Internet forums are full of self-proclaimed experts spouting absolute crap about things they know nothing about. Unless you were hiding out in a cave with Osama Bin Laden or not visited the home of countless pointless photographic brand based crusades that is otherwise known as dpreview.com you maybe aware of this hot topic that has spawned a thousand fevered posts and countless character assassinations. If not it goes like this: equivalence is the idea that the size of the camera format effects focal length, depth of field, and exposure, and that when talking about equipment one should express this in terms relating to 35mm equivalents. These arguments became particularly prevalent when Olympus and Panasonic started to have some success with their micro four thirds format. The people express this idea most fervently are those who have sold two of their children and a kidney on Ebay to fund the purchase of a so-called full-frame dSLR and they want to let everyone who uses a smaller format that they are using an inferior product. Usually these arguments go like this:

m4/3 user: Hey I’ve just got my self a new lens, it’s the Olympus 45mm f1.9. It’s a lovely lens, wide open it produces very portraits wide open.

Full framer: It’s not a 45mm f1.8 it is a 90mm f3.8. It cannot render out of focus backgrounds and the smaller sensor needs more light and therefore longer exposures or higher ISO. A proper photographer would use an 85mm f1.2 on a full frame camera.

The argument will typically degenerate and lead to insults and other full frame users diving in to bolster the argument with lots of spurious mathematical equations, graphs and claims of mental deficiency on the part of any smaller format user and maybe even casting doubts on the mental faculties of their parents as well. So what is all this about then and why are people getting so hot under the collar?

Equivalence is not a new thing, it didn’t come with digital, it was a known fact back in the dim dark ages of film, and then funnily enough 35mm film was seen as the miniature inferior format. In those far off times a lot of professional work, and when I use the term professional I mean done for money, was shot using large format, the smallest was 5×4 inches and the largest practical was 10×8 inches. The next step down was medium format where a roll of film was used that was nominally 6cm wide, the popular sizes were 6×7, 6×6 and 6×4.5 cm. Small format, or miniature format was 35mm and smaller. It wasn’t until the advent of modern electronics  and a significant breakthrough with tabular grain film  in the 1980’s that 35mm really took off as a format. The ease of use that automation made with sharper finer grained films that made many professional photographers and advanced amateurs take up using 35mm and by the end of the 1990’s the larger formats had become niche products for specialised uses.

Professional photographers were well aware of equivalence.  A 10×8 camera with a 300mm lens has the same angle of view as a 5×4 camera with 150mm, a 6x6cm camera with 80mm and a 35mm camera with a 50mm. As most working photographers ran more than one format and some three or more favourite angles of view would be duplicated across the systems. So for 5×4 a photographer may have a 90mm, a 150, and a 300. His/her 6×6 system would then have a 55, 80 and 150mm lenses, and the 35mm system  35, 50 and 100mm lens all giving roughly the same angles of view.

Factors that influence depth of field are:

• format size
• focal length
• distance to subject
• magnification
• lens aperture

From this we can, therefore assume that the larger the format size , the longer the focal length, the closer to the subject, the larger the lens aperture, and the greater the magnification the less the depth of field. So that is clear then. Well no because when we talk about depth of field it is also important to remember that what we are really talking about is acceptable sharpness because a lens can only render one single distance in focus (the plane of critical focus) and objects at other distances to the front and rear will have diminishing sharpness the further they are away from that point. The reality of this is that for most normal usage part of the scene will be acceptably sharp both in front of and behind the point of critical focus, and this zone will be formed that 1/3 of it is in front of the subject and 2/3 behind. This zone can be shallow with only a small part of the scene appearing to be sharp, or it can be deep with the apparent sharpness seemingly covering everything. Seems simple so far, well it gets more complicated because you can break these relationships. As magnification increases the depth of field decreases and the zone of apparent sharpness also shifts from the one-third in front and two-thirds to the rear to an equidistant amount front and rear. Also once you start playing with the plane of the lens in relation to the sensor and subject then everything can go a bit strange. Typically one of the problems that plagued large format shooters was that of getting enough depth of field. Large sensor, small depth of field, not much chop if you’re a landscape shooter. Well using the Scheimpflug rule you can place the plane of focus wherever you want in a scene by positioning the plane of the lens to be parallel with the plane of the subject by tilting the camera front, if that does not completely achieve what you want then you tilt the sensor plane as well  so that the planes of the subject, lens and sensor form an intersecting point and this will mean that the subject plane will then be completely sharp while shooting wide open. Working the other way round you can diminish the zone of sharpness  by swinging and tilting the camera front. This is not the sole prerogative of large format camera, tilt and shift lenses are available in medium format, 35mm, APS and even m4/3 . The other thing that throws a spanner in the works for depth of field is that the size of the output and viewing distance plays a big part. Simply put big print viewed at a distance and apparent sharpness is reduced.

To test the theory I decided to print out an A3 sized lens resolution test chart and this was then set up on an easel in my garden. A tripod was then set up 1.8 metres away from the easel. The cameras tested were the Olympus EP-2 and the Canon EOS5d, both having roughly the same number of megapixels. Choosing lenses was somewhat problematic although I have a set of professional grade fast aperture constant zooms for my 5d I don’t for the EP-2. The lenses used were for the Canon an EF 24mm f2.8, a Sigma 105 f2.8 macro and  an 75-300mm f/4-5.6 IS USM. For the Olympus the lenses were a Zuiko 12-50mm f3.5-6.3, a Zuiko 60mm f2.8 macro, and a Zuiko 40-150mm f4-5.6. Each camera was mounted on the tripod in turn and set to shoot large quality jpegs in aperture priority, image stabilisation where appropriate was switched off and the self timer was used to prevent any camera shake. The lens was focused on the chart. None of the images have been processed in any way and the full-sized images can be seen by clicking on any of the composite shots below where you’ll be taken to my Flickr account. At the 24mm and equivalent view the DOF was pretty much equal, although I suspect that there maybe more apparent differences at apertures of f2 and wider, but I wasn’t able to test that. The comparison between the two short telephoto macro lenses highlighted the most difference and at wide open at f2.8 the Olympus manages to resolve some background detail while the Canon doesn’t at all. At the equivalent of 300mm at f5.6 there is very little discernible difference, but that changes at f8 and smaller where the m4/3 camera starts to resolve more detail.

So what happens to exposure. Doesn’t that change? The short answer is no. When I got my first medium format camera, a secondhand Mamiya C330 Professional , it had no built in metering and the  handheld Weston meter that came with it had obviously been dropped and no longer worked properly so I used my Olympus OM1n  as a meter and guess what? It worked! Much later when working in the studio with studio flash I determined the exposure with my flash meter and would often make exposures with different format cameras but the exposure value remained the same and the images were correctly exposed. Memory is a fine thing, but I wanted to make definitely sure so I decided to run a quick test comparing full frame, to APS-c, m4/3 and compact camera with a sensor measuring 8.07 x 5.56 mm. The test was simple each camera would use a lens of as near as possible field of view equivalent to 105mm on full frame. The ISO was set to 200, and the shutter speed was set to the maximum synch speed so as not to let ambient light interfere with the result. White balance was set to flash, and pictures settings were natural and the images recorded as fine jpegs. A Canon 550EX speedlite was mounted on a light stand, set on manual to 1/2 power and fired at the test scene through an umbrella diffuser and was triggered by a synch cord. The flash output was measured with my Minolta flash meter which is accurate to 1/10 of a stop, the aperture required for correct exposure was f7.7, each cameras lens was set to the nearest corresponding value which was f7.1. All images are straight out of the camera without any processing. Each cameras image was opened up large in Lightroom 3 and screen shot was taken of the image showing the histogram. Looking at the histogram shows that there’s barely any discernible difference between each cameras recording of the scene.

So what does all this mean? Well the full framer is right in that a 45mm lens on m4/3 does have an angle of view equal to a 90mm on a 35mm sensor. As to the inability to have shallow depth of field that is wrong, it is definitely possible to minimise depth of field but we are talking about subjective differences here as there is that nebulous term “acceptable sharpness” and then it depends on how big you print, and how close you are when you view it. Personally when I take a head and shoulders portrait of someone I like to have from the tip of the nose to the ears in focus as I like to be able to see who I’m looking at. Shooting at 85mm f1.2 the depth of field can be measured in millimetres and so the iris of the eye might be in focus but the rest is just blur. This is just a matter of taste. If you were shooting landscapes using wide-angle lenses then there is little to choose between m4/3s and full frame, and in fact I might go as far to say that the smaller camera has the advantage being smaller and lighter. As to the matter of the smaller format needing more light to form an image that is just plain wrong.

So is the 35mm dSLR superior to m4/3? Yes and no. Infuriating aren’t I! It all depends upon your photographic priorities. If you want to squeeze every last ounce of image quality out of a scene then 10×8 or whole plate large format film cameras either contact printed or drum scanned cannot be beat. Hopeless though for getting shots of the rug rats playing at the park, expensive to run, not very portable and definitely not conducive to spontaneity. Want something you take anywhere and be able to upload images to the web, well your phone cam is your best bet. My Olympus EP-2  is my favourite go anywhere camera. I can rig it up to be like a dSLR by adding its optional electronic viewfinder when I want to use a long lens, put on the 17mm f2.8 pancake lens and it fits in my jacket pocket, put on a mic,  a loupe, put it in its cage or rig and its a great little video camera. It perfect for lightweight travel. Out walking the dog or shopping then chances are I’ve got my Panasonic LX-5. My Canon EOS5d  is the original 13Mp version and I still keep for certain things and for certain lenses that I just don’t want to give up. There are no absolutes, pick your camera for what you need not what other people think that you should use.