November 28th, 2007, 01:11 PM
Small Format Deep DOF Advantage - Fact or Myth?
Most of us take for granted that large sensor digital cameras have better image quality than small sensor digital cameras. Why else would someone spend $30K on one of these? Though some disagree, I feel that it is best to use equivalent images when comparing image quality from one format system to another. The term "equivalent images," as used throughout this post, refers to images with the same output (print) size, angle of view (AOV), perspective, apparent exposure, and depth of field (DOF). Joe Mama, whom I know from the DPReview forums, has written an extensive essay covering the issue of equivalence between different digital formats. There have been many discussions in the DPR forums about "equivalent" images, and the general consensus has always been that large format systems always match or exceed the image quality of small format systems when equivalent images are compared. Yet, there has been one niggling exception to this. Image quality from digital medium format systems seems to compare poorly to that of 35mm digital systems when compared at high ISO or in very long exposures in low light. This exception has been largely dismissed as evidence that medium format digital is not yet as mature a technology as 35mm digital.
Conventional wisdom suggests that digital small format systems have an advantage in deep DOF photography. However, a number of parties have debunked this apparent myth. Clarkvision has an excellent technical discussion, titled " The Depth-of-Field Myth and Digital Cameras." Quoting the conclusion of that article, Clark wrote the following:
"Given the identical photon noise, exposure time, enlargement size, and number of pixels giving the same spatial resolution (i.e. the same total image quality), digital cameras with different sized sensors will produce images with identical depths-of-field. (This assumes similar relative performance in the camera's electronics, blur filters, and lenses.) The larger format camera will use a higher f/ratio and an ISO equal to the ratio of the sensor sizes to achieve that equality. If the scene is static enough that a longer exposure time can be used, then the larger format camera will produce the same depth-of-field images as the smaller format camera, but will collect more photons and produce higher signal-to-noise images. Another way to look at the problem, is the larger format camera could use an even smaller aperture and a longer exposure to achieve a similar signal-to-noise ratio image with greater depth of field than a smaller format camera. Thus, the larger format camera has the advantage for producing equal or better images with equal or better depth-of-field as smaller format cameras."
While recently considering a candid portrait I took of my older son Oliver with the Fuji F30, it occurred to me that when a certain shutter speed is required in low light, small format systems may indeed have a significant advantage for deep DOF equivalent images. The Fuji F30 is a compact, small sensor digital camera with a 1/1.7" 6MP sensor. This photo was taken with available light at ISO 1600, 1/38s, f/5:
To get an equivalent image handheld at the same shutter speed using my Canon 5D, one would need to use the 5D at an f-number and ISO value each approximately 4.4 stops [calculated as ln(43.3/9.5) / ln(sqrt(2) using the diagonal dimensions of the sensors since the aspect ratios differ] higher than those used on the Fuji - approximately ISO 34,000, f/22. I am certain that given similar ambient lighting, my Canon 5D would not produce that nice of an image at ISO 34,000 at the same output size. Do we need to debunk the debunking of the myth of small format deep DOF advantage? In response to my questions in the DPR forums, John Sheehy gave a technical explanation for why small sensor cameras should have an advantage in high ISO, low light, deep DOF, shutter speed-limited photography.
John stated the following:
"You're overlooking the fact that the noise down in the deep shadows has nothing to do with photon counts. P&S and FF DSLR sensors vary far more greatly in photon capture than in read noise. Most of the existing Nikon DSLRs have similar or the same read noise as P&S cameras do, relative to RAW saturation (or, in ADUs).
You can't model camera noise in terms of photons collected, until you remove read noise from the equation. Most of the extra noise you'd see with a 5D or D3 pushed to ISO 70,000 is due to read noise. The fact that the most objectionable things in the image are patterned tells you right there that it is read noise that is being boosted to objectionable levels, as there is no patterning in shot noise."
I still wondered whether the much-discussed Fuji F30 6MP Super CCD was doing something special here, or whether other small sensor cameras might do as well. I therefore shot a simple test using the Ricoh GX100 and Canon 5D with 50mm lens to create equivalent images with a deep DOF in low light with shutter speed matched. The Ricoh has a 10MP 1/1.75" sensor. To match the diagonal AOV of the 5D system, I used a focal length of 10.5mm with the Ricoh. To otherwise meet the criteria for equivalent images, the Ricoh was shot at f/3.9, ISO 800, 1/10s; whereas the 5D was at f/18, 1/10s. The 5D cannot natively shoot at the necessary ISO, so I took the image underexposed at the maximum ISO of 3200 and then "pushed" the exposure during RAW processing.
Both images were processed from RAW using Adobe Lightroom 1.3. In fact, all I did was choose "auto expose" in Lightroom, and Lightroom pushed the 5D exposure by an additional 2.4 stops, almost exactly where I would have calculated it (final ISO equivalent of 16,890). The resulting histograms for the two images matched closely. All other values in Lightroom were left at default. The resulting Ricoh TIFF was upscaled by 15% (using Bicubic Smoother) to match the diagonal dimension of the 5D image. At this point, I ran both images through Noise ninja using "Auto Profile" and default settings with "Coarse Noise" checked. This somewhat helped to close the gap, but the GX100 file still has significantly less patterned noise. Click here to see the final, full-res 5D image and here to see the full-res, upscaled GX100 image.
Here are the 25% resized images, similar to what one might see in a small print. I ran both of these through Photoshop's Auto Color before resizing them using Bicubic Sharper.
First the 5D:
Now the GX100:
The 5D held up pretty well here, but the small sensor camera took the day!
If you've read through this long post, you may be wondering, "Why does this matter?" After all, most folks associate serious deep DOF photography with landscape photography, where tripods are generally used and shutter speed limitations do not apply. Low-light street photography is one application in which deep DOF often desirable, and tripods are generally not an option. In addition to obvious operational advantages (discreet appearance, light weight, etc), small sensor cameras would seem to have an image quality advantage in this setting. Another such setting would be macro photography of a moving subject, such as a flower in the setting of a slight breeze.
If anyone would like to try their hand with the RAW images used in this comparison, they will be available for 10 days or 100 downloads by clicking here (sorry for the associated pop-up ad).
Addendum: Some discussion of this post can be found at D-Spot (Hebrew) and at POTN (English).
Originally published on the old Serious Compacts blog. Older comments can be found here: http://seriouscompacts.blogspot.com/...ever-have.html
6/21/12 update: I'm not claiming that all small sensor cameras will have this advantage over all large sensor cameras. However, my own experience with many small and large sensor cameras over the years has borne out the fact that in most such comparisons of small and large sensor cameras of a given technology "generation", there is an advantage similar to the one demonstrated here for the smaller format under low light, deep DOF, shutter speed limited shooting conditions. More research and development dollars go into small sensor development than large sensor development, so it should come as no surprise that square millimeter for square millimeter, a current iPhone sensor offers better technology than a current 5D sensor.
April 23rd, 2011, 05:02 AM
I have a 5DII, but I use a GX100 or GX200 for all my 'product' photographs. I need the combination of a 70mm equivalent AoV lens and reasonable DoF. At full zoom and full wide, I get a DoF that covers anything I'd want to photograph. On the 5DII, I'd have to close down to F/22 to get the same DoF and with 4.5 stops less light and diffraction limitation, I may as well not have bothered.
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April 23rd, 2011, 06:09 AM
Good post. Though this topic is known to provoke heated debate!!
Originally Posted by Amin Sabet
One of the great advantages of m4/3, and to a certain extent APS-C, sensors for me is this DOF advantage. I work outdoors most of the time, usually in good light. I generally walk some distance and have no desire, or the back muscles, to carry a tripod. Since what I shoot is mostly in the landscape/location/travel catergory, I'm usually looking to get the maximum depth-of-field at a low ISO. Since I'm working hand-held almost exclusively I also want a high enough shutter speed to eliminate camera shake.
Because of the m4/3 2x sensor crop I get quite an advantage. I tend to use a lot of wide-angle lenses. So for a shot where I would use a 28mm lens on full-frame, I can use a 14mm on m4/3 for the same angle of view. If I choose the same aperture, say f/8, I will get more DOF on m4/3 because a 14mm lens at f/8 gives more DOF than a 28mm lens at f/8. If I'm happy with the DOF I get from that 28mm at f/8 on full-frame, then on m4/3 I have a further advantage, in that I can use a wider aperture on the 14mm lens on m4/3 and achieve the same DOF. I can therefore use either a faster shutter speed or a lower ISO or both.
When I used medium format film cameras, usually with films like Fuji Velvia, this advantage was reversed. I had to use smaller apertures to get the DOF I wanted, and using an ISO 50 film made it essential to either work in very good light, have very steady hands or use a tripod.
People often complain about the fact that its difficult to get limited DOF on m4/3 but for me its the opposite reaction. One of the reasons I'm so enthusiastic about it and use it so much is because it makes what I do so much easier. By using base ISO I get no problems with noise, and indeed using my GH2, which has ISO 160 as its base means I can shoot high quality images at (mostly) the aperture I want with a decent shutter speed which avoids camera shake and produces crisp sharp pictures. I can also take advantage of using relatively slow, but extemely useful zoom lenses.
With regard to interior work, I switched from using full-frame (5DMkII) to APS-C (7D) to give me a little more "room for manouvre" with DOF. Working quickly in a rapidly changing situation meant my primary aim was to get my subject sharp and in-focus. I never had a client complain about the fact that too much was in focus, or the bokeh wasn't very nice!! First and foremost its important to "get the shot" and while switching to APS-C didn't give me a huge advantage, it gave me enough to make it worthwhile.
One of the reasons I am hoping that Sony can produce the rumoured 24MP APS-C sensor is precisely because of the above. I'd personally love a 24MP m4/3 sensor. I wouldn't care if it was unusable above 400 ISO because I'd probably never move it from its lowest ISO setting. But thats just my particular needs.
I just wish that m4/3 had been around years ago. Then I wouldn't have had to drag myself, cameras, lenses and tripods up mountains, with the current state of my back as a direct result of this. However it does enable me to continue to do what I do in (relative) comfort and I'm extremely grateful for that.
April 23rd, 2011, 07:49 AM
Looks like there may be a little bit of a front focussing problem with the 5D too.
One of the biggest things to come to grips with for me when I went from 1/1.7" sensor cameras (Sony DSC-V3) to the Olympus Pen was the loss of depth of field. Particularly with macro. It was less of a jump going from MFT to the 1.5x crop sensor of the Nikon D90.
Sometimes shallow depth of field is great, but there are other occassions where a lot of depth of field wins.
April 23rd, 2011, 08:37 AM
No front focus, Peter. It's just that the 5D is a much taller camera and therefore with the base of the two cameras at the same height, the Ricoh was closer to the immediate proximal floor (therefore throwing it out of focus). Note that maximum sharpness is in the same location for both, and the background elements are similarly OOF in each case.
Originally Posted by PeterB666
As you can see, I wrote this post in 2007. A lot has changed in sensor technology, and we now have 35mm sensors with substantially lower read noise than the Canon 5D. Meanwhile, small sensor technology has also improved, and this particular debate has continued.
What I've come to realize is that the basic assumptions for the arguments are often flawed. In the DPReview forums, for example, there are a lot of math-inclined folks who love to make the following arguments:
- For otherwise equal sensor technology, more pixels is always better (contrary to what they hold to be the "megapixel myth")
- For equal sensor technology, larger sensor image quality will always match or exceed small sensor image quality (even for matched, deep DOF in handheld, low light conditions.
Even a cursory glance at these statements raises will cause a thinking person to realize that if argument #1 is true and if smaller sensors tend to have a higher pixel pitch (which is true), then the assumption for #2 (equal sensor technology) is unlikely to be satisfied.
The reality today is that the camera offerings out there have strikingly different sensor technologies. The Leica M9 is an example - CCD instead of CMOS, and no AA filter. Dramatically different sensor tech than our CMOS DSLRs. Same for medium format. We have Sigma's Foveon tech, and now other companies seem to be preparing to bring out their own 3-layer sensors. Even within the APS-C DSLR camp, we have cameras with higher read noise than certain MFT cameras and other APS-C sensors which have higher dynamic range than certain current 35mm format cameras! Meanwhile, for small (less than 1") sensor cameras, the pixel density is very high, and the research & development dollars are likewise very high. Very different sensor technologies all around.
Thus, the whole concept of "For equal sensor technology, [argument]" becomes kind of pointless when we are surrounded by mostly unequal sensor technology.
April 23rd, 2011, 09:01 AM
All very true and completely ignored by many.
Just two examples from my experience.
Leica M9 - full frame but very poor dynamic range. The Pentax K-5 APS-C sensor is much better.
Samsung NX100 - 14MP APS-C sensor but no better high ISO performance than 12MP m4/3 cameras.
Add in strength of AA filter and lens "correction" software and you have radically different results, which don't follow preconceived dogma.
Originally Posted by Amin Sabet
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April 23rd, 2011, 04:01 PM
Interesting thread! Will the Pentax NC-1 be the start of a "new area", great DOF, small enough to always take with you, interchangable lens system, though no EVF / OVF ?
Have a nice day, Herman
Username Twitter: @hermandijkhuis
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