When it comes to sensor size, there are basically two kinds of DSLR cameras on the market. One is the full frame DSLR based on the 35mm film format and the other is the smaller APS-C or crop frame DSLR. The APS-C cameras are generally lower cost and lighter weight and generally very good for most photography. The full frame cameras are heavier and more expensive, but you get a larger sensor for potentially better photos under some conditions.

There was a difficult to pass up sale on the Nikon D750 full frame DSLR and 24-120 mm f/4 lens so I ended up buying one. I also have a D5300 APS-C DSLR and I thought that it would be interesting to compare the two.

The Sensor

Below is a scale diagram I made that shows the relative size difference between the sensor of a full-frame sensor Nikon (FX format) D750 and an APS-C sensor Nikon (DX format) D5300. Canon has similar sensors that differ by fractions of a millimeter from Nikon, so this should all apply to Canon gear as well.

The surface area of the full frame sensor is more than twice that of the APS-C sensor, so it has more than twice the light available to work with. So, if both cameras have approximately the same pixel count and sensor technology, the full frame camera will have much better light sensitivity per pixel.

Equivalent Focal Length

One reason this all matters and I am bothering to write about it is that camera lenses are all measured in focal length based on a 35 mm sensor (or film). So, this means that a particular lens will give you a different apparent focal length and field of view when used on an APS-C camera compared to a full frame camera as the APS-C sensor is basically cropping out part of the image that the lens casts and only seeing the middle part. The crop factor is about 1.5x for APS-C meaning that if you put a 50 mm lens on an APS-C camera, the field of view will be about what you’d get with a 75 mm lens on a full-frame camera. To demonstrate this, I set up a little scene and tried to use equivalent focal lengths on the two cameras.

The first image below was taken with a Nikon D5300 (APS-C sensor) with the lens focal length at 34 mm. The second image was taken with a Nikon D750 (full frame sensor) with the lens focal length at 52 mm). Both images were taken using a tripod that I didn’t move. As you can see, both images have approximately the same field of view even though the lens was set at different focal lengths.

Another thing to point out in the above pictures is that in both photos, the aperture is set to f/4 and manually focused on the animal’s face so that the pictures would be as similar as possible.

Note how much more shallow the depth of field is on the D750 image even with the same apparent focal length. The mirror in the background is much blurrier in the full frame shot than in the APS-C photo. So, full frame sensor cameras also can get shallower depth of field, which might be desirable if you really want the bokeh.

Nikon DX Lenses

Nikon also makes DX lenses which are designed for the APS-C cameras. These lenses can be smaller and lighter as they only have to cast an image big enough for an APS-C sensor. The focal length is still measured relative to a 35 mm sensor, though.

The D750 allows you to set it to auto-detect a DX lens and crop the image to use the APS-C equivalent part of the sensor. But you can also disable that and choose to use the entire sensor. The image below was taken using the D750 with a DX lens in crop mode with the focal length set to 35 mm. It basically gives an image similar to the one taken with the D5300, even in depth of field.

When I disable crop mode for a DX lens and use the entire sensor, you can see that the DX lens only casts an image large enough for the APS-C sensor and the rest is not there.

Next I scaled both of the above images to the sensor size using Microsoft Visio. I then placed the first image on top of the second to show the amount of crop for an APS-C sensor, just to demonstrate all of this for myself.

So, there is an apparent zoom using the same focal length lens with an APS-C sensor, but it is really just the sensor only using a smaller part of the lens.

Low Light Performance

Another advantage of full frame sensors is that they have more area exposed to light. So with same number of pixels, the larger sensor should have better light sensitivity and better low-light performance.

The Nikon D750 and the Nikon D5300 both have 24 megapixel sensors, the difference being that the D750 sensor is much larger than the one on the D5300. So, how do they compare at high ISO? To test this I set up the tripod with the cameras photographing a poorly lit book shelf. Both cameras were using an equivalent focal length and aperture and I turned off High ISO Noise Reduction. Below are the pictures from the two cameras at ISO12800. I ,may have had the white balance different on the two cameras.

Now, I can pick a dark area and crop out a section to compare the noise performance.

A lot more noise is noticeable in the D5300 crop. You can click on the images to enlarge them for closer inspection. ISO12800 is kind of an extreme case, but it illustrates the better low-light performance of a larger sensor. This will be a more noticeable difference under extreme low-light conditions, especially with long shutter speeds, like night sky photography.

Summary

Full frame sensor DSLRs have a much larger sensor than APS-C DSLR cameras, but generally come at higher cost and weight for the cameras and lenses. By cropping the center of the lens, the APS-C cameras have an equivalent focal length that is about 1.5 times that of a full frame DSLR. Also as a result of the lens cropping, the depth of field increases for the equivalent picture. A full frame sensor with bigger pixels also can have better low light performance than a smaller APS-C sensor. Which one is right depends on what you want and how much you are willing to spend.

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