DOF and Cropping take 2

Started Feb 11, 2014 | Discussions thread
Ian Stuart Forsyth
OP Ian Stuart Forsyth Veteran Member • Posts: 3,201
What the heck is a cropped image ( with the 645d)

This photo below was taken using the largest image circle that I had being projected with a 55mm lens at F4, taken with a pentax 645d. This would mean that the image circle was cropped with a 44mx33mm sensor with the surface area of 1452mm2. We can use this image and see how applying different sizes of formats cropped from this image circle will affect the DOF FOV of images. I will use several common formats and see what happens to these formats when viewed at the same output size of 960 pixels wide simulating conditions that we would normally use photographic equipment in (Same output size). I have no real idea how far the tomatoes are from the camera so let’s give it a roundabout distance of 50cm away. Using this DOF calculator It works out to be 3.6cm total DOF and looking at the 645d photo it’s in the ball park.


The image below is what a Full Frame camera would crop (35.9mmx24mm total area 861.6mm2) from the image circle of a 55mm lens still using F4 & again using the same output size of 960 pixels wide. As you can see with no surprise that the FOV has changed but if you look at the Full frame image you will see that the DOF has also changed. You may ask, but we cannot change physics or the physics of the lens to have different DOF from the same lens? We didn’t, we changed what portion of the image circle was being used and the physics of the lens is what changed the DOF. Just think about this, we didn’t change the physics of the lens we changed the portion of the image being captured. So let’s use that DOF calculator and see if jives with what we see in the image 2.1cm DOF so it did change interesting.

Full Frame Crop

Let’s move on to APSC size camera (23.7mmx15.7mm2 total area 372.1mm2) and see what portion of the image circle is now going to be used. Now I hope nobody is going to be surprise that the FOV will change and at this time a few more of you should not be surprised to see that DOF also changed. Let’s calculate the DOF 1.44cm DOF

And the last format for today Q ½.3 (6.17mmx4.55mm total area 28.1mm2) skipping the commentary and go to the DOF calculator 0.36cm of DOF.

So what can we come away with this in this little exercise, First off F/2.8 on 645d is not the same as F/2.8 on Full Frame as F/2.8 on apsc is not the same as F/2.8 on the Q. Only thing that remains the same is the exposure as that is derived using F/2.8 but this is irrelevant as all 4 formats capture different amounts of total light and total light is what determines noise in the final image.(if all formats have the same QE)

Now looking at the photos above, myself and many others would say this is about the worse way to compare image across formats we want to compare DOF with image that have the same FOV as this is how we photograph. We use things like DOF, FOV, perspective & focal point as visual aids in conveying our creative work, so why not use these key segments in photography >>> and god said let there be Equivalency<<<< to evaluate formats.

I will work on that tomorrow

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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

 Ian Stuart Forsyth's gear list:Ian Stuart Forsyth's gear list
Nikon PC-E Nikkor 45mm f/2.8D ED Pentax *ist DS Pentax K10D Pentax K20D Pentax K-7 +24 more
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