Depth of field with 2X Converter and 70-200 F2.8

[Sharkman]

Quick question, I know if I add a 2x converter to my 70-200 f2.8 I will lose 2 stops of light, but does it also affect the depth of field by two stops? (ie giving you the depth of field you would get at f5.6 rather than f2.8)

To start with I thought it wouldn't as it is not altering the actual aperture of the lens, but the more I have thought about it the more I have confused myself!

I already own the 100-400 lens so if the 2x does affect the depth of field on the 70-200 there is no point in me getting one. I just occasionally need the really shallow depth of field from f2.8 at the 400mm focal length and as I can't afford a 400 f.8 prime, adding the 2x to my 70-200 f2.8 seems like the best bet

Thanks
Sharkman

[MiG82]

If your focal length has just doubled and your aperture is constant, the f number must double (halve strictly speaking) to f/5.6 by definition, right?

[hmhm]

Depth of field increases with smaller apertures (larger "f-numbers" ) and decreases with longer focal lengths.

If you throw a 2x tele-converter onto your 70-200/2.8, you now have a 140-400/5.6.

The "aperture" is the ratio of the focal length to the diameter of the lens. If you hold the diameter constant, but effectively double the focal length, you will have halved the aperture (thus passing a quarter of the light).

A 200mm lens with a 2x tele-converter shot at f/5.6 will achieve identical depth of field as a true 400mm lens with no tele-converter also shot at f/5.6.
-harry

[Sharkman]

OK, Thanks guys. Looks like the only benefit I would get from adding the 2x converter would be the weight saving on not carrying the 100-400 zoom on trips abroad! Having said that, its probably worth it just for that after the row I had at the check in desk last time as my hand baggage was over weight, by about 3kg!

[JMSetzler]

The 2x converter will not affect your depth of field. It does not physically change the size of your aperture. Depth of field is affected by aperture size, focal length, and distance to subject. The 2x converter does not affect any of these.

The 2x converter will reduce the amount of light that makes it to your film plane / sensor by about two stops, but it will not affect the depth of field.

[EXA1a]

JMSetzler wrote:
The 2x converter will not affect your depth of field. It does not physically change the size of your aperture. Depth of field is affected by aperture size, focal length, and distance to subject. The 2x converter does not affect any of these.

The 2x converter will reduce the amount of light that makes it to your film plane / sensor by about two stops, but it will not affect the depth of field.


Sorry, but you're wrong.
A 200/2.8 lens plus a 2x teleconverter makes a 400/5.6 lens. There is no physical difference to a 400/5.6 prime. Although the diaphragm hasn't changed physically, the written aperture numbers on the lens have effectively doubled. And the effective aperture do influence the DOF, not the aperture written on the lens.

By simply using a DOF calculator, e.g.

http://www.shuttercity.com/DOF.cfm

it turns out that a 200mm lens at f2.8 has about the same DOF as a 400mm at f11. This means, using the same aperture setting, your DOF gets shallower with teleconverter.

--Jens--

[JMSetzler]

The converter does not change the physical size of the aperture though...

[JMSetzler]

Consider this scenario:

We are making two identical photos. The subject is the same distance from the camera in both cases. The first photo uses a 200mm f/2.8 lens and the camera settings for the photo are f/2.8 and 1/250". The second photo is made with a 100mm f/2.8 lens and a 2x converter. The camera settings are f/2.8 and 1/60". The depth of field for these two photos should be identical.

[JMSetzler]

I stand corrected....

I seem to have forgotten that the physical aperture size of an f/2.8 setting on a 200mm lens is twice the area of the 100mm f/2.8 opening...

[mishkin]

for distances much less than hyperfocal, DOF is proportional to F-number and inversely proportional to the square of focal length:

DOF = 2*CoC*F*Distance^2/Focal^2

2x teleconverter increases both F and focal length by factor of 2, which makes DOF 2x less than without TC.

Mishkin