PeterW
Lifetime Member
Member has Passed
Posts: 3,804
|
Post by PeterW on May 21, 2010 7:14:24 GMT -5
Wayne wrote:
Agree with you there, Wayne.
Back when most mid-priced cameras had lenses with maximum apertures of f/3.5 or f/4.5 there was an old rule of thumb that said the best overall performance from a lens was at three stops smaller than maximum aperture.
Don't know if that still applies to modern wider aperture lenses.
With almost all lenses for formats of 6x9 cm and smaller you often find that at the smallest stops like f/22 you get definition falling off at the corners and sometimes halo effects at edges of high contrast which gives fuzzy definition.
With many cheaper and even mid-priced cameras the lens coverage was only just adequate enough for a rectangular format. This was one of the reasons for many makers changing to a square format such as 6x6 cm on 120 and 620 cameras and 24x24 mm on 35mm cameras though this was never so popular as 6x6.
With older large format view cameras and the more modern monorail cameras it used to be common for photographers to fit a "next size up" lens; half-plate lens to a quarter plate camera, whole-plate lens to a half-plate camera and so on.
This had two advatntages: the wider coverage allowed for lens movements and camera movements like swing and tilt without going outside the best "circle of definition" of the lens. Also, with the larger format lens, there was less tendency to introduce aberrations at very small stops.
All this has probably gone by the board with modern short zoom lenses. Most "kit" zooms fitted to 35mm digital SLRs give a performance that would have brought rave reviews 40 years ago.
PeterW
|
|
Mark Vaughan
Lifetime Member
I STILL have a pile of Nikons. Considering starting a collection of Ricoh SLRs and RFs.
Posts: 191
|
Post by Mark Vaughan on May 21, 2010 10:02:01 GMT -5
I use depth of field preview to stop down my old non-AI Nikkors (for example when using the F3 with an old non-AI) to get a proper meter reading, but that's about it.
My favorite depth of field button is on the old SRTs: it must be the easiest one to use ever. That, and some of them had the ability to lock in or out.
Take care! Mark
|
|
|
Post by herron on May 21, 2010 15:16:15 GMT -5
The best depth of field indicators were the scales they used to put on lenses. Unfortunately most don't include depth of field scales anymore.. I never had a lot of use for the proview buttons. BTW, someone mentioned shooting at f22. I don't know of any 35mm lens that is at its best at f22. In fact a lot are pure awful at that setting. You have to get up into large format view cameras before f/22 becomes decent. f/56 to f/11 is the sweet spot on most lenses with f/16 still useful. Wayne You're right ... the "sweet spot" for most lenses is a couple stops less than mimimum ... (around f11 for a f22 lens), but the most DoF will be available at f22.
|
|
mickeyobe
Lifetime Member
Resident President
Posts: 7,280
|
Post by mickeyobe on May 21, 2010 23:30:34 GMT -5
As I understand it the deterioration of an image at small apertures is the result of diffraction caused by the aperture blades. Hence it stands to reason that f22 on a larger lens will undergo less diffraction than f22 on a smaller lens.
What I don't understand is how f22 on two lenses of the same size will offer differences in quality unless the overall quality of one lens is less than another.
Will the number of aperture blades or the shape of the aperture have any affect?
Mickey
|
|
|
Post by vintageslrs on May 22, 2010 6:38:35 GMT -5
Unless I'm going for a special effect, I usually attempt to shoot with an aperture of F8 or F5.6, with my vintage SLR's and lenses for that exact "sweet spot" reason..
Bob
|
|
PeterW
Lifetime Member
Member has Passed
Posts: 3,804
|
Post by PeterW on May 22, 2010 11:58:09 GMT -5
Mickey'
You're quite correct, Mickey, when you say that some of the deterioration at small aperures is cause by diffraction at the aperture blades, but that's not the whole story.
It's a long time since I did any optical theory but most "schoolboy" books on light say that the least diffraction is when the aperture stop is placed in front of the lens. Trouble is that these elementary books deal only with simple meniscus lenses or, at best, a simple double convex lens, and always use a round hole in a metal plate as an aperture stop.
With multi-element lenses I think I'm correct in saying that the aperture stop should be at the point where the rays of light cross over (to give an upside-down, left-to-right image at the focal plane).
One problem here is that light rays from different ends of the spectrum "cross over" at slightly different points. This didn't worry early lens designers because the emulsions of the time recorded only light from the higher end of the spectrum.
With the introduction of orthochromatic emulsions, later with panchromatic and even more so with colour emulsions this did become a problem.
To take as an example one of the longest-living lens designs, the Tessar, Paul Rudolph designed it when emulsions were almost soot and whitewash, and limited the maximum aperture to f/6.3. I've forgotten what the smallest stop was.
The design and the types of glass were later modified by Ernst Wandersleb (twice) and Willy Merte, ending with a lens having a maximum aperture of f/2.8 which could cope not just with panchromaric emulsions but with colour as well, bringing all the rays of light to focus at the same point at the focal plane.
The exact placing of the centre element of a Tessar is critical in the design, and many of cheaper "Tessar layout" lenses which appeared when the Tessar patent expired don't have the centre element placed with such care as the Tessars made by Carl Zeiss. Nor is the placing of the aperture stop controlled with as much care. But then many of these cheaper lenses have a minimum aperture of f/16 so the problems with very small apertures don't arise.
One feature of the Tessar is that it can be focused by moving the front element without very much deterioration of the image. Zeiss got away with this on the Super Ikonta because it was medium format, either 6x9 or 6x6, and negatives weren't often enlarged all that much, but it did affect the effective placing of the lens aperture
Camera designers like Nagel, who designed the Pupille and the Retina, and the designers at Rollei, didn't like introducing unncessary aberrations and moved the whole lens to focus it. Similarly the Leica (the Elmar is a Tessar Layout) and other precision 35mm cameras moved the whole lens to focus it.
The whole ball park changes when you get to six-element Gauss-type lenses, and changes even more with zoom lenses.
So lenses that are the same size, look similar and have a similar layout don't all perform the same any more than apples of the same size that looks similar don't all taste the same.
With regard to the shape of the aperture blades, yes this does have an effect which is most noticeable at small apertures. You've probably noticed that the higher grades of leaf shutters which incorporate an iris aperture use more aperture blades than cheaper versions so that the aperture is closer to a circle which gives the least diffraction at small apertures.
You'll also have noticed that some cheap cameras, and even some better automatic point-and-shoot cameras use just two iris blades which open and close to give either a square or rectangular shape aperture. It's cheaper to make, and they get away with it because few of these cameras stop down smaller that f/11 or f/12.5. The automatics increase the shutter speed instead of stopping down the lens. Most of them also focus the lens by moving it bodily rather than just moving the front element.
Once you get below the higher-priced cameras where designers use as little compromise as possible, camera designers are constrained by cost, not just prime cost but by arguments with production engineers who want the camera to be built as simply as possible, by automatic machinery if possible.
They prefer to spend money on precision machines rather than on skilled labour, gambling on large volume production and high sales to show a profit. So then the advertising people have to dream up effective campaigns to boost sales.
Long life of a product comes quite low on the list as do cheap repair costs, so if the prime cost can be kept down we get the throw away and buy another one consumer mentality.
Also, the bulk of camera users are quite happy if they get reasonably sharp, recognisable likenesses of their friends and family on a 6 x4 inch print.
As always, I'm prepared to be shot down over anything I've written, so if anyone can correct it or add to it please do.
Sorry if I've rambled a bit, but the answer to your question, Mickey, isn't a simple yes or no. There are far to many factors and compromises to take into account.
PeterW
|
|
Deleted
Deleted Member
Posts: 0
|
Post by Deleted on May 22, 2010 13:00:42 GMT -5
Great info, Peter. I just push the button and hope something happens
|
|
|
Post by herron on May 22, 2010 13:30:45 GMT -5
Peter -- I knew we could count on you to give us a good definition. Thanks! ;D
|
|
mickeyobe
Lifetime Member
Resident President
Posts: 7,280
|
Post by mickeyobe on May 22, 2010 14:49:45 GMT -5
PeterW,
Thank you for that wonderfully detailed and lucid explanation. Schoolboys like me are always thrilled when an extraordinary teacher helps them to see the light. We are never too old to learn. Indeed, as I grow older, I find that learning can be very pleasurable when presented in a palatable manner as you have done here and always do.
Mickey
|
|
PeterW
Lifetime Member
Member has Passed
Posts: 3,804
|
Post by PeterW on May 23, 2010 17:03:15 GMT -5
Thanks, fellas. If anyone wants to see cross-section drawings of some Zeiss lenses, Rick Olsen has the pages from a Zeiss lens manual on his website. Have a look at and scroll down to Zeiss Lens Manual, parts 1 to 8. It's a Zeiss Factory Technische Reparaturunterlagen (Technical Repair Document) so if you read German it will help you take apart and put together some quite exotic Zeiss lenses. It backs up what I wrote about some of the dimensions being critical in a top-quality lens. Much of this critical positioning is taken care of by ultra-precision enginering in making the parts of the mount. All parts of the mounts of lenses from Carl Zeiss in Jena are (or were) interchangeable. On some lenses there are one or two dimensions which should be checked on assembly, and you find the plus or minus limits on these dimensions are very tight - typically 0.02 mm, that's 0.0007 in or plus or minus seven tenths of a thousandth of an inch. That's not the sort of tight tolerance you normally meet in camera and lens tinkering. I for one haven't got the equipment to measure to that accuracy. After looking at these drawings I would be very careful who I entrusted with stripping and cleaning something like an f/4 20mm Flektogon. PeterW
|
|
mickeyobe
Lifetime Member
Resident President
Posts: 7,280
|
Post by mickeyobe on May 24, 2010 13:06:48 GMT -5
After looking at these drawings I would be very careful who I entrusted with stripping and cleaning something like an f/4 20mm Flektogon. PeterW And I would very definitely exclude myself from fiddling with any lens at all. Mickey
|
|
|
Post by herron on May 25, 2010 11:30:32 GMT -5
I don't mess with them much anymore, either. It strains my eyes, and makes my knuckle joints hurt.
|
|
|
Post by John Parry on May 26, 2010 16:00:55 GMT -5
But didn't the DOF preview button star high in the advertising? I couldn't see much point but it at least was an indication that the lens was opening and closing as per...
Regards - John
|
|
|
Post by vintageslrs on May 26, 2010 16:36:22 GMT -5
Right you are John, on both accounts..... ;D
Bob
|
|
daveh
Lifetime Member
Posts: 4,696
|
Post by daveh on May 30, 2010 5:20:38 GMT -5
Never found them any use. They always seemed rather optimistic (as to what was in and out of focus) to me.
As regards what f-stop to use: surely it depends what you want to achieve. There is always a trade off of one thing against another.
|
|