DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
Ian Stuart Forsyth
OP Ian Stuart Forsyth Veteran Member • Posts: 3,375
Re: Cropping from within a format (101) correction

Ian Stuart Forsyth wrote:

So let’s take the 4 different crops (formats) of the image circle from the previous post I have made and bring them over here

We have the 645 crop (format)

Full Frame Crop (format)

Apsc Crop (format)

And the Q crop (format)

As an experiment let’s take each format and crop them to the same FOV and scale them to the same resolution ( magnification).

Here is the 645 with the same FOV

Now the Full Frame with the same FOV

Now the APSC with the same FOV

And finally the Q with the same FOV

As you can see that they share the same FOV and DOF now why is this? (bear with me if you Know) they were taken using 4 different formats using the same focal length of 55mm and shot at F4. If you sit back and think about for a minute you will see that they are all made using the same portion of the image circle that was projected by the 55mm lens at F4 so why would they not be the same.

At the end of this exercise what have really done? We have taken the Q and cropped that image and made what ? a new format size the same was done to the APSC as well the Full Frame and also the 645.

This does not prove F4 on any of the 4 format sizes is the same but rather that if we crop an image from format and compare it to another format at the same FOV that the DOF will be the same only because the share the size portion of the project image circle from the 55mm F4 lens (same format size). So at the end of the day we still have F2.8 is not equal to F2.8 across formats.

The test showed this.

Taking images with 4 different format sizes and cropping them all to the same FOV and viewing them at the same magnification is a practice that many use to show that F4 is the same as F4 across format. But this is not correct and will not prove that F 4 is the same. Here’s why, the size of the final viewed image is in fact another format size with a different FOV. By cropping the 645 to this FOV you are using another format,…. And cropping the Q to the same FOV you are using the same format size as the crop from the 645 image. So when a person uses this practice as proof of F4 is equal to F4 all they have done is shown that yes they have the same DOF only because they have been using the same final format size of 1.06mmx0.76mm with surface area of 0.81mm2( same portion of the image circle)

This is why I called this cropping within a format

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