WHY REVERSE LENSES?
For magnified imaging lenses are better used reversed – the diagram shows how, in normal use, the subject is larger than the on-sensor image and normal lenses are designed and corrected for the angles the light rays make on exit and entry under such circumstances. When the image on the sensor is larger than subject then lens corrections are more effective when the lens is reversed: biggest item closest to larger lens surface.
above - lens corrected to handle subject larger than image on sensor : below – reversing the lens allows the lens to handle an image greater in size than the object
EFFECTIVE APERTURE
With a 24mm lens used in this way you achieve a magnification (M) of 105/24 = 4.4 In basic terms, light rays from a small area of the subject have been spread out by the lens system to cover a bigger area in fact (4.4)2 times greater. Thus, there is a much smaller “effective aperture” than the one marked on the lens, and this not just affects image brightness (and thus exposure) but also the onset of diffraction as the effective aperture gets smaller
With TTL metering there is no need to make calculations, the camera looks after that but it is useful to know a rough effective aperture (compare with the marked one) so you don’t inadvertently close down to where diffraction softens anything. In practice, I find that it is it is better to keep the effective aperture to around f16 – f20 and not stop down more: a rough mental calculation working backwards is what is needed.
Within the practical realm of x1 – x5 photography the effective aperture is roughly (I stress this is ‘ball-park’ and here is not the place to go into this) obtained by multiplying marked aperture by magnification on the front lens. So, with the 24mm lens set to f4 and a magnification of 4.4 you’d have an effective aperture smaller than f16 , before you even considered making further adjustments on the prime lens.
the 'hyperdermic needles' of a stinging nettle are revealed by using an old Canon FD 50mm f3.5 macro bought in 1977...reversed on to a Nikon 105mm f2.8 VR AF macro . M =2 x (2:1)
here an old Sigma 24mm f2.8 with jammed AF was given a new lease of life reversed on to the Nikon 105mm f2.8 macro and used with the nettle stem as subject. The sharpness is every bit as good as the specialist 'true' macro lenses I have in my collection. M =4 x (4:1)
Going Deeper – the problems
The practical approach above is the best one when dealing with lenses even if, in theory, one can solve any lens problems from a series of equations. These become nightmarishly tedious to solve, with numerous variables when there are even a few lens elements, let alone the number in modern lenses. In practice, lens designers use the mathematical equivalent of a ‘suck it and see’ principle when tracing the path of different rays through a lens system from edge to centre by using calculations that can be done in their millions every second by computer until they get an ‘optimal’ compromise.
Little, if any lens optics remains in any school physics syllabus and most universities courses have abandoned it. This may be why there are some mistaken ideas floating around the internet but voiced with great certainty (as always). Coupling lenses creates a far-from-simple system where entrance pupil of the prime lens and exit pupils of the front lens do not coincide so assumptions about light going through are just that – assumptions. In this branch of physics ‘intelligent guesses’ help with ideas of what might be happening and we often need no more…the empirical approach works best (ie ‘suck it and see’)
Diffraction and the perception of sharpness
At some future stage I shall write a detailed post on this for it seems there is more confusion than ever thanks to wrong assumptions about sensor site sizes and separations mixed with a liberal helping of old ideas about resolution and line-pairs. Sharpness is subjective: resolution can be measured if criteria are set for what you mean by ‘resolved’ for two points close together.
I have lived for decadeswith attempts to circumnavigate the softening effects of diffraction at small apertures- mainly by fooling the eye . Nowadays, with sharpening algorithms one can, to all intents and purposes, reverse the perception of image degradation due to diffraction by around 2 stops worth (my visual estimation) …note this is the APPEARANCE for these algorithms make edges more clearly defined by adding darkened pixels at edge transitions. The diffraction phenomenon and sharpening modes have no relation whatsoever. It is what you see that counts.
sharpness is an impression you can accentuate by your control over the angle of lighting, choice of position to maximize depth of field, reduction of vibration and a good lens...
One thing that becomes important is the difference between depth of field and depth of detail in this kind of imaging. This concept is important in Photomacrography covered in Kodak publication N.128. It looks dated (B&W) but contains information that would be difficult to find elsewhere and might be of interest to some readers.
Image Composites – stacked images
A more recent solution (also to be the subject of a future post) is to create stacks of images shot at optimum lens aperture (f5.6-f8) with focus changed slightly between shots. These can be merged with Helicon Focus software: wonderful for stationary subjects but a challenge with living insects. Sadly, many of the pictures one sees on line are of ‘recently deceased’ specimens and it shows…maybe people think “they’re’ only insects”…I saw one questioner write how can I best kill insects for stacking shots ? Is it back to the days of killing bottles and pins or should someone just try harder and be more patient perhaps…?
an example of the depth of field that can result from stacking images - here images were stacked with Helicon software. I used a shutter speed of 1/500th sec at f/8 with seven images and a lot of patience on a windless morning
Lighting
This will inevitably be flash (covered here)– a single gun held close acting as a broad source works amazingly well if it is lightly diffused. You might have to make a reverse lens hood – a rear lens cap with the centre punched out because reflections can result from some of the chrome surfaces on the lens rear and create flare. You don’t need DTTL control of flash – experiment and look at the LCD system. Exposure is can be controlled via the camera – though flash durations this close will be so short that the length of cable (if used) matters since pulses are nanoseconds in duration (which is in the realm of the time electrical signals take to move down cables). You just dial in compensation – reducing flash exposure if too bright: increasing if too dark.
Focusing and Vibration
A 2x-5x magnification is all that is practical (just) in the field where the slightest movement of subject (or camera and lens) becomes an earthquake. Thus, switch off the autofocus and focus either by moving camera plus lens or moving the subject (often easier) to focus.
I tend to operate on the principle (used in the best microscopes where camera and add-ons are fixed) of moving the subject, fixing the camera assembly to a focus slide or support and clamping that to a table: you can use a very rigid tripod as well. With your specimens on twigs etc (and whilst looking through the viewfinder) slide them in holding with both hands steadied on the table. You can also use a bean bag which is excellent at ground level for absorbing vibration. With shots of moving creatures let them come into view (ants on a stem) where you have pre-focused and use a cable or wireless release.
It is not the easiest photography you will ever have done – don’t be too ambitious and work first with a 105mm macro or a 135 telephoto or a zoom and a 50mm lens so you get the feel for what 2x on the sensor is like. Finding the subject through a stacked system is sometimes the main problem – but the results are worth it.
I have been experimenting (the only useful thing to come out of the worst weather locally in years) with a new focusing slide…take one already cannibalised microscope stand and an angle grinder (gently). I’ll show some pics when I talk about making your very own optical bench…almost portable and photographing small life forms including those in water – take two glass mounts from 6cm x 6cm slides and make a micro aquarium.
Next macro-post: reversing lenses onto bellows or extension tubes for better macro performance
Hi Paul,
I really enjoy your posts -the info and the style
I own a 105mm vr macro – and an 18-55mm nikon kit lens- could I reverse this onto my macro lens?
It is a zoom lens and a different diameter- the macro or base lens being 62mm thread- the zoom 52mm.
Is this workable?
If so would twisting the zoom alter the magnification between 5:1 to 2:1 ?
Would vignetting be a problem?
Im guessing you will say “try it and see” , but i dont have the adapters/rings yet.
Be great if i knew it was workable before investing in them.
Also im off to southern italy this year – can you recommend good field guides-ie books for native insects and flora?
regards,Richard B
Hi Paul,
The first image listed in this article is not posting.
This is great reading and I think it should be a constantly evolving/moving/living dynamic blog. I know that i, for one, would love to be able to log on and read all the time instead of fits and starts. I am also fully aware that this would take away all your spare time and is not pracical. . .but would be nice.
Also, a “new book”? . . .hmmm, how long do we have to wait for this gem?
Kindest regards,
Bruce
Hi Bruce,
Thanks for the alert – it should load now. I had not realised that I used a TIFF where the byte order was for a Mac. I have saved it as a JPEG and re-loaded it.
The ‘fits and starts’ thing is, as you say, an ideal but it is a time thing really. Posts take a long time to construct and I enjoy doing them a lot (probaly too much!) but there are pics to be taken, articles to write and tons of stones to move…. It would be tempting fates to say too much about a project where what you hint at will become a reality. Just let’s say ‘one is working on it seriously’ and, at least in the interim, material is there even if it is piecemeal. I am always nervous about posting something of too great a length inducing the yawn factor and driving down ‘hits’!
The new book is a reality (did I really say that?) the contract is signed. I have to deliver all text and pics by the early part of 2011 and am working on gathering material now. Publication is scheduled for a year this autumn (around September/October 2011). The delays are about par for the course!
best regards
Paul
Hello Richard,
Firstly, thank you for your comments about the posts.
Second, yes you anticipated my response…some zoom lenses do work well and it is impossible to say without trying. You can get a good idea by fixing the camera with the 105mm VR on it and holding the zoom reversed in front of it. I have just tried the same with the 18-85mm VR that I have and get an image with everything wide open so with your 18-55mm you should get that range of magnification you suggest through at the short end the assembly will be very close to the subject. One problem( and minor irritation) is that these lenses have no proper manual diaphragm so you have to tape the little lever on the lens mount in a position where the diaphragm is open. It is easy to do it wide but stopping down you’ll have to try and see where vignetting sets in. In the past I have taken a rear lens cap – punched out the end (neatly!) and then fitted a small screw or similar to act as a lever to shut the diaphragm. Insulating tape works just as well if you are not happy with this. It used to be possible to buy a 50-55mm standard’ lens for a pittance (Canon, Nikon, Olympus…even) because no-one wanted them. At least then there is not the diaphragm problem: a lens hood made as suggested is a good idea, for the first time I tried this years ago I had flare from chromed parts at the rear of the lens.
The problem with a lot of zooms is you never quite know where the exit pupil is when you turn them around and if this is not very close to the surface of the camera lens you lose light and get vignetting.
best of luck
Paul
Hi Richard,
Sorry I missed out the last part of your email re guides to Italy. The best guides available here tend to be translations of UK/European Field Guides. Thus I would recommend the following:
PLANTS
a. Wild Flowers of the Mediterranean (Blamey & Grey-Wilson) will cover most plants you find
b. Field Guide to Wild Flowers of Southern Europe by Paul H Davies & Bob Gibbons… if I may be immodest and then for INSECTS
Collins Pocket Guide: Insects of Britain & Western Europe (Michael Chinery) – a very good choice of things that will put you on the right track.
The Pocket Guide to Butterflies of Britain & Europe (Paul Whalley and Richard Lewington) this Mitchell Beazly guide goes with me everywhere.
I also like Butterflies of Europe (Tristan Lafranchis) which has lots of points for accurate identification.
You might also look at the excellent visual resource being built up on the site Natura Mediterraneo (http://www.naturamediterraneo.com/forum/gallery.asp) its not yet complete but the insects are excellent. It is in Italian but you’ll get the Latin names.
hope this is helpful
Paul
Paul,
The other downside to the 18-55mm lens is its plastic end- seems very snappable and not ideal for reversing- a 2nd hand 50mm looks tempting. Im just looking for the cheapest way to increase magnification with the kit i have ideally.
anyhow cheers for the advice will try the lens and check out those books.
re insects:
I have the Chinery book which is brilliant- however- i went to tuscany last year and found this tiny critter on a number of occasions:
http://www.movementpattern.com/movementpattern/Photos.html#119
I still am unable to identify exactly what it is..I have more photos- can u shed any light- its unlike any other insect i have seen- it moved more like a horse than an insect! maybe some kind of assassin bug?
cheers,
rich
Richard,
I agree with you about Michael Chinery’s book – it is hellish difficult to select species for a field guide and in photographic ones you tend to get those of which the author has shots (and people often pass the common by) he has worked wonders with this. I wish there was a guide to southern Europe but, when I was in the business of co-authoring flower guides it was very difficult to get any editorial interest in anything other than N. Europe.
It is an intriguing insect and I would say the Assassin bug guess puts it pretty near- particularly with the comment about movement: the proboscis is immense compared with body length and it is a nymph and thus much harder to place. I’ll pass on the link to some Italian friends who are great with the small obscure stuff. My guess would be one of the Nabidae (Damsel bugs) – the rostrum looks 4-segmented rather than 3 as in the Reduviidae (Assassin bugs)…maybe a stage of the tree damsel bug (Himacerus apterus)…a guess, for I am struggling, too!
By the way if you tell me where you are going in S Italy I might be able to point you in the direction of a few good sites
best
Paul
Hiya
we will be near Naples mostly, any advice on anything in that area – not just macro – very much appreciated Paul.
rich.
Hi Richard,
I’ll send a more detailed response direct to your email address in the next few days with some ideas. Apart from the intrinsic chaos of La Bella Napoli the city has some good bits (fantastic museum and amazing archaeology)…Lois loves it but it is not to my taste (too anarchic).
North along the coast lies the Parco Nazionale del Circeo and just south is the Amalfi coast – I have stayed in the hills at Cava de Tirreni and walked from there onto the Amalfi pensinsula – spectacular views and lots of Mediterranean ‘stuff’ (butterflies, mantids etc). SE of Napoli you get into the Parco Nazionali del Cilento which not only has a lovely coast but is virtually empty with tiny villages and small, quiet roads. At the southern end lies Palinuro which has sea grottos and good diving. Matera lies an easy day trip east along the autostrada and that is an amazing place – very evocative clustered houses and rock dwellings and the most amazing population of lesser kestrels that do their stuff against the evening light when you can see the translucent tail feathers. In most parts of Italy it is so easy to get away from crowds by going in land and just taking small roads.People tend to flock coastwards where they love to be close together (strange how an antisocial misanthrope like me has ended up here!). It gets very hot down south which means you can get images early morning and evening and leave the day to the sunseekers!
best for now
Paul
all sounds good to me
v much looking forward to it- wildlife, weather, food and history,
thanks paul, if you get any more news on the bug or other italian info please let me know,
rich.