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02-28-2008, 12:17 PM   #1
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A35-70 question...

I have what I think is a very nice copy of this lens. I went on a LBA binge a while back and this was one of the gems. I scooped it up on eBay for $30 (along with a P5 camera that I subsequently sold for $20). It has quickly become my favorite walk-around lens and couples nicely with the A70-210/f4 zoom. I could use a little more reach (A35-105?), but overall I'm pretty happy with it.

My question is about the max aperture. Every source I've seen lists this lens as 35-70mm/f3.5-4.5, which led me to believe the max aperture would be f3.5 @ 35mm and f4.5 @ 45mm. It turns out that this lens is a constant f3.5 at all apertures. Is that right or is there something screwy here?

I just took some shots with it here in my office. EXIF data should be intact (I can't tell at work). All pics were taken at wide open at f3.5 (at least that's what the camera is showing. No PP, jpegs straight from the camera.

Here is a resized macro shot at 70mm (taken at 7" to the front of the lens):


100% crop of above shot:


50mm


35mm


02-28-2008, 12:36 PM   #2
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Nice lens and pics too!

Just to be totally clear: the front bezel on the lens tells you that it has a variable aperture of 3.5-4.5? Isn't it the constant f/4 version? How did you find out about the f/3.5 at the tele end of the lens?

I have the F version of the lens you have. Maybe we should compare
02-28-2008, 01:19 PM   #3
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I assumed was a variable aperture because the front bezel of the lens reads "SMC PENTAX-A ZOOM 1:3.5~4.5 35~70mm"

The information I'm getting from the camera (and EXIF data) says that it can do f3.5 at all focal lengths, but I don't think that's correct.
02-28-2008, 01:40 PM   #4
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I have tried it with my F 35-70. If the camera is on, I can see the F value change on the top LCD when I turn the zoom ring. Please try that.

02-28-2008, 02:04 PM   #5
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Yep, that's what I'm referring to. The aperture stays at f3.5 as I zoom in, both in the viewfinder and on the top LCD. I just switched over to the DA18-55mm kits lens and it does go from f3.5 to f4.0 to f5.6 as I zoom in from 18mm to 55mm. The A35-70 doesn't do that...
02-28-2008, 03:29 PM   #6
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There were two versions of the A 35-70; a f/3.5-4.5 version and a constant f/4 version.
If the barrel markings indicate the former, then you have the variable zoom version. Like Asahiflex I have the autofocus F version and it does show the aperture value changing when zooming on the long end.

My guess is that this info is not transmitted to the camera for your lens and doesn't show up in the exif. At any rate, if you're shooting in aperture priority, the camera will compensate with a slightly slower shutter speed. Looking at the exif for your images at 35, 50 & 70mm, the shutter speed has progressively increased in duration. So the lens works but it is not relaying the exif info for the aperture correctly.
02-28-2008, 07:08 PM   #7
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The first shot was at 0ev while the latter 2 were at -0.3ev. That's why the shutter was a little faster in those shots. BTW, it doesn't show aperture at f3.5 unless I set it there. I did it for these shots just to show that it's giving me what I think is f3.5 at all focal lengths.

I'm pretty sure there's no problem with the lens info making it to the camera. I've got plenty of shots taken with this lens where the lens was stepped down and it showed on the camera and in the EXIFs.

Examples:

f5


f7.1


I guess I'm going to have to take a closer look to see if there's something wrong with the lens.
02-28-2008, 07:12 PM   #8
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A lenses only show fixed apertures. So even if your lens is actually variable aperture, you have to adjust the displayed aperture in your head.

Eg if you have a 4-5.6 zoom, you can select 4.0 at the tele end, but it's actually 5.6, 5.6 is actually 8, etc.

F and newer lenses have a CPU which adjust that max aperture readout in suitable bodies.

02-28-2008, 08:12 PM   #9
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Ahhhh.....enlightenment!

Thanks!

That explains everything. I feel better now...
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