DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
James O'Neill Veteran Member • Posts: 4,661
Re: Cropping from within a format (101)

Ian Stuart Forsyth wrote:

James O'Neill wrote:

Ian Stuart Forsyth wrote:

As you can see that they share the same FOV and DOF now why is this? (bear with me if you Know)

They are all the same because you have not achieved the same FoV by changing focal length but by cropping, In one you have cropped once and the rest you have cropped twice.

The test was not to change the FOV by FL see below

The smaller the fragment of the image you zoom in to, the easier it is to see what is out of focus. The cereal is just as out of focus in the whole frame as it is in the cropped-down-to-Q frame, but at such you can't see it in the whole frame.

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.

That's not the clearest phrasing in the history of English. A small image from the camera - whether it started small or was cropped from a bigger one - has to be magnified more than large one to fill the same size sheet of photographic paper or computer screen. As you magnify what was in the D.o.F zone because you couldn't see it was slightly out of focus is revealed to be blurred.

D.o.F isn't some magical "What's in is perfectly focused" zone , but "What's in is so slightly out of focus you don't notice". The same focus "error" with the same lens and same aperture will produce the same distance of blurring on the film or sensor: 0.02 mm of blur is harder to see on a 33x44mm image than a 3x4 mm one. Hence all d.o.f. calculations use a circle of confusion size which depends on image size : and that is image size after all cropping has been done.

The test showed this.

Taking images with 4 different format sizes and cropping them all to the same FOV

You didn't though. You took an image and cropped it to 4 sizes. Then you cropped the large size a second time, so cropped the same image down to the same image in 4 different ways.

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.

You can argue it both ways - if you magnify a section of the image that has a smaller FoV but it is the magnification - or more accurately the change in workable Circle of Confusion size - that has the impact on DoF. If you change the FoV by changing image size and not focal length , it reduces the circle size.

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)

It has nothing to do with the image circle of the lens. It doesn't matter if the 50mm lens is for a Q or for 645 . And I don't know what fragment of an image was 1.06mmx0.76mm

When people show a crop from a big image and whole of a small image made with the same focal length and aperture and say "look they are the same" they are: they must be otherwise the lens would be doing something different to the light depending on what was behind it. But that is like saying the pixels that represent the shape of this letter > O < is the same whether you look at normal resolution or whether you magnify the screen 10 times, but it looks Circular, and black on white , right ?  But if you magnify it you get this.

Think of film. I shoot with with a 10"x8" plate camera, (254mm x 208mm) and something a certain amount out of focus turns into a circle 0.07mm wide that's 3/10,000th of the width. Too small to see . The I crop down to a Q sensor-size at 7.4mm X 5.6mm that same circle size is 1/100th of the image width, plenty big enough to see. The same thing has different APPARANT sharpness depending on the magnification.

 James O'Neill's gear list:James O'Neill's gear list
Pentax K-5 IIs Pentax K-1 Pentax smc FA 50mm F1.4 Pentax smc DA 18-250mm F3.5-6.3 Pentax smc FA 43mm F1.9 Limited +2 more
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