Focal length? Effective focal length? Actual focal length? Crop factor? Do you get more reach with a crop sensor? Is it equal to a longer focal length? There are some misconceptions surrounding what happens when a lens is used on cameras with different sensor sizes. Let’s have a look at the differences between a full frame, APS-C and micro 4/3 sensor.
We hear about crop factor, effective focal length or angle/field of view in 35mm or full frame terms. Simply put, a lens’ focal length is the same no matter what camera it is on, as the focal length is a physical measurement, but the angle and field of view that a particular focal length provides varies depending on the size sensor the lens is projecting the image onto.
Focal length is measured by the distance of the focal point from the lens, or the distance from the principal point to the focal point more precisely. The distance from a camera lens’ focal point to its capture medium be it film or a digital sensor, with the lens focus set to infinity determines it’s measured focal length.
The field (or angle) of view that it provides depends on the size of sensor that is capturing the light from the scene in front of said lens which is where we get our “crop factor” or commonly referred to as “effective focal length.”
For our first test, I will be using the same lens on three different cameras. First, a full frame Canon 5DII, next a Canon 40D (APS-C) and finally a micro 4/3 Panasonic GF1. Keeping each camera in the same exact position on a tripod I want to explore two things. Number one, the “crop factor,” how it crops into the image circle projected onto the sensor and the apparent magnification a cropped field of view provides, and number two, the difference in depth of field at the same aperture in these three different sensor formats.
In terms of a full frame or 35mm standard angle/field of view with any particular focal length, a full frame presenting a value of 1, a Canon APS-C sensor provides a crop factor of 1.6x and the micro 4/3’s sensors provide a crop factor of 2x. To determine the crop factor, simply divide the longer side of the full frame sensor (36mm) by the long side of whichever sensor you need to determine the crop factor for. For instance, based on the measurements for a Canon APS-C (22.2mm) it is 36 / 22.2 = 1.621… or more simply 1.6x. To start off with, here is the physical size of each sensor we’re working with in relation to each other.
Using my 35mm f/1.4 lens on the 3 cameras all following images are sized to fit into a 500 pixel wide file. As we are not exploring resolution or native file size on this one, we won’t be looking at 100% crops compared with each other or anything, so this should be pretty straight forward. Let’s begin.
First, we have our field of view. All shots are at f/5.6 taken with the 35mm f/1.4 Canon lens from the same exact location on a tripod roughly 6 feet away from the point of focus.
Full frame field of view 35mm:
APS-C field of view = 56mm equivalent:
And finally the micro 4/3 field of view = 70mm equivalent:
Pretty substantial difference in field of view. All of these show a 35mm focal length, but differ largely regarding field of view provided. The flowers in the center of the frame fill a lot more real estate the smaller the sensor. The magnification, based on focal length, from subject to focal plane, is unchanged, but the magnification of the subject created by the cropping of the image circle is increased.
Now, let’s look at depth of field. Same set up but adjusting the aperture of the lens to f/1.4 to emphasize a narrow DOF:
Full frame at f/1.4:
APS-C at f/1.4:
Micro 4/3 at f/1.4:
Now, I’d always assumed that the larger the sensor, the shallower the depth of field. This is true, but only when the subject is framed the same way which requires one to change the focal length to achieve the same angle of view from a fixed location, or alter the distance between the subject and camera which will change the angle of view, but can allow for your subject to occupy the same physical space in your frame while relatively “framing” your subject as it may appear on a different format. For instance, the above set of pictures were all taken from the same exact location roughly 6′ away from the flower in focus to demonstrate the field/angle of view on the different cameras. To determine the acceptable area in focus I used the online DOF calculator. (bookmark it, it helps out quite a bit) Inputting my aperture as set for the recent 3 pictures above at f/1.4 and the subject distance at 6′, according to the online DOF calculator, the different sensor sizes gave me this information:
Full frame – near limit(in focus): 5.65 ft far limit (in focus): 6.4 ft. total area in focus: .75 ft (9″)
APS-C – near limit: 5.77 ft far limit: 6.25 ft total area in focus: .47 ft (just under 6″)
Micro 4/3 – near limit: 5.82 ft far limit: 6.19 ft total area in focus: .37 ft (a hair over 4″)
The final question I want to look at regarding the cropping of a lens’ image circle is how that magnification affects distortion. If I’m using a 35mm lens on a 2x crop factor micro 4/3 camera, I have the same angle of view as I would with a 70mm lens on my full frame camera, but would I see the decrease in barrel distortion I would going from a 35mm lens to a 70mm lens on my full frame? Let’s see. For this comparison, I’m getting rid of the APS-C to simplify things. This is between the 5DII and the GF1.
Using my trusty Martha Stewart self healing cutting surface, which I mounted to the wall by way of some Scotch tape®, and the 24-70f/2.8, zooming the lens while on the GF1 to 35mm (70mm FOV equivalent) and the 5DII shot at both 70mm and at 35mm (and cropped the 35mm shot to the same frame as the GF1 @35mm/70mm fov), I wanted to see if the center of the lens exhibited the same characteristics as a 35mm focal length or a 70mm focal length for the GF1.
First, the 5DII @70mm:
the GF1 @ 35mm (70mm equivalent FOV):
and finally, the 5DII @35mm and roughly cropped to match the frame from the GF1:
My framing is slightly off, I know. What I can see though, is when the 5DII is set to 35mm and then the image is cropped, it behaves just like the GF1 does at 35mm effectively cropping to a 70mm field of view by way of the sensor showing the same performance and distortion control. At 70mm, the 5DII shows a slight pincushion distortion ” ) ( ” while at 35mm cropped to the same field of view as the GF1 it shows a slight barrel distortion ” ( ) ” as does the GF1 at 35mm. This makes sense when you consider that the 35mm focal length is the same no matter what cropping takes place, the only thing that changes is the field of view as the smaller sensor uses only the center of the lens’ image circle as shown here:
Please excuse my horrific set up shot here. I hadn’t really anticipated using it like this. It is however the full frame from the 5DII shot at 35mm before I cropped into it as shown above. The white circle represents the full image circle from a full frame compatible lens. The lighter, larger yellowish rectangle represents the full frame’s use of that image circle while the darker, smaller portion in the center is the area that the micro 4/3 sensor “crops” to and uses. This is why, even though it has the same field of view as a 70mm lens would have on a full frame camera, it is utilizing the center of a 35mm lens and subject to any optical distortion that the center of the lens has functioning as a 35mm field of view on the full frame camera. So, in short, the “crop factor” does give extra reach by way of this crop, but it does not act the same way as a lens twice the focal length would on a full frame camera in regards to optical distortion. Some argue that they would rather have a full frame and just crop it themselves after the fact. That is fine and dandy, but where that can get troublesome is when resolution comes into play. Cropping into a 12 megapixel image file captured on a full frame for example to the same field of view as the micro 4/3 sensor provides already at 12 megapixels, is going to suffer quite a bit regarding overall resolution as you are taking those 12 million pixels and throwing nearly 70% of them away leaving you with a 3 or 4 mega pixel image file where you would start with a 12 megapixel file from the smaller, more pixel dense m4/3 sensor. So, a smaller sensor can offer a benefit to those looking to maintain resolution while gaining “reach” by way of cropping the image circle. While you do get a cropped portion and the benefits to the sharper area of the center of a lens, it still is subject to any of the optical characteristics of that shorter focal length.
So, while you get reach by way of this crop, you are still using the same focal length, you’re just getting an altered field of view. You artificially get “reach” by way of this crop, but lose out on the wide end of the scale when using a crop sensor camera. I say artificially because you’re still using a shorter focal length and do not necessarily gain the same distortion characteristics that longer focal lengths employ.
I hope this has helped define crop factor for you, I know that going through this exercise has certainly helped me understand it more clearly.