Ok so I have worked out (with lots of help from here) roughly what i am after. Now I want to go test some lenses. First I want to test my assumptions. I'd like some advice on doing a test in the street on a 24-90/135 zoom. Ta. A.

-- anna edgar (anna.edgar@lineone.net), February 18, 2002

Answers

Anna, do you know what qualities you are looking for in a lens?

If you are looking for sharpness, put the camera on a tripod and shoot architectural details (buildings, bridges, etc.) at 90mm, 50mm, and 24mm. Do this at the widest aperture; then at one stop down, two stops, etc. until you get to f/11 or so. Start at the longest focal length; get a careful focus, and keep it there as you come out wider.

If looking for speed of handling, shoot handheld and try focusing and zooming in on faces in the crowd, etc.

Try some shots such as portraits at maximum aperture and see how the out-of-focus background looks. Lenses are not all the same, that way.

Shoot some color and black-and-white. Is the lens contrasty, or soft?

Focus on a line drawn across a newspaper, with the camera 45 degrees to the paper. Is the line in focus? or the print in front or behind the line?

Finally, do you like shooting with this lens?

-- Bob Fleischman (RFXMAIL@prodigy.net), February 19, 2002.

>>well yeah I want everything!!! I want it to be both as clear as poss and with as little barrel distortion as the compromise allows. I do know there’s a compromise involved.

I wasn’t happy with the 28-200 as the thing seems to not allow the camera to focus properly, but have since looked at some others and by comparison speed of focus (apparent/untested) it isn’t bad at all. So I guess to answer the question I want a lense that works as well as possible: clarity speed/accuracy of focus and distortion. I also want a 'convenient' lense to tote, So part of ‘convenient’ is weight. Look I know it wont be perfect! I am in the process of working out what ‘acceptable’ is, $ for result so to speak.

’If you are looking for sharpness….Start at the longest focal length; get a careful focus, and keep it there as you come out wider.’ >>>do you mean dont touch the focus just leave it there? this is tricky as my camera will re focus each time i use the shutter release unless I either hold the focus by holding the shutter button or getting the focus and then switching to MF. this latter hasnt worked well for me even at infinity. I was checking a panorama sequence that i took recently and discovered that half of them are out of focus. THis is nasty as i was trying to preserve the settings for the sky. I think actually that my camera (MZ5n) will preserve the settings so perhaps i should look at the manual again. The other thing I always assumed was that you got the best clarity if you shot at the smallest App with the slowest speed. (with slowest film poss) If this is the case then it shouldnt really matter what the max F/ is ???? Except perhaps with AF, with low light even if with fast film??? Or do 2.8 constants usually have better glass and it’s a quality thing that 2.8 constants predicates??? I try not to shoot with an F/ 2.8 on my primes unless i have to, or want to blur background.

’Focus on a line drawn across a newspaper, with the camera 45 degrees to the paper. Is the line in focus? or the print in front or behind the line?’ >>>>>>>>>>At what F stop? Minimun distance? At right angles to the line?

-- anna edgar (anna.edgar@lineone.net), February 21, 2002.

Anna: just a partial answer tonight, more later. Zooms usually are focused more accurately at the longest focal length. When shooting manual focus, the best practice is to focus long, then hold that setting as you zoom out wider. But with an autofocus, it may be that your camera can get just as good a focus at other settings. It's probably OK to do it that way.

The other answer is that the focus test with the newspaper is usually done at the widest aperture, to minimize depth of field. That way, you can see any departure from accurate focus.

Oh, and you might evaluate distortion by photographing some structure that has lots of straight lines at right angles, like an office building, and looking for any curvature of lines that are supposed to be straight. Might be a good idea to take the building also with a prime lens, like your standard 50mm, so you can compare the prints.

Later.

-- Bob Fleischman (RFXMAIL@prodigy.net), February 21, 2002.

"The other thing I always assumed was that you got the best clarity if you shot at the smallest App with the slowest speed. (with slowest film poss)"

Okay, Anna, let's look at that part next. The answer is no, the smallest aperture isn't the sharpest. The smallest aperture merely gives you the greatest depth of field, thus maximizing the range of distances over which objects appear acceptably sharp when they are closer to, or farther away from, the point of sharp focus.

Otherwise, the sharpest image in the plane of focus will occur not with the smallest aperture, but with the optimum aperture. The optimum aperture is different for every lense, so we can't just it's f/4, or f/8, or whatever. For prime lenses, it's often two stops down from maximum; so for example, it could be f/4 for an f/2 lens. Once you stop down much smaller than that, the picture actually starts losing sharpness, owing to diffraction limiting. This happens when light rays bend as they pass over the sharp edge of the diaphragm blades. If this sounds like science fiction, take a look at the way the light from a streetlamp at night bends where it passes over the edge of a building.

Here's another reason why small apertures are less sharp. You have to settle for a lower shutter speed, so you get more camera shake to blur the shot.

Slow film? Yes, if you use a tripod. Otherwise the slower shutter speeds are again a problem. Fast film, like Ilford Delta Pro 400, can give very crisp results, even though grainier. Graininess isn't the same as unsharpness.

Since you are looking for quality and want to put all this effort into testing, I might mention that lenses are with a long zoom ration lika a 28 to 200 are harder to design and build to the same standard as ones with a more modest ratio, like a 35-105 or a 28 to 90. I like zooms that hold a constant f/stop, but they are bulkier than those that go something like f/3.5 to 4.5. It's all a matter of compromises and trade-offs. If you want the sharpest possible pictures, you don't want a zoom at all. (Witha couple of exceptions)

So keep the questions coming, and let us know how you're progressing.

-- Bob Fleischman (RFXMAIL@prodigy.net), February 22, 2002.