Teleconverters and extension rings...

[ffureel]

Hi all.

I am trying to figure this out but struggling. Ok.

Lets say we have a 50mm f/2 lens.

Question 1/

If I use an extension tube, is the only effect that it has is in reducing the minimum focus distance? An example, If I take a portrait shot with the lens as standard, at minimum focusing distance and wide aperture. If I added an extension tube and focussed the lens further back to take essentially the same photo, would the extension tube photo have shallower depth of field? Or would the focusing further back on the lens negate the closer focusing of the extension tube?

Question 2/

2 x teleconverter. If i added one of these to my 50mm lens I know you have to multiply the focal length by 2 and also the effective exposure. So the lens would become a 100mm f/4 lens roughly. Now the f/4 bit is where I am struggling. I know the exposure is effected to an equivalent of f/4 but what about the depth of field? Would the depth of field be like a 100mm f/2 lens or like an f/4 lens?

Another question. Is it possible to reduce depth of field with a lens? Say we are at widest apertures. the camera is not allowed to move, change focus, subject does not move etc... Is it only possible by using a lens with a wider aperture?

Thankyou.

[ChrisLilley]

Thanks for bringing the subject up! I'm not totally clear on the effects, especially on depth of field.

For comparison, an ND filter has no effect on closest focus distance or on focal length, it affects the amount of light entering the lens (so for exposure, the aperture is smaller) but for DOF calculations and for considering the effects of stopping down on resolution loss by diffraction, the aperture set on the lens is the one that is used, not the effective aperture caused by the filter.

OK so thats the easy case considered, as it has no optical effect other thsan darkening.

My understanding is that an extension tube will also result in darkening (effective aperture decreasing, ie more light needed). I'm not sure of the relationship between number of mm oc extension and number of stops of light lost.

Similarly for a teleconverter, a 1.4x loses one stop of light and a 2x loses two stops (so your 50mm f/2 becomes a 100mm f/4). Thats 2 stops, so you need 4x the amount of light.

However, for depth of field - stopping down (on the aperture ring) increases the depth of field, which is important for macro. Stopping down too far decreases resolution, due to diffraction. So my questions are:

DOF
does a teleconverter result in more depth of field, because the effective aperture is less; or less depth of field, because the focal length increased; or no change, because the two effects cancel out?

Non infinity distance
for macro, the effective aperture decreases as one gets closer to the subject and closer to 1:1 magnification. I read that my f/2.8 macro is actually at f/5.6 at its closes focus distance. For DOF, should I use the aperture as set on the lens, or the effective aperture? Since the actual size of the hole in the iris has not changed, is it correct to assume that diffraction effects are taken from the aperture as set, not the effective aperture?

[poilu]

1) extension tube don't change properties of a lens as they don't have optical element.
2) the depth of field will be a 100 F4
3) you can change dof properties of a lens by shift & tilt, this is used in large format where they have a very thin dof
if you want larger dof, the smaller sensor is the solution; with a p&s you have infinite dof

[ChrisLilley]

[poilu]

[ChrisLilley]