Gfyuki

As I know the focal length of macro and tele lenses are the same, which is around 200mm. Now I wonder, why we can not use tele lens for macro photography?

There is no direct relationship between Telephoto focal lengths and Macro capability. There are some fixed focal length prime lenses that fall into the Telephoto range in terms of focal length and also are capable of close enough focus to be Macro lenses. But a lens doesn't have to be a telephoto lens to have Macro capability and there are many Macro lenses that have shorter-than-telephoto focal lengths.

Macro lenses allow closer focusing than most lenses. By allowing you to get the subject closer to the camera, it allows you to increase the size of the subject in your photo. Macro capability is measured in terms of Maximum Magnification (MM) that is only indirectly related to focal length. Magnification is expressed as the ratio between the actual size of the subject and the size of the subject's image that is projected onto the film/sensor. A 1:1 Macro lens, which has an MM of 1.0x or 100%, means if the subject is 15mm tall, the lens can get close enough to project a properly focused image of the subject on the focal plane that is 15mm tall. A 1:2 lens would have an MM of 0.5x or 50% and would project an image 7.5mm tall of the 15mm subject. This is because if both lenses are the same focal length the 1:2 lens would require twice the distance to properly focus on the 15mm subject.

Macro lenses also tend to be optimized to perform best at their minimum focus distance (MFD) and are usually highly corrected for field curvature to give a fairly flat field of focus. They're great for shooting images of flat test charts at close distances and for doing two-dimensional art reproduction work.

In contrast, many other lenses, including the vast majority of telephoto lenses, are designed to perform best at longer subject distances and may or may not have a high degree of correction for field curvature. Some of the best "portrait" lenses tend to leave field curvature uncorrected or undercorrected as this tends to give them very smooth out of focus highlights (bokeh). Similar lenses with flatter fields of focus tend to have bokeh that is "harsher" or "busier."

Most Telephoto lenses are designed to focus on distant subjects, not to reproduce nearer subjects at high magnifications. A 600mm lens will do very well at taking a 6 foot tall human at very large distance (a little over 400 feet) and filling the 36mm tall sensor frame (full frame is 36mm x 24mm) in portrait orientation. I've never seen a 600mm lens that can get close enough to a 36mm subject to fill the same frame and properly focus on it. By the time you are close enough to the subject, you are inside the lens' Minimum Focus Distance (MFD) by several yards/meters. Most telephoto lenses have very large MFD and thus small MM numbers. That is what a Macro lens is designed to do: by reducing the MFD you can focus on a much closer object and get a higher MM.

There are some telephoto zoom lenses on the market, usually in the 70-300mm range, that claim to be Macro capable. But if you examine the specifications of such lenses, you see that at best they are 1:3 in terms of magnification. They can only focus close enough to project a 15mm image of a 45mm subject. That gives them an MM of .33x or 33%. While it is theoretically possible to design a telephoto zoom lens with 1:1 Macro capability, it is not practical. Most true Macro lenses have a fixed focal length designation that allows them to be simpler, cheaper than a comparable zoom lens would be, and produce better image quality at closer subject distances.

1. You can. In fact people do. Longer focal lenghts have the advantage, that you can (for same magnification ratio) remain further away from the object you take the photo as with a shorter focal. This is usefull as you will have more light on the object. (Sufficient light is one of the problems in macro photography. Why this is so needs a longe explanation.)


2. Macro is not tied to the focal lenght.
   A macro lens is a lens that is calculated to give best performance for short film/sensor-object distances. In fact you can take any lens and insert macro rings to get nearer your object. But a lens not calculated for short distances will deliver a subpar image.

Out of the box a normal telephoto lens won't be able to focus close enough to work as a macro. Extension tubes will take care of that of course. A more subtle point is that macros tend to be very well-behaved optically, with very little distortion and a very flat field of focus, and very sharp. These are not such vital design parameters for a general-purpose lens, where more attention may have been paid to things like flare resistance and focus speed which are not so relevant for a macro.

Canon has a 180 mm L macro lens, Nikon makes a 200 mm. But these are way outside the norm, something like 100 mm is the mainstream.

Tele lenses tend to be great for macro photography if you add a closeup lens in front so that they can actually focus on stuff reasonably closely (a +3 closeup maps infinity to 0.33m distance). The focusing ability is the main differentiator. If you want reasonable image quality, the closeup lens needs to be an achromatic lens (consisting of two elements that cancel in chromatic dispersion but not in optical strength) and of course combining two separate lenses in that manner tends to lead to worse overall quality than having a single lens designed to have the same net effect from the start.

Also out of focus background tends to be "beyond infinity" for the tele lens and thus not a priority for graceful degradation and/or avoiding vignetting of bokeh. It helps somewhat that you'll need to stop down usually in order to get tolerable depth of focus.

TLDR: the close focusing ability tends to be the main issue for macro/closeup use of tele lenses, and a closeup lens will help with that. The quality of the closeup lens is significantly involved in overall image quality: forget about $10 sets of single element closeup lenses in strengths +1, +2, +4, +10. In fact, with a strong tele lens, you'll likely want not much more than +2 (which brings infinity down to 0.5m and 1m down to 0.33m) since otherwise handling depth of focus becomes too awkward.

Several closeup lenses produced as accessories for various cameras don't even mention their strength (since it would look disappointing compared to cheap single-element ones readily available on the market) but you can take their largest focusing distance in meters (which usually is specified) and invert it for the strength in dioptres.

If you have a tele lens with a minimum focusing distance of 5m and a +2 closeup lens hauls this in to 0.45m, that's an 11x factor in size you gain over using the tele lens without closeup lens and subject at minimal possible distance. If you have a lens that already focuses at 0.1m, you'll need ridiculously strong closeup lenses to get the subject noticeably larger.

Of course, if you have a tele lens with 100mm filter threads, getting a reasonably quality closeup lens (which has to go at the front) will be prohibitively expensive, so closeups usually provide most value for fixed zoom lenses on cameras that have a moderate filter size, typically in the superzoom or by now travelzoom category.