# DoF - 300mm f2.8 vs 420mm f4



## whothafunk (Feb 5, 2017)

I hope I'm in the right part of the forum. 

I know there are some sites that give you mathematical numbers about depth of field when you punch in different numbers, but I don't really understand what I'm seeing.

I'm interested in buying an extender because I do need the extra reach. I'd use it only in good day light, so my only concern is the DOF. 
Does a 400mm f4 produce about the same, more or less depth of field compared to 300mm f2.8? Is there anyone that maybe has a 300 2.8 and a 1.4x extender to show real life examples?


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## candc (Feb 5, 2017)

with the same subject framing: 420 f/4 will have a greater dof than 300 f/2.8


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## privatebydesign (Feb 5, 2017)

candc said:


> with the same subject framing: 420 f/4 will have a greater dof than 300 f/2.8



Whilst that is true, for the OP''s shooting situation, focal length limited, the 420 f4 will have less dof than the 300 f2.8. 

That is, same subject from the same place, i.e. a bird from a hide, the 420 f4 has less dof.

Now if you move back from the subject so the subject is the same size in the viewfinder then the 300 f2.8 will have less dof than the 420 f4.

It is important to remember that dof is subjective and the actual shooting conditions need to be taken into account to make the correct calculation.

Incidentally, you don't need the TC to do the assessment, just crop a 300 f2.8 shot to the fov of the 420 f4 shot and you have that 420 f4 shot. As you increase magnification you reduce dof, so cropping reduces dof and that crop is consistent with the results you will get.

So you might argue what is the point of a TC? Well it gets you more pixels on subject, that's it.


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## IglooEater (Feb 5, 2017)

privatebydesign said:


> candc said:
> 
> 
> > with the same subject framing: 420 f/4 will have a greater dof than 300 f/2.8
> ...



Thanks Private I was interested in the answer too. While I figured out candc's point, I hadn't figured out the rest yet.


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## whothafunk (Feb 5, 2017)

primary use will be in outdoor sports, mainly soccer. thank you for the answer!


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## Jopa (Feb 5, 2017)

DoF calculator http://www.dofmaster.com/dofjs.html
Bokeh simulator: http://dofsimulator.net/en/

Put your numbers and take a look.


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## privatebydesign (Feb 7, 2017)

I had the chance to illustrate this earlier.

First shot is 300 f2.8 @ f2.8.
Second shot is 300 f2.8 with 1.4 TC for a 420mm f4.
Third shot is the first shot cropped to the second shot fov.


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