Extension tubes

[agb]

I have a L 100mm f2.8 macro, which I have been using for my macro photos, however as we all know these lenses have a very narrow depth of field when used at close range, eg at 30cm,unless stopped down to very small apertures.
On this particular lens for instance on the 7D at 30cm the depth of field is 0.25cm.
As I have a 70-300mmL lens I was considering using this with extension tubes so that I could be further away from some subjects, but I do not know what happens to depth of field when using these with long focal length lenses. Can it be calculated?
Can anyone enlighten me?

[OzzieTraveller]

G'day Graham

Yes mate - DoF does reduce dramatically as we get closer & closer

There is one law of optics / physics here that we need to respect ... "when the image size is the same, the DoF is the same"

ie- it matters not whether you / I am using a 50mm or a 200mm lens ... each will have differing camera to subject distances to make the image size identical, and when the image sizes are the same, the DoF will be the same also

What does occur tho is the front-of-lens to subject distance alters considerably with a longer lens, so less likely to "scare the natives" and also to avoid shadows falling across the subject when one gets horribly close to the subject

You can google DoF tables for various image sizes & apertures ... I haven't done it, tho I guess it might be of some help on some occasions. I prefer to work to my age-old formula for my equipment and the degree of DoF that I am comfortable with for each particular subject

Regards, Phil

[crafty1tutu]

I don't know how to calculate DOF with macro extension tubes, but I did use the Kenko macro extension tubes with the Canon 70-300 lens for a couple of years whilst saving for a macro lens. I was so happy with the results and you can see some of my photos on my Flickr account.

http://www.flickr.com/photos/83646071@N00/sets/72157606413538473/

[fabian628]

I have a L 100mm f2.8 macro, which I have been using for my macro photos, however as we all know these lenses have a very narrow depth of field when used at close range, eg at 30cm,unless stopped down to very small apertures.
On this particular lens for instance on the 7D at 30cm the depth of field is 0.25cm.
As I have a 70-300mmL lens I was considering using this with extension tubes so that I could be further away from some subjects, but I do not know what happens to depth of field when using these with long focal length lenses. Can it be calculated?
Can anyone enlighten me?

you will be further away, but longer focal length so dof will be the same.

If you are having trouble with shallow dof maybe use narrower aperture (however not too narrow maybe). Many people belive you need to shoot macro at apertures of f/16 and narrower, I dont think this is necessary if you can get nice angles of the subject. Use shallow dof as an advantage to your pictures. when i shoot natural light i usually shoot f/4-f/5.6 at 1:1 :th3: