Leica M8 — Infrared Imaging
Related: black and white, digital sensor, exposure, flare, focusing, infrared, Leica M, MTF and Micro Contrast, sensor cover glass, shutter
Updated: January 10, 2008
This article explores using the Leica M8 for infrared photography.
The 10.3-megapixel Leica M8 digital camera offers outstanding image quality for visible light imagery. Lacking the anti-aliasing filter found on digital SLRs, and with unusually thin sensor cover glass (0.5mm), sharpness is reported to be outstanding, even compared with digital SLRs of significantly higher megapixels. The M8’s designers wanted to exploit the very high image quality of Leica rangefinder optics and they succeeded, at least according to numerous M8 users.
One “gotcha” to the thin sensor cover glass of the M8 is inadequate blocking of deep red and short-wave infrared (650 - 730nm), which can lead to color shifts (see Infrared Contamination). Leica addresses this by offering infrared filters to be placed onto the lens.
This infrared weakness is also a strength that makes the (unmodified) Leica M8 a dual-use camera for color and infrared photography. By comparison, today’s digital SLRs block infrared so effectively that trying to use them for infrared photography is an exercise in frustration; a digital SLR must be permanently modified to achieve reasonable shutter speeds to be of practical (or at least convenient) use. For example, the new Nikon D3 attenuates infrared by about 15 stops!
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Leica USA loaned diglloyd.com a Leica M8 camera body along with the new 35mm f/2.5 Summarit-M and the 16/18/21mm Tri-Elmar for testing—thank you Leica.
In addition, Zeiss USA provided the full line of the Zeiss Ikon “ZM” lenses. All of the ZM lenses are designed for use with the M8 (and other “M” mount cameras), though they are not “coded” for digital use as Leica lenses are.
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The section discusses general image quality issues.
The Leica M8 produces RAW files in the DNG format. The images on this page were processed into 16-bit TIF files using Adobe Photoshop CS3. Appropriate post-processing was done based on extensive experience with a variety of other infrared cameras (see the Guide).
The Leica M8 uses the Kodak KAF-10500 sensor; see the datasheet for details on IR transmission. In particular, refer to page 14 of the datasheet (“KAF-10500-DX Spectral Response (with 0.5mm BS-7 cover-glass)”, which shows the spectral response of the red/green/blue photosites. (For copyright reasons that graph is not shown here directly).
Beyond 650nm (very deep red), the spectral response graph shows almost no response from the green and blue photosites, but substantial response in the red channel from 650-700nm and continued response out to about 725nm. In short, the M8 shows potential as a near-infrared camera, but offers limited potential for infrared in the 750-1000nm range. Only removal/replacement of the sensor cover glass would allow true infrared while excluding very deep visible red light (see Digital Infrared). One can argue where infrared really begins, but 700nm is a reasonably accepted wavelength.
Because the green and blue photosites offer almost no spectral response beyond 650nm, digital noise and image detail are concerns, since nearly all data is going to come from the red photosites, which comprise only 1/3 of the total photosites. The images on the page were made using the B+W 092 filter, which has a steep cutoff at 700nm, with 50% transmission at that point (see Filtration below), and visible light completely blocked below 650nm.
Based on using half a dozen other cameras for infrared use, including the Canon EOS 5D and Nikon D70, the Leica M8 images look relatively grainy and slightly impaired in terms of crisp, clean detail; this is almost certainly due to the very low response of the blue and green photosites caused by the sensor cover glass filtration.
However, compared to other unmodified cameras, the Leica M8 looks exceptionally good and in fact can provide very satisfying images, especially when rendered in grayscale (black and white). Those looking for higher performance and true infrared-only imaging will need to acquire and modify a camera for dedicated infrared use, such as a Nikon or Canon DSLR.
For best results, the base ISO of 160 should be used. However, higher ISO values such as ISO 320 may be used, and for high-key images ISO 320 provides satisfying results. Images with dark areas might be fairly noisy at ISO 320 however, so prefer ISO 160 when possible. This is less of an issue when processing for grayscale (black and white).
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The false color-effects with the Leica M8 are quite good, but digital noise is high enough that results are generally more visually acceptable when converting to grayscale. In fact, the results in grayscale are pleasing, and similar to film grain, and so should be very appealing to those wishing to make black and white prints. False color can render many different variations. Many/most are not suitable for many/most images, but for some subjects the results can be quite appealing.
This is not a camera review of the Leica M8, so only a few points relevant to digital infrared on the Leica M8 will be covered here.
Unlike a DSLR, placing an infrared filter over the lens has no effect on one’s ability to compose an image—the Leica M8 is a rangefinder, with a separate viewfinder for composition. Therefore, the infrared filter can be left on at all times, but it is best to use manual exposure.
For this test, the dark red B+W 092 filter was used. It has a 50% cutoff at 700nm so that deep red visible light (near-infrared) passes, as does nearly all infrared. However, this means that the resulting images are really “partial infrared”, since much of the spectral response of the sensor will be in the 650-700nm range, which is very deep red or “near infrared”, easily visible to the naked eye (which makes one wonder why it’s considered infrared at all!).
The practical effect of the M8’s spectral response declining rapidly beyond 700nm is that the tonal response for some subjects will be significantly different from that of other infrared cameras; whether the differences are significant will depend on subject matter. The author’s subjective impression is that the (unmodified) M8 produces less strong contrast between clouds and sky and less bright foliage than with a dedicated (converted) infrared camera. Direct A/B comparisons were not made, but considerable experience with half a dozen converted infrared cameras teaches that each camera responds somewhat differently.
Specific results depend on the sensor and filtration used. But perhaps most underestimated is how much the lens coatings affect infrared transmission, so the foregoing should be understood in the context of quite a bit of variation (the author has tested over 80 lenses on the Canon EOS 5D and Nikon D70 IR, and there can be considerable differences in how much infrared is attenuated). And in the end, what matters is whether the results are satisfying to the M8 owner.
According to the M8 sensor spectral response chart, there is almost no response beyond about 725nm, which suggests that the stronger B+W 093 (850nm) filter might be of no practical use with it (ditto for the 900nm Hoya RM90). Unfortunately, the B+W 093 filter was not tested for this review before returning the M8 to Leica USA. One reader anecdotally reports that the 093 filter is quite usable with the M8, so M8 users might want to try that filter if it is available to them. However, even on dedicated (converted) cameras, the author’s B+W 093 filter cuts off 2-3 stops of exposure time, and creates highly monochromatic images.
Filters with a cutoff in the 680nm to 715nm range might also work well with the Leica M8, but these were not available for testing, and would add significantly to the exposure time, making handheld shooting more of a challenge.
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It is feasible to shoot the Leica M8 handheld, but this generally means shutter speeds in the 1/60 to 1/200 sec range and apertures in the f/2.8 - f/5.6 range (at ISO 160 - 320). With a strong summertime sun and bright conditions (beach, etc) these figures improve. But in the wintertime, hand-holding is frequently a challenge. (The foregoing applies when the B+W 092 filter is used).
Only the 75mm and 90mm Leica M lenses are apochromatic, and none of the Zeiss ZM lenses are. So like most lenses made today, infrared focus will diverge substantially from visible-light focus, and the single biggest challenge to obtaining sharp images is achieving accurate focus in infrared (focus and what to do about it are covered in extensive detail in the diglloyd Guide to Digital Infrared).
Focusing the M8 in visible light (using one’s eye) is the only choice, since there is no “Live View” feature as in the latest crop of DLSRs. There is always substantial lens-dependent backfocus error in infrared when doing this. For example, focusing the lens at a distance of 10 feet (~3 meters) might result in an actual focus distance of 15 feet or even 20 feet for deep red/infrared. Unless focus is adjusted to compensate, images will be blurry at the desired plane of focus and sharp behind it. Although small apertures such as f/11 and f/16 can mitigate the error, the entire zone of focus is shifted to a non-optimal location, and peak sharpness will not be achieved as intended.
The images below illustrate the problem. In visible light, the large leaf at bottom center was in sharp focus (images below are reduced-size crops, the leaf was in the exact center of the frame). But in infrared, there is a strong focus shift to the rear (at least several meters). By adjusting focus, the leaf becomes sharp in infrared (right).
Controlled testing can establish how much backfocus occurs with each particular lens, allowing a manual focus adjustment that can reduce the focusing error by 90% or better.
For this review, an approximate focus adjustment was determined for the 35/2.5 Summarit-M and a few other lenses; this range was from 2-6mm of “throw” of the focusing ring, being lens dependent. Leica M8 users who intend to shoot infrared will want to establish a precise adjustment for each lens using a controlled test setup, keeping in mind that it can vary with focusing distance. It might be helpful to mark the lens once this is determined, perhaps with a strip of millimeter-ruled tape so precise adjustments can be made reliably and repeatably.
Hot spots and flare can be troublesome in infrared. The Leica M and Zeiss ZM lenses for the Leica M8 handle both issues exceptionally well as compared with digital SLR lenses, but both hot spots and flare can be problems in some shooting situations. For more details on how to deal with both issues, see the diglloyd Guide to Digital Infrared.
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Flare shows up primarily in the blue channel. For false-color images, this can make the image problematic, but for grayscale (black and white) images, a nearly flare-free result can often be obtained using the red channel (or green channel).
The best advice for avoiding flare is the same as for visible light photography: shield the front lens element from direct light such as sunlight, and use a lens hood at all times.
Testing showed that hot spots were exceptionally well controlled with the Leica 35/2.5 Summarit-M and about a dozen other lenses. Even stopped down to f/11 nearly all of the lenses showed respectable behavior. In many cases, subject matter masks the hot spot and a small loss of contrast is the main negative.
However, the 16/21/21 Tri-Elmar showed more hot-spot issues, especially when stopped down to f/16. As with flare, the hot spot can be dealt with by avoiding the blue channel of the RGB image. The behavior is really quite acceptable compared to most Canon and Nikon lenses on digital SLRs.
The Zeiss ZM lenses were also tested for hot spots (15/2.8, 18/4, 21/2.8, 21/4.5, 25/2.8, 28/2.8, 35/2, 50/1.5, 50/2). (The 15mm and 18mm are problematic for infrared because there is no easy way to attach a filter to the front of the lens). Like the Leica “M” lenses, all the ZM lenses can be considered strong choices for infrared, but can also exhibit faint hot spots, especially when stopped down to f/8 or smaller apertures.
All images below were taken using the B+W 092 deep red/infrared filter (700nm cutoff). Click each example to see it at a larger size. Unless otherwise noted, images were taken at the M8’s base ISO of 160.
Unfortunately, holiday-season time limitations precluded shooting a wider variety of subjects, but the varied lighting conditions in the examples shown below provide an accurate view of the Leica M8’s capabilities.
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In this image, a slightly different camera position resulted in significant flare. By paying close attention to light striking the front of the lens, most flare can be avoided.
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One of the first shots taken with the evaluation M8. Image quality is high, but a bit noisy at ISO 320.
Provided that accurate focus is achieved, image sharpness is excellent with the Leica 35/2.5 Summarit-M.
False-color works well for this image. In addition, part of the appeal of false-color short-wavelength infrared is the tonal differentiation; any of the three R/G/B channels may be used for a grayscale image and/or Lab mode may be used to extract the luminance and/or a mix of any of these may be done.
The Zeiss 50/2 performs very nicely here. No hot spot appears to be present in spite of the strong contrast and black foreground. Image sharpness is very high.
The blue channel often provides the most appealing contrast, but can also suffer from the worst flare and/or hot-spot trouble (in infrared). In this case it reduces the contrast at the left side of the oak tree. The red and green channels do not have the flare problem, but also do not offer the same pleasing contrast between clouds and sky.
Actual shooting conditions exert considerable influence over flare so comparing lenses on different scenes should not be used as a basis for comparison. These two different images (above and below) of the oak tree are shown as a matter of interest, not comparison.
False color and a hue adjustment were used to create a blue sky consistent with visible light, while rendering the trees in an unusual hue. Other cameras that respond to infrared in the 700-1000nm range can sometimes offer greater tonal distinction than seen here, but this capability in an unmodified camera is rare.
The Zeiss ZM 15/2.8 appears to be an excellent performer in infrared, but there’s no easy way to get a filter over the front of the lens. Probably with the right filter size and some tape it could be done though. The ZM 18/4 has a similar complication.
All of the Zeiss ZM lenses are good performers in infrared. Here the red channel yields a very nice image with no hot spot whatsoever at f/11.
The Zeiss ZM 21/4.5 provides excellent results with little evidence of hot spots or flare, and its tiny size is a bonus.
The Leica M8 is unique among pro-caliber digital cameras in that it can be used at reasonable shutter speeds for infrared imagery without modification, and is even practical for handheld use under bright conditions. All it takes is a filter on the lens, and the camera is transformed from a visible light camera into a capable infrared camera.
Image quality is very high. Lens performance of both Leica M and Zeiss ZM lenses is excellent, and optical issues such as flare and hot spots are very well controlled as compared with digital SLR lenses. With the addition of a B+W 092 filter (or equivalent), Leica M8 users enjoy a versatile camera capable of both visible and infrared photography simply by changing the filter.
No one should rush out to buy the US$5500 Leica M8 for its infrared capabilities, since a digital SLR can be purchased and modified for dedicated infrared use for $1200 or less. But for existing Leica M8 owners, or prospective owners “on the fence” the additional flexibility is a nice bonus.
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