Okay, much like the “effective focal length” discussions, there seem to be many “effective aperture” discussions going around as many of these mirrorless interchangeable lens compact system cameras begin to really round out their lens offerings. Well, I guess the micro 4/3 system has while the Samsung NX is getting there and Sony NEX system users still have a small handful of options, but not really here nor there. The fact that so many lenses can be adapted across various formats can complicate the discussion. Along with those who shoot with APS-C DSLR cameras, all references for focal length and depth of field seem to fall back to a full frame/35mm sized sensor as the benchmark. There are some misconceptions as to how the aperture, or focal length affects the exposure or depth of field in these varying formats.
C’mon in, we’ll straighten this all out.
So, here is the simple breakdown:
- Firstly, focal length is focal length. 14mm is 14mm, 50mm is 50mm. A lens’ focal length is a physical measurement, it doesn’t matter what the sensor or film measurements are that it will focus light onto.
- Next, aperture is aperture. f/1.2 is f/1.2, f/3.5 is f/3.5, etc. Much like focal length, aperture is a physical measurement, it is a ratio (1:2.8 for example) relative to the focal length and, regardless of focal length, will allow the same amount of light through a lens (any lens) at that aperture setting regarding exposure. For all intents and purposes, and to not complicate this with minutiae, if you can achieve a shutter speed of 1/100 second at ISO 100 when shooting at f/2.8 in any given condition that meters this as the proper exposure, it doesn’t matter the focal length or sensor size, any focal length on any camera at that aperture will allow for the same exposure settings.
- Crop factor affects the angle of view as it relates to another format, not the actual focal length. (XXmm in “full frame” terms, etc) We’ll call it the Equivalent Angle of View (EAOV). Also referred to as Equivalent Field of View (EFOV).
- Crop factor affects the relative depth of field at any given aperture compared to a different format (ie: full frame provides shallower DOF at f/1.4 than it does on a micro4/3 camera, etc) only when we take into consideration “framing” to match the relative framing of a different format at the same focal length (smaller the sensor, deeper the depth of field at a given focal length when framed as it would be with how a focal length “looks” on a larger format necessitating more distance between subject and camera to achieve the same framing.
Much of the conversation, or debate, revolves around the perceived ability, or inability to achieve shallow depth of field enabling a separation of subject and background elements (see bokeh, etc), or conversely enabling a deeper depth of field at larger apertures to enable a greater ability to capture a full scene in focus with faster shutter speeds for say, handholding (see street shooting, landscape, etc).
- DOF Example:
In relative terms, to equal DOF and relative framing, you need to open up one stop for APS-C or two stops for Micro 4/3 comparatively speaking in relation to full frame, (you have the same DOF with a full frame @ f/4 as you would with an APS-C @ f/2.8 or a m4/3 @ f/2 when the focal length is fixed and the angle of view/framing is equaled by way of adjusting the distance between the subject and sensor plane). Example: a 50mm lens at f/4 on a full frame shot at 5 feet will have the same DOF (and relative framing) as a 50mm lens at f/2.8, on an APS-C sensor shot at 7.5 feet, or a 50mm lens at f/2, on a micro 4/3 sensor shot at 10 feet. If adjusted to these settings, you’d have three very similar looking shots regarding subject framing and DOF. Keep in mind that the angle of view is still tighter when shooting on a smaller format which will affect the actual angle. By “relative” framing, in this case, we’re discussing the desire to have your subject occupy the same amount of space in the frame.
*You can check to see what the DOF, CoC and acceptable focus would be using online DOF calculators, like THIS ONE. Input your camera, focal length, distance to subject and it will let you know what you’re working with.
From a fixed location, using a singular focal length, regardless of the size of the sensor, if all is equal, so too will the depth of field be, or area in acceptable focus. If shooting on a smaller format (sensor) you will gain the crop factor as well, which if using the focal length example above, a 50mm f/2 lens will operate with a field of view like a 100mm lens while on a 2x crop micro 4/3 format camera, with the depth of field of a 50mm f/2 lens on a full frame camera from that fixed location.
Digital crop factor, in this way, is not going to exhibit the same image if you were to merely crop a full frame image in post as the physical measurements will actually alter the depth of field, altering the appearance of the image, depending on the DOF, all other elements being equal. However, if you were shooting so that everything was in focus for instance (think a distant landscape shot), and cropped into a full frame image file to replicate the digital crop factor, the two images would appear the same (barring resolution disputes, etc) as if you would have shot it at the same focal length, but recorded on a smaller format.
Now, you’ll notice that I’ve used the term “relative framing” quite often above. I say this, because the angle of view, when altered by way of crop factor when used on different sensor sizes, cannot be directly replicated when using the same focal length. For example, you’ll have a different amount of the scene in view in the background depending on the originating angle of view, but can relatively frame a subject to match at the focal point. You can (assuming you have enough physical room) accomplish this at the focal point regardless of how far away that focal point is, but you’ll always have a difference in the angle itself. To achieve the same angle of view from a fixed location between a full frame and micro 4/3 sensor, you would need to either double the focal length on the full frame, or halve the focal length on the micro 4/3 camera. At this point, because the physical focal length is the variable, it will affect the DOF by increasing it for the smaller format, and decreasing it in relation by way of the larger format.
- Exposure Example:
Much like a 50mm lens being a 50mm lens regardless of the size sensor it focuses light onto, an f/1.8 lens is an f/1.8 lens as well, period. It doesn’t matter what size sensor it projects an image onto, a lens’ physical attributes are unchanged and the focal length is a physical measurement as is the aperture in relation to that physical measurement by way of a mathematical ratio. That being said, there’s not really any need to understand the math involved to grasp that the aperture of a lens can be a powerful, creative tool as well as a huge aid in effectively capturing certain images.
Simply put, there are two major factors that the aperture affects, firstly, the light allowed through the lens enabling a faster shutter speed and secondly, the depth of field, or area parallel to the sensor that is in focus. Very simply, the larger the aperture, the more light allowed through the lens, the faster the shutter speed and the shallower the depth of field. (click here for a more in depth article on lens aperture) We hit on the DOF above, now here’s the exposure side of it.
Let’s look at the light transmission, or rather dispel any misconception that a lens with a large maximum aperture works any differently on any given format regarding light. The light let through a lens, any lens, at a stated aperture will allow for the same shutter speed if all other exposure elements are equal. This is important for many shooters who don’t want to use flash, or bump their ISO too far when shooting in less than good light. For instance, a 14mm lens at f/2.8 will allow for the same light and shutter speed as a 200mm lens at f/2.8 (of course the subject, and field of view may change how the light interacts with the scene (potentially affecting the metering), like getting the sun, or bright light included in frame for instance, but let’s not get hung up on that for now). Also, a 14mm lens set to f/2.8 on an APS-C, or micro 4/3 camera will allow the same light, and relative exposure as that 14mm f/2.8 lens would on a full frame (or any other) camera. Exposure differences on digital cameras can be equated with incorrect listings on exposure variables on different cameras or formats, wherein the amplification is incongruous with the listings via other formats (i.e.: one company’s ISO200 isn’t the same amplification as another’s, etc) or may be a variable in the processing. For all intents and purposes, you should be able to essentially use one camera to meter for another camera (I use a 5D2 to meter for my meter less medium format Hasselblad 500cm film camera all the time) as exposure is exposure and if there are sample differences, it is down to the translation by a particular camera and its processor, of the physical makeup of a lens, not the numbers.
For example, if you were to shoot a uniformly lit scene with a micro 4/3 camera and a 50mm f/1.4 lens wide open, shutter speed manually set to 1/100 second at ISO 200, it will show the same exposure as that same lens, opened to f/1.4, shutter speed set to 1/100 at ISO 200 shot on a full frame camera. On the micro 4/3 camera, the angle of view will be cropped to the same angle of view as a 100mm lens would appear on a full frame camera, but assuming that the light hasn’t changed, the two images will show the same basic tonal and luminosity values for any congruous subjects in the images. There may be firmware or processing variables, jpeg compression, etc, that could affect the way the final image is rendered, but the exposure will be the same. If you have your metering set up differently, and attempt to adjust the exposure manually based on what your light meter shows, you may come up with differing exposure settings based on how, and what your camera may be metering, but this is beside the point. Exposure is exposure. You may see slight variances from one format to the next, or even two cameras with the same sized sensor, but this is more down to differences in how sensors, or pixels are able to gather light, S/N ratio, or other factors in how any individual electronic device translates analog data, through a lens, onto a sensor, and into a digital file. For all intents and purposes, they should register and expose light near identically (numerically speaking).