Monday, May 22, 2017

Why Sensor size has nothing to do with the Depth of field?

Yesterday i posted a fact on FB that "Sensor size has nothing to do with the DoF". And then i was waiting for the replies becuase i knew that some one will reply. But surprisingly one of very dear Photographer who is a Doctor in States and i have so much respect for this gentlman replied which i was expecting. And his reply was spot-on right. He said what i said is not correct and Sensor size does effect the DoF if we shooting same subject from same distance.

What he said was right and 99% of photographers, I am sure you too agree and endorse what he said. In general understanding the “Depth of Field” works like this which he mentioned ……. BUT only if you try to achieve the similar “angle of view” using different sensor size.

Allow me to go deep and unfold the mystery which not many people know. The first thing that we need to understand that different sensor sizes have different field of view at a given (fix) focal length and distance. 5x4 got bigger field of view, then medium format got smaller angle of view then 5x4, similar goes for full frame, then APS-C & least angle of view is for Micro4/3.

I am going to use FF & APS-C sensor sizes to explain why Sensor size has NOTHING TO DO with the Depth of Field (DOF). But before we go any further, another thing that we need to understand that any lens …… say 16mm focal of a crop body will be 16mm regardless it is used on FF or APS-C or even Medium Format. It always remains the 16mm. But as we understood earlier that angle of view for every sensor is different so to achieve the “similar angle of view” from a 16mm lens at a “Given/fix” distance it wont be possible. Why? Because 16mm is 16mm ….. and always remain 16mm. Its physical focal length never changes no matter you use it on FF or APS-C so the angle of view “from that lens” will remain the same. Its like you small tap/nozzle in the garden to sprinkle water. No matter how much pressure/volume of water is behind, the amount of water coming out from the nozzle will be totally depends on how much tap/nozzle you opened. Same applies here. The DoF will remain the same because we are assuming that we are shooting using a fix focal length, fix aperture and from a fix distance no matter what type of sensor is beneath it.  So what will be the output “angle of view” if you use 16mm crop lens on the follow digital sensor?

APS-C --> 16mm --> 16mm angle of view

FF --> 16mm --> 16mm angle of view

Medium Format --> 16mm --> 16mm angle of view

The angle of view will remain the same no matter what sensor size you used the lens on. Now if ever used the crop body lens on a full frame body or full frame lens on a Medium format body (using special adaptor) then you will be noticing that it creates heavy vignette. Why? Because optically the 16mm that was used above for an example was design for crop sensor which is 24mm x 16mm in size. So the circle of project of light from that 16mm lens was meant for 24mm x 16mm crop sensor. Now if you use the same lens on FF which is 36mm x 24mm sensor it means there are extra area on the sensor which is not covering by APS-C lens. That’s where that “Extra Angle of View” kicks in of the larger sensor format. Now we need to redesign the lens which covers the extra area of the 36mm x 24mm FF sensor. Now to cover the whole FF sensor & to achieve the similar angle of project of 16mm crop, optically you have to design a lens of 24mm and only by then you will be having similar Angle of View between FF & APS-C. So what focal length of FF & Medium format we need to get equal angle of view of 16mm APS-C?

APS-C --> 16mm --> 16mm angle of view

FF --> 24mm --> 16mm equivalent angle of view

Medium Format --> 30.5mm --> 16mm equivalent angle of view

Now many of you will wonder from where that 30.5mm focal length comes for Medium Format comes? It came just like crop factor of 1.5 came ☺ . Now divide the focal length of the lens by the following factors and you will get the equivalent angle of view.

APS-C = 1.5
FF = 1
Medium Format = 0.79

BTW besides the lens multiplication factors, these lens manufacturing companies when they redesign the lenses for different digital formats, they redesign the Aperture opening accordingly which is based on focal length “f/1.4” that “f” is focal length. So ones the aperture value is changes, it also effects the DoF. In general there is a “one stop” value difference of DoF value between the different sensor format. For example

M4/3 f/1.2 = APS-C f/2 = FF f/2.8 = Medium Format f/4  

But these are just practically assessed values and 95% of lenses are made on this.

As you have seen that focal length is changes to achieve the similar “angle of view” which ultimately changes the focal length of the lens, which also changes the Apeture opening of the lens, which ultimately “indirectly” effects the DoF. So what I stated above is correct that sensor sizes has nothing to do with the DoF ..... directly. 
So I hope after reading all this lengthy details, it will be cleared now.  


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