[Squeed]

Just got this lens which is in reasonable nick. Don't seem to be able to see the aperture changes when off the camera but when on my ME Super in manual mode the light meter is registering changes as the aperture ring is moved and slight clicks can be felt. In Bulb mode with lens on camera and back open no change visible from fully open. In Bulb back closed no resistance or clicks when rotating the aperture ring.

As I can't find a user guide for this lens I am unsure of use of constant aperture. I am just a tad more than real beginner and new to the forum - kind regards to everybody Mark

[microlight]

Hello Mark, welcome! I've had one of these for a long time, although I haven't used it for quite a while. As far as I know (and I'm sure others will chip in), constant aperture only really means that the maximum aperture doesn't change throughout the zoom range, in contrast to a variable aperture zoom like the DA55-300PLM, which is f4.5 at the short end and f6.3 at the long end. Once you stop down a little then the constant aperture 'advantage' disappears since f8 is f8 regardless of maximum aperture. Practical use? Slightly more light gathering power at max zoom range, but tend to be bigger and heavier as a consequence. In normal shooting, you probably wouldn't see a difference between this lens and the 80-200 M lens, except that the latter is slightly smaller and lighter.


If you use digital as well as film, one place where it's easy to tell the difference between constant and variable aperture zoom is if you're using video - for example with my DA20-40 (variable) wide open on the K-3II then the exposure changes slightly as you change zoom level, but not with the 16-45 which is a constant aperture.

[aslyfox]

Welcome to the forums

you will find friendly members and knowledge here

yes, microlight is correct, you will be able to use F2.8 at any point in the lens' focal range since this is a fixed aperture lens

you might want to do some reading:

Some higher-end lenses can maintain the largest aperture throughout the entire zoom range, so only one number is detailed. (f/2.8, below right).

Fixed aperture lenses utilize more sophisticated lens elements than variable aperture As such, these lenses are generally more expensive than variable aperture zooms.

Zoom Lens Maximum Aperture: Fixed and Variable Apertures from Nikon

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A fixed aperture lens means it can remain at it's largest aperture throughout it's zoom range, i.e., the f/2.8 is available at 55mm, but any aperture at any focal length you decide on, like f/22 at 17mm, can be used.

fixed aperture lenses: Beginners Questions Forum: Digital Photography Review.

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These lenses are usually heavier, better constructed and more expensive but it’s a higher quality of glass in lens.

The benefit is that you will have the larger aperture all throughout the focal range – and more light means you will be able to shoot in low light situations.

But don’t think that constant aperture lens can only shoot at one aperture, say f/2.8.

They are able to switch to all the apertures that are normally available – the entire range of apertures all the way up to f/22 or f/32.

What is a Constant Aperture Lens? - Improve Photography

here is a link to the " owners review " for that lens -

https://www.pentaxforums.com/lensreviews/SMC-Pentax-A-70-210mm-F4-Zoom-Lens.html

[Benz3ne]

Squeed, try nudging the aperture lever on the reverse of the lens when it's off the camera and set on something other than wide-open (f/4).
You should be able to see the aperture open and close there.

Edit: you might want to ignore me. There's too much blood in my caffeine system. Should be the other way around, should be stopped down and open up when you nudge the lever.

[Squeed]

Thanks so much guys for that wonderful and quick help. I will get my head around the constant aperture with this kind help, so cheers. And fantastic, having nudged the lever when off the camera I can see the other "fs" - just great. I bought my first Pentax around 1980 a lovely K1000 with a f1.7 50mm lens, and later bought a 135mm - stupidly I traded in the lot, some 10 years ago, for a compact digital! I have made amends during the "lockdown" and raided Ebay for 2 Spotmatics a SP and SP II an MX and an ME Super which came with the Pentax-A ZOOM 1:4 70-210mm so having fun.

[aslyfox]

. . . so having fun.

you have found the secret