Digital, Film, and Resolution: A Test

Canon's 50D's Resolution Approaches that of High-Definition Film

By Neil Lipson Back to

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It’s been f ive years since I tested the resolution of the Canon 10D digital SLR against that of f ilm. At that time, the 10D was about $1,500 for the body, and I found that film offered about twice the resolution of digital, depending on the lens. Times have changed. I just had the pleasure of testing the new Canon 50D against the same f ilm, the same lens, and the same resolution chart. The results were impressive.

The film camera was a Canon 630 with a Canon 50mm ƒ/1.8 lens; the digital camera was a Canon 50D, hot off the shelf. I used the Canon 580EX f lash. The resolution chart was the 1951 USAF, purchased from Edmunds Scientific.

Before I discuss the analysis, I want to point out that resolution is only one of many parts that affect a camera’s performance. Those elements include, but are not limited to, color accuracy, grain (or noise), and contrast range. When comparing film and digital, additional factors are the ability to process the photo, ease of manipulation of the resulting photo, and the ability to transmit the photo electronically—along with the cost of the equipment versus the number of photographs taken compared to the cost of the film and processing, and any delays the processing may take.

For most photographers, however, the main deciding factor is resolution. The type of photography that you do dictates which parameter is the most important. For example, for scientific, architectural, or aerial photography, resolution is the overwhelming factor. For surveillance, photojournalism, and high-contrast situations, contrast range may be the deciding factor. For portrait photography, color and low grain (or digital noise) may be the major consideration.

The equipment

I decided to choose the Canon 50D for a few reasons. The first is the reasonable price range, which at this time is about $1,100 street price. The sensor size is 22.3×14.9 mm, slightly smaller than the 22.7×15.1 mm size of the Canon 10D that I originally tested 5 years ago. I could have tested the f ilm against the new Canon 5D

Mark II, but that is much more expensive, usually out of the range of most photographers, and it is a full-size sensor instead of the APS- size one for the 50D.

Because the sensor is a smaller size than the 35mm negative, the lens focal length effectively increases by a factor of 1.6. This means that the 50mm lens has an effective focal length of about 80mm. How does this affect the resolution testing?

When you use only the center portion of the lens, you are taking advantage of the highest resolution that the lens is capable of achieving, which I call the sweet spot. The outer edge of the lens offers a lower resolution than the middle portion. There are digital lenses that are meant for the APS-size sensor, and these, obviously, will not work on a 35mm f ilm camera (or a digital full-sensor camera too). It would be prudent for anyone buying a lens from now on to purchase one that will work on both the APS-size sensor as well as the full-frame digital sensor, given the possibility that you may upgrade your APS body to a full-frame body. Even if you don’t, you get better resolution from the center of the full-frame lens, and don’t have the drop-off at the edge, because you’re only using the center portion.

I used the Canon EOS 630 to analyze film-camera resolution, but any of the Canon bodies would have done the job since the only thing that mattered was the f ilm and the lens. With the powerful Canon 580EX f lash, I had no problems with any blur created by the mirror moving. I pointed the flash strategically at the chart to avoid the ref lection hitting the chart on the resolution test pattern.

To make sure I got the best results, I made all distance measurements with the Fluke 416D laser tape measure. It completely eliminated all distance inaccuracies, unlike a metal tape measure that can flex during the measurement.

For the film analysis, I used Technical Pan exposed at ISO 25 and developed in Technidol. Once I had accurate measurements from the film plane to the chart, I used that as the main focus, and used bracket focusing in 1/16-inch increments to eliminate focusing errors caused by the camera. I also used this technique for the 50D measurements. While Technical Pan can be developed in a few different developers, I used only Technidol. All the temperatures from beginning to the end of the film developing were within 2°F to minimize reticulation and grain.

To read the line pairs on the film, I originally tried making extreme blowups, but that proved to be very tedious, so I switched to using my Propper Micropere 6094 binocular microscope set at 40× magnification.

I want to elaborate on how you read the line pairs per millimeter (lp/mm) from the chart. The chart has vertical and horizontal lines in the dead middle, on all edges, and in strategic places in-between. You examine the chart captured on f ilm through a microscope, or with digital, using Photoshop or any photo viewer, and blow it up to see lines on the chart. The exact method is printed on the chart and calls for examining the chart to determine the limiting group- element that can just be resolved. This gets combined with the focal length of the lens and the distance from the chart to the film plane into a formula that calculates the lp/mm.

Results

I made hundred of exposures, both in film and digital, at various f-stops and bracketed focusing points. The results were:

• Digital body resolution: 58 lp/mm center; 52 lp/mm edge
• Film resolution: 90 lp/mm center; 55 lp/mm edge

If you compare this with my original test of the 10D, which was 36 lp/mm both center and edge, you can see the substantial improvement of the 50D over the 10D. This is going from 6.3 megapixels to 15.1 megapixels, an increase of 2.40. However, you cannot multiply the resolution by that number because the megapixels numbers are derived from multiplying the vertical pixels by the horizontal. The correct method to calculate the increase follows. I checked the pixel resolution comparison with the 10D and the 50D, and the pixel ratios were 4770/3152 for the horizontal and 3177/2068 for the vertical, so the factor was 1.51 for horizontal and 1.52 for the vertical.

The original article found 36 lp/mm, and that multiplied by 1.51 comes out to 54.36, which closely matches the actual measurement I found during the tests. This verified that I was doing the testing correctly and consistently.

I should point out that Technical Pan has a maximum resolving power of 150 lp/mm at ISO 25 of developed in Technidol, so it would outperform just about any lens on the market. Even if I had done the testing at an ISO 100, it would not have affected the resolution, as the lens lacked the resolution that the film has. With the 50D, the resolution of the sensor is much closer to the resolving power of the lens. Therefore, improving the sensor beyond that of the 50D may not do much more for you if you use the standard lenses you already have. To see a real improvement, you would probably have to buy very expensive lenses. The only possible improvement you might have with a much higher megapixel body would be in other areas, like low-light performance at high ISO settings, or using it with top-end lenses. Another improvement of the expensive bodies would be in the dynamic range.

As part of the testing, I did a series of shots on the 50D at ISO 100, 400, and 1600. I then did the same ISO tests on my XTi, and found that the 50D had the same or better noise quality than my XTi. You have to take into account the better resolution, but I did not find any of the noise problems that some people have mentioned recently with the 50D.

Conclusions

There were a few surprises with the 50D, and not all of them have to do with resolution. The 50D needed no tweaking right out of the box, for instance. I did not have to adjust the f lash settings for underexposure like I did for my Canon XTi, and the histograms on the 50D were dead-on in every shot with no adjustments necessary. I also found that the Canon EFS 17–85mm image stabilization lens that I tested with the 50D, focused more accurately than my old 50mm lenses, which didn’t focus at the exact same place on repeat exposures.

The 50D features improvements beyond the resolution. The blowups of the 50D images possessed a higher- than-expected image quality—the photos looked like they were closer to 80 lp/mm. I found I could easily create 16×20 enlargements that looked as good as, or better than, 35mm enlargements. How can this be if the actual resolution is slightly less? Apparently the 50D’s improved color balance, low noise at low ISOs, and the high dynamic range of its sensor improve the image more than the jump to 55 lp/mm–resolution alone would lead you to believe. Resolution is easy to measure using the 1951 USAF chart, but I believe those other factors also act synergistically to give an even better looking image. Given the fact that digital post-processing is easy, in my book, the 50D wins hands down.

Keep in mind that super-high resolution is not necessarily best for some types of photographs, such as portraits, where you would show every tiny detail on an aging face. However, the digital photo can be “dumbed down” in post-processing if necessary, to reduce blemishes, for example. I’m not going to tell my film friends to abandon their f ilm cameras, as people still do classic f ilm and paper photography for a variety of reasons, such as making platinum and palladium prints. While film may become a smaller portion of the market, it still will be around for a while.


About the Author

Neil Lipson
NLipson
Neil Lipson is a computer and advanced image editing consultant and has been doing serious photography for almost 50 years. He has written for numerous photography magazines during this time. He can be reached at lipson@att.net.