Non-constant aperture zooms

Michel Hardy-Vallée · Oct 29, 2007

A small beginner's question that I can't fathom how to answer: when using a zoom that does not have a constant aperture (say, 3.5-5.6), is the aperture change for ALL f-stops or only for the widest aperture?

In other words, if I use such a zoom at f/8, will it remain at f/8 when I zoom?

David A. Goldfarb · Oct 29, 2007

No, all the stops change as you zoom. If you're metering through the lens, the meter will compensate. If you use a handheld meter or non-ttl auto flash, then you have to figure it out.

Chris Breitenstein · Oct 29, 2007

The numbers (3.5, 5.6, 7, 11, etc) are a ration for the size of the opening in relation ship to the focal length of the lens. to make the math easier I will use f/2 as an example. if the lens is a 50mm lens, and it is at f/ 2 then the aperture opening is 25mm. if it happens to be a zoom lens, say 50-100, when you zoom in the focal length changes. the 25mm opening that was f/2 when zoomed out (50mm) is now f/4.

Now to answer the question you asked. No. f/8 will change when you zoom in.

Ian Grant · Oct 29, 2007

That's a very good question but I just checked my Canon and Tamron 17-35mm f2.8-f4 metering off a plain evenly lit wall and the metering stayed constant as I zoomed at f5.6 and f8.

I think the answer is with older mechanical lenses the apertures all vary as you zoom, but with most modern autofocus lenses where the camera body sets the aperture electronically it stays constant.

Actually the constant aperture was a sales feature, now I think about it, with Canon lenses.

Ian

Michel Hardy-Vallée · Oct 29, 2007

Thanks for the answers, it makes more sense now.

I was planning a worst-case scenario at work that would involve a Nikon manual body, an AF zoom, and non-TTL auto flash. Mismatched trio, I know, but I'm waiting for an FM2n kit to arrive in the mail.

Like a good young imbecile I did not take the shipping option bearing a tracking number (save money, yeah right!), so I'm anxious for it to arrive soon enough for a job I'm doing in mid-November (stupid! stupid! stupid!).

Clearly, should the scenario be worst case, I will rent a body/flash that can deal with the zoom I have (as per Ian's explanation), unless my manual body and prime lenses arrive on time.

BTW, I can't believe how cheap it is to rent an F5 for a week! 120$, plus a little extra for a lens. Rental is really the unsung hero of analog photo.

John Koehrer · Oct 30, 2007

With regard to Ian's suggestion that the variable aperture is compensated for by the electronics of the camera. It's not gonna happen unless the shutter speed changes.
You've got a mechanical limit imposed by the design of the lens ie:variable aperture and unless there's some compensation somewhere.
I guess the conclusion should be if it's working OK it doesn't matter how it does it.

Ed Sukach · Oct 30, 2007

It is a *FAR* more difficult design problem to keep the aperture constant as the focal length changes - more difficult, more elements/groups in question, but IMHO, indicative of a greater committment to quality.

The one lens I have that seems to perform the *best* is an old(?) Olympus - Zuiko 75mm - 150 f/4 - with a CONSTANT aperture. That has fifteen (15) elements in eleven (11) groups.

Somewhere along the line, it was decided to sacrifice lens design efforts and rely instead on "automatic" exposure, assuming that most (everyone?) would be deliriously happy not even knowing what the aperture was.

Possibly the start of the "dumbing down" trend so prevalent in recent years?

copake_ham · Oct 30, 2007

mhv said:
.....

BTW, I can't believe how cheap it is to rent an F5 for a week! 120$, plus a little extra for a lens. Rental is really the unsung hero of analog photo.

I dunno.

As I write this, with 20 minutes to go - there's one selling on eBay for $620. Doubt it will go much higher than $700 so $120/week rental is m/l one-seventh of the sell price.

If you got the need for seven weeks worth of F5 - you're better off buying one.

Michel Hardy-Vallée · Oct 30, 2007

copake_ham said:
If you got the need for seven weeks worth of F5 - you're better off buying one.

I can't know I've got the bug before I get infected... That's how I found out about the FM2n, at any rate!

unohuu · Oct 30, 2007

copake_ham said:
I dunno.

As I write this, with 20 minutes to go - there's one selling on eBay for $620. Doubt it will go much higher than $700 so $120/week rental is m/l one-seventh of the sell price.

If you got the need for seven weeks worth of F5 - you're better off buying one.

There is one on Craigslist for the princely sum of $400...not that I need another film camera, but that surely is tempting. Two F100s and the F5 and an N90s, along with the F4. I might never need to buy another film camera.

Ronald Moravec · Nov 2, 2007

BTL metering will compensate. What screws you up is if you use a hand meter or change focal length without a change in marked aperture. I don`t use them any more for that reason except for Nikon DX.

donut · Nov 2, 2007

i would say that setting the camera to f8 then zooming in the aperture would still be f8 . maybe f8 wasnt a good example to use . now if you had said starting at the widest aperture for a given lens , lets use a 70 - 300 f4 - f5.6 zoom ., if you started off at 70mm and f4 then zoomed in , at a certain focal lenth the apeture would change and by the time you got to 300mm you would be at f 5.6 . even at f8 on this lens the aperture would stay the same throughout the entire focal range .
nikon with above lens 70mm f4
100mm f4.2
135mm f4.5
200mm ff5.0
300mm f5.6 ( roughly )

donut · Nov 2, 2007

what you would have to watch out for is if you are using an in between focal length , actual f stop might be a bit hit/n/miss

Ed Sukach · Nov 2, 2007

What happens to the numerical value of the f/stop depends om the placement of the aperture itself - the iris diaphragm. In a "constant aperture" system, the diaphragm is stationary - it will not move with the lens elements. If the diaphragm moves forward and back, the f/stop value will change accordingly.

Remember the formula for f/stop:

f/ = focal length (distance from diaphragm to film plane) / actual diaphragm diameter.

As an example:

Distance for diaphragm to film plane = 50mm
Diaphragm aperture diameter = 25mm

f/stop = 50(mm) / 25(mm) = f/2

The placement of the diaphragm in a "zoom" system, is NOT universal; in some systems it may move more than in others - and I can't think of any way to determine where it is, other than referring to the design itself - or - (shudder, shudder) disassembling the lens and making many measurements....

mawz · Nov 2, 2007

The answer is 'it depends'. If you're setting aperture from the aperture ring, yes it does vary with length as the aperture ring's settings are only accurate at the wide end of the zoom. With a f4-5.6 lens, setting f8 on the aperture ring will give you f8 at the wide end and f11 at the long end. In-camera metering and TTL flash handle this nicely, handheld metering for flash or ambient will have issues.

If the body is setting aperture, it may or it may not, depending on the body (and that body's configuration, as some can be set to do either). The camera will most likely tell you the actual aperture though, as long as the lens and camera communicate electronically (exceptions include variable-aperture Pentax KA mount zooms, which can only tell the body the absolute max aperture, not the apertures through the zoom range)

Michel Hardy-Vallée · Nov 3, 2007

OK, I got my answer in person: using a Nikon FE body and a 28-80 f/3.5-5.6 D zoom, I can confirm that regardless of the f/stop set on the lens, zooming will change the effective aperture of the zoom.

Chan Tran · Nov 3, 2007

In your case that you using a Nikon FE then yes. The aperture changes as you zoom at all aperture. If you use the built in TTL meter then the meter does compensate for this but the aperture does change as you zoom.
On the other hand if you use the same lens on a newer camera like an F5 or F100 and you set the lens at mininum aperture and then selecting the aperture via the sub command dial then the aperture won't change unless you set it near maximum or minimum aperture.

Michel Hardy-Vallée · Nov 4, 2007

Chan Tran said:
In your case that you using a Nikon FE then yes. The aperture changes as you zoom at all aperture. If you use the built in TTL meter then the meter does compensate for this but the aperture does change as you zoom.
On the other hand if you use the same lens on a newer camera like an F5 or F100 and you set the lens at mininum aperture and then selecting the aperture via the sub command dial then the aperture won't change unless you set it near maximum or minimum aperture.

Interesting, it looks like an offshoot of the shutter-priority mechanism developed for the FA. I've always been amazed that AI lenses had a provision for adjusting aperture from the body.

Non-constant aperture zooms

Michel Hardy-Vallée

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