DOF and Cropping take 2

Started 7 months ago | Discussions thread
Ian Stuart Forsyth
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Re: DOF and Cropping take 2
In reply to edspen, 7 months ago

edspen wrote:

A lot of verbiage for a photography forum. I'm suprised no one has posted pics illustrating many of the fine points being addressed. How's that old wag of the tongue go, oh yeah " a pics worth a heck of a lot of keyboard hits ". The batteries are rechargeable, get out those dust collecting toys and snap some examples to go with all that verbiage (you know, to show the kids).

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Ed

Here you go, original post was here

http://www.dpreview.com/forums/post/52545757

So during the weekend with the aid of some engineering equipment I was able to create a 3d grid in the air that I was able to move camera and lens along this grid and able to hold the X & Y axis while moving the camera along only the Z axis and ran the test of cropping an image to affect perspective. I was able to keep the camera on this axis with in +- 5mm throughout the whole test so it should be accurate enough for what I have planned. All photos are taken with the K5 and 70-200mm F2.8 sigma lens.

First I will revisit cropping for the use of increasing the reach factor of a lens first just to confirm what I had done previously.

The first photo is taken @ 70mm F4.5 and cropped to a 2mp image ( the original uncropped image is labeled DCIM_0025 70mm F4-5orig if you want to compare). Being that I used a crop factor of 2.86 giving the image a FOV of a 200mm lens. So let’s now take a photo with a 200mm lens at F2.8 but first to do a side by side comparison one must scale that photo such that they are view at the same 2mp size( the original unscaled image is labeled DCIM_0027 200mm F4-5orig) and compare it to the cropped 70mm F.45 cropped 2mp image.

View: original size

View: original size

As you can see when we use cropping to increase the reach factor we have to give up something in return and in this case its DOF ( & light ) so as one can see that cropping a 70mm F4.5 lens not only has an effect on the FOV but also the DOF. So let’s work out what F stop would be needed by that 200mm lens to share the same DOF as the cropped 70mm F4.5 image. Let’s take the crop factor I used to make that 70mm image 2.86 X F4.5 = F 13. So let’s throw up the image taken at 200mm F13 that was also scaled to a 2mp image (original DCIM_0034 200mm F13orig) and see how it compares to cropped image.

View: original size

View: original size

As we can see that they are almost exact match so what can we conclude from this? I would have to say that a 70mm F4.5 image that is cropped by 2.86 to give it the FOV of 200mm has the same DOF as an image shot with a native 200mm FL at F13, contrary to what some people believe.

Let’s move over to cropping to change perspective, this was a lot harder to move the camera back on only the Z grid but here it is. So the first photo was taken using 70mm F4.5 and because we will be comparing it to a cropped image one would to scale it to 2mp image ( original DCIM_0025 70mm F4-5orig). For the next image I held the 70mm at F4.5 but moved the camera back until the cropped 2mp image of the 0 (as this was our subject) was the same size of the zero on the scaled photo taken at a much closer distance (original DCIM_0004 70mm f4-5 moved orig) .

View: original size

View: original size

So as one can see they still have the same usable DOF but the image shot at the closer distance has a much different property to the DOF as its compressed due to the difference in perspective.

What can we concluded with this test?

We can use cropping to alter a lenses perspective by cropping the FOV to a FOV narrower than the native lens allowing us to photograph at a greater distance away from the subject and will in turn give us the same useable DOF but due to perspective the visual properties look difference ( compressed DOF). Due to moving the camera further away we give up the Reach factor that cropping gives us but in return it gives us the ability to alter perspective.

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The Camera is only a tool, photography is deciding how to use it.
The hardest part about capturing wildlife is not the photographing portion; it’s getting them to sign a model release

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