Zoom lens apertures

RalphLambrecht · Dec 16, 2017

David Lyga said:
One thing that I always wondered about was zoom lenses with FIXED maximum apertures. In other words, zooms, say 70 - 200, can have a maximum aperture of, for example, f4 to f5.6; i.e., the actual maximum changes with focal length.

However, there are some zooms that retain the SAME aperture throughout the focal length range. How can this be achieved? - David Lyga

by starting with a large opening for the short focal lengthwise and progressively closing it down for the longer focal lengths?

E. von Hoegh · Dec 16, 2017

Nodda Duma said:
Performance of zooms benefited significantly from computer aided design. Prior to the 80s, the design process was just too complex to directly optimize using the traditional fixed-focus hand-calculated design methodology which minimized the ray trace calculations to keep design time down to a few months. Now it's not a big deal, and the designer has been able to focus on deviating from the first order approximation zoom layouts of the pre-computer era.

The underlying theme is that optics are almost always designed to be "good enough" and not beyond because of the signifucant design effort required. I've always found that aspect to be fascinating.

Yes, I remember that from when I calculated the Doppel Anastigmat back in the 90s. 1890s that is.
But seriously, I was given an odd zoom, a Yashikor 90 -190 f:5.8 42mm screwmount push pull, a long skinny thing that is actually pretty good - and has a high "funk factor".
Do you have any familiarity with this beast?
Picture - https://www.ebay.co.uk/itm/90-190MM-90-190-5-8-YASHICA-AUTO-YASHINON-106241/362188130438

cooltouch · Dec 16, 2017

DWThomas said:
Was that while metering off a large monocolor evenly lit surface? That is a heck of a zoom range, and on a conventional scene such a large change in range might result in some contrasting object becoming a larger percentage of the field of view. But it could indeed be a shady setup. Can't say I've actually tried such a test myself.

It was several years ago when I discovered this about my Kalimar. Knowing me, I was probably metering a blank wall or some other similarly evenly lit surface to confirm this condition. I'm sensitive to how a subject's exposure value can change when zooming in and out due to exactly what portion of the subject is being metered along the way.

AgX · Dec 18, 2017

At a current flash manual from Metz I found this puzzling remark:

Nodda Duma · Dec 18, 2017

AgX said:
At a current flash manual from Metz I found this puzzling remark:

View attachment 192081

Translation: We assume you are using cheap zooms.

AgX · Dec 18, 2017

Not quite... I just checked current tests. A Canon EF 70-200mm F2.8 was tested at T3.3, thus half a stop loss.

Other current zooms were tested for losses of over one stop.

David Lyga · Dec 19, 2017

AgX said:
Not quite... I just checked current tests. A Canon EF 70-200mm F2.8 was tested at T3.3, thus half a stop loss.

Other current zooms were tested for losses of over one stop.

With so many elements it would really be unusual for 'no light loss' to occur. Much reason for of the stated f stop is due to marketing, and there is a legal leeway allowed. It is amazing that they perform as well as they do. Flare is the main culprit and it diminishes the speed on zooms more than on fixed focal lengths. - David Lyga

bernard_L · Dec 19, 2017

AgX said:
A Canon EF 70-200mm F2.8 was tested at T3.3, thus half a stop loss. Other current zooms were tested for losses of over one stop.

Interesting. I thought the distinction mattered only for movies to ensure perfect homogeneity between scenes. But this means that with a zoom, you have better use the in-camera meter, that will be fooled into thinking that the scene is not quite sunny after all, and expose accordingly. OTOH, I don't use zooms.

Zoom lens apertures

Privacy & Transparency

Privacy & Transparency