This might be a silly question, but would it be possible for lens manufacturers to come up with a consumer zoom lens that has a decreasing variable aperture (i.e. 28-105mm f/4.5-3.5)? I mean, the bigger aperture is not needed as much in the wider focal length than the narrower one.

-- Ronald R. Gregorio (gregorio@ksc.th.com), July 04, 2000

Answers

Ronald, The f stop number is a math generated number based on the retio of the lens opening compared to the focal length. For example, on a simple 50mm lens, f2 represents a 25mm lens opening... f4 is 12.5mm, and so forth. The problem with your question is that the reason that f stops traditionally decrease on variable aperture zooms, is that as the focal length increases, the diameter of the aperture usually increases only slightly, within the confines of the physical dimension of the lens barrel. Take a look at a 28 to 70mm constant 2.8 and a variable 3.5-4.5... the size differance is dramatic.

To have a zoom increase the maximum aperture when increasing focal length would require 1. a very large lens and 2. a complicated cam action to proportionately winden the opening in conjunction with the zoom action. In theory, this could be done, but nobody could afford it. The best compromise is a constant aperture zoom, still larger and more expensive than most care to either carry or pay for. Also think about it... since the larger opening is already there at the long end... it is a waste not to use it at the short end... thus the constant aperture zoom and not the increasing aperture version you asked about.

-- Al Smith (smith58@msn.com), July 04, 2000.

Ronald, what makes you think that a larger aperture isn't needed at shorter focal lengths? A wider lens is more useful indoors, and that's where you'd need a bigger aperture. And you need a wider aperture to get a shallow depth of field with a shorter lens. I don't follow the logic of shorter lens - smaller aperture.

-- "Photoscientia" (photoscientia@beeb.net), July 04, 2000.

Thanks for the replies. I asked to see if it was possible and practical to do it at all. Even if it's possible, it may not be practical size-wise and cost-wise. I guess what I'm looking for is already out there, which is the fixed aperture zoom lens.

On the second reply, I think that photographers would need a wider aperture at the long end of the zoom than the shorter one because they need more speed to handhold the camera plus and a nicer background blur for portrait use. Most pictures at the short end normally benefit from high depth-of-field where you want the foreground and background focused.

-- Ronald R. Gregorio (gregorio@ksc.th.com), July 04, 2000.

Ronald, Another possible option, one that works for many, is simply to use your small variable aperture zoom, and get a single focal length lens in the 85 to 105 range to accomplish your portraits when that is your goal.

I don't know what system that you use, but all the major ones have an 85mm lens of about f1.8, which will blow your zoom out of the water. At the widest stops you get razor thin depth of field that can elevate your shots above the standard fare... your friends with their slow zooms won't be able to emulate the effect. At the same aperture as your zoom at the 85mm setting, while the zoom is wide open, the prime will be closed down at least 2 stops, and will be about as sharp as it is going to get... compared to the zoom's weakest setting.

These lenses are not too expensive, compared to the constant aperture zoom, and a lot smaller and lighter... If you spend 5 minutes with one, you will not know how you did without one. While I own several zooms covering this range, but when I know I am shooting portraits, it is the prime one my camera. Al

-- Al Smith (smith58@msn.com), July 09, 2000.

Thanks Al. I'm using a Nikon F90 with some primes and one zoom already. I have the 80-200/2.8D ED that I use for portraits, although it's a bit big, and heavy. I use to have an 85/2 AIS but sold it when I converted to AF and matrix metering. I'll probably get an 85/1.8D AF later on for portrait use. I also have a 20/2.8D AF, 35/2D AF, and a 50/1,8 AF. I'm looking for a zoom covering 28- 70mm that will give me pictures close to those I'm getting with my 80- 200/2.8 zoom, but the prices for pro zooms are prohibitive so I thought that maybe a consumer zoom would do. I'd just be using the zoom for event photography when changing lenses is a bit of a hassle.

-- Ronald R. Gregorio (gregorio@ksc.th.com), July 10, 2000.