+1 These are my two carry around lenses.

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All a dream
E-M5(black&gripped), P14, PL25, PL45, Oly9-18, Oly12-50, P45-200, FL-14, MAL-1,
OM1, OM28/f2.8, OM50/f1.8, OM100/f2.8, Viv135/f2.8, OM35-70/f4, OM75-150/f4, T20

Well I have tried exactly that in some very well controlled experiments at work. The results did not follow what you say. The depth of field in the 200mm macro was shallower than the 100mm. Enough that we could not keep everything in focus that we needed to be in focus with the 200mm. The 100mm had 40% more depth of field at identical magnification. This was for digital image correlation measurements to quantify strains in some very small parts, so we know the dimensions of everything very precisely. When it is out of focus we get data dropout, so the results were clear.

You must have re-written the laws of physics. Maybe you could post the results. I agree with fredlong. Many years ago I took some photos at various distances/focal length but same aperture and magnification to prove the point. The results depend on a rigorous definition of what is in focus and what isn't and the results can be difficult to interpret because the rate of blur is different with shorter/longer lenses away from the point of sharp focus.

Keith

PS there are plenty of discussions on the internet about this, some with all the maths to back it up. http://toothwalker.org/optics/dof.html and scroll down to ' DOF at constant magnification' and 'macro depth of field' for example. This link has some numerical examples and a DOF calculator http://www.cambridgeincolour.com/tut...h-of-field.htm

Lens design can have a big effect on what is in focus or out of focus. There's field curvature, various aberrations and the way a lens transitions from in to out of focus. Just changing the structure of the diaphragm can change the diffraction characteristics.

CPWarner's experience proves that what "should" be isn't what always happens in the field.

Fred

The more likely cause of the apparent difference is using a less than rigorous criterion for deciding the DOF. "... the way a lens transitions from in to out of focus ..." is different for shorter and longer focal length lenses, but if a small circle of confusion is used as the definition of what is and isn't out of focus then that is mostly irrelevant, as the examples in the links I provided show.

Keith.

Keith,

As the data I am referring to is is confidential and can not be shared. That said, there is a difference between theoretical math/physics and reality. Considering that I do theoretical modeling and experimental validation of that modeling every day, I see discrepancies all the time. You are probably correct that the difference between the lenses that I saw was due to how we defined what was in focus. I would say that it was highly rigorous, with numerical evaluation of the pixels we could continue to track. That assessment, as it was a practical one, probably does not follow the theoretical physics, and thus the difference.

Why should practical work not follow theoretical physics? What you seem to be trying to say is that while the accepted theory is that for a given magnification the focal length does not affect the depth of field, your tests say everyone is wrong! I did my own tests a long time ago and to prove the theory is correct to someone else who didn't believe the maths. I guess nothing changes even when faced with overwhelming evidence to the contrary from numerous sources.

Keith.

There are a lot of portrait shots earlier in the thread.

I think all we can infer from his experience is that for those two particular lenses in that particular circumstance an unexpected result was obtained. The testing was apparently rigorous but the differences in the lenses were uncontrolled. Sometimes the universe just isn't simple.

Fred

There are confounding factors that aren't immediately obvious, as well. Many lenses aren't actually the focal length they are marketed as. Some lenses change focal length when you rack focus... some severely. Measuring magnification with a ruler is fraught with hazards; viewfinders are rarely exactly 100% of the sensor. Quoted sensor sizes aren't always the nominal used area of the sensor. Some IBIS systems even crop the sensor somewhat.

As a quick example, my Canon 100/2.8 macro (internal focus) is effectively 60mm when focused to 1:1, for example. You can't expect that lens to give the DOF of a 100mm lens at full magnification, but how would you know this without testing?

You can't accept any markings on your lens barrel as gospel, so finding results don't quite match what you expect shouldn't be cause for alarm.