Who hasn't wondered what the hubbub is all about. All this "crop factor" talk. Full frame, 1.3x, and 1.6x (well – for the Canon-ites out there). I think we all understand how it effects the reach of our lenses, but how does it effect the bokeh of our images?
A Quick Re-wind…
For those who may not know the first part of that, it is pretty straight forward. The crop factor, multiplied times the focal length of your lens, yields an "effective" focal length number, or "35mm film equivalent" number. A full frame sensor – sized the same as a negative – has a no crop factor, or really a value of one. For Canon, this would be the 5D, and the "s" flavors of the 1D series – the 1Ds, 1Ds Mark II, and the 1Ds Mark III. Most of the entry level (and again – I'm typing "Canon" speak here) and mid level cameras are 1.6x, (Digital Rebel, 400D, 20D, 30D, 40D, 50D, etc) while the Canon 1D (non "s") cameras toe the line at 1.3x (the 1D, 1D Mark II, and 1D Mark III).
So – in practical numbers (and an easy one at that) if you put a 100-400mm zoom on each of the different crop factor cameras, set it as wide as it could go (at the 100mm position), you would get photos out of them that appeared to be taken at 100mm (the full frame), 130mm (the 1.3x), and 160mm (the 1.6). This can be a bonus if you are looking for extra reach out of your lens, but can be a detriment if you are looking to get wider angle shots.
Back to the Point of Today's Post…
Okay – the sharp end of today is what about those middle zones… what really is the difference? Does the crop factor make any difference to the photos that would be framed the same in all three ratios?
Let's say three people, one with each crop factor type, see the same subject and want to photograph it. We'll also say that all three people stand the same distance away from that subject and frame their shots the same way through their viewfinders. Okay – and – miraculously enough, all three are using the same lens and decide upon the same settings for f-stop and shutter speed. How will their pictures be different?
Sure – we need to throw out the differences in the sensor's sensitivities, color saturation, etc… those really are not a factor of the crop factor itself, just a part of the construction of the sensor elements and image processing software and firmware.
How the Test was Done…
I have a 1D Mark II N and a 50D, so I had the 1.3x and 1.6x covered; luckily a friend – Kevin – just picked up a 5D. Voila – gotta love a plethora of bodies to test with. Now that the gear was covered, it was pretty easy to configure a test that would minimize differences and allow for valid comparisons.
A few of my vintage cameras soon were on a table with a magazine propped up against the wall to serve as a more defined thing to get out of focus in the background. I mounted my 70-200 f/2.8 L IS to my tripod, and switched off the IS. To get the same net image, the effective focal lengths had to be the same. For the FF and 1.6, the math was easy, shoot the 1.6 at 100mm shown on the lens, and shoot the FF at 160mm. Some quick math (160/1.3) yielded 123mm for the 1.3x. We ended up shooting w/ the 50D first, so we metered with it and chose the exposure of 1/10th at f/2.8 (we chose the f stop to minimize the depth of field). Other tidbits – all the cameras were set to "neutral" picture style; RAW; I shot gray cards to set custom white balance for all three cameras; all were set for mirror lockup and a remote trigger (wired) was used.
After shooting, the images were brought into Lightroom, I did boost the vibrance on the shots from the 50D as the colors looked very different from the other two bodies (more on that later - as in another post). They were then exported as 800px wide images, 70% quality, sharpen for screen (for the "full image" pictures posted here); and as 100% quality, scaled to match the smallest (the 1D) image dimension so that when cropped, they would all be the same size, no sharpening.
The three photos below (click to enlarge) are the three "as they filled the viewfinder" images. The 1.6x (50D) at the top; the 1.3x (1D Mark II N) in the middle, and the full frame (5D) at the bottom.
Okay – to sum it up – there is a very noticeable difference between the three photos. Now the distance for these was about 7 feet from the camera to the cameras, and another 8 inches or so to the magazine. The two areas that seem to illustrate the bokeh difference for me are the forearm of the rider and the lettering of Velo News.
On the forearm, the word GARMIN is sorta readable in the 50D (1.6x crop), is a bit blurrier on 1D (1.3x crop) and is darn near un-readable on the 5D (full frame). In the lettering of the N, you can see how much the orange color expands and blurs as the crop factor gets smaller.
Now, some might think – wow – what if I don't want that much bokeh… I should avoid full frame… Nope – just don't shoot at f/2.8. You can add depth of field by going to f/4 or f/5.6.
The key thing here is, if you are looking for MORE bokeh, moving to a full frame sensor camera body will do the trick. It isn't just buzz… it is very noticeable.
Thanks for reading!