F stops converters?

I've asked this question before and never did get a definitive answer.
When you add a 2x converter this makes an F stop of 5.6 into F11. BUT does this increase the depth of field as it would if the F stop was just altered on a lens without the converter?. OR does the DOF remain static?. The smaller the F stop the shallower the DOF the bigger the F stop the greater the DOF. Hence my question.
CHEERS Vic.

I believe it does double the depth of field Vic.

That was quick! Are you absolutley sure?
CHEERS Vic.

That's wrong
It's halved

You're both wrong. The depth of field is the same, but due to the enlargement ratio, it seems less. If you put a 2X converter on a lens, and go backward to get the same size for the main subject, the DOF will be the same.

But you don't step backwards when you put a converter on..

Putting the converter on is like stepping in closer, so
the DOF will always be less.

Putting a 2x converter on doubles the focal length, so depth of field AT THE SAME APERTURE will be shallower. This would be comparing say an actual f11 on the lens alone with f5.6 set on lens + converter which are both an effective f11 and give you reduced DOF but the same image brightness.

Now if you leave the lens set at f11 and put the 2x adapter on you will be working at effective aperture of f22 which increases the DOF compared to f11 on the lens alone without converter. This means you have 2x the focal length (decreasing DOF) and half the aperture (increasing field) which will cancel out giving you the same DOF as before - but a darker image.

So you can choose how you use the 2x converter, giving you either reduced DOF or not (this assumes you don't complicate matters further by walking backwards)

Still "arguments" still no difinitive answer. I thought someone would find an article on the subject somewhere but it seems the jury is still out as you all have different answers. Who is right and who isn't?
CHEERS Vic.

If you're feeling confused by contradictory answers, there's only one answer - have a go, try it and see what the result is.

DOF is purely a function of magnification, so if you decided that you wanted a picture of a coin to fill the frame is makes no difference if you use a 50mm or 100mm macro lens. One will be twice as far away, but the DOF is the same because the magnification is the same.

The perspective will change of course as you are now at a different distance, but that's something else.

Well you get different answers according to what the question is, correct answer depends on what actual aperture you use, whether you change camera position to keep magnification the same or maintain camera position increasing magnification etc. We are guessing differently as to what case you would like explained so the answers are different.

As you ask for something more definitive the starting point for answers is usually wiki http://en.wikipedia.org/wiki/Depth_of_field but expressed less mathematically perhaps http://www.cambridgeincolour.com/tutorials/depth-of-field.htm .

Quoting John's case from the second article:
" Note that I did not mention focal length as influencing depth of field. Even though telephoto lenses appear to create a much shallower depth of field, this is mainly because they are often used to magnify the subject when one is unable to get closer. If the subject occupies the same fraction of the image (constant magnification) for both a telephoto and a wide angle lens, the total depth of field is virtually* constant with focal length!"

And quoting my case from the same article:
"On the other hand, when standing in the same place and focusing on a subject at the same distance, a longer focal length lens will have a shallower depth of field (even though the pictures will show something entirely different). This is more representative of everyday use, but is an effect due to higher magnification, not focal length."

Good luck and happy reading,
Rob.

I don't think you can ask that question without factoring in the effect of the converter on the focal length of the lens.

My understanding is that putting a 2x converter onto (say) a 200mm f5.6 lens effectively converts it into a 400mm f11 lens. DOF will be as it would be for any lenses of those configurations.

However I believe the closest focussing of the lens in use is retained.

That's my belief too.

John

Closest focus point moves away proportionally, unless you also add an extention tube.

Because the near point is a product of the aperture (unchanged), focal length (increased), circle of confusion (unchanged).

You'll find all the maths you require in Wiki under the above topics.
Alternatively, as John says, try it and see

regards
Bernard

This well known manufacturer seems to think not, and a quick Google shows a lot of people think the same.