Help on Kenko 1.4x on a 100mm 2.0 (no extra pins)

I put my Kenko 1.4x on my 70-200 f4 IS and because it has the 3 extra pins, it is able to compensate for the one stop loss thus widest aperture is 5.6

I put it on my 100mm f/2 (which doesnt hav the 3 pins) and as expected my camera can go down to f2.

However, do I have to do anything here? Exposure compensation of one stop? What really happens when I shoot at f/2? or should I only shoot at widest f2.8?

[QUOTE=Foggy;4066557]I put my Kenko 1.4x on my 70-200 f4 IS and because it has the 3 extra pins, it is able to compensate for the one stop loss thus widest aperture is 5.6

I put it on my 100mm f/2 (which doesnt hav the 3 pins) and as expected my camera can go down to f2.

However, do I have to do anything here? Exposure compensation of one stop? What really happens when I shoot at f/2? or should I only shoot at widest f2.8?[/quote]

you're really at f2.8 because the TC isn't reporting the change. you don't need to do anything.

ed rader

Thanks for the insta response

If it is really shooting at f2.8 then i'm curious if I am on Aperture priority, f2 would give me twice as fast shutter speed than f2.8, in this case wouldn't my image at f/2 Av mode be under exposed by one stop? I ran some shots at f/2 amd f/2.8 with shutter speeds being say 1/30 and 1/60 respectively, but both exposures look very close?

Nope...when the camera tells you you're at f/2 with the TC, you are really at f/2.8, and the shutter speed will reflect that. Add a stop to all apertures displayed when shooting with your TC on the 100. (the same is true on my 80-200L).

Remember, the camera meters the scene based on your wide open aperture, then determines the shutter from there...since the wide open is really f/2.8, the calculations will be correct, just the display of the aperture will be wrong. (It doesn't say 'this amount of light requires this aperture + shutter speed'. It's 'this amount of light is entering the lens at wide open...what's the shutter speed to expose properly' (in Av)...and for other apertures, it adjust stops from there.)

To restate Jman's points in a slightly different way, for added clarity...

The 100mm f/2 max aperture lens with 1.4x convertor is truly an f/2.8 max aperture. But the camera erroneously displays it as an f/2 lens

The sensor sees less light with the f/2.8 lens than with an f/2 lens, so its meter is registering less light. So, for a given aperture (reported as f/2) in Av mode it would choose a slower shutter speed than it other wise might. So the meter compensates for the aperture number lie, by virtue of it seeing less light coming thru the lens.

For example, ISO100 1/100 sec. f/16 is the true amount of light on the scene.
With the 100 f/2 lens alone, the meter registers ISO100 1/100 sec. f/16 and thinks it has an f/2 lens in place.
With the 100 f/2 lens with 1.4x, the meter registers ISO100 1/50 sec. f/16 and thinks it has an f/2 lens in place. What it THINKS is f/16 is truly f/22, but the amount of light it sees is -1EV, so it says '1/50' with the pseudo f/16 (truly f/22) and that results in the proper exposure for the scene!

Ah i see makes sense.

Thanks for the explanations!