I recently came across a discussion online about how a Speedbooster changes the field of view, depth of field and exposure. There was so much misinformation and conflicting statements about how they affect your lens or camera that the users who were giving correct advice were virtually drowned out. Therefore, I hope to dispel some of these mistruths by boiling down what a speedbooster actually does and how, before busting some common Speedbooster myths at the bottom.
Update 7/6/21: Accompanying video embedded; originally uploaded 13/2/21. It covers many of the same topics as below, along with visual demonstrations.
I've tried to explain things as clearly as possible, which has the effect of making this article a bit of a lengthy, wordy one. If you want the TL;DR, it's as follows: "Speedboosters make a lens wider and faster, but don't change the depth of field". If you want to know how or why, read on.
First thing's first: Speedbooster® is actually a registered trademark of Metabones, the company arguably responsible for popularising them almost ten years ago with the advent of the Sony E mount and the accompanying E mount Super 35 video cameras. A Speedbooster is just a brand name for a focal length reducer: that's what it's proper name is. Metabones didn't invent the focal length reducer, but in the same way we say 'Hoover' to refer to any vacuum cleaner, the moniker has become the defacto term for such a device in colloquial terminology. I'll continue to call it a Speedbooster - but bear in mind I'm referring to any focal length reducer from any brand (the original online discussion mentioned above was in reference to Canon's EF to RF focal length reducer).
Let's start by addressing one of the most commonly misunderstood aspects of using a Speedbooster: it fundamentally changes the lens' optical design. It's literally adding in extra elements. With this extra glass comes a change in the lens' focal length, but it does not change the lens' entrance pupil.*
(*Quick note: a lens' entrance pupil is not the same as its physical aperture opening, ie how wide the diameter of the iris blades open. It's a related but slightly different concept, though for the purposes of this explanation, assume they're the same and don't worry about it.)
The final note before we begin is reminding ourselves of what an f-number is. A lens' f-number is a basic fraction: focal length over entrance pupil, for instance a 50mm f/2 lens would have a 25mm entrance pupil (50/25=2). With that in mind, let's cover three aspects of a lens: field of view, aperture and depth of field.
Field of View
Let's say we add a Speedbooster to our 50mm f/2 lens. It has a magnification of 0.71x, therefore, we can multiply the focal length by this factor: 50*0.71=35.5. The lens is now a 35.5mm lens (I'll just say 35mm from now on for the sake of simplicity), but the entrance pupil has not changed. This bit is important to understand and is so often overlooked or misunderstood. For the rest of this article I'll stick to using this example lens - when I say 50mm, I mean the original lens without the Speedbooster and when I say 35mm, I mean the same lens but with the speedbooster attached. (Additionally, we're assuming the 50mm is a full frame lens and we're 'speedboosting' it onto a Super 35/APS-C format camera. Speedboosters for other formats are available.)
Because the crop factor of Super 35 versus full frame is ~1.5 (this varies slightly camera to camera), a 35mm lens on a S35 camera will have (approximately) the same field of view that the original 50mm lens does on a full frame camera. 35.5*1.5=53.25.
I've already explained above that the entrance pupil does not change. However, given that we've now reduced the focal length (from 50 to 35), the formula for calculating f-number has changed. The entrance pupil is still 25mm in diameter, so it now gives us 35/25=1.4. So a 50mm f/2 lens, speedboosted, becomes (not effectively, but actually) a 35mm f/1.4. That's how Speedboosters 'magically' create an extra stop of light. To clarify, our 35mm f/1.4 will give you an exposure one stop brighter than the original 50mm f/2, whether that 50mm is used on full frame or S35.
If it helps, a Speedbooster (a focal length reducer) is the exact opposite of a focal length extender (aka teleconverter), like the Canon 1.4x or 2x. All an extender does is increase the focal length, the byproduct of which is the f-number drops and you lose one or two stops of light. This is evident to anyone who's used one, it isn't a hypothetical or theoretical change, it's an actual, visible change in exposure.
One of the side effects of using a Speedbooster known to most users, is that it changes the projected image circle of the lens. A full frame lens speedboosted will reduce the image circle, enough to only cover S35. Again, compare to an extender. They're literally magnifiers: they 'zoom in' on the image by literally stretching the projected image over a larger area, so that using the same capture window (ie the sensor), you're sampling a tighter part of the image = narrower field of view. Therefore a speedbooster is just a demagnifier. It makes the projected image smaller.
With the above in mind, let's consider that a lens lets through a certain amount of light, limited by the aperture. Our 35mm f/1.4 lets through the same number of photons as the original 50mm f/2, albeit in a smaller image circle. All this is akin to is concentrating that same amount of light over a smaller area, increasing the brightness. Extenders project the same amount of light, but over a larger area, hence, a darker image. (I like to think of an analogy from Bilbo Baggins: “I feel thin, sort of stretched, like butter scraped over too much bread.”)
Depth of Field
Depth of field seems to be another one of those topics that's often confused. The only things that affect depth of field are focal length, aperture and distance to subject. Format size does not affect depth of field. Put it like this: the image has already been formed in the lens before it reaches the sensor. You can hold a lens close to a wall and it will project a discernible image. No matter how big the wall is, it doesn't change the depth of field!
All of the following assumes the same distance to subject, leaving only focal length and aperture affecting depth of field. We'll also assume the lenses are used at maximum aperture. Larger formats are associated with thinner depth of field simply because of the need to use longer focal length lenses for any desired field of view (compared to smaller formats). A speedbooster gives you the 'full frame look' or 'full frame field of view' simply because you're using a wider lens on a smaller format as demonstrated above: a 35mm on Super 35 is about the same field of view as 50mm on full frame.
Our speedboosted lens has now widened its field of view by decreasing the focal length. With shorter focal lengths comes deeper depth of field - but, we've also decreased the f-number, which makes the depth of field thinner. Both of these combined cancel each other out: a 35mm f/1.4 will have the same depth of field as a 50mm f/2.
Using larger formats does not automatically make depth of field thinner. The depth of field is only thinner when comparing the same field of view and the same aperture. Whilst our 50mm f/2 (on full frame) and 35mm f/1.4 (on Super 35) share the same field of view and the same depth of field, using a 50mm f/1.4 on full frame will yield a thinner depth of field.
Our original 50mm f/2, used on any format, will give you identical depth of field. By using smaller formats, all you do is sample a smaller image and therefore a narrower field of view. You can take a photo on a full frame camera, then crop it to the same perspective as a photo taken with a smaller format and the depth of field will be the same.
Myth 1: When using a full frame lens on a Super 35 camera, the image is darker. Using a Speedbooster gives you the same exposure as if you were using the lens on full frame.
False, a lens used on any format will yield the same exposure. Using a Speedbooster will give you an image brighter than when used on full frame.
Myth 2: When using a full frame lens on a Super 35 camera, the focal length is longer due to the crop factor. Using a Speedbooster changes the lens back to its original full frame focal length.
False, a lens' focal length does not change no matter what camera it is used on. A crop factor is simply a tool to compare the relative sizes between formats. A full frame 50mm lens does not become a 75 or 80mm lens when used on Super 35, it is still a 50mm. The crop factor simply illustrates that it would be necessary to use a 75 or 80mm lens on full frame to match the same field of view as the 50mm when used on Super 35. Using a Speedbooster reduces the lens' focal length, thereby widening the field of view on Super 35 to match what would be expected if the lens was used on full frame.
Myth 3: The depth of field gets thinner when using a Speedbooster.
False. Whilst the aperture does get faster which, in itself would reduce the depth of field, the lens also becomes wider in focal length, which increases the depth of field by the same factor. Assuming distance to subject remains the same, depth of field remains the same when using a Speedbooster. However, if you were to move closer to your subject to offset the wider field of view, depth of field would get thinner, but this is a result of reduced distance to subject and not the Speedbooster.
Myth 4: Using a Speedbooster improves the quality of the lens.
Sort of. By widening the field of view, you're making use of the whole image circle of a lens, thereby 'shrinking' any inherent optical flaws in the original lens. A lens may appear sharper or have less aberrations, but this is only because we're not looking through a smaller portion of the lens so closely. Equally, consider that a lens does not perform as well towards the edges of its image circle compared to the centre - we may start to see vignetting that we didn't before. A Speedbooster however has optics of its own which, no matter how well it's made, will inevitably introduce some flaws of its own. This probably negates any optical improvements it may offer; a speedboosted lens on Super 35 won't necessarily be better or worse, but any imperfections may be slightly different.
Myth 5: Using a Speedbooster is better for low light shooting.
Maybe. Making any lens one stop faster (with an actual, measurable and visible effect on exposure) will definitely help. Compared to using the same camera without a Speedbooster, it is definitely better for low light, but compared to a full frame camera without speedbooster, well that depends on the cameras being used! Full frame cameras generally perform a little better in low light than smaller format cameras. This essentially boils down to two reasons: with a larger sensor it is possible to either use larger pixels or fit in more pixels. Larger pixels means more photons being captured, therefore lower noise. More pixels means that, when presented at the same size (on a phone, computer or cinema screen) any grain or noise is comparatively smaller and therefore less noticeable. Even when the original higher resolution is downsampled to a lower resolution, any noise is also shrunk and is effectively suppressed. So, whether a speedboosted Super 35 camera is better for low light than a full frame camera, that's probably a close call and further testing would be necessary to determine the most effective system for low light.
Myth 6: I can Speedboost Super 35 lenses.
Technically, yes, but not if you want to use them on a Super 35 camera. The most popular use for Speedboosters is to use full frame lenses on Super 35 cameras. This compresses the larger image circle onto the smaller format. Speedboosting a Super 35 lens is totally possible, however trying to do so on a Super 35 camera would result in hard vignetting (called portholing) as the image circle is now too small to cover the sensor. You can make use of speedboosted Super 35 lenses on even smaller formats such as Micro Four Thirds however.
Myth 7: Using a Speedbooster turns my camera into full frame.
No - you can't change the size of the sensor. A Speedbooster changes the lens' focal length to make use of its full image circle by compressing it onto your smaller format sensor. It gives you a wider field of view when using the same lens and sensor.
Bonus misunderstanding: "I want more depth of field."
I've worked with a number of directors who, after reviewing a composition, request more depth of field. What does that really mean? A greater depth of what's in focus? I stop down, and the director corrects themselves - "No, no - I mean I want less depth of field." What they're really asking for is a more pronounced background blur. Describing depth of field in terms of 'more' or 'less' is often confusing. Depth of field should be described in unambiguous terms like shallow, narrow or deep!