Simple problem: 0.20 V drop

The latest pro lenses (Nikon and Canon and others) are pretty good - especially longer lenses. They can make lenses as good as Leica / Zeiss, when they want to.

I like old cameras and film - but I don't think one can say that digital is "much inferior" - at least when talking of pro DSLRs.

Canon, for example, have quite a spread of products from "consumer" to "pro". Some are rubbish, others very good. Certainly, thay are made more cheaply - more plastic etc, and everyone seems to be skimping on quality control these days.

What I like about my old Zeiss Planar (Contax) 85 mm f/1.4 for example, is the minimal depth of field one can get. You can't get that with an AF zoom lens, no matter how good it is.

colin

So how come the performance of your circuit is so good? Or why isn't Win's better, with the feedback? I guess Win's is less temperature dependent (and has a lower quiescent curren)?

Thanks,

colin

Simplicity (besides, I AM the master ;-)

Feedback is good... when you have sufficient loop gain. I knew, when I saw Win's Iq, that loop gain would be low, thus the swipe ;-)

Yes.

Yep, mine is 90.5uA versus Win's 22.3uA

I doubt that there is a market, but it would be trivial to make a CMOS chip to do this function perfectly ;-)

...Jim Thompson

Exactly. I have a 250mm fixed mirror lens which is the perfect match for wildlife photography. Haven't seen anything like it on digital cameras.

My Sony CyberShot has a macro focus setting, so I can do closeups. I can also force focus to my own desires or turn on AF.

...Jim Thompson

On Mon, 22 Jan 2007 16:53:21 +0100, Colin Howarth wrote in Msg.

Depth of field is a function of aperture and imaging scale, nothing else. So it has to do neither with AF nor zoom but only with the maximum aperture (and of course no zoom lens for 35mm film goes up to f/1.4)

robert

Sure, you can also turn AF off with regular film cameras. However, the geometrical distortions and color aberrations of a AF zoom lens can be an order of magnitude different from a lens that is calculated to a single focal length.

Or in chip designer speak AF zooms are like the attempt to build a low noise 5GHz CMOS amp with microamps of quiescent current. You have to take shortcuts and compromises. The lenses my sister calculates are often remarkably "featureless" but professional sports reporters pay four-digit for them.

Is there a ranking here? What about Spehro? :-)

Well done.

It would be a small market :-) But it would be interesting, to see how things improve as one is allowed to use more transistors.

Once again, thanks. This has been very educational.

colin

yup.

That's exactly the point I was making. Just a couple of examples, from Canon:

EF 28-105 / 4-5.6 189 EUR cheap plastic zoom lenses EF 100-300 / 4.5-5.6 USM 349 EUR which can't do 1.4, 2, 2.8 or even f/4 EF 100-400 / 4.5-5.6 L IS USM 1699 EUR expensive zoom lens which can't ... EF 70-200 / 2.8 L IS USM 1999 EUR expensive one. better

EF 85 / 1.2 II USM 2099 EUR they can do it, if they want (no zoom) :-)

colin

AF is irrelevant, except that it's easier to wooosh(*) if the lens has less mass (ie. made of plastic).

An order of magnitude is exagerating somewhat.

The problem with zooms is that they don't have really big apertures like f/1.2. This means that for a portrait, for example, you can't make the background extremely blurred (which looks nicer, if it's a crummy background :-)

Amateurs often spend more on equipment than pros....

colin

(*) photographic technical term

Nah, Jim is Dr. Vbe.

Last time I did something like this (to replace a large Hg cell, D, IIRC, in a thermocouple calibration box "aka 'portable potentiometer'), I used something like a TO-5 LM317 and stacked enough alkaline cells in there to get the voltage required. The instrument was calibrated at each use with a standard cell, so accuracy was not an issue, but the voltage stability of alkaline cells is really, really crummy compared to Hg cells. The LM317 is probably typically as good as the original Hg cell (something like 50 or 100 ppm/K, IIRC).

If I was doing a Leica, I might throw something together with an LM10. I'm not sure a simple active "voltage divider" circuit is going to be stable enough with an alkaline cell. Maybe some other battery technology would be better.

Best regards, Spehro Pefhany