John,

I think the gist of what you are saying is that any difference in aperture as you zoom is small. This says to me that the lens designer is trying to control the effect, but is not always 100% successful. I can't argue with that. All lens designs are a compromise, and there is no such thing as a perfect lens.

So let me re-phrase the question. In practice, do modern zoom lenses adhere to their stated apertures closely enough to be usable in manual mode?

I believe the answer is generally yes. However, my experience is limited, and it would be interesting if anybody has any real-world evidence of aperture-shifts that are sufficiently large to make it impracticable to use a particular (modern) zoom lens in manual mode?

G

Yes, yes, yes, but......

My contention is that most modern zooms are in fact constant-aperture zooms. Some may have a variable maximum aperture, but at smaller apertures the effective aperture is constant. In other words, f8 gives f8, whatever the focal length

None of you are addressing that point!

G

John,

You're being very woolly. I still don't know what you are trying to say.

"Lenses are more complex than this". Complex than what?

The question (which relates to Pentax K mount zoom lenses) is whether the effective aperture changes as you zoom. I say, from limited experience, that IF it does so, it does so very little, and far less than you would expect if no effort was made in the design to compensate for the fact that a longer focal length should theoretically reduce the amount of light coming through the lens.

The question is a practical one; it doesn't matter whether the effect is achieved mechanically or optically; what matters is whether you can put a zoom lens on a camera in manual mode and expect to get consistent results at different focal lengths.

G

Just picking up on John's point, a 50mm prime that was miraculously converted to a zoom, would change from f2 to f4 (2 stops). At 200mm it would lose a further two stops, and become f8.

Yet we don't see anything like that change in the max aperture range in practice. Pentax's new 50-200 has a max aperture range of 4-5.6, which is ONE stop, not four.

Clearly, there is some compensation going on or these lenses would not function like this.

G

Well, I've just done an experiment with the following lenses:

1 M 100mm 2.8
2 FA 100mm 2.8 macro
3 M 75-150mm 4 zoom
4 F 35-150mm 3.5-4.5 zoom
5 F 70-210mm 4-5.6 zoom
6 VS1 70-210mm 2.8-4 zoom

Camera to manual, zooms all set to 100mm, apertures 5.6, then 8 and then 16. All lenses tested in order at the same aperture, then the next, etc. Speeds fixed at start of each test to give a correct exposure for the first lens to be tested.

Result: Very consistent exposures for all the Pentax lenses; no discernible differences between primes and zooms at any of the tested apertures. No sign of any aperture problems with the zooms.

Surprise. The Vivitar (a proper Series 1 by Komine, said to be the best) was a good one stop down at each aperture, compared to the Pentaxes.

I should of course have tested the same zooms against primes of different focal lengths, but I didn't have time.

What this tells me is that, on the evidence of this limited test, so-called variable aperture zooms adhere closely to their stated apertures.

I think the point may be that these lenses are not what a textbook might call a variable aperture zoom. They are zooms which have a different max aperture at different focal lengths, but have a consistent aperture otherwise.

G

John,

Sorry, I didn't find that very helpful. What are you trying to say?

The question is this: when a zoom is set to f8, is it f8, or is it something else? If it IS f8, regardless of focal length, then some compensation is occurring. If it is NOT f8, then:

1 how doyou know what the aperture is?
2 why do they bother to inscribe apertures on the barrel if they're not correct?
3 how do you know what shutter speed to set on a manual camera if you don't know the aperture?

It seems pretty obvious to me that ALL proper zooms contain a compensating mechanism in them to ensure that the aperture is what the lens says it is, whatever the focal length (except within the stated max aperture range).

That being the case, the "variable aperture" bit only refers to the range between the largest and smallest max aperture stated in the lens description (eg: 28-80mm f4.5 - f5.6), and only at longer focal lengths.

Provided you use an aperture no larger than the smallest max aperture (f5.6 in my example) the aperture will not change.

Is that correct or not? Logic says yes.

G

Arthur,

I am not an expert, but I believe any decent zoom lens will increase or reduce the size of the actual aperture as you zoom so as to maintain a constant aperture value.

In any event, whatever the focal length, if the lens says f8, it must be f8; it can't be f16. If that were so, the markings on the aperture ring would be useless, and photography with a zoom lens on a manual exposure camera would be impossible.

The only problem arises when you are shooting wide, with the lens on max aperture. Zoom in, and the lens WILL alter the aperture to whatever is the maximum posible at that focal length.

G

Arthur,

I find that in the studio I'm usually using f8 or f11, so the fact that a zoom is not fixed-aperture doesn't matter. It only matters when you are using it wide-open.

If you ARE shooting wide-open in order to minimise DOF, then surely a fast prime would be a better choice.

George

There's also the 35-135mm zoom, which on film is a very good "tourist" lens if you don't want to weigh yourself down. The 24-90mm has an equivalent range on digital, and I find it very useful.

G

Arthur's advice is fine, but whilst Photoshop CS2 may well be the best image editor available, it is very expensive.

There are several cheaper alternatives if your finances are in delicate health the moment.

For RAW, Phase One Capture One is excellent and not too expensive in the LE edition. Some people think it is superior to RAW conversion in Photoshop, and it is very quick to process a batch of pictures.

A free RAW converter is RAWshooter Essentials.

For photo editing, Picture Window Pro requires much less of a learning curve than PS CS2, and does pretty much all you need. Again, the price is reasonable.

Finally, the bars do show in Manual mode. You have to press and hold the On-Off switch in the Illuminate position. When you do this, the back (Aperture) button changes function and controls the shutter. You can then use the camera like an old match-needle SLR. This is a crafty camera.

By the way, you CAN use the camera in Av mode. However, the only aperture that will work is fully open.

Good luck. You've made a sound choice of both body and lens, in my view.

George