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An analysis of camera sensor size and depth of field.

Yellow represents a full frame sensor, green is a crop sensor.


If we keep the distance of the subject from the camera constant, then the actual size of the subject on the sensor is also constant. That is, sensor size does not affect the size of the subject on the sensor.


However, with a crop camera, the subject appears larger when the whole image is viewed on screen/print at the same size as the full frame image. This is the same for A and B (close, and further away).


In both A and B, the larger subject size of the crop image is not magnification. It is only a result of cropping the sensor size, and then enlarging the image on screen/print to view it at the same size.


Magnification is a product of focal length of the lens and distance of subject to camera - NOT sensor size.


The DOF is higher in B than in A because of the greater distance of the subject.




i. if we compare a top of the range crop and full frame camera, is the sensor so much better in the full frame (as well as other qualities such as low light handling), that full frames win out? Full frames are far more expensive, so we may expect this to be true. However a very good crop camera should still have an excellent sensor.


ii. is there an advantage to being further away with the crop camera? Yes - you are less likely to scare the insect, or to cast a shadow on it.


iii. you will save a lot of money by getting a crop camera - even a very good one.

1. To get the subject the same size on the computer screen/print with both cameras, we need to shoot further away with the crop camera.


This is because the subject will now fill the same proportion of the sensor, so when the crop image is enlarged, the image looks the same.


This greater distance gives us more depth of field with the crop camera image. Not the sensor size!


However, in this case, assuming the same pixel density on both sensors, the subject covers fewer pixels on the crop camera, so we will have less resolution and a lower quality image.


Also, the pixel (photosite) size may be higher on the full frame camera; this may mean a similar number of pixels covered as with the crop camera, but more light gathered per pixel - so again the full frame has higher quality.


But this advantage has to be weighed against the lower DOF.

2. Alternatively, if we compare a full frame and crop camera shooting at the same distance, we can crop the full frame image in processing to give us the same proportion of subject to frame size as in the crop image.


As the distance was constant, the depth of field will be the same. The number of pixels is also likely to be the same, so we have an image of equivent quality - assuming similar sensor quality overall.


Conclusion: based on just the factors mentioned here, there is no benefit either to a full frame or a crop sensor camera:


a. The DOF advantage of shooting further away with the crop camera is lost in terms of image quality.


b. If shooting at the same distance, the DOF is the same, and cropping the full frame image reduces the number of pixels, so the advantage of a larger sensor is lost.


Therefore, the quality of the sensor (i.e. the camera) and the lens are much more important than sensor size.

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