“If entrance pupil is what matters for blur, that’s interesting and new to me. A quick compare of a couple lenses I have sitting around makes a startling point of this that may contradict what I have so far thought about DOF. For example, my 100mm f/2.8 for my Mamiya Press has an entrance pupil a bit smaller than the 135mm f/2.8 el cheapo M42.Does this mean that the Mamiya would have more depth of field at that aperture than the el cheapo? I always thought it was a complex interplay of distance to subject, distance from background, format size, lens length and aperture. Is it possible that lenses for smaller formats, in shorter focal lengths, just typically have a smaller entrance pupil, and therefore the aforementioned factors automatically come into play? Or is it really that simple, small entrance pupil = more depth of field?

Just mind blowing that my 6×9 Mamiya lens may have inherently less background blur than a $15 Ricoh M42 screwmount. ”

The real mental breakthrough for me was realizing that there’s TWO separate things we’re talking about.

The one we’ve always paid attention to is literally the depth of the IN-focus zone. This is what you learn from one of those online calculators like,
http://www.dofmaster.com/dofjs.html
And you’re right that several factors play into to that. The one too often forgotten is that the tolerable blur diameter increases with larger image formats. That’s the “circle of confusion” number, where something like 1/1500 of the image diagonal is conventional.

But in many cases, the quantity we actually care about is something different: It’s the AMOUNT of blur in areas far OUTSIDE focus. That calculation is entirely different from the traditional DOF figures. This is when the entrance-pupil diameter really starts to matter.

With the two lenses you mentioned, the cheapie telephoto would need to be used at 3 times the distance of the Mamiya to give the same subject framing. Adjusting for that, it looks like the DOF at e.g. f/2.8 is about half as deep when using the Mamiya lens. However you may still find that the blurriness of the far background is greater with the 135mm.

Practically speaking, that lens will also make it easier to maneuver to avoid some distraction in the background, since its angle of view is smaller.

The bigger the image format, the longer the focal lengths that go with it. So the entrance pupil to achieve the same f/ratio must also be larger. This is one reason why large format lenses are rarely faster than f/5.6, since otherwise they’d need to be ginormous and even more expensive than they are already.

Just to clarify, what Dwayne’s developed was a roll shot by photographer Steve McCurry. Working with National Geographic, McCurry had made special arrangements to receive the final roll to come off Kodak’s Kodachrome production line.

There are plenty of other Kodachrome rolls still in circulation, and Dwayne’s will continue to develop them through December 30th 2010. (I believe the date “December 10th” mentioned in the article is just a mistake.)