DOF and the use of extenders

[Edward]

If i shoot my 400 mm f 2.8 at f 4, adding a 1.4X makes it an f 4.  Is the depth of field the same with and without the extender at f 4?

[Skirball]

If you shoot your lens at f/4 with the 1.4, it will be effectively f/5.6.  The lens will behave exactly as a 560mm f/5.6 would behave, as far as DoF is concerned.

Edit: Note that I was careful not to mention that technically focal length doesn't affect DoF, to avoid that whole argument for the 1000th time.  Oops, there I go again...

[Edward]

IF I SHOOT THE 400 MM  F 2.8 WITH A 1.4X IT BECOMES AN F4; NOT AN F 5.6.  

MAYBE REWORDING MY QUESTION AS FOLLOWS WILL CLARIFY:

A.  IF I SET MY 400 MM F 2.8 AT F4 THE DOF WILL BE X.

B.  IF I USE THE 1.4X WITH THE 400 MM AT F 2.8 IT BECOMES AND F4 AND THE DOF WILL BE ?

WILL THE DOF IN SITUATIONS A AND B ABOVE WILL BE THE SAME OR DIFFERENT? 

[Skirball]

I'm well aware of that.  Funny how people asking for other's advice are frequently so strident in response. Perhaps you should re-read what you said in your first post:

"If i shoot my 400 mm f 2.8 at f 4, adding a 1.4X makes it an f 4."

If you shoot a 400 mm f/2.8 AT f/4 - implies that you moved your aperture to f/4.  Adding a 1.4X will make it f/5.6.   I get what you're trying to say (after your second post), but that's not what you said originally.

If you simply read my post instead of fly off the handle that you don't think you got the response you want, you'll see that I answered your question regardless of which aperture you select.  The lens with extender will behave as the effective focal length and focal ratio after multiplication. 

[Edward]

1.4 EXTENDER TIMES F2.8 LENS APERTURE WIDE OPEN= F 4 (3.92 TO BE EXACT)  NOT 5.6.  SO THE LENS BECOMES AN F4 WHEN USING THE 1.4X EXTENDER.

THE QUESTION IS IF THE THE LENS PROVIDES THE SAME DOF WITH OR WITHOUT THE USE OF THE EXTENDER?

[Stephen]

Hi Edward!

Please remember to avoid using ALL CAPS per the Forum Guidelines.

Thank you!

[Skirball]

Again, I'm aware of that (f/2.8 + 1.4X = f/4).  I have answered your questions several times; perhaps you could work on your reading comprehension - and tact, while you're at it. 

The example you posted in your second post is poorly stated, hence I explained myself instead of simply answering - you're comparing a 400 mm @ f/4 to an effective 560mm @ effective f/4.  Technically speaking, the depth of field are practically the same - but not exactly - if you move back with the 560mm to get the same magnification factor.  But we could discuss a couple pages on that topic alone - actually, 100s of pages with how slow you are.  So I left it simple.  A 400mm at f/2.8 with a 1.4X will behave exactly as a 560mm @ f/4 would with regard to depth of field.  Why is that so hard to understand? 

[hsbn]

Skirball is correct, but here a quick run down using a quick DOF calculator, full frame camera, object distance 100ft
400mm @ F2.8 DOF ~ 3.37ft
400mm @ F4 DOF ~ 4.82ft
400mm @ F2.8 + 1.4x extender = 560mm @ F4 ~ 2.44ft

[Skirball]

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@hsbn wrote:
Skirball is correct, but here a quick run down using a quick DOF calculator, full frame camera, object distance 100ft
@400mm @ F2.8 DOF ~ 3.37ft
@400mm @ F4 DOF ~ 4.82ft
@400mm @ F2.8 + 1.4x extender = 560mm @ F4 ~ 2.44ft

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@And 400mm @ f2.8 + 1.4x extender @ 140 feet to object (same magnification ratio): 4.81 ft

[ebiggs1]

Edward the confusing part of this is the words used. Just like using a certain lens on a crop camera does not change the focal length of that lens. Adding a tele extender does not change the focal length of that lens.

The thing that does change is the angle of acceptance.  The focal point of that lens does not change and can not change.

Also any time you increase the length of the lens tube, you will sacrifice some f-stop value.

Adding the 1.4x to your 400mm lens dose not turn it into a 560mm lens. It only gives you the angle of acceptance of a 560mm lens. It will behave similarly (effectively) to a 560mm lens with the f-stop penalty added.