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Full frame vs crop camera comparisons part 2.


Claim 3 Full frame sensors provide better image quality

The theoretical pixel-density advantage of crop cameras discussed in Part 1 should have given a victory to the crop-sensor cameras. Surprisingly, that didn't happen, so let's see if the claim that larger-format sensors provide better image quality is true.

A similar experiment was set up to that in Part 1, and again I enlisted the services of a very co-operative chimpanzee. The lens used was a Canon 70-200 f2.8 IS (with IS turned off) mounted onto a sturdy Gitzo tripod .

However, this time, instead of shooting each camera from the same distance to the subject, I attached each camera in turn and moved the tripod backwards and forwards until the magnification/field of view was the same for each camera. To judge this, I simply placed two memory cards on the table, which enabled me to match the frame width for each camera.

This methodology should put the full 16.7 megapixels of the 1Ds mk II onto the subject - so should give this camera a huge advantage - not only because it has a full frame sensor, but also because it has 40% more pixels than the other cameras. The 1D mk III and 40D each have 10.1 Megapixel sensors and Canon's third generation digic processor - but the sensors are of different sizes of course, so it will be interesting to see if the the 1Dmk III with it's larger sensor can beat the 40D in this test.

NB As in Part 1, I had to compensate for the different output sizes from the 16.7 and the 10.1 megapixel cameras. To do this, I opened the images in Photoshop at 100% (actual pixels) and then used the generally-accepted practice of interpolating up the smaller images and interpolating down the larger image to a mid point value (actually 13.4 Mp). This method is not perfect, but each camera is similarly slightly disadvantaged. In reality, it is a small price to pay in order to make direct comparisons without having to compensate for image size differences.


Starting image : Co-operative chimp. He stayed still the whole time (unlike my cat that was my original chosen subject!).

100% crops (identically post-processed)



1D mk III 100% crop


40D 100% crop


1Ds mk II 100% crop


Just to show how good the 1Ds mk II is ....this is the 100%, sharpened crop at original size with no interpolation.Look again at the starting image of the chimp on the table to see what a tiny proportion of the original image this represents.

CONCLUSIONS

As should be expected considering it's lofty original price tag, the 16.7 Mp 1Ds mk II was the clear winner in this test. It shows better resolution and the extra megapixels will enable much larger prints to be made when compared to the 10.1 Mp cameras. However, it is not possible to judge how much the full frame sensor is contributing towards this performance and how much is simply due to the extra 40% of Megapixels present. I must also confess to having to search quite hard to find a subject that clearly demonstrated the differences that I routinely see on my monitor. There is nothing like feathers and fur (even if it is a bit synthetic) to show up camera and lens differences.The quality differences also have to survive the drastic downsizing that web-viewing necessitates.

The 1dmk III shows a small but definite and reproducible increase in image quality over the 40D and this is in agreement with my general feeling of having used these cameras for some time. However, the differences only show up in the 100% crop. As the cameras share Digic 3 processors, number of megapixels etc, it is reasonable to assume that the increase in image quality is due to the larger sensor benefits previously discussed. If you extrapolate from the 1.3x crop sensor results then it is reasonable to expect that a similar improvement again is present within the 1Ds II full frame image.

All cameras tested produce stunning images, and it is only when you start pixel-peeping at 100% crops that the differences become apparent. The 1Ds mk II can clearly produce the best image quality of the three, but at a price ! Whether you think that the extra quality justifies the cost is a matter of personal opinion. I bought my 1Dsmk II second hand after in was superceded by the 21Mp 1Ds mk III. The 1Ds mk III is essentially the same camera as a 1D mk III other than it has the full frame sensor, so how Canon can justify the massive price premium I don't know. At the time of writing this in the UK, a new 1Ds III is £4700 compared to £2376 for a 1D mk III - that's £2324 more, essentially just for the better sensor !

The Canon 40D is currently only £498 and is therefore an incredible bargain - as it can still compete with the other two cameras on image quality. The 1Ds mk II feels like an old camera by comparison, as it lacks the high-quality rear screen, liveview, much improved menu system and requires a lot of complicated multiple button-presses to operate it. The ol' gal can't half still deliver on image quality though - it is a bit of a classic.

Finally I should give a big dollop of praise to the Canon 70-200 IS zoom lens (this is beginning to sound like the Oscars) as it delivered incredible image quality throughout these test. It is my favourite zoom lens as it can compete with most primes.

Claim 4 Full frame cameras have shallower depth of field, so it is easier to blur the background

To put this to the test, I used my trusty chimpanzee assistant again. The tests were set up as before with the 70-200 f2.8 IS lens. I excluded the 1DIII from this test as the objective was simply to determine whether reducing sensor size increased depth of field. I had placed a newspaper at a fixed distance (two feet) behind the chimp in order to help gauge depth of field.

The first test was conducted with the cameras set up at an identical distance from the subject, with the identical lens set to the same focal length (78mm) and aperture (f8). Due to the 1.6x crop factor, the image from the 40D below appears much larger than that from the full frame camera of course.

40D 100% crop


1Ds mk II 100% crop

CONCLUSIONS

These are interesting results. Full frame cameras have a reputation for having shallower depth of field - and this makes it easier to get a smooth background in portraits for example. But this is clearly not happening here - in fact it looks like just the opposite is happening. What we are seeing is both the subject and the background sizes being multiplied by the crop factor of x1.6 in the case of the 40D. This makes the background lettering appear more blurred. In the next image, I have interpolated the 40D image down by a factor of 1.6x to match the background size (the text) of the 1Ds mk II image.


40D image reduced

Compare the depth of field again to the 1DsII image - now we can see that there is actually no difference ! So let's say that again - when shooting from the same distance, using the same focal length lens and the same aperture there is no difference in depth of field between a full frame and a crop camera.

This is not actually the end of the story though, as another test must be conducted, in which we will match the magnifications of our chimpy subject. To achieve this, keeping everything else the same, I moved the tripod/lens/1DsII closer to the subject until the magnification was the same as the 40D image. I used a couple of CF cards as crude markers of the width of the 40D field of view, and then matched it with the 1Ds II in the viewfinder.


40D 100% crop


1Ds II image matched in magnification by moving the camera closer to match the field of view of the 40D

CONCLUSIONS

When the full-frame camera is moved closer to the subject to match the magnification of the crop camera, with the same focal length and with the same aperture selected, then we can finally see that the full frame camera does show a marked decrease in depth of field. i.e. blurring of the background. This can be a help or a hindrance, depending on whether the requirement is to achieve greater or lesser depth of field. Depth of field is at a premium in macro photography, so a crop camera may be preferable to maximise depth of field. However, when trying to separate a subject from it's background, such as in portrait work, the full frame sensor's decreased depth of field may be an asset. Horses for courses.

NOISE COMPARISONS

There are other comparisons that I have not carried out that would be of interest if time were not an issue. Noise performance vs ISO setting is certainly one. However, there are plenty of such noise tests on other websites if you need this information:

Digital outback

DP review


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