Lifesize

Lifesize

This blog follows on from one of Professor Bob Newman’s columns in Amateur Photographer, about the idea that smaller sensors give greater depth of field, which he started by saying is ‘not exactly’ true. He then went on to discuss macro lenses, and it’s here that the nugget – in my eyes, anyway – was concealed.

Now, I have the impression that Professor Bob Newman’s monthly technical articles in Amateur Photographer are sometimes a little confusing. Certainly I find them so, and I believe this is the result of three things. First, Prof Newman actually knows a lot of stuff. This means that he wanders easily into the long grass of physics, in a way that leaves the rest of us wondering what just happened. But he writes about interesting things, and important ones, so I find it's worth the effort of persisting, even if i come out knowing something new that is not the something new I was expecting to know.

And he’s a controversialist. That means he often asks a contentious question, just to make a point – and, in my judgment, he doesn’t always answer the question so much as pose another one! And finally, he’s a competent rather than great writer of the language.



Anyway, three-quarters of the way through the article is something that is absolutely obvious, but which I hadn’t thought about. Macro lenses are sold on the basis that they give one-to-one reproduction. Irrespective of focal length, they cast an image on the sensor that is precisely the same size as the real object. A one-inch object will occupy an inch of sensor.

Ahhh… There’s the thing. If you’re comparing full frame with micro four thirds, an inch of sensor changes from ‘fitting comfortably’ to ‘won’t fit’.

So, to get as (apparently) close with my Olympus OM-D as with my Alpha 7, I need only go to 1:2 to fill the frame as well as the alpha 7 does at 1:1. Simples…

But the article was about depth of field, and that may take me a while longer to sort out in my mind. Follow this space.