Equivalence. It’s the bugbear of anyone who reviews Micro Four Thirds lenses. You are being conned says the incoming mail. Your f/1.4 lens is really an f/2.8. And your so called shallow depth of field is commensurate with f/2.8, too, not f/1.4. It’s an argument I’ve heard so many times and while factually true, is pointless and irrelevant. The only rational response is -so what?

Put simply, a native Micro Four Thirds lens is just that. A native Micro Four Thirds lens. It isn’t a Full Frame lens. It won’t fit a DSLR and if it did it wouldn’t cover the whole frame. I’ve tried more and more to describe lenses according to their angle of view since that is universal. If you know what angle of view you want, you can choose a lens to get it. Thus, I know that I like as a standard prime a lens with a moderate wide angle, around 54° horizontal. A quick calculation at Points In Focus Photography tells me that for a Micro Four Thirds sensor it would be 17mm, for FF 35mm and for Medium Format 55mm. Easy.

Not so easy for depth of field. I hear from people who are quite obsessed with getting shallow depth of field. Medium Format is the real answer as anyone who has used Hasselblads will know. The Sonnar 150mm f/4 short tele let you photograph someone full length and splat the background totally. But for most with that obsession, Micro Four Thirds is the wrong choice of format. A more practical and adequate choice would be a DSLR with its 36x24mm sensor. But human nature doesn’t work that way. Come to think f it, nor does animal nature. I remember photographing some sheep grazing in a grassy field. I was there a couple of hours and noticed how some of the sheep would try to get under the wire fence to get to the next field. If they couldn’t, they would stretch themselves out and eat the grass the could reach under the fence.

The point was, the field next door was exactly the same as the field they were in. It was just as grassy and it was just as green. That I reckon, is why photographers buy Micro Four Thirds and then complain about too much depth of field. What do I want? Whatever I can’t have!

The difference with photography is that to some extent you can have what you want. But only at the cost of making it what you didn’t want. The Olympus Pro series primes are some of the best lenses ever made for Micro Four Thirds. With their f/1.2 aperture they are not only great in low light, they are an effective way of getting really shallow depth of field on a 17x13mm sensor. An f/1.2 for 36x24mm would give even shallower depth of field but starts to look like a gimmick. If you are shooting a portrait with an 85mm f/1.2 lens wide open, you’ll likely have far too little depth of field for a pleasing result and you’d stop down to f/2.8 or so.

Here comes my point – a Nikon 85mm f/1.8 for full frame will weigh a little less than an Olympus 45mm f/1.2 and be half the price. Most of the time, you will be stopping it down to between f/2.8 or f/4 to get adequate depth of field. In other words, it is doing the a very similar job to an Olympus 45mm f/1.8. My point more exactly is that in trying to emulate a full frame lens, you have ended up with what is in all except focal length essentially a full frame lens. The laws of optics and light are unfortunately not optional. In its simplest expression, the maximum speed of a lens is the ratio of the light gathering front glass to the focal length, focal length/front element diameter.

A 45mm lens with a 37.5mm front element is an f/1.2 lens. For the same 21 degree horizontal angle of view on a 36×24 sensor we need an 85/90mm lens so that 37.5mm front element will have an {85/37.5} f/2.4 maximum aperture(these are ballpark figures!). Since depth of filed is governed only by the size of the hole the light passes through and distance, the 37.5mm front element will give the same depth if filed on both lenses. Some photographers make a lot of equivalence but it is really only sensible to consider a lens in the context of the sensor size of the body it is mounted on and usually designed for.

So what’s the upshot? Simply, if you want similar performance from a Micro Four Thirds camera and a FF camera you will need a similarly sized lens. Given a 20Mp sensor, if it is 1/4 the area of another one, it will need to be fed twice the light intensity – twice the aperture – to get the same noise levels. Ditto depth of field. The trouble with using Micro Four Thirds to ape the results from FF is that it nullifies the advantages of the Micro Four Thirds which have always been mainly in the size of the lenses for a given angle of view. The body size is a moveable feast, from a Panasonic GM1 to G9 via Olympus E-M10 and is a straight choice.

The big mistake in emulating FF performance is that you are trying to emulate the FF camera’s ultimate performance. You will never do that and you do not need to, any more than a FF system camera needs to perform like a medium format one. It ain’t gonna happen. Micro Four Thirds takes advantage of the fact that modern imaging technology has outrun the needs of the photographer, making a 36x24mm sensor superfluous for normal non professional needs. Viewing results on tablets, even large monitors and prints at 20 inches /300dpi inches, the extra IQ of the FF sensor is all but superfluous.

That sheep may be right, perhaps the grass on the other side of the fence is greener. But even if it is, does it taste any better? The Olympus 45mm f/1.2, like the Panasonic 42.5mm f/1.2, is a great lens. But every shot you take at f/1.8 and lower represents unnecessary weight and outlay. Plus, given the size and price of the Pro lens, if I were going to have a set of these primes, wouldn’t it be cheaper and more effective to buy a a small bodied DSLR and a set of relatively cheap FF/1.8 or f/2 primes for it? If I am going to carry FF size lenses around with me, might it not make sense to use a FF body too?

I am not offering these thoughts as what I personally favour or a recommendation of any sort but offering the argument that if I’m looking for FF performance, wouldn’t it be better to use an outfit where it comes as part of the nature of the system rather than force a system designed for compactness into an alien role? What has brought all this on is the Panasonic 50-200mm f/2.8-f4 I have been using. I am a big admirer of the Olympus 40-150mm f/2.8 Pro zoom but there’s one thing that irks me. Its size means that it has to sit lengthways in my camera bag. I am totally unprepared to buy a bigger bag since in the city in which I mostly lurk and on the bicycles I mostly use for transport, a bigger bag spoils both my enjoyment and mobility. I fit my gear to my bag, not my bag to my gear. The Panasonic 50-200 fits my bag nose down and in spite of all its disadvantages vis a vis the Pro zoom, I find it in my bag all the time while the Olympus sits on a shelf. In particular, the vertical storage of the lens means I can add my 45mm f/1.8 and Meike fisheye to the bag with no increase in weight.

It hurts me to see such the beautiful Pro zoom lens sitting on the shelf but a Lowepro sling bag with a G9 and lenses from fisheye through to 200mm with a wideish aperture portrait lens as well is a seductive thing. I can understand and agree with every reason put forward for those big, expensive optically superb f/1.2. And yet, in my heart, ever since I bought into Micro Four Thirds I’ve retained my original reasoning. Put a an Olympus 17mm f/1.8 on a Panasonic GX9 body and go outstreet shooting in Soho. Now go out with a 17mm f/1.2 on the front. What can I say? Little and good, big and bad.