Many photographers, digital or not, believe that an SLR offers more convenience and flexibility than other types of cameras. While generally this may be true (and I do not want to go into the whole argument on this), in the digital domain there are areas where a non-SLR will be a hands-down winner. Two applications come into mind: macrophotography and photography in infrared.

Macrophotography

When taking pictures of small objects (say, a few centimeters across) with a 35-mm camera, we are used to some technical complications and inconveniences. Most of them stem from the fact that the subject-to-lens distance becomes comparable to the focal length of the lens. This carries three implications.

1. The lens, in order to focus on the subject, has to be greatly extended forwards. For example, to frame a field of 36 mm across, a "standard" 50-mm lens has to get an extra 50 mm extension as compared to focusing at infinity. This complicates the design, and therefore such extension is usually achieved with extension rings; lenses allowing to do it without rings (referred to as "macro" lenses"), as few of them as there are, are larger, heavier, and more expensive.
2. It is easier to correct the lens optically under an assumption that the subject distance is much greater than the focal length. By "much greater" I understand at least one order of magnitude, i.e., a factor of ten. This is why most 35-mm cameras focus down to 50 cm or so. Using extension rings does not solve the problem - it only provides the extra extension. Once again, this is why the specialized "macro" lenses cost more, again.
3. A relatively minor point: the nominal aperture is defined for the lens focused at infinity. The effective one decreases inverse-proportionally to the lens extension. In out 36 mm across-the-field example the aperture denominator (F-stop) should be multiplied by two.

No wonder that SLRs dominated the macro field in film photography: they offer interchangeable lenses (addressing point 1 above), and most have through-the-lens metering (addressing point 3).

But how different are things in digital cameras? Aren't laws of optics laws of optics?

There is just one difference here, but a very significant one: the digital camera sensor is smaller than a 35-mm film frame.

The difference is not so large in case of digital SLRs: most of the advanced-amateur models have a sensor about 1.6 times smaller, in terms of linear size, than a film frame, while the Four Thirds standard, introduced by Olympus, has a factor of two. For most practical purposes, digital SLRs are not so much different from 35-mm ones (and even less so from half-frame models, like the Pen F).

On the other hand, non-SLR models use sensors from four to seven times smaller than a film frame. To achieve the same image angle, the lens focal length has to be also four to seven times shorter (we will say that the lens equivalence ratio, N, is between 4 and 7).

Suddenly shooting a scene 36 mm across (our example above) becomes not so different than shooting one which is a meter wide.

The extra extension is no longer prohibitively large, the same subject distance (the same angle, remember?) becomes much larger than the focal length, no special correction necessary, and the exposure metering in these cameras is done through the lens (usually by the image sensor itself). All three points above become non-issues.

The shorter focal length has one more implication: much greater depth of field. Skipping the details: if you are using a digital camera with N=5.6 (see above) at F/4.0, you will have the same depth of field as shooting with an OM-1, with a lens with the same viewing angle, stepped down to F/23 (4*5.6=23). While shallow depth of field can be used creatively, try to tell this to someone chasing a bee on a flower, with a camera hand-held. In macro applications DOF is your friend.

This is why many non-SLR digital cameras allow you to focus quite close: down to ten, or even three centimeters from the lens front, for a field of view until now available only to SLR users, and that with special equipment.

In particular, most Olympus models not only provide this feature (allowing you to get a field three centimeters wide), but the macro performance is just great - and this includes even many of the least expensive, entry-level models like the bottom-shelf, $250 Olympus C-460 clamshell (a.k.a. D-580), with the FOV just 28 mm across and a very pleasing performance.

Last but not least, all non-SLR digital cameras offer real-time viewing on the LCD monitor; this not only takes care of the parallax effect, but the preview is usually more convenient than through the SLR eyepiece. SLRs do not allow for real-time electronic viewing.

(Just for the record: the E-10 and E-20 Olympus SLRs of a few years ago are an exception here; not only do they have real-time LCD preview, using a beam splitter instead of a "normal" SLR mirror, but their CCD sensors are also much smaller than those in all other models, with the equivalence ratio, N=4.)

Infrared

I very much enjoyed Geoffrey Tyrrell's piece on infrared photography with the OM-1, published in the December issue of Quest. There were three reasons I liked reading it: (a) a great, atmospheric picture accompanying the article; (b) rekindling of old memories of my own IR experiments back in the Seventies, with really awful results; and (c) the sweet feeling that I can now do digital IR without ever, never, having to go through all that hassle again. (The listing is in no particular order.)

First, a disclaimer. Digital cameras were not designed with infrared in mind. While the CCD sensor is naturally sensitive to wavelengths well above 700 nm (and even above 100 nm), camera manufactures do everything to cut off infrared from the sensor, as it messes up the visible-light image. All digital cameras have a special anti-IR filter in front of the image sensor.

Still, if you cut off all visible light with use of a "black" infrared filter, whatever remains above the visible range is enough to create an image, except that the exposure needs to be about 2000 times longer, at least in Olympus cameras I've used. This translates into 1-2 seconds in full sunlight, therefore necessitating the use of a tripod.

(Ironically, some earlier digital cameras were equipped with anti-IR filters which were more IR-friendly; the two-megapixel Olympus C-2020 and C-2000 from 1999 had infrared sensitivity almost 100 times as high as that in the present-day models.)

This is not an article on IR photography (see here for more on that subject), so let me jump straight to the point: why is digital IR different between SLR and non-SLR cameras?

The main difference is that once you put an IR filter on an SLR, you shoot blind, without seeing anything in the viewfinder. On non-SLR cameras you can still use the optical finder; but even more: the LCD monitor is still active, showing the image as "seen" by the sensor, mostly in a red monochrome rendition. On an SLR I have to take the filter off and then to put it on every time I want to recompose the frame; no such need on my compact camera!

Second: the exposure metering in non-SLRs uses the signal from the very CCD sensor which records the image. I usually rely on the camera's metering, with just some compensation based on experience. In SLRs the exposure sensors are independent; while they use the light passing through the lens (and IR filter), their spectral sensitivity is different than that of the image sensor, and they are not protected by the anti-IR filter mentioned above. The same can be said, at least in many compact cameras, about autofocusing.

While not really critical, these differences make infrared photography easier and more hassle-free if you use a non-SLR camera. This is why I carry my old C-5050Z along on all trips - think of it as of a second body, loaded with an infrared film.

Other uses

Since writing the original version of this article I've received some feedback, urging, just for the record, to mention other uses where a non-SLR camera may be a preferred solution over an SLR. This section has been added to the article as a result.

Some people would like us to think that you cannot be a "serious photographer" (whatever that means) if you are not using an SLR. Well, I have seen some ignorants sporting $5000, 6 kg Nikon outfits, and some outstanding photographers doing their best work (or even all work) with an almost-pocketable Leica equipped with a 50 mm standard lens. Ever heard of Gary Winogrand? (On the digital front, you can meet a Magnum photographer shooting with non-SLRs here.)

An SLR is big, heavy, noisy, and because of all that it attracts attention. There are circumstances when you want to avoid all these attributes. This is about your convenience and your shooting style.

Don't get yourself pushed into buying an SLR by people calling everything else a "point-and-shoot camera". Seriously, after almost fifty years in photography I've met some such opinions — something about the quality of our school system. Maybe it is a good idea to read some old-fashioned (i.e., printed) books on photography and at least get the terminology straight before starting to share your wisdom with others on the Internet?

Anyway, my advice is that if you take your pictures seriously (and you are not on a shoestring budget), you should equip yourself with both types of cameras, and use one or the other or even both at the same session — depending on your current needs, the type of assignment, and possibly the phase of the Moon.