There is a perpetual online discussion amongst measurbators about “equivalence”.

Equivalence is a very real phenomenon which can be predicted and measured.  The idea is that when comparing cameras with different sensor sizes, smaller sensors tend to have greater depth of field at “equivalent” focal lengths.

Why “equivalent” focal lengths?  Ok, it gets technical pretty quick, but rather than bore you with esoteric technobabble, lets just look at some images.  (Photography is about making images, right?!)  If you feel that the differences are drastic, then by all means read on.  But if none of these images strike you as dramatically different from one another, just know that you can safely ignore anybody who ever mentions “equivalence” or “equivalent apertures”.  Because they’re either trying to waste your time or sell you something.  (hopefully I’ll have saved you from the clutches of some forum troll)

Here they are:

This is what the war is about.  A mountain of a mole hill, idn’t it?

So what are these earth shattering differences due to?  Sensor size.

Sensor size affects the final image in many ways–the least dramatic of which is probably depth of field!  Generally speaking, the larger a sensor is, the shallower the depth of field is.  The top image was shot with a Nikon D700 (36x24mm sensor), the second was with a Nikon D7100 (~24x16mm), and the last is shot with a Panasonic GX1 (~18×13.5mm nominally–however this was shot in 2:3 aspect ratio, so really about 18x12mm).

Why is this important?  Because to get the same framing with cameras with different sensor sizes, one needs different focal lengths.  The first image was shot at 200mm; the second at around 125mm, and the third at around 95mm.  These were all shot with the same zoom lens, wide open at f2.8, adjusting only the focal length.  In fact, I didn’t even move the lens–I just mounted it directly to a tripod and swapped cameras with the lens in place.  (sorry, it does appear to have wiggled a little from shot to shot).

Focal length is one of the main ingredients in determining depth of field.  The other is aperture (Aperture is…a complex topic.  For more, just read here: https://en.wikipedia.org/wiki/Aperture).  Since aperture was fixed from shot to shot, all the difference in DOF is due to focal length.

Because of the differences in sensor size, to get the same framing with each of these cameras, one needs to utilize different focal lengths.  For instance if one is using an 85mm lens on a “full frame” camera (aka 35mm, aka FX, aka 24x36mm sensor), one would need to use a 42.5mm lens on a Micro 4/3 system camera to get the same framing.  But if you used the same aperture setting on both, the 4/3 camera would look a little different–a little bit more would be in focus.

If one knows anything about photography, one knows that if you want shallower depth of field, one just “opens up” their lens.  You use a larger aperture.  If the full frame camera is shooting at f4 and you want exactly the same look, you shoot at f2 on 4/3rds.  No big deal!  Right?

Well the issue is that all lenses are subject to the same design limitations, and that means that the largest apertures commonly seen on lenses are 1.8, 1.4, and a rare 1.2 here and there.  And this is regardless of sensor format/mount.  85mm lenses for FF and 45mm lenses for 4/3rds are both readily available, but only at the same apertures.  And to reproduce the look of an 85mm f/1.4 lens on a D700, one would need a 42.5mm/f0.7 lens, which is practically non-existent.

In short: you can’t buy lenses for the the 4/3rds system that will give you exactly the same look that you can get with a “full frame” camera.  You’re not likely to see a 35-100/f1.4 lens for 4/3rds, which would replace your 70-200/f2.8 on your full frame camera.  Not in the near future at least.  Maybe in the distant future.  If ever.

And that is a reason people constantly use to deride smaller sensor cameras and their users as less than professional or inferior in some way.  And by “people” I mean the the marketing departments of companies that don’t make small sensor cameras that are very competitive (aka Nikon and Canon).  And measurbators in online forums.  (aka TROLLS)  Well, maybe they’re not trolls per se.  Just people with inadequacy issues, trying to justify a purchase (maybe one already made?).  Or sufferers of GAS.  (Gear Acquisition Syndrome)

There are many reasons that the Micro 4/3rds system can’t replace my larger sensor DSLRs today.  But “equivalence” ain’t one of them.  m4/3 is actually competitive for a lot of good reasons right now: price, lens selection, features, image quality, video capabilities.  Even the weakest points–autofocus and resolution–are really not so far behind the best gear.

Is there a difference in DOF between these three cameras?  SURE!  Is it a big difference?  I dunno.  You tell me…