QUESTION: IQ+DOF + extenders

Is the IQ and DOF the same as the lenses widest aperture still when you use extenders?

(e.g. on the 300 mm f4 lens as an example)

i.e. When you use a 1.4 or 2* extender with your chosen f4 lens does the DOF now work at the new working aperture i.e. 5.6 or f8 instead of the f4

Presumably ytou still have to stop down to get beter IQ that the lens would oterwise offer at one or two stops down?

Wow Simon, you really have to stop thinking so hard!

I assume the DOF is as seen by the camera - ie f2.8 to f4 with 1.4TC reflects DOF at f4. As for best IQ? Not sure, I would guess that the sweetspot is still somewhere in the middle but not sure how IQ degradation of the TC plays into that.

EOSAddict wrote in post #2176284
Wow Simon, you really have to stop thinking so hard!
I assume the DOF is as seen by the camera - ie f2.8 to f4 with 1.4TC reflects DOF at f4.

ahh.. but is it??? because now we also have the extra FL to take into consideration, so are we getting the same relative DOF from a 300+1.4 ext (=420mm lens) at the working aperture of 5.6 (f4 set - can you set the widest aperture now that the aperture is no longer controlled by the lens as it used to be?))
- as we would from a fixed FL lens of 420 at 5.6? (ok I know there isn't one - but if there was)

EOSAddict wrote in post #2176284
As for best IQ? Not sure, I would guess that the sweetspot is still somewhere in the middle

somewhere in the middle?,... somewhere in the middle? !!


I see you're on the dreaded crop camera

EOSAddict wrote in post #2176284
but not sure how IQ degradation of the TC plays into that.

That's the most complicated part of it theoretically but in practice I guess you just stop down and make do

Simon King wrote in post #2176333
somewhere in the middle?,... somewhere in the middle? !!


I see you're on the dreaded crop camera

Well I was talking approximate as it's clearly different for each lens, but since I have great IQ from all my lenses at any aperture ( ) I just set f8 and forget it! (don't have a heart attack I'm joking!)

Dreaded? Hmmm. Although I have been hankering after a 5D lately.....

From what I understand, a 143mm f/2.0 lens with a 1.4xTC at f/2.8 will render the same DOF as a 200mm f/2.8 lens at f/2.8, given the same focusing distance.

As for IQ, I noticed that my 70-200 f/2.8 is sharper with a 1.4xTC at f/4 than without it at f/2.8. I put it down to user error with the more shallow DOF. It's still hard to throw the IQ degredation of the TC into the equation since many of these 1.4xTCs don't even degrade quality at a noticeable level.

Not sure if that even answers your question.

Calzinger wrote in post #2176370
From what I understand, a 143mm f/2.0 lens with a 1.4xTC at f/2.8 will render the same DOF as a 200mm f/2.8 lens at f/2.8, given the same focusing distance.

As for IQ, I noticed that my 70-200 f/2.8 is sharper with a 1.4xTC at f/4 than without it at f/2.8. I put it down to user error with the more shallow DOF. It's still hard to throw the IQ degredation of the TC into the equation since many of these 1.4xTCs don't even degrade quality at a noticeable level.

Not sure if that even answers your question.

neither am I but it was interesting

I have been dying for a 143mmf2 lens for quite a while now , where did you get yours?

(only Joking! )

BTW might it be that you used the IS (more lilely to turn it on) with the TC?

Otherwise that is a tad bizarripoos to get sharper images with the TC on

Simon King wrote in post #2176282
Is the IQ and DOF the same as the lenses widest aperture still when you use extenders?

(e.g. on the 300 mm f4 lens as an example)

i.e. When you use a 1.4 or 2* extender with your chosen f4 lens does the DOF now work at the new working aperture i.e. 5.6 or f8 instead of the f4

Presumably ytou still have to stop down to get beter IQ that the lens would oterwise offer at one or two stops down?

The DOF will be for the extended focal length and effective fstop, so the 300 f4 wide open with a 1.4X will have the DOF of a 420mm lens at f5.6.

There is an IQ loss using extenders (teleconverters), this would normally be less than the loss of IQ in upsizing. The extended lenses often sharpen up by being stopped down, 1 stop helps a lot with the 300 f4 + 1.4X or 2X. There are other issues also such as the reduction in AF speed and reliability or reverting to manual focus in some cases.

I have done tests with the 300 f4 and both Canon TCs here. If you work your way through the menu you can see the crops as well as the parametric plots.

A comparative plot of the centre sharpness figure of merit for example.

Lester Wareham wrote in post #2177596
The DOF will be for the extended focal length and effective fstop, so the 300 f4 wide open with a 1.4X will have the DOF of a 420mm lens at f5.6.

There is an IQ loss using extenders (teleconverters), this would normally be less than the loss of IQ in upsizing. The extended lenses often sharpen up by being stopped down, 1 stop helps a lot with the 300 f4 + 1.4X or 2X. There are other issues also such as the reduction in AF speed and reliability or reverting to manual focus in some cases.

I have done tests with the 300 f4 and both Canon TCs here. If you work your way through the menu you can see the crops as well as the parametric plots.

A comparative plot of the centre sharpness figure of merit for example.

WOW man! That's comprehensive!

very impressive


thanks you smelly much!