pupil magnification of Canon lenses

paddler4

Anyone know the pupil magnification values for the Canon 60 and 100mm macro lenses?

I went back to reading up on DOF in response to a criticism that out-of-focus material in the foreground can be distracting in some cases. This criticism was provoked by this shot, which I made with the 60mm (the only macro I own now):


I just found an unusually clear explanation of DOF in general, including the implications for macro photography: http://toothwalker.org/optics/dof.html

After dealing with the usual misconceptions about DOF and focal length, the author describes two things relevant to this particular kind of problem:

--given the same lens design, longer focal lengths generally shift DOF forward relative to short focal lengths, while (under some circumstances) not much affecting the total DOF

--telephoto lenses often are not designed the same in terms of one parameter, pupil magnification, and the smaller P for some telephotos will increase DOF.

The moral of the story would seem to be that to lessen the problem with this shot, use a longer macro lens, particularly if it has a lower P value.

So, I wrote Canon to check whether the 100mm f 2.8 really does have a smaller pupil magnification than my current 60mm, and they told me:

"pupil magnification is not a specification that
Canon publishes for our lenses"

Nice of them. Anyone know whether the 100 mm really does have a smaller p than the 60mm? Or anyone think my interpretation of all this is wrong?

Thanks

Dan

Lord Vetinari

AFAIK for all practical purposes the DOF will be the same at the same magnification and aperture using different focal length macro lenses. What is different is the OOF areas which will be more OOF and less recognisable in a longer focal length macro lens than a shorter macro lens.
ie longer macro lenses tend to have a better bokeh. I'm not sure this would really affect your picture in a positive way though as the flowers in the foreground might appear even more blurred.
So if anything I would have said the flowers in the foreground would be less blurred with a shorter focal length macro lens then a longer one but they would both still be OOF.

The other thing to remember is the important thing is whether you like a shot for whatever reason rather than other people's opinions !

Brian V.

paddler4

Brian,

Many thanks.

Re whose taste matters: absolutely. However, I do agree with the critic in this case. I think it is more pleasing in shots like this to have more of the foreground in focus. I think the third of the ones you posted today--the last of the hoverflies--is a great comparison to the one I posted in this respect. BTW, they are all magnificent.

Re DOF: In general, my understanding is the same as yours. However, the anonymous poster at the site I mentioned, http://tootwalker.org/optics/dof.html, has more detail than I knew before. He explains that the similarity of DOF across focal lengths has two exceptions: when the focal distance is not small relative to the hyperfocal distance (not an issue here!) and when lens designs differ in the degree of asymmetry, measured by pupil magnification. That's what I hoped to get from Canon. Instead of answering my question about the 100mm 2.8, they suggested I buy the 180, which is nearly 3 times the price. I'll probably end up with the 100 anyway because it is too hard to chase bugs with a 60mm lens.

BTW, the posting also has the clearest explanation of the background blur difference that I have seen. He draws a distinction between absolute and relative blur, the former being what you get if you enlarge portions of the images to the same size. The former is similar across focal lengths, while the latter is not. If some of the other posters on this site are as new to this as I am, they might find that posting helpful.

Dan

Lord Vetinari wrote:

AFAIK for all practical purposes the DOF will be the same at the same magnification and aperture using different focal length macro lenses. What is different is the OOF areas which will be more OOF and less recognisable in a longer focal length macro lens than a shorter macro lens.
ie longer macro lenses tend to have a better bokeh. I'm not sure this would really affect your picture in a positive way though as the flowers in the foreground might appear even more blurred.
So if anything I would have said the flowers in the foreground would be less blurred with a shorter focal length macro lens then a longer one but they would both still be OOF.

The other thing to remember is the important thing is whether you like a shot for whatever reason rather than other people's opinions !

Brian V.

jpc

Hey Dan,

Here's a similar shot taken at 180mm:



My increased working distance allows my subject and the stamens in the foreground to both be relatively close to the focal point. Since the focus goes away gradually between the focal point and the camera, the closer an object is to your camera, the more out-of-focus it is. (I know you know this) In your case, the little flower bits were probably very close to the end of your 60mm lens, while mine were over 12" away from my camera.

So, I'm thinking that a longer lens would have resulted in less blur of the objects in your foreground.

paddler4

Thanks. (and a nice shot!). I think it is time for some experimentation.... it would be easier if Canon would provide the one missing #, but they won't.

jpc wrote:

Hey Dan,

Here's a similar shot taken at 180mm:



My increased working distance allows my subject and the stamens in the foreground to both be relatively close to the focal point. Since the focus goes away gradually between the focal point and the camera, the closer an object is to your camera, the more out-of-focus it is. (I know you know this) In your case, the little flower bits were probably very close to the end of your 60mm lens, while mine were over 12" away from my camera.

So, I'm thinking that a longer lens would have resulted in less blur of the objects in your foreground.