Fixed aperture zoom with handheld exposure meter: I have an old Nikkor 80-200 mm F4.5 zoom. F-stop by definition is focal length divided by aperture opening diameter. At 200 mm the...

Elmerviking

I have an old Nikkor 80-200 mm F4.5 zoom. F-stop by definition is focal length divided by aperture opening diameter.
At 200 mm the opening is 200/4.5 = 44 mm. At 80mm the opening is supposed to be 80/4.5 = 18mm. The opening does NOT change when you zoom! As I understand it more light is coming in through the lens when you chose a shorter focal lens, which is confirmed when I put the lens on my DSLR. I shoot in manual mode with auto ISO. The ISO varies when you zoom in or out.This COULD be explained by more photons reaching the sensor when the focal length is shorter. I don’t know. But what about using the lens on my old film camera, a Nikon F with Photomic Ftn meter, and metering with a handheld exposure meter? Should I use the actual F-Stop or adjust it when zooming out?
How is the maximum F-stop on a fixed aperture zoom? At which focal length? A bit confusing to me..

/Bosse

BebuLamar

Elmerviking wrote:

I have an old Nikkor 80-200 mm F4.5 zoom. F-stop b... (show quote)

Do you notice that the ISO always go down when you zoom to the shorter focal length?

Elmerviking

Yes.

BebuLamar

Elmerviking wrote:

Yes.

While I can't explain or confirm whether the aperture would change size when you zoom when using a fixed aperture zoom but I know they are not supposed to change exposure when you zoom. I first thought that when you zoom the scene is now a different scene and it can be either brighter or darker but if you notice that it's always darker at long zoom and brighter at short zoom then I wouldn't know.

Elmerviking

BebuLamar wrote:

While I can't explain or confirm whether the aperture would change size when you zoom when using a fixed aperture zoom but I know they are not supposed to change exposure when you zoom. I first thought that when you zoom the scene is now a different scene and it can be either brighter or darker but if you notice that it's always darker at long zoom and brighter at short zoom then I wouldn't know.

The size of the aperture does not change on my old Nikkor. F 4.5 at 200 mm is 44 mm aperture diameter. The same opening corresponds to F 1.8 at 80mm...80/44=1.8!

Elmerviking

My question is: If you use a handheld exposure meter can you use the meter reading on all zoom range, or do you have to adjust for different zoom range? When you use the built in meter in the camera of course it doesn’t matter...the camera will adjust ISO if you shoot with auto ISO.

BebuLamar

I have to check a few of my fixed aperture zoom and see that they do change. However, with them I never compensate. With variable aperture zoom I do.

RWR Loc: La Mesa, CA

Elmerviking wrote:

I have an old Nikkor 80-200 mm F4.5 zoom. F-stop b... (show quote)

The lens is a constant aperture zoom, not fixed. The maximum aperture is f/4.5 at all focal lengths. Just use the setting your hand-held meter reads.

Elmerviking

RWR wrote:

The lens is a constant aperture zoom, not fixed. The maximum aperture is f/4.5 at all focal lengths. Just use the setting your hand-held meter reads.

Nikon actually calls it “ fixed aperture lenses”.
At 80mm F 4.5 should be 80/4.5 mm diameter, which is 18 mm diameter. It is NOT...the aperture is still open like 44 mm and does not change when you zoom!
My variable zoom...the 18-140 mm changes the opening when you zoom. I checked this now. Hmm...

RWR Loc: La Mesa, CA

Elmerviking wrote:

Nikon actually calls it “fixed aperture lenses”.

While the two terms may be interchanged, “fixed aperture” more commonly refers to a lens with a non-changeable aperture, as with a mirror lens. At any rate, your lens maintains the same effective aperture setting throughout the zoom range. Incidentally, though the F/4 and F/2.8 versions do produce better image quality, the f/4.5 can hold it’s own quite well, even at today’s standards. It’s a genuine Nikon classic.

Elmerviking

RWR wrote:

While the two terms may be interchanged, “fixed aperture” more commonly refers to a lens with a non-changeable aperture, as with a mirror lens. At any rate, your lens maintains the same effective aperture setting throughout the zoom range. Incidentally, though the F/4 and F/2.8 versions do produce better image quality, the f/4.5 can hold it’s own quite well, even at today’s standards. It’s a genuine Nikon classic.

It is indeed a very good lens, according to Ken Rockwell one of Nikon’s top 10 lenses EVER!
I still haven’t used it on my film camera, only on my Nikon D7100. As I posted earlier I bought it on eBay for $3.25!
I still don’t understand how it can be F4.5 at 80 mm with the aperture approximately 44 mm diameter.???

Shutterbug57

If the lens is working as described, which is unusual in my experience, you would have to compute the actual f stop (aperture as a ratio of the focal length) and enter that into the meter to get accurate meter readings.

TriX Loc: Raleigh, NC

Most constant aperture zooms that I am familiar with have a variable aperture internal diaphragm that changes as you zoom getting smaller as the FL gets shorter. If you look through the unmounted lens as you zoom, can you see the internal aperture change? If not, that would account why you’re seeing a different meter reading as you zoom (and why it cost $3.25). Just some thoughts...

GoofyNewfie Loc: Kansas City

That’s a constant aperture Zoom, where the maximum aperture doesn’t change throughout the range. A variable aperture Zoom lens’ maximum aperture changes as the focal length changes:
A fixed aperture lens has only one aperture. Mirror lenses and the Portragon are 2 examples.

Elmerviking

GoofyNewfie wrote:

Thatâs a constant aperture Zoom, where the maximum aperture doesnât change throughout the range. A variable aperture Zoom lensâ maximum aperture changes as the focal length changes:
A fixed aperture lens has only one aperture. Mirror lenses and the Portragon are 2 examples.