Circle of Confusion with 1.4extender

[AndyB1976]

Hi

Drawing up my Canon wishlist, I'm pondering a 70-200 f2.8 II with 1.4extender or a 70-200f4 with 300mm f4 prime.

Im quite impressed with the ISO crops from the-digital-picture.com with f2.8+extender vs the 300mm f4 prime.

I understand the prime will out perform most zooms even without the extender, not withstanding the image degradation and loss of a f-stop.

So my question is, does the f2.8 with the extender attached (to f4 equiv) provide a circle of confusion greater than a prime f4 lens, or in other words is f2.8 still f2.8 with the extender attached with regards to CoC/DoF albeit with the speed of a f4?

Sorry for this type of rambling question lol



Andy

 

[HoppyUK]

Welcome to TP

With a 1.4x converter, the 70-200mm f/2.8 actually becomes a 98-280mm f/4 and behaves like one in every respect.

 

[AndyB1976]

Thanks Richard!,

I was trying to get my head around the CoC/DoF at 280 with the extender at f4 compared to say the 300mm prime at f4

My initial thoughts were the CoC/DoF/blur effect would be unaffected entering an extender resulting in the more pleasing effect leaving the extender onto the sensor from an initial f2.8 aperture.

I guess I was expecting the extender to magnify the original image onto the sensor rather than becoming integral to the focal length:bonk:

Thanks again for your help!


Andy

 

[HoppyUK]

Thanks Richard!,

I was trying to get my head around the CoC/DoF at 280 with the extender at f4 compared to say the 300mm prime at f4

My initial thoughts were the CoC/DoF/blur effect would be unaffected entering an extender resulting in the more pleasing effect leaving the extender onto the sensor from an initial f2.8 aperture.

I guess I was expecting the extender to magnify the original image onto the sensor rather than becoming integral to the focal length:bonk:

Thanks again for your help!


Andy

That's what it does. That's what changing the focal length does

The DoF effect is a function of image size (however it's arrived at) and f/number.

 

[Lornholio]

This explains it well: http://moosepeterson.com/blog/?p=13633

200mm f/2.8 + 1.4x lets you shoot at an effective 280mm f/4. The f/2.8 to f/4 difference isn't caused by the lens's aperture closing down, it's caused by light being lost within the teleconverter. The lens is still wide open, so your DOF is more equivalent to 280mm f/2.8.

So 300mm f/4 has more depth of field than a 200mm f/2.8 + 1.4x converter.

 

[HoppyUK]

This explains it well: http://moosepeterson.com/blog/?p=13633

200mm f/2.8 + 1.4x lets you shoot at an effective 280mm f/4. The f/2.8 to f/4 difference isn't caused by the lens's aperture closing down, it's caused by light being lost within the teleconverter. The lens is still wide open, so your DOF is more equivalent to 280mm f/2.8.

So 300mm f/4 has more depth of field than a 200mm f/2.8 + 1.4x converter.

Not so. And that's a porkie talking moose on the link.

The telecon doesn't 'suck up' light at all, it spreads it as a fundamental part of the magnifying (focal length increasing) process, raising the actual f/number. Basic inverse square law.

The whole lens is changed. Focal length is changed. F/number is changed. AND DoF is changed.

 

[Lornholio]

I stand corrected, checked on the calculator I have that has a teleconverter option. Shame on Mr Peterson.