Rongnongno wrote:
First the myths...
DoF is different
That is false. Since the lens has the same DoF given the same distance/aperture values the field of view does not change, period. Once again, some may think the DoF is 'longer'. It is not.
Ron,
You are correct! The DoF doesn’t depend on the size of the sensor.
I see that there are 13 pages of claims and counterclaims. Since this is a technical discussion, there should be only one correct set of facts. I’ve been reading this thread, and trying to understand the logic from both claims. Both are valid, each in their own way.
First, let’s be sure that we understand, that in photography, CoC describes the largest blur circle that is indistinguishable from a point, not the size of the of the blur circle.
If you are going to compare DoF between two photographs, then the lens needs to have the same focal length, aperture, and plane of focus.
Also, the print needs to be the same size, be viewed from the same distance, and
the objects in the photo must be the same size. How does that happen?
a) The size of the blur circles on the sensor doesn’t depend on anything but the lens settings and the distance from the plane of focus. There is a different size blur circle for each point, depending on the distance from the plane of focus. That is, when you take a photo of an object, both the size of the object and the size of the blur circles on the sensor will be the same, regardless of the sensor size.
b) Therefore, a FX lens used at 10 meters with the same f11 aperture (example) has the same size blur circles on the sensor, regardless if the sensor is on a FX or DX body.
c) When the image is transformed from the sensor to a viewable image, whether it is a print, projection on a screen, or displayed on a monitor, there is a magnification factor. In the case of using a 4x5 negative to produce an 8x10 print, it is 2:1. This will magnify the size of the object and the blur circles by the same amount.
d) If you produce a print (of the same size) from each of two different size sensors, such that
the object is the same size on both prints, then the magnification factor and the size of the blur circles will be the same size on both prints. When viewed from the same distance, the CoC and the DoF will be the same.
Congratulations Ron, you are correct. Does that mean that all of the DoF calculator apps are wrong? Not at all!
The DoF calculators expect that you will make a print that includes the entire image from the sensor. If you produce a full image print (of the same size) from each of two different size sensors, then the print from the larger sensor will have a wider field of view.
The objects will be smaller, and the blur circles will be smaller. When viewed from the same distance, the CoC and the DoF will be the different. The DoF calculator doesn’t know that you want to compare two photos that need to have the same object size.
I hope that this helps everyone involved.
Gary Minor