extension tube query

[jasonpgc]

Thanks,

Base on the web page

if I use extension tubes,

to get 1:1 we loss 2 stops of light

to get 1:2 we loss 1 stop.

 

[MaGixShOe]

ext. tube dun cause to lose any stop of light

 

[Sgt. Pepper]

Originally posted by MaGixShOe
ext. tube dun cause to lose any stop of light

pls clarify..

 

[MaGixShOe]

Originally posted by Sgt. Pepper
pls clarify..

if a lens is a F2.8 lens, add the ext. tube it will still be a F2.8 lens

 

[Sgt. Pepper]

Unlike teleconveters/extenders, ext. tubes wont change your aperture value. You have to compensate the light loss by slower shutter speed or opening up the aperture.

 

[jasonpgc]

Now I'm really confused, by you two.

For Teleconvertors,

Suppose you take a meter reading and have a exposure of f5.6 1/125s.

After you add the 2X teleconvertor, the aperture value shown on the lcd still remains at f5.6 1/125s. However the meter now tells you that it is two stops underexposed. What I usually does is to reduce the shutter speed to 1/30s and bring the meter back to the center again.

Now, it this the same procedure I should take if I add a 50mm extender tube to a 50mm F1.8 lens?

 

[munfai]

Originally posted by jasonpgc
Now I'm really confused, by you two.

For Teleconvertors,

Suppose you take a meter reading and have a exposure of f5.6 1/125s.

After you add the 2X teleconvertor, the aperture value shown on the lcd still remains at f5.6 1/125s. However the meter now tells you that it is two stops underexposed. What I usually does is to reduce the shutter speed to 1/30s and bring the meter back to the center again.

Now, it this the same procedure I should take if I add a 50mm extender tube to a 50mm F1.8 lens?

you can rely on the exposure meter to give you a correct indication of how much compensation you need.

 

[munfai]

Originally posted by Sgt. Pepper
Unlike teleconveters/extenders, ext. tubes wont change your aperture value. You have to compensate the light loss by slower shutter speed or opening up the aperture.

some teleconverters on some camera makes do indicate a change in relative aperture value (minolta for example).

most (if not all) SLRs nowadays use TTL metering, so you can reliably count on the camera exposure meter to give you a correct reading.

 

[jasonpgc]

Thanks munfai,

I've yet another question,

Going back to the example on 50mm extender for 50mm f/1.8 lens.

If I set the aperture for the 50mmf/1.8 at f8. After adding a 50mm extender, since the light falls by 2 stop, I presume the effective aperture is now f/16. However, in this case, is the DOF the same as without adding the extender?

 

[munfai]

Originally posted by jasonpgc
Thanks munfai,

I've yet another question,

Going back to the example on 50mm extender for 50mm f/1.8 lens.

If I set the aperture for the 50mmf/1.8 at f8. After adding a 50mm extender, since the light falls by 2 stop, I presume the effective aperture is now f/16. However, in this case, is the DOF the same as without adding the extender?

DOF should be shallower

 

[jasonpgc]

Do you mean this?

If I used to shoot marco with f/8 at 1/125s with a 50mm lens without extender,

Now if I want to achieve the same exposure with the 50mm extender, I must set the exposure to f/8 at 1/30s.

And I also want to get back the DOF lost, assuming 2 stops lost, therefore I must set the exposure to f/16 at 1/8s?

The conclusion? If you want 1:1 with extender while keeping the correct exposure and same DOF, you lost 4 stops of light? :dunno:

 

[MaGixShOe]

wah... u say till like so cheem..

ok if u shoot at F8, and get 1/250, after adding the ext. tube u should get the same exposure also, DOF is shallower coz the focusing distance is shorter.

if u want to get 1:1 with a 50mm lens and ext. tube, the focusing distance will be very very close, it may block off alot of available light

 

[jasonpgc]

Thanks MaGixShOe,

If the focusing distance is shorter to get 1:1, I understand that the DOF is almost zero, and it is impossible to get back the same DOF (without the 50mm extender. )

Can you estimate the focusing distance for 1:2 and 1:1 for a 50mm lens with extender?

 

[MaGixShOe]

ok.. if the 50mm is set at the min. focusing distance 45cm, then the magnification will be 0.645 on a 25mm ext. tube

focusing distance will be reduced down to 12.7cm

taken from some leaflet that comes with my ext. tube. got the forumla to calculate focusing distance and image magnification too

 

[jasonpgc]

Thank you for all your replys,

I've bought a 25mm extender just now.
I hope I can contribute a little to those interested.

I've taken some rough measurement with the new gear.

***

For 50mm f/1.8 lens with 25mm ext tube,

At min focus Setting, Subject to lens filter ring=8.3cm, Size= 1:1.43
At Infinity, Subject to lens filter ring=9.7cm, Size= 1:1.86

With the 25mm ext tube,
I think the working distance is really too close for comfort.
It is really not practical to add a 50mm ext tube for a
50mm lens for any field job.

***

For 80-200mm F4.5-F5.6 with 25mm ext tube

80mm, At min focus Setting, subject to lens filter ring=28cm, Size= 1:2.71
80mm, At infinity, subject to lens filter ring>30cm, Size=1:3.14

I notice the magnification stays relatively constant throughout 80 to 200mm.
However, the wondeful thing is that by varying the focal length,
I can actually manipulate the focus distance
without changing the magnification too much.
The Zoom ring effectively becomes the focusing ring

***

For 80-200mm F4.5-F5.6 with 25mm ext tube, with 2 element +2 close up lens

80mm, At min focus Setting, subject to filter ring=18cm, Size= 1:1.71
80mm, At infinity, subject to filter ring=21cm, Size= 1:2

135mm, At min focus Setting, subject to filter ring>30cm, Size= 1:1.43
135mm, At infinity, subject to filter ring>30cm, Size= 1:2

200mm, At min focus Setting, subject to filter ring>30cm, Size= 1:1.29
200mm, At infinity, subject to filter ring>30cm, Size= 1:1.86

Adding the +2 filter shortern the working distance
and increase the magnification ratio.
The 2 element filter generates significantly less distortion than
single element ones.
In this case, Zooming increases the magnification ratio.
The combination of a 25mm tube and the 2 element +2 lens manage to bring
a mid zoom lens down to almost life size without compromising too
much on image quality.

 

[Falcon]

Great info, Jason.

Originally posted by jasonpgc
Thank you for all your replys,

I've bought a 25mm extender just now.
I hope I can contribute a little to those interested.

I've taken some rough measurement with the new gear.

***

For 50mm f/1.8 lens with 25mm ext tube,

At min focus Setting, Subject to lens filter ring=8.3cm, Size= 1:1.43
At Infinity, Subject to lens filter ring=9.7cm, Size= 1:1.86

With the 25mm ext tube,
I think the working distance is really too close for comfort.
It is really not practical to add a 50mm ext tube for a
50mm lens for any field job.

***

At min focus setting, it is possible to achieve that kind of magnification? I tot it can only achieve 1:2. Also r u able to vary your focusing distance? I believe closeup filter has a fixed working distance i.e +4 will give u approximately 25 cm.

 

[jasonpgc]

Originally posted by Falcon
Great info, Jason.



At min focus setting, it is possible to achieve that kind of magnification? I tot it can only achieve 1:2. Also r u able to vary your focusing distance? I believe closeup filter has a fixed working distance i.e +4 will give u approximately 25 cm.

1) The magnification given by standard calculation are base on the lens set to inf. When set to the min focus distance, your image magnification is naturally bigger than at inf.

2) 2 element +2 close up have a greater focus range than the cheap hoya single element type. Canon and nikon and sigma made 2 element close up lens.

 

[Sgt. Pepper]

I dont think adding 25mm extension tube to a 50mm or more focal length will give you more than 1X magnification.

80mm with 25mm extension = 1:3.14 magnification

 

[jasonpgc]

Did I mention 1:1, I thought I write

For 50mm f/1.8 lens with 25mm ext tube,

At min focus Setting, Subject to lens filter ring=8.3cm, Size= 1:1.43
At Infinity, Subject to lens filter ring=9.7cm, Size= 1:1.86

Despite having a longer focal length, the 80mm min focus distance is around 0.9m. The 50mm on the other hand is around 0.45m. Can try it on your 50mm and see :bsmilie:

 

[Sgt. Pepper]

May I know what is your definition of "size"? e.g. Size= 1:1.43