F stops converters?

Here you go

http://photo.stackexchange.com/questions/4928/how-does-a-teleconverter-affect-de...

So, it is my understanding that the 2x converter works the same way as the crop factor of an APS-C sensor. Thereby the focal length of a lens is the same, but the angle of view of the lens differs taking into account the crops factor of either the sensor or converter. Therefore you have to multiply the aperture by the factor of the sensor/converter to give you the depth of field equivelent.

The only way is to confirm this is to do for example

75mm with 2x on at F4 on a full frame
100mm on APS-C at F5.6
150mm at F4
All from the same position.

Confused? I know I am! LOL

On the other hand, if your converter gives you what you want, then just use it.

Closest focus remains the same.

They're not confusing at all.

Add a 2x converter to a 50mm f2 lens you effectively now have a 100mm f4 lens; the closest focus remains the same. Add a 1.4x converter to that 50mm f2 lens and you have the equivalent of a 70mm f2.8 lens. That's all there is to it! Depth of field would be same as any lens it has become the equivalent of.

Forget crop factors, you don't suddenly take that into special consideration when swapping from a 50mm lens to a 100mm lens on the same camera or altering the focal length of a zoom lens.

What might have an impact on the resulting image is the magnification of lens aberrations - eg any purple fringing might be more obvious.

John

An example (by using a depth of field calculator) for a 1.5 crop factor APS-C camera:

50mm lens
f/5.6
Subject 10m away
Closest = 6.8 m
Farthest = 18.85 m

50mm lens + 2x Conv = 100mm
f/5.6 + 2x Conv = f/11
Subject 10m away
Closest = 8.13 m
Farthest = 12.98 m

Is that OK?

Philip

Ssssshhhhhh - everyone likes to make it more complicated than that.

Which it is easy to do

I reckon you're right.

Clearly some of those pontificating previously have never used one but are practising to get a job in customer support!

John

OK I concede - I simply did the maths given by Wikipedia. and got a similar answer to MrB

It's been a while since I had a converter (it was M42) so have not been able to do a practical test.

regards
Bernard

Ive never met a teleconverter with zero length. They all incorporate a degree of extension. I'd guess that explains how the close focus remains constant.

Look at this site which calculates depth of field and hyperfocal distances: http://www.dofmaster.com/dofjs.html and you will see that with a 2X converter, at the same distance, and at the same aperture set on the lens (so that, say, f/4 is set, but the aperture is f/8 because of the converter) there is less depth of field.

For example:
K20D, 100mm lens @ f/4 and 10 feet: Depth of field is from 9.77 to 10.2 feet.
(Add the 2X converter, change nothing else)
K20D, 200mm lens @ f/8 and 10 feet: Depth of field is from 9.89 to 10.1 feet.

To agree with others, the closest focus doesn't change when a teleconverter is used.