Digital SLR Cameras, Lenses and Film

(From: photo.net)

Do you need good lenses with digital SLR cameras?

I often see comments that run along the lines of "Since digital cameras don't have the resolution of film, you don't get any benefit from premium lenses since the digital sensor limits image quality". I also see statements to the effect that digital sensors need the very best lenses to give good results. True? Well from a theoretical viewpoint I can make a convincing argument based on convolution of MTF curves that better lenses should give better images even on small sensor, limited pixel count digital SLR camera. But what do real tests show?

Well, I took some digital shots using a Canon EOS 10D 6MP DSLR and both a Canon EF 75-300/4-5.6 IS lens (IS off) and a Canon EF 300/4L lens. Both lenses were used at f5.6 and ISO 100 sensitivity was used on the camera. Shutter speed was around 1/750s and the camera (or lens) was mounted on a sturdy tripod. Note that these images are 400% blowups from the original digital file. We are looking in close detail at the very limit of resolution so everything looks somewhat blurred.

Here, in the center of the image, you can see that the better lens not only resulted in higher contrast, but also greater perceived sharpness. Resolution is limited by the sensor, not the lens, but the better lens does give a better image. The difference is less than you'd see on film under optimum conditions. Still, you can score one for the better lens.

At the corner of the image the difference is more obvious. Again contrast with the better lens is higher, but in this case the cheaper lens is clearly showing much more chromatic aberration (green/purple fringes on the tangential lines). Score another one for the better lens.

So it's certainly true that better lenses give better results even on a 6.3 MP 10D. Does that mean you should only use "L" lenses - of course not. There are many excellent non-L lenses. For example the 50/1.8, the 24/2.8 and the 28-105/3.5-4.5 are all good lenses. In fact the 75-300 is a very good lens at the shorter end, but as you can see above does suffer somewhat when zoomed to 300mm and used wide open. It's still not bad, just not as good as the 300/4L. What is equally true is that you can't just say "Well, the 10D doesn't have the resolution of film so it really doesn't matter what lens I use". Just like with film, choice of lens is still a factor in image quality, though probably not quite as big a factor.

Just as an aside, one very interesting aspect of using a digital sensor is that, to a large extent, the image resolution is independent of the ISO setting. Above are resolution patches shot at ISO 100 and ISO 800. While the ISO 100 patch might not be quite as good as an equivalent film test and high resolution scan, the ISO 800 patch looks just as good! There's a bit more noise, but not enough to have any effect on resolution. You can't say that about film!

What about film. Won't I get better results shooting film and scanning?

In theory, yes. It's not difficult to show that film with 80 lp/mm resolution scanned at 4000dpi should give higher resolution than a 6.3MP digital image. 100 lp/mm film scanned on a drum scanner should be even better. But those are just numbers. Also, resolution is not sharpness. Sharpness is a subjective quality that depends on contrast, acutance (edge sharpness) and resolution, moreover the perception of sharpness is related to some spatial frequency filtering that the eye/brain system does. Our perception of sharpness depends more on how well certain spatial frequency bands are reproduced and these aren't always the highest spatial frequencies. Grain and noise probably come into the equation too. There's much more to sharpness than just high lp/mm numbers. The only real way to judge which of two images is sharper is to look at them under the conditions that they will actually be looked at!

Test Targets: Here's a comparison of a two digital and one scanned film shot of a cropped section of an image of a resolution test target (all corrected to the same size on screen). The rightmost image is a section of a shot of a test target taken with a 300/4L lens at f5.6 on Kodachome 25 film and scanned at 4000dpi with a Canoscan FS4000US scanner. The leftmost image is a shot taken with the EOS 10D 6.3MP camera with the same lens from the same distance. Since there's a 1.6x "digital multiplier", the field of view of the 35mm image was cropped. The leftmost image compares the native resolution of the 10D sensor with Kodachrome 25 scanned at 4000dpi. As you can see, there's really not a lot of difference. Note that the 6.3 MP sensor on the 10D is about 15 x 22mm. If we had a full frame 24 x 36 mm sensor with the same pixel density it would be a 16 MP sensor, so you can think of the 10D as having a cropped 16 MP full frame sensor! That's better than any current DSLR, better than the 11 MP Canon 1Ds and even better the 14MP Kodak 14n. Yes, it's true, the 10D has a higher pixel density than either the 1Ds or the 14n - but of course the sensor is 1.6x smaller so the total pixel count is less. Given this and all the hype (some deserved, some not deserved) about the 1Ds and 14n being better than 35mm film it shouldn't be surprising that the left and rightmost images are quite similar.

However comparing the native resolution of the sensor to scanned film isn't really a fair comparison in real world applications. To get the same digital shot with the same magnification and field of view as the film shot we need to use a lens 1.6x shorter in focal length, i.e. 187.5mm, so the center image was shot with a 70-200/4L zoom set to 187.5mm and f5.6. Now the field of view of the digital camera is exactly the same as the field of view of the film camera. As you can see, under these conditions the resolution from the 10D is clearly lower than the scanned film

A second way to look at these images is to say the left and rightmost images represent what you might see if the made equal prints from the 10D and from a 15 x 22 mm cropped section of the 35mm negative (the 10D sensor is 15 x 22 mm). In this case resolution is very similar. The center and rightmost images represent what you might see if you made the same sized print from the 10D and from the full frame 35mm slide. The 10D image needs 1.6x more magnification to get to the same print size and thus resolution suffers

But we don't spend our time taking pictures of test targets and looking at the scans under high magnification - well, most of us don't anyway. We shoot real objects and look at prints. In the real world, differences you can easily see in scientific tests may not be so obvious. Nor may these differences be as significant as they appear to be from scientific tests. Most real objects don't have detail in the form of high contrast black and white bars with sub-mm spacing.

Real Images: It we want to look at real images we have the problem of what to compare with what, but for this test I chose to shoot 35mm film with a 500/4.5L lens on Sensia 100 and scan at 4000dpi with a Canoscan FS4000US scanner, since that's a pretty typical example of what I use. The comparison digital shot was made with an EOS 10D set to ISO 100 and a 300/4L lens at f5.6. I know from previous testing that the 300/4L and 500/4.5L give very similar - and very high - image quality. Both are capable of putting over 80 lp/mm on film in the center of the frame. With the "digital multiplier" of 1.6x the "effective focal length", or more correctly angular coverage, of the 300/4L on the 10D sensor was equivalent to that of a 480mm lens on 35mm film, so I didn't need to change my shooting position much to get the same image as with the 500mm lens. The full frame shot is shown below (it's the digital shot) and the red box shows the area enlarged,

Below is the first enlarged image. It's a 100% crop from the scanned film. On my 17" monitor with a screen resolution of 1280x1024 this would represent a "real life size" section from a 40" x 60" print. It's approximately a 3.25mm square section on the slide. Your monitor and screen resolution will give a different scaling factor of course (unless it's a 17" using 1280x1024).

Now here (below) is the same section reproduced to the same scale from the 10D image it had to be upsized by about 200% to match the scale of the 4000 dpi scan since it started out around 2000 x 3000 pixels whereas the 4000 dpi scan started out around 4000 x 6000 pixels. Actual lp/mm resolution is lower, as evidenced by the less smooth diagonal lines, but viewed from a distance it's hard to tell (remember, on my monitor this is equivalent to a 40" x 60" print!). Also, the shadow detail is better AND the highlight detail is better, even the color balance is better. Now maybe I could do a more optimized scan (maybe not). Maybe I could have shot on Velvia rather than Sensia 100 - but I wouldn't normally do that. Maybe my exposure could have been tweaked a little, but this is what I got in real life and represents what I'd be likely to get under typical shooting condiions in the real world. This scan probably took 5-10 minutes including loading the slide, running the preview, running the final scan and tweaking it slightly to try to color correct. The digital image is straight out of the camera, saved as a JPEG (not even as a RAW file) with the default camera parameters and aperture priority autoexposure with multizone metering and aufofocus. I pushed the shutter and this was what was recorded.

Again remember that though this image shows "jaggies" on some of the diagonal lines, this image represents a section of a print much larger than anyone would attempt to make from a 6MP DSLR file. Below is a representation of a 20" x 30" print and a 10" x 15" print (as displayed on my 17" 1280x1024 monitor). These are just approximate sizes of course, but I don't want people to get a false impression of image quality (or lack of it) by looking at greatly enlarged images without realizing just how enlarged they are.

20" x 30"

10" x 15"

Conclusions

Based on MY particular film and digital workflow:

• Even on a 6MP Digital SLR like the EOS 10D, better lenses give better results
• Digital scans of high resolution film (Kodachrome 25) at 4000dpi on a FS4000US scanner yields higher resolution images than those shot directly with an EOS 10D. Not surprising, but confirmed by experiment.
• While 10D resolution is somewhat lower than ISO 100 film, it's very little affected by ISO setting so it's quite possible that high ISO digital resolution may be better than scanned high ISO film.
• From a practical viewpoint, 10D images printed on an inkjet from digital files are probably equal film up to maybe 11x14. They are certainly good enough to be very hard to distinguish from film.
• As an aside, if you want the ultimate in sharpness do it the right way. Get a large format camera and shoot film!

Of course it's possible that if I shot everything on Velvia and had professional drum scans made of my slides, film would look better than it does when I shoot Sensia 100 and scan on an FS4000US. In fact I'm sure it would. If I regularly made 20x30 prints, maybe drum scanned Velvia would be significantly better than 10D digital images. Again, I'm pretty sure they would. However the point is that I don't get professional drum scans done, and for most of my work I don't use Velvia and I don't think I've ever had a 20x30 print made from a 35mm slide, so such comparisons, while valid in the abstract, aren't really valid for me and my workflow. I'm just not going to use nothing but Velvia at EI 40, I can't afford to get everything drum scanned and I'm not in the business of making 20x30 prints. So while film may be better in the abstract case, for me it's not.

For me I think digital has now replaced film. Not 100% but certainly 90%. I'll still shoot some film, but the first camera I'll reach for is digital and I'll only use film when I think it can do something digital can't or if I really need the higher resolution that scanned fine grain high resolution film can give. Most of the time I think the digital images will be good enough and for most of my applications they won't look much different than optimized film scans.

Note I'm not saying "digital is better than film" or even "digital is as good as film". I'm just saying that for me, most of the time digital (from an EOS 10D) will likely meet my needs for image quality.

(C) Copyright 2003 Robert M. Atkins All Rights Reserved