We hear a lot about lenses and their 'equivalents'. How such and such a mirrorless camera lens and aperture is 'equivalent' to such and such a lens and aperture on a 35mm sensor camera. I do it too, so I'm not innocent, but I've always disliked using it, because in the real world it just doesn't work like that.

I set up a tripod and my Stratocaster to photograph. First I photographed the guitar with my D800E and 50mm lens at f/3.5. Then without moving either the tripod or the guitar I put my Nikon 1 V1 plus 18.5mm onto the tripod and photographed the guitar at f/1.8.

This is what I got.

The first thing to notice is that its not even the same picture in the frame. The camera / lens combinations are, of course, different sizes which affects how far the lens is from the subject. Now I could of course have very carefully taken ages to make sure exactly the same composition was in the frame. But who on earth does that when they are taking pictures? Only very sad people, I would suggest. 

Add in to this, the fact that the Nikon 1 has slightly overexposed, the D800E has slightly underexposed, the cameras are focused on different things because I used centre spot focusing on both cameras and again the different sizes of the cameras has affected that, the depth of field is different, the shutter speed is different.

Now in the Photoshop file info it quite clearly states this but just how useful is this, and indeed any other notion of equivalence? What we have above is two different pictures taken with what looks like two different lenses at different focal lengths. Its not that close, even when you think of it as an approximation. Also I see no point whatsoever in setting up some test where everything is changed around so that exposures, positions, apertures, focusing etc. are set up so that each picture is identical. Again how sad is that? Do any of us when we go out shooting try to emulate the performance of a similar lens on a different format? Some might I guess, but thats somewhat down my personal list of priorities of what to do with a camera. Yes I decide on the aperture I want, yes I decide on my composition, shutter speed etc. But all my decisions relate to what is best for the image and I make them based on the lens / camera combination I have. I see little point in doing anything else.

It can be useful to get a rough guide as to what lens sizes are on formats we are unfamiliar with. Incidentally it is the case that there are lots of people using cameras these days who have never used 35mm, either film or digital, and exactly what use a 35mm equivalent is to them I cannot imagine. 

I watched a great programme on the photographer Norman Parkinson a few days ago (He even got a Google homepage). If you are in the UK you can see it here. While he was obviously technically proficient, when he asked his assistant for a camera to use he decribed them as 'The Hassleblad with the fat lens' and 'The Nikon with the zoom'. He did of course know exactly what he was getting and what the combinations would offer him, but was much more concerned about the rapport with his models and the images he was creating.

And thats the way we should approach it surely. I know what kind of pictures I'm going to get with my Nikon 1 V1 plus 6.7-13mm zoom, and I know what kind of pictures I'm going to get with my Nikon D800E and Sigma 12-24mm zoom. Somewhere in there is some kind of 'equivalence' but I see little point in thinking about it. 

I think in future I'm going to to describe lenses of different formats as 'being roughly equivalent to' each other. It strikes me that its not useful to go any further than that. There are just too many variables to confuse the issue.