The Bokeh Effect: How Sensor Size Affects Background Blur
Of all the things that photographers argue about in our secret monthly meetings, sensor size and its impact on our work is perhaps one of the most heated topics that can come up. From the true “bigger is better” snobs (“Sensors? Bah! 8×10 film is where it’s at!”) to the ones who prize portability above all (“Micro Four Thirds rules!”), the debate between advocates of MFT, APS-C, and full frame sensors often reaches religious fervor.
Contentious topics related to sensor size include resolution, high-ISO performance, and dynamic range but the quality and characteristic of bokeh, or out of focus backgrounds, is perhaps the most fiercely debated. While there’s no contest that the bigger sensors can clearly produce much smoother and, well, blurier (not a word, I know), it’s also an unfair statement that the smaller sensors like the ones in Olympus and Panasonic Micro Four Thirds cameras can’t produce good bokeh.
The Prerequisites for Understanding Sensor Sizes (Updated)
Now, before you get into this article, if you have questions about what crop sensors are, how they work, etc., you want to read a few of these articles:
- New DSLR Owners: What You Must Know About Full Frame vs Crop Frame Sensors Before Choosing a Lens
- Understanding Sensor Crop Factors
- Transitioning from Point-and-Shoot to DSLR: Understanding Full Frame vs Crop Frame Sensors
- Best Wide Angle for a Crop Sensor Camera
- Introduction to Full Frame vs Crop Frame Sensors Plus Great Sensor Comparison Resources
- Sensor Size and Filmmaking: Choosing the Right Camera for Your Project
- Aspect Ratios and Sensor Sizes for Video Beginners
- Metabones Canon EF to Sony NEX Speed Booster Adds Full Frame Versatility in the Crop Sensor Video World
- Intro to Depth of Field in Filmmaking
These articles will give you a good understanding of what crop sensors are, and what using a crop sensor camera implies, for the most part. In this article, we’re going to drill down to one specific thing. We will take a look at just how the size of your camera’s sensor affects the bokeh characteristics of your image. To do this, we devised a pretty simple test.
Test Process and Gear for Bokeh and Sensor Size
We took a resolution chart printed on standard matte paper and stuck it on a couch in our San Carlos office’s lobby. In the foreground are a couple of glass terrariums, and our focus point is on the top of one of them. The distance between the subject and the camera is about 3-4 feet, while the distance between the terrariums and the back of the couch is about 4-5 feet. Here’s the equipment list we used.
The list is fairly straightforward. We used three cameras:
- Canon 5D Mark III (full frame sensor)
- Fuji X-E2 (APS-C sensor with 1.5x crop factor)
- Olympus OM-D E-M1 (Micro Four Thirds, MFT, sensor with 2x crop factor)
Now, focal length and aperture are the other two factors (besides the sensor size) that determine how your bokeh is going to look. In our case, we wanted to have a consistent equivalent focal length through all three cameras. A 90mm lens on a 5D series camera isn’t a 90mm lens on an Olympus, so we basically need to make sure that our angle of view is more or less the same. With that in mind, here’s our lens selection.
All three lenses now have more or less an equivalent focal length of 90mm. There are some small differences in actual execution – the aspect ration of the MFT sensor, for example, is 4 x 3 instead of the 3 x 2 of the Canon and Fuji, but they were all shot from the same spot, fixed onto a tripod so that distance between camera, subject, and background is all the same.
The Test Using 3 Sensor Sizes
With that set, we proceeded to take pictures in Aperture Priority mode at every 1/3 f-stop. f/2.8 was the widest that all three cameras/lenses could go, so let’s take a look at that.
Right off the bat, we can see that f/2.8 apertures aren’t the same on every sensor. Shot in Aperture Priority, all three cameras exposed within 1 stop of each other as far as shutter speed goes – though the Fuji likes to overexpose by a stop, which it did here. But the resolution chart in the background rendered completely differently in all three shots.
Now let’s look at them at f/8. Once again, clockwise from top-left: Canon 5D Mark III, Fuji X-E2, Olympus OM-D E-M1.
Again, the three cameras render the res chart in the background very differently. Finally, let’s go all the way to the narrowest aperture all three lens/camera combinations can support: f/22.
The results are exactly as expected, though at f/22 they are more similar than at most other apertures.
So – what did we learn here? Well, we knew that the full frame camera would give us the most blurred and smooth-looking bokeh, and that’s pretty much what we see at all f-stops, from f/2.8 to f/11, to f/22 and everything in-between. We know that the MFT chip, at half the size, doesn’t do nearly as good a job of blurring the background as the full framer or the APS-C sensor.
That much we already knew, but it’s handy to see just what f/2.8 on a full frame camera compares to f/2.8 on an MFT camera in visual terms.
What’s f/5.6 on a Full Frame Equivalent to on an Olympus/Fuji/etc.?
Let’s take this one step further though. We know that f/2.8 isn’t the same in so far as depth of field goes. But what does f/2.8 on an MFT camera equate to on a full framer? How about f/4 on an APS-C?
Well, the first thing to note is that while you can calculate the conversion mathematically, the results aren’t going to be exact. That’s because there are too many variables here – sensor design, resolution, optics, in-body processing pipeline, and more. What we’re trying to do here is to get you as close as possible to some answers about the effect of sensor size on DoF.
The best resource we found for this comes to us via a tutorial on Cambridge in Color. There’s a nifty DoF calculator called “Depth of Field Equivalents” on that page, and we can see that f/2.8 on a full frame sensor is f/1.4 on a Micro Four Thirds sensor and f/1.8 on an APS-C.
Our lenses on the smaller cameras in this experiment don’t go that wide, so let’s start with the widest they can go – f/1.8 on the Olympus 45mm f/1.8 lens.
As we can see, that equates to f/3.6 on the full frame sensor camera, and according to the same tool, that equates to f/2.4 on the APS-C sensor camera.
With that in mind, let’s look at three images with those aperture settings.
Clockwise from top-left: Canon 5D Mark III at f/3.5 (the closes we could come to f/3.6), Fuji X-E2 at f/2.4, and Olympus OM-D E-M1 at f/1.8.
Not bad! The blurred resolution chart looks pretty close in all three image. That Olympus image, though, has a resolution chart that looks a bit… sharper than the rest.
After much experimentation, I happen to know that the Olympus has a tendency to underexpose by as much as two-thirds of a stop. The slightly underexposed image has a tendency to look a bit sharper than it actually is.
Let’s look at one more set. Here we have, clockwise from top-left: Canon 5D Mark III at f/5.6, Fuji X-E2 at f/3.6, and Olympus OM-D E-M1 at f/2.8.
I adjusted exposure on the Olympus file to make it look more like the rest in terms of brightness.
The Fuji file looks just a bit blurrier (again, I know that’s not a word) than the rest, which reinforces my point that the equivalents aren’t exact. But it’s pretty close, and you now have a good foundation to judge the impact of your camera’s sensor size on background blur.
Here’s how to do the math in your head – or in a calculator.
Going from MFT to full frame is easy. Divide the f-stop on the full frame by 2 to get the equivalent depth-of-field (DoF). So, f/8 on a full framer would be give you the approximate DoF of f/4 on an MFT camera.
For APS-C, you divide the full frame f-stop by 1.5 for Nikon and Fuji, and 1.6 for Canon. So, f/5.6 on a Canon 5D Mark III would be f/3.7 on a Fuji, or f/3.5 on a Canon 7D.
The key thing to take away from this experiment is this: If a shallow depth of field is your end-all, be-all, go for a full framer. That said, it’s not impossible to get a decent equivalent DoF on smaller sensor cameras. For example, f/1.8 on a Canon 5D Mark III would be f/0.95 on an MFT camera would be f/1.2 on a Fuji, and f/0.95 on an Olympus.
Pretty crazy numbers – until you realize that both Fuji and Olympus have glass that opens that wide. MFT users have f/0.95 lenses at 17.5mm, 25mm and 42.5mm from Voigtlander, while Fuji has the 56mm f/1.2 R lens for their cameras.
Yes, getting the equivalent full frame DoF of f/1.2 isn’t quite possible on the smaller cameras, which is why we here at BorrowLenses are huge fans of the “right tool for the right job” approach — and why we rent so many different camera and lens models.
I’ll leave you with this impromptu portrait I made of Michio with the Voigtlander 42.5mm f/0.95 at f/0.95. Notice how his ear is out of focus while his eyes are tack-sharp.