[Canada_Rockies]

A 70-210/4

and

M 80-200/4.5 (version 1) and M 80-200/4.5 (version 2)

look like push pull zooms, to me..

The A 70-210/4 is a one touch zoom. It is also an internally zooming lens. The lens length does not change while zooming. Sometimes terminology can really hang up a discussion!

[OniFactor]

The A 70-210/4 is a one touch zoom. It is also an internally zooming lens. The lens length does not change while zooming. Sometimes terminology can really hang up a discussion!

quite so! can you explain how the one touch zoom mechanism works?

[Canada_Rockies]

how is it a push pull, without changing length? the push-pull's ive had (2 sigmas for Minolta Alpha mount), there were 2 barrels, one slid over the other, and changed the total length. does the pentax work any differently? it looks the same as the sigmas, since it has the aperture lines on the barrel, like the ones i've used..

The zoom/focus ring on the A 70-210/4 moves back and forth to zoom without changing the lens length. You have more barrel in front of the collar/ring and less behind and vice versa. The lens is an internally zooming lens. The mechanics are such that the ring moves some of the lens groups when pushed and pulled, and when it is turned it moves a different collection of lens elements (which may include some of the same elements as the zoom movement uses). In the case of the lens mentioned, the focus control does change the length of the lens slightly, so it is not internal focus.

Not all one touch lenses are internally zooming, nor are all two touch zooms internally zooming. An example is the DA* 16-50 which changes length as the focal length collar is turned. The 12-24 is shortest in the middle, as is the FA 24-90. An internally zooming lens is simply one where the last piece of glass towards the subject stays the same distance from the film plane while zooming. The lens designer must take into account a zillion variables. Sometimes the choices are different.

Are you perhaps trying to say that you prefer two touch to one touch? Separate collars for focus and zoom? I know I do, but the A 70-210/4 was far more attractive to me, at a premium of $10 over the F 70-210/4-5.6 two touch lens that was beside it in the showcase when I bought the lens. I'll take f/4 at 210 over f/5.6 any day of the week, including Sunday, even if I have to have a one touch zoom/focus collar. I find it annoying, but I like the optics, so I use the lens anyway.

[OniFactor]

The zoom/focus ring on the A 70-210/4 moves back and forth to zoom without changing the lens length. You have more barrel in front of the collar/ring and less behind and vice versa. The lens is an internally zooming lens. The mechanics are such that the ring moves some of the lens groups when pushed and pulled, and when it is turned it moves a different collection of lens elements (which may include some of the same elements as the zoom movement uses). In the case of the lens mentioned, the focus control does change the length of the lens slightly, so it is not internal focus.

Not all one touch lenses are internally zooming, nor are all two touch zooms internally zooming. An example is the DA* 16-50 which changes length as the focal length collar is turned. The 12-24 is shortest in the middle, as is the AF 24-90. An internally zooming lens is simply one where the last piece of glass towards the subject stays the same distance from the film plane while zooming. The lens designer must take into account a zillion variables. Sometimes the choices are different.

Are you perhaps trying to say that you prefer two touch to one touch? Separate collars for focus and zoom? I know I do, but the A 70-210/4 was far more attractive to me, at a premium of $10 over the F 70-210/4-5.6 two touch lens that was beside it in the showcase when I bought the lens. I'll take f/4 at 210 over f/5.6 any day of the week, including Sunday, even if I have to have a one touch zoom/focus collar. I find it annoying, but I like the optics, so I use the lens anyway.

i didn't realize that a push pull zoom could be internal zooming, as i've said, every one i've actually used had the front element on the same barrel as the zoom ring, and when you pushed the zoom ring out, it pushed that element out, as well. i've never used a one touch lens, as compared to a two touch, so that's not really the problem, it's just that i didn't realize that there was a mechanism that allowed you to slide forwards and backwards on a lens, to adjust zoom, without changing it's physical size! i'll have to keep my eye out for an A 70-210, for sure, now.

[Canada_Rockies]

i didn't realize that a push pull zoom could be internal zooming, as i've said, every one i've actually used had the front element on the same barrel as the zoom ring, and when you pushed the zoom ring out, it pushed that element out, as well. i've never used a one touch lens, as compared to a two touch, so that's not really the problem, it's just that i didn't realize that there was a mechanism that allowed you to slide forwards and backwards on a lens, to adjust zoom, without changing it's physical size! i'll have to keep my eye out for an A 70-210, for sure, now.

Before you jump, check the lens database for all the flaws and make sure you can live with them. I have not put it on the K10D for a test routine yet, but suspect I will find some chromatic aberrations that will need post processing, and some of the dreaded purple fringe as well.

I know that the M 400/5.6 doesn't like sharp edges, but it seems okay when it is not facing directly into the sun, so I'll keep it until I can save up for the DA* 200/2.8 which has gotten rave reviews in Chasseur d'Images.

[jbarnhart]

The constant aperture is achieved by throwing away light at the short end. When I rotate the zoom ring on my FA 28-70mm f/4 I can see the aperture blades begin to close at the 28mm end and open all the way at the 70mm end.

Constant aperture was convenient back in the day when we sometimes used our rigs with external light metering because it was easy to zoom without having to take readings or do the math in your head for the changing aperture. Does it still have relevance in the digital age?

[Geekybiker]

Constant aperture was convenient back in the day when we sometimes used our rigs with external light metering because it was easy to zoom without having to take readings or do the math in your head for the changing aperture. Does it still have relevance in the digital age?

Well, I find it annoying when I'm in Av or P mode using aperture priority and it keeps changing my aperture while zooming (and doesn't reset it lower when I zoom back in)

[Canada_Rockies]

The constant aperture is achieved by throwing away light at the short end. When I rotate the zoom ring on my FA 28-70mm f/4 I can see the aperture blades begin to close at the 28mm end and open all the way at the 70mm end.

Constant aperture was convenient back in the day when we sometimes used our rigs with external light metering because it was easy to zoom without having to take readings or do the math in your head for the changing aperture. Does it still have relevance in the digital age?

It sure does.

• When your 200mm setting is f/5.6 instead of f/4 you need to slow the shutter or increase the ISO to get accurate exposure. You don't want a slower shutter speed at 200mm compared to 50 mm if you can avoid it.
• If you are using flash, the flash range decreases as you zoom, just when you need it to increase.
• If you want to use multiple flashes in a studio setup, then you must have a constant aperture to maintain the flash exposure your flash meter indicates.
• If you want to use auto flash rather than TTL or P-TTL, or your flash is incapable of doing TTL or P-TTL, you need a constant aperture to match the flash settings.
• If (and I don't, except for the flash meter mentioned above) use an incident light meter for more accurate exposure you need to know the exact aperture.
• If (and I do) you use the sunny 16 exposures, you need to know the aperture accurately.
• If you use an M, K or M42 lens, you need to know the aperture (except wide open when you can use Av mode).

No doubt others on the forum can add more situations where a varying aperture will not work.

[Jerry Thirsty]

Before you jump, check the lens database for all the flaws and make sure you can live with them. I have not put it on the K10D for a test routine yet, but suspect I will find some chromatic aberrations that will need post processing, and some of the dreaded purple fringe as well.

The -A 70-210/4 works great on digital, and is the main reason I decided not to pursue any of the new 70ish-200ish/2.8s. The only real flaw is the tendency toward zoom creep when you're carrying it or pointing it up or down, but that can be adjusted (or so I've read, mine are still nice and tight as is).

[RBellavance]

Well, I find it annoying when I'm in Av or P mode using aperture priority and it keeps changing my aperture while zooming (and doesn't reset it lower when I zoom back in)

Hmmm, the aperture should come back down when it can. At least, that's how my K10D behaves in Av mode with my two non-constant aperture zooms...

Ex. with the Sigma 17-70mm: I dial f/3.5 @ 17mm, and zoom to 70mm. The aperture changes to f/4.5. When I come back to 17mm, the aperture returns to f/3.5.

[Lowell Goudge]

The -A 70-210/4 works great on digital, and is the main reason I decided not to pursue any of the new 70ish-200ish/2.8s. The only real flaw is the tendency toward zoom creep when you're carrying it or pointing it up or down, but that can be adjusted (or so I've read, mine are still nice and tight as is).

I own the origonal sigma 70-200 F2.8 and it does not suffer at all from zooom creep. It is really a fine lens.

[AndrewG NY]

An example is the DA* 16-50 which changes length as the focal length collar is turned. The 12-24 is shortest in the middle, as is the FA 24-90. An internally zooming lens is simply one where the last piece of glass towards the subject stays the same distance from the film plane while zooming. The lens designer must take into account a zillion variables. Sometimes the choices are different.

For what it's worth, I don't think this is completely accurate. The FA 24-90 f/3.5-4.5 is shortest at 24mm and longest at 90. I think you're right about the 12-24 though it is fairly subtle since its whole zoom length is covered in about 1cm.

[Canada_Rockies]

For what it's worth, I don't think this is completely accurate. The FA 24-90 f/3.5-4.5 is shortest at 24mm and longest at 90. I think you're right about the 12-24 though it is fairly subtle since its whole zoom length is covered in about 1cm.

Oops! you are absolutely correct. It is the 28-80 FA that has the in and out at both ends. I must be a politician. I only open my mouth to change feet.

[OniFactor]

I own the origonal sigma 70-200 F2.8 and it does not suffer at all from zooom creep. It is really a fine lens.

i think he meant the 70-210 f/4 exhibits zoom creep..

[Lowell Goudge]

lowell, from what i've seen, of the Pentax models, only the FA 80-200 2.8 is internal zoom, the rest have barrels that change length while zooming.. but thanks for pointing out the constant apertures

Sorry, I kinda mmissed your second requirement