Create 2019-02-23-matching-dof.md

_posts/2019-02-23-matching-dof.md

@@ -0,0 +1,57 @@
+---
+published: true
+layout: post
+date: '2019-02-23 19:37 +0100'
+title: Matching DOF
+tags:
+  - video
+---
+[https://www.youtube.com/watch?v=hi_CkZ0sGAw](https://www.youtube.com/watch?v=hi_CkZ0sGAw)
+
+Of bigger sensor to smaller one is done by using crop-factor and aperture (f-stop). For example:
+
+Full frame camera with f4 will have the same dof as camera with crop factor of 1.6 if we divide f value with crop factor. 
+
+    f4 / 1.6 = f2.5. 
+
+For micro 4/3 the crop factor is 2 so 
+
+   f4 / 2 = f2. 
+
+This will change the exposure, so we can counter that with changing the ISO.
+
+    Full-frame ISO
+    --------------- = Equivalent ISO
+    (crop factor)^2
+
+If full-frame ISO is 400 then for camera with crop factor 1.6
+
+    400 / (1.6)^2 = ISO 156
+
+or crop factor 2
+
+    400 / 4 = ISO 100
+
+so
+
+    full-frame focal length / crop factor = Cropped focal length
+    full-frame aperture / crop factor = Cropped aperture
+    full-frame ISO / (crop factor)^2 = Cropped sensor ISO
+
+## Limitations
+
+For a lens that says 105mm / 1.5 you would need asp-c lens 70 mm with f0.9 (there is no such lens)
+
+### 1/2" sensor
+
+For a small camera with 12.7mm sensor (5.41 is crop factor), what lens would be the same as asp-c with f2 and 50mm lens at ISO 200?
+
+    So f2*1.6 = f3.2 to get the full frame aperture. 
+    Full frame ISO would be 1.6^2 * 200 = ISO 512
+
+    Small camera ISO would be 512/5.41^2 = ISO 17 (impossible?)
+    Small camera aperture would be f3.2/5.41 =~ 0.6 (impossible?)
+    And the lens would be 50/5.41 =~ 9mm (possible)
+
+Small required ISO could be bitten with build in ND filters, but aperture on for example sony PXW-Z280 stock lens is F1.9 - F16 (5.6 - 95.2 mm zoom). 
+