I've done some research into constant aperture lenses, and I can tell that they are better than variable, but I can't find out how or why. Can anyone offer help? Thanks!

-- Frank McKay (squanklin@yahoo.com), December 20, 2000

Answers

What do you mean with better ? Compared to what, better resolution, easier to handle, contrastier, cheaper... ?

-- Marc Leest (mmm@n2photography.com), December 21, 2000.

Exactly my point - I don't know!

-- Frank McKay (squanklin@yahoo.com), December 21, 2000.

The only thing I can see is that an optimal aperture should be completely round, to avoid lightbending effects that can be observed with apertures that are formed as an polygon as variable apertures. Another thing possible is the rendering of the out of focus area, that is also very dependent of the form of the aperture, since blurred highlights take the form of the aperture. Last thing possible is the use of very small (pinhole) apertures that render a very good depth of field but will not have maximal sharpness (diffraction effect). Hence my first question.

-- Marc Leest (mmm@n2photography.com), December 21, 2000.

On manual (or manual mode) cameras a constant aperture lens is much easier to use as the exposure settings required do not change as you zoom. If you're using a program mode (either full auto, aperture priority or shutter priority) the camera will make the adjustment required itself, although you then must be wary that the 'other' exposure variable (the aperture or shutter speed) doesn't change to a value you don't want to use (eg too slow to hand hold)

-- Nigel Smith (nlandgl@unite.com.au), December 21, 2000.

Supposedly, the more round the aperture, the better the bokeh. It is a feature touted on a lot of digital cameras, but my opinion is they are trying to turn a liability into an asset. Spin control. Constant aperture lenses may be better for people who don't know anything about photography, but for the creative photographer they represent a limitation of choice. The better lenses have irises that produce a near-round opening at any f-stop.

-- Ed Buffaloe (edb@unblinkingeye.com), December 22, 2000.

I'm not sure what you mean either, but I think you're refering to either zoom lenses that have a constant maximum aperture throughout the zoom range, or close focussing lenses that vary the aperture as the focus is changed.
Most decent lens makers have top-of-the-range zooms with constant maximum aperture, and these are obviously easier to use with manual exposure control. The viewfinder image doesn't dim at the long end of the zoom range either.

AFAIK, the only lens range that incorporated automatic focussing compensation were the lenses made by Zeiss Jena. They had a cunning cam mechanism coupled to the focusing helix that opened up the iris slightly as the focus was brought closer, thus retaining a constant effective aperture number.
This mechanism was incorporated in the 180mm f/2.8 'Olympic' Sonnar, and the close focussing 35mm flektogon wide angle lenses.
I've never seen it used in other lenses, but it was a simple and useful mechanism, and I'm full of admiration for the much-maligned East German Zeiss company for their ingenuity and practicality.

-- Pete Andrews (p.l.andrews@bham.ac.uk), January 03, 2001.

I use all-manual cameras (Nikon FM, FM2) and if youv'e ever attempted fill-flash with a variable aperture zoom, you're sort of in the dark, in this window of uncertainty. Constant-aperture zooms make fill very easy, as you can freely zoom to frame without changing exposure. Granted, this may be a minor point in today's automated world, but I don't believe that there is a decided optical advantage to discourage use of variables. I think that when zoom started to hit the scene, Pros gravitaed towards the faster, constant-aperture zooms. I think that partially explains the dominance of such (constant)zooms now.

-- Mike DeVoe (cameraservice@gci.net), January 22, 2001.