Why do macro lenses extend when getting closer?

Discussion in 'Macro' started by mark_stephan|2, Dec 12, 2017.

  1. My question has been answered by Ben. I know that the AF lenses which has IF shorten its focal length when focus at close distance. I just wonder if that a good thing or bad thing. The obvious bad thing is that at close focus you have a lens with shorter focal length than you bargain for. However, by doing so you don't need an extension tube to get 1:1 magnification and lose less light at such a magnification as well.
     
  2. Working distance (front lens to subject) is neither good nor bad, but important.

    Working with living things, you want as much distance as practical to avoid spooking them. For copying photos and documents, while not "macro" but using similar equipment, you want the shortest enough working distance so you son't bump the ceiling (or need the floor). A longer lens also has a narrower field of view, which helps avoid distracting background elements in nature. With a 50 mm lens, it's hard to keep the lens from casting a shadow on the subject, and a lens hood is out of the question at 1:1.

    I have a manual 50 for copying documents as small as slides. It's compact, sharp and inexpensive. I find a macro lens on the order of 100 mm is a good compromise for home and nature. Photos of small objects for sale (or bragging rights) have better perspective when the camera is further away.

    I have rever used 1:1 magnification in the field (yet). It's very fussy to get things right, impossible when there is wind without patience and blind luck. A 200 mm micro has a minimum working distance of about 8", which is ample. I don't own one, but for magnification of 1:4 or less, extension tubes work well, even with a zoom lens (if you don't zoom). A 300 mm prime lens gives you over 2 feet for flowers, butterflies, or similar sized objects. The same is true for MF cameras. I use a 120 Macro with my Hasselblad, or more often, a 180 with an extension tube or two, giving about 18" of working space for typical subjects.

    Not all IF lenses change the field of view (breath) when focus is adjusted, and even non-IF lenses change FOV because the relative distance from the camera changes. A focusing rail is very useful in achieving fine focus (and essential for focus stacking) indoors or out. A gear rail is best for macro work, but a screw thread rail is better as you get closer (or exceed) 1:1.
     
  3. Ed, I have thousands of shots at 1:1 done handheld out-of-doors with KM and 55 or 105 MicroNikkors on Nikon SLRs. The subjects were flowers, insects and such. The trick was to use flash illumination to stop motion, both mine and the subjects. And that many handheld shots of un- or lightly constrained live fish in aquaria, some out-or-doors in a portable photo tank, others inside. Also with KM and flash.

    It all worked because with KM I could shoot between f/11 and f/16 set (that's f/22 to f/32 effective) at 1/125 (Nikkormat) and, later, 1/250 (FM2n). There's a delicate balance between overpowering ambient, loss of image quality in the plane of best focus to diffraction, and loss of image quality outside of the plane of best focus to diffraction (again) and shallow depth of field. My setups killed ambient, forced stopping down a little too far. Tradeoffs, tradeoffs.

    This is uncomfortable to do when the image capture device, film or chip, is too sensitive. With ISO 100 a two stop ND filter is needed if ambient is to be overpowered. Three stops might be better. I dislike this so much that I've stopped shooting flowers and such.

    Turning up a digicam's ISO isn't what's needed for full control, low ISO with flash is better.
     
  4. Our local aquarium (Shedd Aquarium, in Chicago) frowns on the use of flash. Turning up the ISO is the only alternative. Denizens at the Chicago Botanic Garden are not likely to be disturbed, except by the inevitable wind. I haven't bothered with flash, much less a tripod, but my results have been less than stellar. It's time to break out the Benbo. On my last visit, I found A9 autofocus will track a honeybee.
     
  5. Ed, that your results have been less than stellar is a message. Its time to break out the KM and learn how to use flash closeup.

    Yes, I know, Kodachrome processing is gone and won't come back. So its time to grit your teeth and break out the ND filters. Alternatively, go up in format, shoot ISO 100 E6 with cameras that have leaf shutters that will sync at, ideally, 1/500.

    And learn how to use flash closeup. Guide number arithmetic adjusted for magnification works well.
     
  6. Ed, I haven't been in the Shedd for decades. I spent time there centuries ago when I was a student. Anyway, if they still have their small aquarium room, that would be a good place to shoot fish with flash. Larger tanks are difficult, the flash-to-subject distancesare usually too large.

    About modern conveniences, e.g., TTL autoflash. It doesn't always work consistently when working closeup. I prefer to use pre-calibrated flashes. This is equivalent -- think about it -- to metering incident, which gives more consistent results than metering incident. I also adjusted flash power or aperture for especially dark or light subjects.
     
  7. I was asked not to use a flash at Shedd a few years ago. Like tapping on the glass, it supposedly upsets the fish. The galleries are quite dark, so the fish can't see the visitors very well.

    There are extended flash brackets which get around the problem of shading the subject with the lens. They're popular at the Chicago Botanic Garden. LED "flashes", both on closeup brackets and ring type, are an interesting alternative. You don't need a big power supply, and with high ISO so easily attained, probably ample light. Whether they help freeze action is questionable.
     
  8. Ed, low powered electronic flashes have very short flash durations and stop motion -- photographer's and subject's -- as long as there's no significant exposure from ambient. That's why slow film or the lowest possible ISO plus ND filter are essential.
     
  9. That makes sense. An ND filter would make all the difference if the flash turndown were limiting. A filter degrades the image a little, but not as much as subject or camera movement (camera movement is magnified at close range).
     
  10. From experimenting with a Nikon D1(which can sync at any speed) my Normans on an 800 w-s pack firing at full power(2 at 200 w-s, one at 400 w-s) the flash duration is around 1/750. When I go all the way up to a 2000 w-s pack, that goes to around 1/500-I think that's probably intentional as Norman would have wanted someone using a leaf shutter camera at 1/500 to be able to get all the flash power.

    In any case, I've handheld a LOT of stuff in the 1:2 to 1:1 range using this same strobe set up and get perfectly sharp results. The issue with handholding is that I often don't get what I wanted in focus or the framing I wanted.
     
  11. Studio flashes for closeup work? In general, not the best idea. In some situations, though, large flashes far from the subject can give better results than the small flashes close to the subject that I use.

    No one talks about depth of illumination, but it is a consequence of the inverse square law. With my rigs a background far behind the subject is always much darker than the subject. This doesn't make good sense for every situation. Large flash far away can fix this. In the field, however, large flash far away can be difficult to set up. I've tried this approach, with trepidation, for shooting hummingbirds. Never had good success, my most powerful flash wasn't bright enough to overpower bright ambient.

    Ben, if you're working in the studio with immobile subjects, why do you shoot handheld? If you suffer from tremor, using a two-axis focusing rail will give better control over focus and framing than shooting handheld. For mobile subjects, though, its handheld or don't bother.
     
  12. The photo I posted on P1 was done with the set up I described, albeit on a tripod.

    Most of my macro work is on watches, and for a long time I did them on a tripod with continuous lighting. A watch collector friend of mine who is a professional photographer in his day job has sort of guided me on improving my watch photos. He does strobe, which was his advice, and he also shoots handheld mostly. He kind of surprised me when he said that, but it can be so difficult to "catch" the patterns and really show them off that hand holding frees you up a whole lot.

    For publication work, he usually uses a PC-E lens from a tripod, but for routine stuff he gave my 105 Micro with the strobes a solid endorsement.
     
  13. I use a folding tent to photograph things ranging from miniature model cars to firearms. Since shutter speed is immaterial for inanimate objects, I've been using continuous light (mainly daylight LEDs). I have a set uf studio flash units, but that takes up the better part of a room instead of a small table. In lieu of a PC lens, I use focus stacking, for which a focusing rail is essential. Firearm fans like to see reflective surfaces handled well and prefer intensely sharp images. The biggest non-technical problem I have is dust. No matter how well cleaned, there's always dust. Renaissance Wax and a blower helps, but I'm thinking of using a tack rag. Another trick is a shaving brush. It picks up enough oil over time to act as a nearly perfect "tack rag." That tip comes from my brother. It seems drill sergeants really loathe dust and fingerprints ;)

    Oddly, people want to see the flaws in antique firearms. They can greatly affect the value, in either direction depending on the provenance.
     
  14. Ed,

    Most of my gun pics are taken on the back porch railing.

    With that said, I've found both blued Colts and nickel in general to be a pain. And, yes, showing the flaws can be a pain-I always struggle with getting good roll mark photos since that's often the give-away to original or refinish.
     
  15. Ed. Ben thanks for reminding me that there's more to macro than flowers, insects and such, mainly in the field. Also for reminding me that although my preferred lighting techniques work well with my subjects there's an art to lighting that I usually don't think of.

    Funny thing that in the lab I use a digital Nikon with focus stacking to shoot preserved specimens. For some reason even though I'm convinced that where it is practical focus stacking is preferable to "one shot and done" I have trouble thinking of it as macro.
     
  16. This perhaps isn't an outstanding example, but was a quickie when I had the lights out to list some stuff on Ebay. It was a recent purchase.

    I used 3 lights total off a Normal P1250 pack(actually the first time I'd used that pack). Two were in shoot-through umbrellas on the 750W-s circuit, or 375 w-s each. The third was hooked to the 250 w-s circuit in a 10" polished reflector firing at the ceiling to help fill in shadows. This was hand-held using a 105 Micro on my D800. I didn't pay attention to the magnification ratio, but I think that an 18 size pocket watch shot at full frame on 35mm/FX works out to 1/3 lifesize.

    _DSC2051 copy.jpg
     
  17. The camera lens acts just like a projector lens, in that it projects a miniature image of the outside world onto the flat surface of film or digital sensors. First we focus the lens on a far distant object. By far distance we are talking about a distance about 3000 times the diameter of the lens opening (aperture). This distance is called infinity (Latin for as far as the eye can see). This distance, symbol ∞, is significant because light from objects at this distance or further, arrive at the lens as parallel rays

    Light rays that traverse transparent things like glass or water or air are altered as to their direction of travel. This action is called refraction from the Latin to bend backwards. The camera lens forces these image- forming rays so that they trace out the shape of a cone of light. We focus our camera by adjusting the distance lens-to-cone apex until it just kisses the film or digital sensor.

    When the lens is imaging an object at infinity, we measure the distance from lens to cone apex and label this the focal length. Because lenses have a limited ability to refract (bend inward) light rays, should we image an object closer than infinity, the cone of imaging forming rays elongates. We now change the name of this distance to “back focus” distance; it is no longer the focal length.

    The back focus distance elongates and continues to do so as we challenge the lens to work in closer. As we move in closer, the size of the image of the objects enlargers. When we are sufficiently close so the image of this object is life-size (magnification 1 or “unity”), the distance lens-to-image (back focus) will be twice (2X) the focal length. Thus to achieve focus, the lens will be extended 2X further forward from the camera body.

    Now ordinary camera lenses can achieve close focus images. The problem is, ordinary camera lenses are designed to image a 3D world, In other words, most objects we image will be at different distances from the camera. The ordinary camera lens is optimized for this task. When ordinary lenses are tasked to work close, they are compromised. 1. Most close subjects are 2D objects like stamps or objects with minute differences in thickness. For this work, the lens needs to be optimized to work a “flat field”. Thus the macro is optimized to work flat surfaces and compromised when tasked to work on object with uneven elevations.

    2. As ordinary lenses close focus and the back focus elongates, the f-numbers engraved on the lens barrel become inaccurate. This is quite severe; at magnification 1 the error is 2 full f-stops. Historically, this has resulted in a plethora of underexposures. Modern cameras with through-the-lens metering are immune. Nevertheless, this circumstance, called “bellows factor” is a plague.

    The micro lens circumvents “bellows factor”: The front group of lens elements of the macro design magnify the entryway (iris diaphragm) of the lens. This magnified view serves to allow more light energy to enter. This is a clever adaptation, as you focus closer and closer, the diameter of the aperture is magnified more and more. This action counters the “bellow factor”.

    Because of the above mentioned construction of the micro, it is optimized for close-up work and slightly compromised when tasked to work as a “normal” lens.
     
  18. To add a bit to what Alan has said; the rays emanating from distant objects enter a lens in a parallel fashion but rays emanating from objects at macro distance are diverging. These diverging rays push the focus back further and behind the focal plane, thus requiring compensation by using one of several methods to bring the focus to a usable place.
     
  19. The focal length gives the ratio of the size of the image to the angle (in radians) of the light coming in for an object infinitely far away.

    The object angle is the object size divided by the object distance.

    When you focus closer, by increasing the lens to film distance, you don't change the focal length, but you do change the ratio between the image size and the object angle. This change also decreases the effective f/stop, as the f/ values are based on the actual focal length, not the image distance.

    So, yes, if you can decrease the focal length of the lens, without decreasing the image distance, you can focus closer without the change normally seen in the image size and effective aperture.
     
  20. A lot of newer lenses with IF tends to shorten their focal length when focus close. Older lenses don't do this and thus needed a lot more extension.
     

Share This Page