini’ format of its day. Ansel Adams predominantly used 8×10 and 4×5 view cameras, with a film size many, many times that of 35mm. In fact, if people used ‘crop factors’ back in that day, they would consider 4×5 to be a 2x crop factor of 8×10 and 35mm to have a crop factor of 7.5x!

While there certainly was some snobbery about the size of 35mm film vs a large format or medium format film, the size differences there are many times what we are talking about between full frame and smaller format interchangeable lens cameras of today. The ‘normal’ lens on 645 medium format camera is 75mm. On 35mm it’s 50mm, on APS-C it’s 30-35mm, and on 4/3 it’s 25mm.


Full Frame Advantages

Now, before I tell you why worrying about ‘equivalence’ is irrelevant, you need to first understand that I am not saying that full frame cameras don’t have some very real advantages over smaller format cameras. They do, but only ONE of them relates to aperture. Full frame cameras have the following advantages over smaller format DSLRs and Compact System Cameras (CSCs):

  1. Lower Noise. Full frame cameras of similar sensor technology to their smaller counterparts will yield lower noise images at its baseline, and continuing throughout the range. Generally, it’s about half to one stop advantage over APS-C and about 1-2/3 stop advantage over 4/3.

  2. Richer tonality. Full frame cameras generally have higher color bit depth and therefore a little richer tonality and colors. This can give them a somewhat intangible look that gives that little extra something to an image.

  3. The ability for shallower depth of field and extremely blurred backgrounds. While you can certainly achieve plenty of background separation and shallow depth of field with APS-C and 4/3 sized sensors, you aren’t going to match the ability of a full frame camera with a hyper fast lens like an 85mm f/1.2 or a 300mm f/2.8 to obliterate the background.

And, that’s pretty much it. It used to be that you could add Dynamic Range (the ability to capture a wider range of shadows and highlights in a single image) to the list above, but in recent times, this isn’t the case. While a full frame sensor of the same sensor technology will still generally have a little better dynamic range, this has much more to do with the individual sensor than the size any more. For instance, the Olympus OM-D E-M5 has better dynamic range than ANY Canon DSLR ever made .


The Fallacy of Aperture Equivalence

While crop factor has a use simply to compare focal lengths between formats and such, the constant comparison of a smaller format lens to its full frame ‘equivalent’ aperture is largely unevenly applied and misunderstood. It’s often used to show that a smaller format is inferior or not capable of the same things as a larger format.

Here’s the better one. If you have chosen a smaller sensor camera, and are an advanced enough photographer to have mastered the relationship between aperture and focal length and such, and own several lenses and such, then you’ve made that choice understanding the tradeoffs, and ‘equivalence’ talk is useless. It’d be the same if every full frame photographer heard that their 24mm f/1.4 was really ‘only’ equivalent to a 180mm f/11 on an 8×10 view camera. Yes, that would be the ‘equivalent’ lens in focal length and aperture for the same depth of field, but who cares? It’s irrelevant to the discussion on the camera the person is actually using. Just like a 35mm format photographer has made the tradeoffs from large or medium format for a more responsive, smaller kit, so too have APS-C and 4/3 format photographers made the choice to forgo a little noise performance and some ability to shoot at ultra shallow depth of field in exchange for smaller size or more affordability.

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© 2019 Tathagata Mukherjee

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