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With my Canon 550D and Canon EF-S 15-85 1:3.5-5.6 IS USM lens, I have been taking close-ups of Harrison clock parts at Greenwich. When using f36 to get good depth of field, the sharpness is very poor indeed. Subsequent experiments show that the sharpness is OK at f8, but going off at f16 and bad at f36. Using Canon's EF 100 2.8 L IS USM Macro lens gives similar (but better) results. Any advice on close-ups and depth of field to get good sharpness?
Quote: Subsequent experiments show that the sharpness is OK at f8, but going off at f16 and bad at f36.
This may help to understand things a little better... (Damn! www.cambridgeincolour.com. is off-line!!)
I had intended to point you at a most superb gentleman - Sean McHugh - whose website (as identified above) is an 'Aladins Cave' of photographic wonderment and information. (Very strange since I had an email from him just a couple of weeks ago.)
So... in an effort to assist, try a Google Search for the phenomenon of 'Airey Disks' and hopefully - (sans Sean's excellent piece on the subject) you will understand why you are seeing the results you are getting.
(Do keep an eye on that URL... 
)
If you have Photoshop you can use what they call 'focus stacking' to improve depth of filed - you take a series of pictures with the camera fixed in one position but between shots you rotate the focus ring a bit. Then the software works it s magic to pick the 'in focus' bits from each shot.
If you "focus stack". it helps to use a macro lens that doesn't "breathe" - so that image scale doesn't change as the focus is adjusted.
Many modern macro lenses use internal focussing. This means that, when we focus closer, the focal length is reduced and at lifesize, the focal length has been halved. Some lenses, like the Zeiss Makro-Planar have floating elements to correct for this effect but it comes at a price.
The loss of sharpness on stopping down depends on the millimetre size of the aperture rather than the f stop. This makes even the best 100mm macro lenses noticeably fuzzy below f11 and are best not used below f8 if you want then to be sharp. The only cure is to avoid stopping down too far.
There are several challenges in close up and macro photography.
The main problem getting sharp images is a combination of faster shutter speeds being needed because of the increased magnification, combined with any camera or subject movement also being magnified in close up.
As you cannot get very good results at f11 and good results at f16 with either lens a major factor is likely a lack of camera or subject stability at the longer shutter speeds that go with smaller apertures.
A second issue is a lens aberration called diffraction. Basically this confirms a "perfect" lens at f4 has only half the resolution of f2, f8 has only half the resolution of f4 - and so on. There are no "perfect" lenses - all have other aberrations which are stronger at wide apertures. Generally high quality lenses do not reach peak resolution until around f9.5-f11 and lower priced lenses until around f11-f13.
With a good lens differences one stop wider or smaller than optimum aperture are hard to detect outside an optical test bench environment, and differences 2 stops either side are minor or moderate.
As you are not getting good (as distinct from theoretically perfect) resolution at f11 and f16 your lack of quality is likely to be "camera shake" and not diffraction at the maximum magnification an 15-85 lens achieves.
The better results with the specialist macro lens are because it is optically better at macro 
Disagreeing with thewilliam saying lenses become "noticeably fuzzy" below f11 - this is an exaggeration. f16 is often no worse than f5.6, and f22 is sometimes no worse than f2.8.
Even so unless you resort to focus stacking you are not going to get sharp images and f36 depth of field 
Focus stacking requires several pictures at less than a millimeter focus difference with good alignment and using software to blend the sharp part of each image.
Hi all. Thanks, Mike & JJGEE - Sean's website is excellent, and and I think that the diffraction phenomenon is my answer - it fits exactly to what I have been getting. I also found a review of the macro lens at
http://www.dpreview.com/lensreviews/canon_100_2p8_is_usm_c16/page3..asp
which clearly describes the effects.
To lawbert & LensShepherd thanks also. I did indeed spend considerable time producing series of images as I altered the variables, and at all times with a tripod and delayed shutter release, locked-up mirror and using a radio remote - very important points.
To mikehit and thewilliam, thanks for making me aware of focus stacking - something I was not aware of. Will have a look at that in due course.
What a marvellous forum Ephotozine has - thanks again.
I'd disagree with Len about the amount of stopping down for optimum aperture:-
Rodenstock advise 1-2 stops for the 50mm and 80mm Apo Rodagon enlarging lenses.
Leica and Zeiss recommend 2 stops for lenses like the Summicron and Planar designs.
The makers know their own products and this is consistent with my experience as a user.
Yup. 'Diffraction' was where I was intending to take you on Sean's site Ken. Your 'symptoms' are classic.
Quote: I'd disagree with Len about the amount of stopping down for optimum aperture:-
You are entitled to disagree - my testing plus this web site disagrees
http://www.ephotozine.com/article/canon-ef-100mm-f-2-usm-interchangeable-lens-re... shows the Canon f2 peaking by a whisker 4 stops down from wide open at f8 - with f8 being near identical to f11. The lens scores above or almost above excellent by a whisker from f4 through to f16, and good at f22.
A "mistake" working only with resolution numbers is that they are invariable measured at 1000:1 contrast - rarely encountered in real world photography. Resolution figures are much lower at everyday contrast levels - which can be as low as 5:1.
At these lower contrast levels the diffraction resolution limit for each smaller aperture becomes less of a constraint because of the lower resolution figures at contrast levels less than 1000:1.
Focus staking is certainly the way forward if you can't get what you need at a suitable aperture whilst retaining sharpness - however whilst it is tricky its not quite so hard as it sounds, esp when working with a nice still subject.
First up is to make sure you've a stable surface to shoot on and from - carpet is a pain, so a good solid floor that won't shift as you move your weight and body is a great help. From there you can add a cheap (yet good) focusing rail such as one of these make:
Ebay Focusing rail
Focusing rail in ebay will get you many more vendors of the same design.
You then just need to make a small turn on the focusing wheel rail to move the camera a tiny bit closer for each shot in the series. You can then use free focus stacking software to stack the shots, such as CombineZP.
There are a few other stacking software options such as Helicon Focus and Zerene Stacker as well as the stacking option in Photoshop - in general there isn't a market leader and (when used correctly) they are all capable problems (even the freeware listed above). There are select cases where one has the edge over the others, however they are generally not conditions repeatable as a method for shooting in general (which is why focusing software is one example where those who do it a lot tend to end up collecting the software options)
EDIT: As for optimum apertures I've a further series of test shots (displayable at fullsize) from 3 macro lenses at various apertures to show diffraction and sharpness changes Macro test shot series
Quote: You then just need to make a small turn on the focusing wheel rail to move the camera a tiny bit closer for each shot in the series.
I wondered about that technique - I have briefly tried using the focus ring but it was so difficult to get a series of closely-enough spaced focussing points.
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