by Ian Burley
Some help on using Four Thirds lenses and apertures
There is still a lot of uncertainty concerning how Four Thirds (and Micro Four Thirds, which is -in this context - exactly the same as Four Thirds) compares to other camera systems, especially so-called full-frame, when using one's camera. Most of us know that the field of view of a Four Thirds camera and lens is the same as a full frame camera with a lens of double the focal length. This is the 50% cropping factor or 2x full frame focal length multiplier. But what about apertures and depth of field? This article should provide some answers. It also highlights some of the inspired and brave thinking that Olympus brought to the table when it developed the Four Thirds platform.
Brightness and exposure
One thing we can clear up straight away is that f/whatever is the same in terms of brightness and exposure no matter what the lens is and the camera's cropping factor. Actually, that's not strictly true as some lenses will transmit more of the light that enters the lens than other lenses; a transmission efficiency that is determined by the quality of lens coatings and the number of lens elements used (more elements can mean less efficiency). But for our purposes, the aperture value is the same, so a full frame 50mm f/2.8 will have the same exposure value as any other lens at f/2.8, no matter what sensor or focal length.
Depth of field
Now this is where it gets interesting. Depth of field describes how well-sharp your view will be in front of and behind the subject that your camera has focused on. Actual depth of field is the result of a calculation that determines what the viewer will perceive as being in focus. The greater the depth of field, the more of the near and far distance will appear sharp. The smaller the sensor (and so the larger the cropping factor) the more depth of field you will get for a given aperture.
Have a look at these two pictures. The first was taken on an Olympus Pen E-PL2 using the m.Zuiko 40-150mm kit telephoto zoom. The second was taken using a Ricoh CX5 compact camera. The camera to subject distances were approximately the same and the field of view for both cameras was in the same ball park, if not perfectly the same. The lens apertures set for both shots was pretty close, but you should be able to see a big difference in the blur of the background.
Olympus Pen, (2x cropping factor), m.Zuiko 40-150mm f/4-5.6, @ISO 200, f=150mm (300mm equivalent) , 1/800th, f/5.6
Ricoh CX5 compact camera, (5.7x cropping factor), 4.9-52.5mm f/3.5-5.6, @ISO 100, f=44.5mm(254mm equivalent) , 1/620th, f/5.2
The compact camera, with its very small sensor and much higher cropping factor defines the background more than the Pen. In this case the Pen result is more desirable because it's good to blur-out distracting background details when taking pictures like this. you may ask is it desirable to use a lens at its maximum or widest aperture like this? With many older lenses the answer is probably no. Such lenses were designed to work optimally at the middle of their aperture ranges. But many modern lenses are designed to work well at the widest aperture, although most will sharpen usefully by reducing the aperture size slightly. Don't go too far - diffraction softening starts to erode sharpness progressively from f/8 onwards with Four Thirds cameras.
But it can also be very desirable to have as much as possible of the scene being photographed in-focus. This requires stopping down the lens (using a smaller aperture or larger f-number) and a wider angle lens.
Those of us that have used 35mm 'full frame' film cameras are familiar with camera settings used back then. f/8 was quite a sensible aperture to use for general purpose snapping, for example. You might stop down to f/11 or f/16 to expand the depth of field, etc. So how do aperture settings on Four Thirds relate to settings on full frame cameras? First of all you need to deal with the cropping factor. If you are using a 50mm focal length on Four Thirds, that means a 100mm focal length for the same field of view on a full frame camera.
To match the depth of field of an aperture used on a full frame camera with a Four Thirds camera, simply halve the full frame aperture number. So, f/8 would give you the same depth of field as f/4 on a Four Thirds camera with a lens that has the same field of view. Here are some examples:
- Full frame 100mm f/16 would equate to Four Thirds 50mm f/8.
- Full frame 300mm f/11 would equate to Four Thirds 150mm f/5.6.
- Full frame 28mm f/8 would equate to Four Thirds 14mm f/4.
And while we are at it, how about comparing depth of field and apertures with cameras that have APS-C sensors (from manufacturers like Nikon, Canon, Pentax, and Sony). They have a cropping factor of 1.5 (actually, the Canon EF-S platform has a cropping factor of 1.6), so if the aperture is f/8 on full frame, it's f/4 on Four Thirds and 5.3 on APS sensor cameras. f/5.6 on full frame means f/2.8 on Four Thirds and f/3.7 on APS. And don't forget to divide the full frame focal length by 1.5 as well for the equivalent APS focal length.
Olympus has developed some exceptionally fast lenses for their field of view. These include the 150mm f/2, 35-100mm f/2, 14-35mm f/2, 90-250mm f/2.8 and 300mm f/2.8. Double up the focal lengths and you get their equivalents in full frame. So a 150mm f/2 has the same telephoto view as a 300mm f/2. I see comments that say, well, actually the 150mm f/2 is more actually a 300mm f/4 equivalent. If you mean in terms of minimising depth of field, yes, but in terms of brightness, absolutely not.
With the Zuiko Digital 150mm f/2 you are getting a whole extra stop of light, which crucially means a faster shutter speed. I have never felt this lens could do with less depth of field but I have been in many situations where more depth of field was useful, so I have stopped down a click or two. With full frame you would start with the prospect of slower shutter speeds and this would be compounded much more if you needed more depth of field. With telephoto lenses you can't do without fast shutter speeds.And then look at the Olympus Zuiko Digital 300mm f/2.8. You can't even buy a 600mm f/2.8 for full frame cameras. The nearest thing is a 600mm f/4 and these lenses are the best part of twice as large and heavy as the Zuiko 300mm f/2.8.
Photographers with full frame cameras and larger may have to stop their lenses down severely to get adequate depth of field. This means longer exposures, perhaps the need to use a tripod or to use a higher ISO speed.
With Four Thirds, you can achieve greater depth of field using brighter aperture settings and so you get shorter exposures. By not needing a tripod or similar support, the basic concept of a lighter and less bulky camera system, which is central to Four Thirds and Micro Four Thirds, is reinforced. It also means you don't need to use higher ISOs as much, which helps address the perception that the relatively small sensors in Four Thirds cameras are noisy (which is a totally different debate!).
But what if you deliberately want to use creative blurring of the background (and foreground) through limited depth of field. Well, the example picture at the top of this page shows that even with a lens that has a maximum aperture of f/5.6 (at the long end of the zoom), it's not difficult to get nicely blurred backgrounds or 'bokeh' . It's even easier if you have a faster lens like an f/3.5. f/2.8 or f/2. It's not quite as easy with a full frame camera, but it's not that difficult. It's certainly a lot easier than with a compact camera, as the sample photos demonstrate.
It's true that if you are photographing a very bright scene, because of the problem of diffraction softening from f/8 onwards, you have less scope with Four Thirds for stopping the lens down to reduce the brightness. A 12 megapixel Four Thirds camera starts to register the effects of diffraction softening from about f/8. On a full frame digital camera with the same number of pixels that threshold is f/13. There is not much difference in threshold between Four Thirds and APS-C digital cameras, especially because many APS sensor cameras stuff more pixels onto the sensor and pixel density is one of the determining factors for diffraction softening. But in any case, you could just use a faster shutter speed or, if necessary, neutral density filters.
So, overall, the lot of a Four Thirds photographer is a pretty good one if you think about it carefully.
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