The Pentax K-mount SMC-A 28-135 f/4.0 zoom lens has caught my eye. It's two-touch, and supposedly has a constant f-number. From the design, I believe it should be parfocal, though I was not able to confirm that by searching the Internet. The 4.8x zoom ratio is about the largest you can find for a photographic lens with those properties, making it a bit more like a TV or cinema zoom lens.

But the focus goes the wrong way, so I won't buy one. And I already have faster 5x zoom lenses, though they are they're huge. I can tell you that 28-135 on the GH2 makes an incredibly useful zoom range.

That's 43 lens. Get MMF-3 adapter from Olympus.

While testing some cheapo adapters from Minolta MD/MC to µFT for their tolerances, I found one (out of four!) that is pretty much spot on (don't ask for the brand – it has none).

I thought it would be a good occasion to find out which older zooms myself and a friend had around are parfocal. I tested the Minolta 35-70mm 1:3.5, which I consider one of the best vintage zooms you can get, it's sibling 24-50mm 1:4 and two Tokinas. BTW, all of these are two-touch zooms, I'd never use a sliding focus/zoom ring for filming.

The Minoltas hold focus just fine, while both the Tokina RMC 28-85mm 1:4 and the newer AT-X 28-85mm 1:3.5-4.5 changed focus considerably. Well, at least the older RMC has constant aperture (like the Minoltas) and the length doesn't change while zooming. The AT-X, while lighter and smaller, changes length and aperture massively and shifts the focus ring so much you'd never use it with an FF.

The focus ring on the older RMC (which is more flare-prone) doesn't move too much, so this pretty heavy lens can at least serve as a variable prime if you like. But two or three faster primes might not be much more weight than this, so it comes down to fast re-framing when you can't move and have to react quickly.

Both Tokinas can be found quite cheap, but I dig the Minolta 35-70mm for it's optical quality – really close to a prime. It can still be found for a reasonable price, but it changes length quite a bit too. BTW, the Minolta was copied for Leica at it's time…

Hope this helps.

Surprised that you say that the Tokina 28-80 isn't parfocal as I have the RMC F4 28-85 and it most definately is. I also don't see any falloff in sharpness at the corners, although the lens needs to be at f5.6 or greater to get a decent image - wide open it is very soft and low contrast. It does suffer from flare if pointed toward a light source wihout decent hooding. See a demo of it here ...

@Energy80 I really like your video, also your lens performance. It is matter of personal taste probably, I don't like too sharp image and too high contrasts. The image has lots of details. Do you still remember did you shoot it in smooth mode, sharpness -2?

@nomad

I use the Zeiss 28-85 some of the time, and it's fine for most production workflow. The Angenieux 45-90mm f2,8 is wonderful for its constant aperture, and sharpness.

Looking to rent out lenses now - if you had all the budget you wanted for a fast GH2 zoom lens to film cinematic looking images....which zoom lenses would you opt for?

Would the Olympus 35-100mm still be one of them?

That lens should be parfocal. If it's not, it's because the lens back focus needs adjustment and/or your adapter is the wrong length. The red lines are focus marks for infrared photography.

After doing some research, I bought a Nikkor AIS 28-50mm f3.5 that I am very pleased with. It is a nice, practical lens for casual shooting.

I'd suspect the adapter.

I have two of the Vario Rokkors 35-70, one with and one without macro. Both great lenses and both parfocal with a Novoflex. But I've seen big differences with cheap adapters, only one out of three was about as parfocal as the Novoflex. But you can still buy 3 or 4 of them, keep the best and dump the rest – it'll still be cheaper than Novoflex. BTW, some adapters can be adjusted, the have three tiny screws which you can loosen, then there is a thread to shorten or lengthen the adapter (sorry, no brand).

They're not the same optical formula. Just similar specs.

Don't go for the 100mm f2, it is easy to brake that. The 100mm f2.5 shouldn't be too hard to find and the 135mm 2.8 is dirt cheap. An alternative to 85mm might be a Tamron SP macro 90mm 2.5, it's a very nice lens for portrait too. Slightly different color rendering to Rokkors, though.