[TwoUptons]

Though if you put the lens on a modern camera with it set to A you may wonder why you could get f4.0 a minute ago and now can only get f5.6 when all you did was zoom.

I know someone who did that once or twice

And it will only catch you for a second before you realize…

-Eric

[UncleVanya]

I remember that in a thread about a year ago I was insisting that this wasn't the case. Then I listened to other members, including you, and actually looked at the evidence. And of course I was wrong and you are absolutely right about this.

Discussion this forum has made it possible for me to eat crow all too often myself. Lol. Perhaps a better way to put it is that I learn from this community even when I am misinformed at the start of a discussion. All it takes is an open mind plus some intellectual curiosity and this wonderful community of photographers and gear heads.

[jerryman]

I will try and explain the science as briefly as possible:

With a constant aperture zoom, as you zoom from short to long focal length, the elements in the lens move and the apparent size of the entrance pupil as viewed from the front, gets larger. It must do this in order to keep the f-stop constant. Remember that f-stop is a ratio. So for example with a 100-200mm f4 constant aperture zoom (set to f4) at 100mm FL the apparent size of the entrance pupil will be 25mm. At 200mm FL the entrance pupil must be 50mm.

With a variable aperture zoom like your 35-80mm f4-5.6, the same thing happens, although not quite as well. At 35mm FL and wide open the apparent size of the entrance pupil is 8.75mm, thus giving you f4. But at 80mm FL and wide open the maximum apparent size of the entrance pupil is 14.29 mm thus giving you a slower f5.6. This is the reason for the second white dot to show you where to read the aperture when at full zoom, for any selected aperture. It does not suddenly jump to a 1 stop slower lens at 80mm FL but will be a gradual progression from about the middle of the zooms range.

This is all down to lens design and variable aperture zooms are invariably cheaper than an equivalent fixed aperture zoom, because of the compromise in design.

If you put your lens on a modern camera and set the aperture ring to "A" then you do not need to worry about any of this as the camera knows the zoom setting and accounts for it.

That's quite interesting, I did not know that "variable/constant" aperture zoom is a thing.
I was just looking down my lens from the front after reading the documents that K-Three had linked, trying to figure out when that switch happened and it does indeed seem to be gradual.

Now I can write my "metadata" at peace, knowing I'm not writing down the wrong values.


Thank you for the explanation!

Regards,
Jerry.

[que es tu]

Great thread! I have 2 FA lenses and have often wondered what the extra markings were for. Thanks for posting!

[mikesbike]

The emergence of the variable-aperture zoom lens concept occurred a long time ago. These are generally less expensive to design and make than are constant-aperture zoom lenses, although this is not always true. For example, my old Pentax FA 28-70mm f/4, a very good lens at a reasonable cost, is not really in the same league as my DA 20-40mm f/2.8-4 Limited. The FA lens was designed for Pentax 35mm AF cameras, therefore is Full-Frame DSLR compatible, while the DA lens was designed for Pentax APS-C DSLR use, and is quite expensive, being of very high quality.

As to the two marks on your lens, as has been said, they are very much related to the fact of your lens being of the variable-aperture variety. I had one or two of each type way back when my first Pentax SLR was the ME Super, which I bought new and used exclusively for some 6 years. I learned then about this difference, and the meaning of the two marks on the aperture ring. As with any of the Pentax camera models requiring use of the aperture ring to set a specific aperture, these two markings can provide some valuable information.

The simplest way to use these markings, is to set the aperture to your desired choice via the aperture ring using the larger of the two markings set on that aperture number. Let's say your lens's given aperture specification is f/4-5.6 (like the one you are showing). Being aware you are dealing with a variable aperture zoom lens, you are then informed to know that these are wide open aperture values, which are f/4 at the shortest FL, and will change to the lesser aperture value of f/5.6 being wide open at the longer FL of the lens's range. This also means the aperture you have selected is only correct (say f/8) at the shorter end of your zoom lens. As you zoom to a longer FL, that aperture value will become not as large, so as you get into the longest FL range of the lens, that aperture value will also be diminished by like amount. But to tell you just what that amount of diminishment will be at any given point of the zoom range, note that when you have set f/8 as your preference, and knowing that will be true at the short end and will change as you zoom, take a look at where the smaller marking indicates while the larger marking is on f/8. It will tell you what your actual aperture value will be when you zoom your lens to its longest FL or getting out near it.

If you were using the ME Super, which is capable of aperture-priority AE operation, you would also see the camera adjusting its shutter speed as you zoom longer, to compensate for the diminished aperture value. But if shooting in Manual Mode, you must re-adjust the shutter speed yourself, according to the meter's indication of that difference. This is why a constant aperture zoom lens is far more convenient for using the Manual mode (the only mode available with the K-1000) with cameras that require use of the aperture ring. The most expensive constant-aperture zoom lenses are those having a large aperture for the FL range. That FA 28-70 mm's f/4 is not an especially large aperture for the FL of this zoom range, so it could be offered at a comparatively low price. I am pointing this out, because if your intentions for using an AF lens on your manual-only camera is for possible future use on a more modern AF camera, one either requiring or not requiring the aperture ring for selecting aperture, a far better lens than the one you are showing for your current use on your K-1000 would be that FA 28-70mm f/4. It also provides f/4 at the longer end, while the one you are showing would likely already be at f/5.6 wide open at 70mm.

For AF cameras not requiring use of the aperture ring to set aperture, the aperture ring's "A" setting allows aperture to be set via the electronic controls on the camera body instead of with the ring. One significant advantage of this is that the reduction of aperture value as a variable-aperture zoom lens is zoomed to the longer end, only pertains to wide open aperture, which HAS to change, while less than wide open aperture can be maintained electronically by the camera's electronic control system, which can continuously compensate for the lens's variable change as it is zoomed. Your choice of f/8 would remain at f/8 value throughout the zoom range, even when shooting in the Manual mode, and your shutter speed would not need to be changed just due to your zooming, as long as the scene's subject lighting in the frame remains constant.

[pschlute]

It may be worth adding a final comment on variable aperture zooms relating to the smallest aperture selectable.

Taking the OP's 35-80mm lens on a K1000 film body, if he sets the aperture to f22 by aligning it with the longer white mark, he will get f22 at short and mid range zoom focal lengths. But zoomed to 80mm it will be f32 (or maybe f27)

You would get a similar (but not identical) behaviour if the lens was mounted on a more modern camera where the aperture is set by the camera e-dial and the lens placed in the "A" position. At shorter FL the slowest aperture selectable would be f22. But zoomed to the longer end of the scale and if desired a slower aperture f32/27 can be dialled in.

I noticed this on my DFA 150-450 f4.5/5.6 lens. From 150-300mm the slowest I can select is f22, but between 300-450 I can select f27 if so desired. So the lens operates between 150-300 as a f4.5-f22, and between 300-450 as a f5.6-f27. Logical when one understands the principles behind this.