For underwater photographers that want to use SLR or mirrorless cameras, one question that needs to be answered is what size camera sensor to use. For SLR's, the choice is APS-C or Full Frame. Mirrorless cameras add additional smaller choices like m4/3 and CX formats.
One way to describe the difference between these different sensor sizes is to talk about "crop factor". Crop factor is just a ratio of how the dimensions of the given sensor compare to the height of a full frame sensor. A full frame sensor, by definition, has a crop factor of 1.0. There is nothing magical about choosing full frame here, but it is equivalent to the ever popular 35mm film size, and is a handy size to compare to.
Crop factor is a semi-useful tool that can describe how the cameras compare with each other given a specific lens. An good example is that the Nikon 60mm "acts" like a 60mm on the full frame Nikon D810, but acts like a 90mm lens on the Nikon D500, i.e. we multiply the 60mm by the D500's crop factor of 1.5. The reason I say is that it is "semi-useful" is that it only behaves like 90mm in terms of the field of view that is captured The lens works the same, we are just capturing a "cropped" chunk of the image out of the center, so the field of view is narrower.
Note that these crop factors are not "exact". There are a couple of reasons that I know about, and maybe more. For example, the m4/3 is a different aspect ratio that an SLR, so do we use the height or the width to compare? And do manufacturers use actual imaging area, or effective imaging area? It's a little bit of a gray area, and they seem to do some rounding (and fudging) to make it "easier".
Here are some crop factors for a few of the sensor sizes we've mentioned, specifically the ones that tend to be used by u/w photographers.
|CX||CX: 2.7||Nikon 1|
|m4/3||2.0||Olympus OM-D E-M5 II, Panasonic GX8|
|APS-C (Canon)||1.6||Canon 7D Mark II, Canon 70D|
|APS-C (Others)||1.5||Nikon D500, Nikon D7200, Sony A6000|
|Full Frame||1.0||Sony A7R Mark II, Canon 5DS R, Nikon D810|
More of the technical stuff below, but let's look at some images first. I recently went to Anilao, Philippines, which is an amazing macro haven. Last time I went there I shot a lot of video, but for this trip I chose to work on still photos. I brought two cameras, the Canon 5DS R (Full Frame), and the Canon 7D Mark II (Canon APS-C), and I shot with both over the course of my trip. Of my two favorite shots that I got, one was with the 5D and one with the 7D. I feel like I would have been totally happy with either camera (and I do really love both of these cameras). But for the purpose of this article, I will try to point out some differences.
Understanding Depth of Field
Let's start with something that may surprise you. The depth of field with a given lens is exactly the same between full frame and APS-C. There is a common perception that full frame has, by default, less depth of field, but that is simply not true. There is a reason for that perception though, but given the same lens and the same subject distance, the depth of field is the same. The field of view is smaller, so if you then enlarge the two images to the same print size, you will find that the full frame camera actually has MORE depth of field.
Now, let's get back the perception that full frame has less depth of field than APS-C. If we use the crop factor mentioned above, and compensate for the diminished FOV of the APS-C camera by using a less with a shorter focal length, and then try to achieve the same composition, now the APS-C will have more depth of field. Let me use an example to make that more clear. Suppose I am shooting a sponge that is two inches tall. With the FF camera, I'm using a 100mm macro lens, and with the APS-C camera, I am using a 60mm lens. I then move the camera so that the sponge fills the frame on both shots. Note that even though we fill the frame with the sponge, the shots are not the same. The perspective is different, because with the 60mm, we are a lot closer. You will see less of what is behind the sponge in the latter shot. So, yes, more depth of field with the APS-C in this case, but it really is a different photo.
Depth of field depends on two things: magnification and aperture. Here's one more tidbit that says that the depth of field issue is less of a problem with macro than people think. You might have heard of the issue of diffraction with small apertures. This is a real thing; stop your lens down too much and your image quality will suffer. The FF sensor size will, to my eyes, tolerate about a stop smaller aperture than the APS-C, (and another stop or two more than m4/3). So for example, in a shot where I might only want to shoot at f/16 with the APS-C, I can shoot with f/22 with the FF.
Bottom line is this... there is not that big of a depth of field difference. When you start shooting super macro, the depth of field will be quite small regardless. Personally, I enjoy shooting both.
Of course there are some differences between Full Frame and APS-C. One of the more noticeable differences between these two cameras is performance - the the 7DII is blazingly fast, and the 5DS R seems quite slow comparatively. Part of the reason for this, of course, is that the 5DSR is recording a whopping 50 megapixels worth of data for each shot. If you are going to shoot this camera, be prepared to buy more HDD and backup space.
All of the images here were shot with the Canon 100mm f/2.8L lens. The super macro shots used the Nauticam SMC, and the even tighter super macro shots added the SMC Multiplier.