Sensor format and lens focal length is one of the most puzzling aspects of digital photography. Everyone probably knows smaller sensors mean increased depth of field for a given focal length and that sub 35mm frame cameras have smaller focal lengths to achieve the same angle of view. This creates the 2x focal length ‘crop factor’ on a Micro Four Thirds format, a 1.6x on APS_C and, well, 1.0x on full frame 35mm. How much difference does this make in terms of depth of field (or depth of focus)? I’ve always wanted to try this out, so time for a play – a test, sorry.
There’s a nice diagram illustrating the difference in sensor sizes here (Wikipedia). All shots taken in RAW, converted to JPEG using DXO Optics 9.
Firstly – field of view. These next three are all shot from the same tripod position at f1.4.
On the 50D it’s a 50mm x 1.6 so an 80mm equivalent :-
On the EPL5 its 50mm x 2 so a 100mm equivalent :-
What’s happening here is that although the effective focal length is changing, the depth of field from the same shooting position is the same for all three lenses – the smaller sensors are just sampling a smaller rectangle of the same 35mm image circle. The EPL5’s image is like an enlargement of the centre of the larger sensors’ images. It’s worth bearing in mind that the EPL5 has more pixels in it’s frame (16Mp) than an equivalent cropped 5DMk2 image (around 12Mp I’d guess).
Now – to try to create the same shot with all three cameras. This isn’t as easy as I first thought! What’s expected is that there will be greater depth of field on the smaller sensor as we’re further away from the subject. The common focus point is the blue reel of cotton with the red spool, focussed using the LCD and focus magnify.
First the 5d Mk2 (50mm) :-
Then the 60D (80mm equivalent):-
Then the EPL5 (100mm equivalent)-
Something of a surprise here – the difference in depth of field between the EPL5 and the 5dMK2 is obvious, but between the 60D and the 5dMk2 it’s not as great as I would have expected.
What this little experiment confirms is that for any given lens – in this case a 50mm f1.4 – the effective depth of field for smaller sensors is deeper than larger sensors when taking the same photograph. It’s still an f1.4 lens for exposure purposes, but for blurring away a background and isolating a subject the large 35mm size sensor is better.
However, not everyone wants shallow depth of field – if you don’t, these results could be seen the other way around! It all depends on what you’re trying to achieve.
For macro, landscape and telephoto photography (where depth of field is at a premium) I can see ‘Micro Four Thirds’ having an advantage.
For portraits and isolating subjects against a blurred away background ‘Full Frame’ is a winner with ‘APS-C’ not far behind it.
For general photography using intermediate focal lengths at medium to infinity subject distances there isn’t that much difference (I’m not taking into account high ISO noise, cost or any of the tens of other differences between sensor formats).
Hope you find this useful – thanks for looking!