Full Frame, APS-C, Micro Four Thirds, what does this all mean?

What is full frame and why is it a reference?

source: https://petapixel.com/color-photography/

In a nutshell - Full-frame was a popular film format, 36x24mm in size. That format carried through to today's professional digital cameras. Lens focal-length equivalence is often used because the same lens will give different compositions on different cameras. Discussion among photographers involving lenses would bring up full-frame equivalent focal lengths. This way, photographers using any format could follow along.

Kodak introduced "135" film in 1934 as a standard film cartridge. This film records images at 36mm x 24mm. By the 1960s, it was the popular standard. This popularity continued till the advent of digital photography. It left us with the common idea of the "professional" format - Full Frame - using these dimensions. The next major competitor is APS-C - a similar rectangle at 2/3rds the side length. Micro Four Thirds is another format even smaller than APS-C. By the time the digital age was upon us, 35mm film/cameras/lenses had dominated the industry for decades. Cost and technical difficulty propelled smaller digital sensors into lower-tier consumer markets, with the expensive "Full Frame" sensors directed to professional or higher-end consumers.

A given lens will create an image when focusing on a medium. The image can be recorded on photographic film or a digital sensor. If you're curious, you can hold a lens near a window, and hold blank paper behind it. Move the paper close to the lens until the image is in focus. You should see an inverted circular image of what's outside the window.

Holding my Nikon 50mm and trying to focus it on this pad while taking the picture proved quite clumsy.

A full-frame camera would render a picture as if drawing a 36mm x 24mm rectangle in that image. An APS-C sensor is about 24x16, and would be that smaller rectangle. A M43 sensor would be a 17x13 rectangle in that image.

FOV Equivalence:

Right away the idea of smaller sensors being crops of the full-frame image should come to mind. It's a tighter field of view - so how can you get a similar picture? The main idea of equivalence should be about framing your shot. 

Let's assume that you liked 50mm lenses on full-frame cameras. If you picked up a micro four-thirds camera, what lens would give you a similar feel? Well because of the smaller sensor, you will need a wider lens. 25mm is quite close. If you were looking for a lens on APS-C cameras, you'd need 35mm.

This is where the idea of crop-factor comes into play. It is the ratio of the sensor's diagonal, comparing the full-frame sensor to the cropped sensor of APS*/M43 and other sensors.

APS-C has a crop-factor of 1.5 (in general - Canon does 1.6). Micro four-thirds has a crop factor of 2. These numbers can be used to quickly figure out equivalent focal lengths on their respective systems.

Most smartphone cameras have a crop-factor of 5 or 6.

Depth Of Field Equivalence:

When your camera is focused on a point, some objects in front of and behind that point are also in focus. This area of sharpness is the "depth of field". 

If I composed a shot on the full-frame A7II, then with the same lens I'd need to recompose on the smaller sensor Panasonic G9. I'd have to step back because the lens has a narrower field of view on that smaller sensor. If you step back, you increase the depth of field.

If instead I switch to a wide lens to compose the image in the same place? Wider lens, increased depth of field. The only way to frame it and get the blur right would be to use a wider lens and faster aperture.

If I wanted to blur the background of an image, and I shot 50mm F2.8 on the full-frame camera, I'd need to shoot 25mm F1.4 on the Micro four-thirds for the same shot. If I shoot 50mm F1.4 on the full frame, I wouldn't be able to copy the depth of field at that composition, because I'd require a 25mm F0.7 lens for micro four-thirds.

This will matter more to photographers using fast zooms, for which there are few equivalent options in crop sensor formats. A 2.8 zoom can still give great subject isolation and background blur on a full-frame camera. The equivalent would be an F2.0 zoom on APS-C or an F1.4 zoom on micro four-thirds. Lenses with those f-stops don't really exist (OK.. Sigma's F1.8 zoom and Panasonic's F1.7!) but some primes can deliver enough blur - or just shoot differently. Nobody says you must take every type of shot, with every camera.

Depth of field equivalence is usually not as important as some make it appear:

When taking pictures it's helpful to know the field of view. Are you shooting wide, normal, or telephoto. Are you trying to compress the image and pick your backgrounds or capture as much of the sky as possible?

I switch between full-frame and micro four-thirds, and rarely think about equivalence for depth of field. Macro excluded - that's always a special case here 😛.

For group photos taken indoors, I will likely need a wide field of view. I may use 14mm for micro four-thirds. I might use 28mm on my full-frame.

I'll probably shoot both kit zooms wide open at F4, and not care about depth of field, because the kit lenses are slow enough not to worry.

I love the shallow depth of field I can get with the A7II and a 50mm F1.4, but I'm also satisfied with what I can get (at least most of the time) with my micro four-thirds Panasonic G9 and some manual glass. I really love to melt messy backgrounds, and some speed-boosted primes are enough to do that.

Relatively cheap speed...

Canon 55mm F1.2 S.S.C. FD remounted to EF on Metabones focal reducer, 39mm F0.86.

Nikon 35mm F1.4 AI-s, adapted to Canon EF, on Pixco focal reducer, 25mm F1.0.

FOV Equivalence Table:

	FF	APS-C	M43
sensor width	36	23.60	17.30
sensor height	24	15.70	13.00
x/FF	1.00	0.43	0.26

	Full Frame		APS-C		M43
Focal Length	FOV horizontal	FOV vertical	FOV horizontal	FOV vertical	FOV horizontal	FOV vertical
12	112.62	90.00	89.04	66.38	71.57	56.89
14	104.25	81.20	80.25	58.56	63.42	49.81
16	96.73	73.74	72.82	52.27	56.79	44.22
20	83.97	61.93	61.08	42.86	46.78	36.01
24	73.74	53.13	52.36	36.22	39.64	30.31
35	54.43	37.85	37.26	25.28	27.76	21.04
45	43.60	29.86	29.39	19.79	21.76	16.44
50	39.60	26.99	26.56	17.85	19.63	14.81
85	23.91	16.07	15.81	10.55	11.62	8.75
135	15.19	10.16	9.99	6.66	7.33	5.51
200	10.29	6.87	6.75	4.50	4.95	3.72
300	6.87	4.58	4.50	3.00	3.30	2.48
400	5.15	3.44	3.38	2.25	2.48	1.86
600	3.44	2.29	2.25	1.50	1.65	1.24
800	2.58	1.72	1.69	1.12	1.24	0.93
1000	2.06	1.38	1.35	0.90	0.99	0.74
1250	1.65	1.10	1.08	0.72	0.79	0.60

How to calculate the Field of view? Trigonometry!

e.g.

Light...:

A 50mm F1.4 lens is a 50mm F1.4 lens no matter what sensor is behind it. The concentration of light is based on the F-stop. If the concentration is constant, then the larger sensor will gather proportionally more light information - and hopefully deliver less noise.

Let's consider a 25mm F1.4 on a micro four-thirds sensor, and a 50mm F2.8 on a full-frame sensor. The 25mm F1.4 makes a smaller image circle that's 4 times brighter than the 50mm F2.8. The full frame sensor gets less light per unit area, but it's four times the area. When looking at light, it's equivalent.

To get the same exposure, you'll need to bump ISO up to four times that of the micro four-thirds camera. 

The question then is "Can it handle four times the ISO?"

That depends!😛... but it normally doesn't matter. Why would I restrict myself to 2.8 when I can get a cheap 1.8? If I buy a 300mm lens on micro four-thirds, I'll need a 600mm on full frame - that's for FOV. If I don't, my subject would be a fraction of the frame. Common lenses for either system though would probably have an aperture of F5.6 or so. I wouldn't need to step down to F11 simply for an equivalence argument.

The question now becomes "How much higher can I push ISO?"

The answer isn't so simple as "4". It depends on several things- the pixel density of the sensor. The technology used. Circuitry...

Take a look at dpreview's comparison tool. The A7iii I've added on here has the same sensor size as the A7Rii. The higher noise on the A7Rii is likely a result of much higher pixel density. Swapping one of these to show how an older micro four-thirds sensor compares to a new one, it's obvious that other than size, technological developments also affect noise. The old E-M10 and slightly newer GM1 have the same sensor size as the OM1, but are far noisier. 

If you're looking at different sensor sizes and considering the idea of "light equivalence" then consider why. If it's for ISO sensitivity, then yes, a larger sensor helps, but it's only a part of the equation.

My shooting habits:

I've recently acquired an A7R.2 (full-frame) and 35mm F1.8 as my main walkabout camera. My Olympus E-M1.3 has been relegated to wildlife and macro. Compared to using the Olympus micro four-thirds with a 17mm F1.8, the 35mm F1.8 is used similarly. I do note the depth of field difference though. I find myself stopping down on the full-frame 35mm F1.8 for more depth when shooting multiple people or if I want to get more of the background. I also note the much better noise handling and the RAW files are easier to manipulate. An Older full-frame camera like this can certainly be worth it, but it's at the expense of newer autofocus, movie capture, touch-screens, buttons for control, more capable image stabilization etc.

But I take a look at a tough shot and work on the RAW file, and the results simply say "worth it".