Extension tubes and DOF

[Ken MT]

Is there any method for estimating DOF when using extension tubes. For example I was photographing a butterfly egg with a 100 mm macro, and 65mm of extension tubes, (it was a very small egg). I tried stacking a number of images, but the best single shot was the best. I would have been close to the minimum focus of the lens. The minimum focus for the Canon 100mm IS macro is 300mm, and on a review does give the magnification for 10mm and 25mm extension rings, but not greater.

[Jim B.]

I did a search and found a Macro Dof calculator. Not going to post a link but the site that came up is called PhotoPills. Also has a field for adding extension tubes.
Hope this helps.

[chauncey]

Not going to post a link

Why not?

[Jim B.]

Because I'm terrible at this internet stuff. Nothing wrong with that site. I forget how to link here without having the entire link show.

[AlwaysOnAuto]

Pretty easy to find, looks like it might be useful too.

http://www.photopills.com/calculators/dof

[Ken MT]

Looks like it is with a teleconverter, not extension rings. There is a calculator in Cambridge In Colour, under tools. Back to my original question, do I add on the extension ring distance to the lens focal length to give 160mm, which drops the DOF from about 0.4 cm to 0.1 which feels about right.

[Dave Humphries]

Try this one instead: http://www.photopills.com/calculators/dof-macro

Looks like a useful site with several other calculators available here:
http://www.photopills.com/calculators

Cheers, Dave

[Ken MT]

That site requires that the focal distance must be 4x the distance from the subject to the sensor plane. Just not sure why.

[Stagecoach]

Is there any method for estimating DOF when using extension tubes.

Yes there is Ken.

There are a number of Macro DoF calculators around for which each gives a slightly different estimate and provides hours and reams of debate and discussion on the methods and accuracy of formulae used.

Here's a calculator I found a while ago https://www.kielia.de/photography/calculator/

The real world reality is that measurements are so small, it's near on impossible to accurately physically measure subject to sensor/node points, sharpness is subjective and final use/size of images so varied that these calculators may not really help.

For example I was photographing a butterfly egg with a 100 mm macro, and 65mm of extension tubes, (it was a very small egg). I tried stacking a number of images, but the best single shot was the best. I would have been close to the minimum focus of the lens. The minimum focus for the Canon 100mm IS macro is 300mm, and on a review does give the magnification for 10mm and 25mm extension rings, but not greater.

You do not say what aperture you were using, I ask because there is often a reluctance to stop down because so often we read and hear of 'diffraction' but it's one of these terms that we rarely ever see someone mention regarding the finished result of a macro shot.

[dem]

That site requires that the focal distance must be 4x the distance from the subject to the sensor plane. Just not sure why.

Because it cannot be less than that according to the thin lens formula that these calculators are using. This formula, however, does not take into account focus breathing and lens design and thickness. Not very useful in practice.

A rather long thread on this topic: Magnification factors for macro lenses

[Ken MT]

Well this was the photo that I took.



Canon 7D Mk 2, Canon 100 mm IS macro lens, 60 mm extension tubes, f/13, exposure 1/8 sec, ISO 100.
Subsequently cropped, upsized, haze reduced and sharpened. The egg was about 1mm long and .05mm wide. Natural lighting.