Extension tubes - DoF question.

[adrianxw]

Some of you may have read my post a while ago about the passive

extension tubes I was bidding for at eBay. I won and they arrived

yesterday. They are simply tubes, no glass.

 

I have had a fiddle with them but not shot any film yet. I wondered,

will the tubes alter the depth of field at all?

[stemked]

I don't have an equation for you, but as magnification increases depth of field really narrows. That's one of the big challanges in macro photography, trying to get as much in focus as possible. There are times when having a really narrow DOF is cool, but usually the battle is the other way. You will often be shooting at f11-f22, extension tubes cause you to loose light so either you need long exposures or some flash. You also will find yourself playing a lot with angles (of the camera and subject) to get as much in focus as possible.

[Jochen_S]

What do you mean by altering? Google for some DOF calculator and play aroundwith it until you recognize that DOF becomes smaller when you get closer to 1:1 magnification. There should be some DOF tables somewhere too. Your lens will care a shit about it if you mount it on a Speed graphic, some Pentax bellows, cardboard tubes or the treasure you recently purchased or even if Pentax happened to mount it in the barrel of on of their Macro lenses. The only thing counting is draw; the distance between film and the lens. If your tubes have levers to keep the iris open, unlock the lens and turn it half way out until it stops down, to preview DOF at a given aperture. - It is no bad idea to close it 2 other f-stops, to gain the previewed DOF on film.

[david j.lee]

you are only working with the indicated f stop when at infinity. as soon as you

move the lens out of it the f stop starts getting smaller. when you use

extension rings , you are actually working with very small f stops. for example

if your lens is at f 22 and you are using 50mm of extra extension on a 50mm

lens, you are actually using f 64. remember that with these kind of f stops you

start to get diffraction and that is worse than narrow dof. i highly recommend

" john shaw`s close ups in nature", you can get it in borders`s or barnes and

noble.

[james_oneill]

The apperture is the same regardless of the distance from the film - so there is no diffraction impact. However there is an effective reduction in apperture ...

How so ? f stop is focal length / apperture diameter. A 50mm lens at f/2 has a apperture diameter of 25mm, at f/4 it's 12.5 mm and so on.

Similarly a 100mm lens has 50mm and 25mm appertures at f/2 and f/4

 

The brightness of the image is a function of the AREA of the apperture and the square of the distance from the image (move the lens twice as far away the light has to cover 4 times the area).

 

When focused on infinity a 50mm lens 50mm away from the image, and 100mm lens is 100mm. So the lens needs an apperture 4 times the area since the 100mm lens has twice the diameter for the same f stop it has 4 times the area. So f/2 or f/4 give the same image brightness regardless of focal length. HOWEVER this only works at infinity. If you put 50mm of extension onto a 50mm lens you've doubled the distance to the film so you need 2 stops more exposure than an off camera light meter would tell you.

 

As for DOF here are 3 equations.

 

Hyperfocal distance = f^2/c*a

f = focal length

c = circle of confusion (0.03mm for film, 0.02mm for digital)

a = apperture (f stop)

 

For a 50mm lens at f/8 on film h = 2500/ 0.03 * 8 ~ 10m

 

for a focus distance d, far point is given by

d*h/d-h

and the near point by

d*h/d+h

 

You get a life size image when d=2*f, (0.1M for a 50mm lens)

 

so the far point = .1*10/10-.1 = 1/9.9 = 0.1010m

and the near point = .1*10/10+.1 = 1/10.1 = 0.9901

 

So focus down to 10 cm and you've got a d.o.f of about +/- 1mm (9.9 cm to 10.1 cm) compated with 5m to infinity at 10m