Doubler and depth of field

If I put a 2x TC on a f4 lens then it will be a f8 but will it change the DOF of the lens.
I'm thinking it will but want to know for sure before I buy it.

not sure if i understand your question. It will give the dof of f8???

btw, I assume you have the 70-200 f4/f4 IS which means you will lose AF with the 2x TC.

kin2son wrote in post #14268047
not sure if i understand your question. It will give the dof of f8???

btw, I assume you have the 70-200 f4/f4 IS which means you will lose AF with the 2x TC.

Thanks I knew it cuts the light but I was not sure if the DOF would change.
With a manual lens would it have to be set at f8 to match or could it be shot wide open.

Black Bart wrote in post #14268061
With a manual lens would it have to be set at f8 to match or could it be shot wide open.

You don't need to set it, as f8 is as fast as it will go with the 2xTC attached.

f8 IS wide open with the TC.

kin2son wrote in post #14268070
You don't need to set it, as f8 is as fast as it will go with the 2xTC attached.

f8 IS wide open with the TC.

How does the TC keep me from setting my manual lens wide open.
Are you saying that it will be f8 even if I set the aperture at f4.

Black Bart wrote in post #14268075
How does the TC keep me from setting my manual lens wide open.
Are you saying that it will be f8 even if I set the aperture at f4.

what manual lens is it?

I thought the 2xTC, at least for Canon afaik, only works with certain lens?

See - http://www.usa.canon.c​om …_ef_2x_iii#Spec​ifications

"Note: This lens is only compatible with fixed focal length L-series lenses 135mm and over, as well as the EF 70-200/2.8L, EF 70-200/2.8L IS, EF 70-200/4L, and EF 100-400/4.5-5.6L. Additionally, please see the lens and/or camera body manual for full compatibility information with the Extender EF 2X III."

Black Bart wrote in post #14268075
How does the TC keep me from setting my manual lens wide open.
Are you saying that it will be f8 even if I set the aperture at f4.

You can't hit f/4 with a 2X TC mounted...

Those two stops of light you're losing have to go somewhere and they come off your max aperture. f/4 without TC. f/8 with TC (two stops slower)

Black Bart wrote in post #14268075
How does the TC keep me from setting my manual lens wide open.
Are you saying that it will be f8 even if I set the aperture at f4.

F stop is a ratio of aperture to focal length. f/4 means focal length/4 i.e. for an 80mm lens, the opening is 20mm. If you add a 2x TC on there, your focal length doubles to 160mm, but you aren't changing the physical size of the aperture which remains at 20mm, therefore the ratio of aperture size to focal length changes, effectively becoming twice as small, resulting in an aperture of f/8.

I guess the question is can you even use the 2x TC with a lens other than what Canon stated? Does it even work?

Sirrith wrote in post #14268113
F stop is a ratio of aperture to focal length. f/4 means focal length/4 i.e. for an 80mm lens, the opening is 20mm. If you add a 2x TC on there, your focal length doubles to 160mm, but you aren't changing the physical size of the aperture which remains at 20mm, therefore the ratio of aperture size to focal length changes, effectively becoming twice as small, resulting in an aperture of f/8.

OK now I think I understand so if I put the TC on wouldn't it help to set my lens at f4.
I understand it would still be f8 but if the lens was set at f4 instead of f8 wouldn't it be brighter.

kin2son wrote in post #14268120
I guess the question is can you even use the 2x TC with a lens other than what Canon stated? Does it even work?

No. It physically will not fit other lenses. There is a portion that protrudes into the host lens, and most lenses cannot accomodate this protrusion.

ettsn wrote in post #14268128
No. It physically will not fit other lenses. There is a portion that protrudes into the host lens, and most lenses cannot accomodate this protrusion.

Thanks ettsn for the clarification. There you go OP

No...if it's a "properly reporting" teleconverter, the camera's aperture cannot be opened wider than f/8. You can keep turned the aperture dial, but it won't open any more.

Now, if you have a "non-reporting" teleconverter, you CAN physically(according to your LCD) open it to f/4, but it is still only f/8. I have this issue with my Sigma 1.4x on my EF 100mm f/2.8 Macro USM. And as it's not reporting properly, it shows the improper aperture and focal length in the EXIF file.... but it's wrong.

Black Bart wrote in post #14268075
How does the TC keep me from setting my manual lens wide open.
Are you saying that it will be f8 even if I set the aperture at f4.

If that is just a scenario to help you understand, then yes, but you can't use any manual lenses with the Canon extenders. It's not like there's some kind of lever that doesn't let the lens shoot wide-open. It's a matter of the lenses geometry changing which changes the values. The lens still shoots shoots wide-open with the TC on.

Black Bart wrote in post #14268122
OK now I think I understand so if I put the TC on wouldn't it help to set my lens at f4.
I understand it would still be f8 but if the lens was set at f4 instead of f8 wouldn't it be brighter.

No. f/4 is no longer possible. Your camera will "see" the teleconverter and interpret this for you. f/8 is wide open on an f/4 lens plus a 2x tc. There is no tricking anything. The lens' focal length is effectively doubled, but the aperture cannot be any bigger than the blades allow. Aperture is represented as a ratio of aperture opening to focal length. That's what the "f" in the fraction "f/4" means. Re-read Sirrith's explanation. It is spot on.