Variable aperture lens question

nwilkins · Aug 24, 2012

Hello everyone, this is my 2nd dumb question of the day. I have a question about variable aperture lenses, specifically a (cheap) Tamron 70-210mm F4-5.6 which came with the Nikon FM2 I just bought. I have only ever used primes and I am not sure how variable aperture works in terms of using an external meter.

So with this lens zoomed to 210mm the largest effective aperture is F5.6. This is presumably the amount of light which is let in when the lens aperture is set to F4 on the aperture ring.

However, if I set the lens aperture to F5.6 on the ring and then zoom to 210mm there is clearly less light coming in than when zoomed to 210mm and set to F4. This is clear when looking through the lens and is confirmed by the camera's TTL meter. So would the effective aperture actually be F8 with the lens at 210mm and the aperture ring at 5.6? Do I have to compensate a whole stop throughout the entire range of the lens?

If using TTL metering then this obviously isn't a problem, but I use an external meter so I need to know what the actual effective aperture is. Also, how do I tell at what point in the lens's zoom range the aperture switches from F4 to F5.6?

Any advice is appreciated!

E. von Hoegh · Aug 24, 2012

Use the aperture numbers on the aperture ring of the lens. The maximum available aperture at 70mm is f4; at 210mm f5.6. It should couple properly with the built in meter of the camera. Any more info should be in the lens' instructions, which may be available online somewhere.

nwilkins · Aug 24, 2012

if I did that, then wouldn't I be underexposing at F4 and 210mm? The lens says its maximum aperture is F5.6 at 210mm

E. von Hoegh · Aug 24, 2012

nwilkins said:
if I did that, then wouldn't I be underexposing at F4 and 210mm? The lens says its maximum aperture is F5.6 at 210mm

F4 admits twice the amount of light as f5.6. If the lens' maximum aperture is f5.6 at 210mm, that will be the maximum amount of light admitted regardless of the setting of the aperture ring. Try to find a set of instructions online.

nwilkins · Aug 24, 2012

as I said in my original post, when the lens is at 210mm it lets in LESS light when the aperture ring is at F5.6 than when it is at F4. This is borne out by using the TTL meter in the camera. That is why I am confused.

If the same amount of light was getting in regardless of the aperture ring's indicating 4 or 5.6 when zoomed to 210mm, then I would not have posted my questions.

E. von Hoegh · Aug 24, 2012

Apparently, f4 on the ring becomes f5.6 effective, and the rest of the settings follow suit. This is why I suggested finding the instructions, or someone familiar with that lens. I do not use zoom lenses on my 35mm (or other) cameras.

tkamiya · Aug 24, 2012

You are over thinking this.

If you set your lens to 210mm, f/5.6 is f/5.6 - just like if you have a prime lens at that focal length and aperture at that size.
If you set your lens to 70mm, f/4 is f/4 - just like if you have a prime lens at that focal length and aperture at that size.

You measure, you set your lens, you are good to go. There is no need to compensate.

E. von Hoegh · Aug 24, 2012

tkamiya said:
You are over thinking this.

If you set your lens to 210mm, f/5.6 is f/5.6 - just like if you have a prime lens at that focal length and aperture at that size.
If you set your lens to 70mm, f/4 is f/4 - just like if you have a prime lens at that focal length and aperture at that size.

You measure, you set your lens, you are good to go. There is no need to compensate.

I've no idea the age of the OP's lens. Some of the older/cheaper zooms behaved as I described,(I guess they didn't expect them to be used with handheld meters?) the fact that the OP's meter indicates an increase in light admitted leads me to think this might be the case with his lens.

nwilkins · Aug 24, 2012

Hi E von Hoegh, thanks for trying to solve this - I have been looking all day for an instruction manual but I can't find anything online. I was hoping someone here had used the lens and would know. I can only conclude, like you, that you must need to compensate throughout the whole aperture range, but that seems crazy.

Tkamiya according to your thinking, what would the F number be if I set the lens to 210mm and the aperture ring to F4? Because that setting definitely lets in more light than when set to F5.6 at 210mm.

Chan Tran · Aug 24, 2012

most lenses of the type you have behave like this. When you set it at the shortest focal length (70mm) then the aperture is what set on the aperture ring. Since it's specified as an f4-f5.5 zoom then at 210mm it's f/5.6 when you set your aperture ring at f/4 and f/8 when you set the ring at f/5.6. I also found that the light loss is proportional to the focal length set. for example if you set the lens at f/5.6 and zoom to 140mm (midway) then the aperture is 1/2 stop less than f/5.6 (f/6.7).

nwilkins · Aug 24, 2012

wow that seems like such a bad design but now I know I'm not completely crazy I'm going to borrow a camera with aperture priority and a similar focal length zoom and run some tests with the two TTL metering systems and the incident meter and a grey card and see what is actually going on at 210mm with this lens. I will post the results here in case anyone is interested.

RalphLambrecht · Aug 24, 2012

So with this lens zoomed to 210mm the largest effective aperture is F5.6. This is presumably the amount of light which is let in when the lens aperture is set to F4 on the aperture ring. However said:
that is my understanding,yes!

MattKing · Aug 24, 2012

The Tamron lens in question was most likely marketed at a time when auto-exposure functions were common in cameras, and directed to photographers who would use them.

So the variable aperture probably went unnoticed by many, at least until they tried to use flash :confused:.

And if the lens happened to be used on a camera with TTl flash, not even the.

cowanw · Aug 24, 2012

This is the best discussion on the topic I can find.
http://neilvn.com/tangents/exposure-metering/variable-aperture-lenses/

Leigh B · Aug 24, 2012

nwilkins said:
wow that seems like such a bad design

Not at all. In fact it was a simple and inexpensive solution to a very complex lens-design problem.

These lenses were designed for use with cameras having through-the-lens (TTL) metering, not with hand-held meters.

Using the lens on a TTL camera, when you zoom to a longer focal length, it's as though the ambient light decreased.

From a metering standpoint there's no difference. The TTL meter thinks the lens is still at f/4 and the scene got dimmer.

- Leigh

cowanw · Aug 25, 2012

This interested me so I put such a 28-200 3.8-5.6 Zeiss Jena zoom on a manual 35mm camera and looked through the open back at shutter speed B. I expected the size of the aperture to stay the same as I zoomed out, resulting in a dropoff of F because of: F= focal length/apparent aperture
I was surprised to find another issue. The apparent aperture decreased in size as the focal Length lengthened.
So the F number decreases not only because the focal lenth changes, but also because the apparent aperture changes. A double wammy.
Serious testing is required to use this lens on manual!

E. von Hoegh · Aug 25, 2012

cowanw said:
This interested me so I put such a 28-200 3.8-5.6 Zeiss Jena zoom on a manual 35mm camera and looked through the open back at shutter speed B. I expected the size of the aperture to stay the same as I zoomed out, resulting in a dropoff of F because of: F= focal length/apparent aperture
I was surprised to find another issue. The apparent aperture decreased in size as the focal Length lengthened.
So the F number decreases not only because the focal lenth changes, but also because the apparent aperture changes. A double wammy.
Serious testing is required to use this lens on manual!

And folks wonder why I don't use zooms, ever.

Diapositivo · Aug 25, 2012

For what I understand, when using an external meter with a variable aperture zoom lens you normally worry only when using the lens at its maximum aperture. When using the other positions in your diaphragm ring, the f/number indicated remains the same regardless of the focal length.

An example: sun high, bright day, front lit subject, you use the "sunny 16" rule, and decide for 1/250 @ f/11 with ISO 125.

Your variable aperture zoom should give you f/11 at every focal length.

That's because when you use the lens at a less-than-full-aperture, you cut away the geometric limit given by your front lens. Your front lens is larger than what is needed to give you f/11, at all focal length.

In the case of the OP I am quite puzzled about what would happen with the second value, 5.6, supposing such a value exists on such a lens as a separate case from full aperture, but that would appear very strange to me.
The most normal case is that the variable aperture lens has full aperture indicated as f/4-f/5.6 and the next aperture is straight f/8.
In this case, f/8 is reliable at all focal length when using an external light meter.

Chan Tran · Aug 25, 2012

Diapositivo said:
For what I understand, when using an external meter with a variable aperture zoom lens you normally worry only when using the lens at its maximum aperture. When using the other positions in your diaphragm ring, the f/number indicated remains the same regardless of the focal length.

An example: sun high, bright day, front lit subject, you use the "sunny 16" rule, and decide for 1/250 @ f/11 with ISO 125.

Your variable aperture zoom should give you f/11 at every focal length.

That's because when you use the lens at a less-than-full-aperture, you cut away the geometric limit given by your front lens. Your front lens is larger than what is needed to give you f/11, at all focal length.

In the case of the OP I am quite puzzled about what would happen with the second value, 5.6, supposing such a value exists on such a lens as a separate case from full aperture, but that would appear very strange to me.
The most normal case is that the variable aperture lens has full aperture indicated as f/4-f/5.6 and the next aperture is straight f/8.
In this case, f/8 is reliable at all focal length when using an external light meter.

What you said is only true with modern lenses and mounted on a camera that control the aperture. The camera interfaces with the lens and know what zoom position you set the lens at and open up the aperture to compensate automatically.

Diapositivo · Aug 25, 2012

Chan Tran said:
What you said is only true with modern lenses and mounted on a camera that control the aperture. The camera interfaces with the lens and know what zoom position you set the lens at and open up the aperture to compensate automatically.

I don't fully understand why it should be so.

The f/number should be, broadly speaking, the result of the focal length divided by the diameter of the front lens.

For instance, 100/50 = 2. In order to have a f/value of 2, a 100mm lens must have a front lens of 50mm of diametre. A 200mm must have a front lens of 100mm in order to have a f/2 aperture. A 200mm with a front lens with a diameter of 50mm has a f/value = 4.

This corresponds to what we observe in our lenses. The ordinary 50mm f/2 lens actually has a front lens with a diameter of 25mm.

We are talking here about the f/value which is just the result of this simple division, and doesn't take into account internal losses of light due to refraction, the T/value or such other measures.

When you "stop down" a lens, you cut out of the picture the contribution of the outer part of the front lens. You are actually using a smaller part of the entire optical system.

The thing, as I see it, is entirely "geometric" and has nothing to do with aperture and focal length communication between lens and body.

The variable aperture zoom has a variable aperture because the same front lens must serve all focal lengths. Given that the diameter of the front lens is invariable, the f/value will vary according to the focal length of the zoom state. At the maximum focal length the entrance pupil will yield less light, a higher f/value.

When you stop down, you artificially reduce the part of the lens that you use. It was so far my understanding that by cutting away the outer part of the entrance pupil, the part which is used varies with the focal length, but you always have "enough pupil" or "enough light" for that closer f/value.

Said in other terms, the front lens is not "wide enough" to give f/4 at all focal lengths, but it is "wide enough" to give f/8 at all focal lengths. The actual part of the front lens which is "used" at various focal length position varies (the lens is used "more" or "less" during the zooming action) but at f/8 there is "enough glass" at all focal lengths.

I do use my external light meter with my 28-85 3.5/4.5 lens, and never considered correcting for the variation at f/8. (It's also true that between 3.5 and 4.5 there is around "only" 2/3 of EV, but I think using slides I would have noticed).

Thinking about it, considering 50mm (the half excursion) the "right" value (f/4, or f/8, or whatever) I would have an exposure mistake which is "up to 1/3 EV" on the wide zone and "up to 1/3 EV" on the tele zone. That makes it not so easy to spot. Yet, I don't understand why what I wrote above, perhaps not very clearly, shouldn't hold valid.

cowanw · Aug 25, 2012

Just in case the web site I linked to disappears in the future.

"Where the desired aperture is mechanically set via the aperture ring around the body of the lens this info is then mechanically transferred to the camera body and then used in conjunction with the shutter speed / ISO setting / metering pattern, etc, to get to a meter reading.

In this case, the variable aperture of a lens will show up during the entire aperture range. ie, f4 becomes f5.6 f8 becomes f11 f16 becomes f22 as you zoom to the longer focal length. In this case you will see the meter reading in your camera change as you zoom in and out.

Where the desired aperture is electronically set on the camera body via a thumbwheel this info is then used to calculate the exposure and to set the lens to the specific aperture needed.

In this case, for a variable aperture lens, the aperture will ONLY vary when the maximum aperture is set at the widest length, and you then zoom in. For any other aperture that you set via the camera thumbwheel, you will get the aperture you requested. ie, f3.5 will change to f4.5 .. but f5.6 will remain f5.6 f8 will remain f8 f11 will remain f11. Therefore you wont see the exposure info change in the cameras viewfinder, except if you zoomed from wide to tele at wide open aperture."

Diapositivo said:
For what I understand, when using an external meter with a variable aperture zoom lens you normally worry only when using the lens at its maximum aperture. When using the other positions in your diaphragm ring, the f/number indicated remains the same regardless of the focal length.

An example: sun high, bright day, front lit subject, you use the "sunny 16" rule, and decide for 1/250 @ f/11 with ISO 125.

Your variable aperture zoom should give you f/11 at every focal length.

That's because when you use the lens at a less-than-full-aperture, you cut away the geometric limit given by your front lens. Your front lens is larger than what is needed to give you f/11, at all focal length.

In the case of the OP I am quite puzzled about what would happen with the second value, 5.6, supposing such a value exists on such a lens as a separate case from full aperture, but that would appear very strange to me.
The most normal case is that the variable aperture lens has full aperture indicated as f/4-f/5.6 and the next aperture is straight f/8.
In this case, f/8 is reliable at all focal length when using an external light meter.

nwilkins · Aug 25, 2012

E. von Hoegh said:
And folks wonder why I don't use zooms, ever.

I have no love for them either, especially after this, but I am stuck with it for now, unless someone wants to send me some nice AI Nikkor primes

Chris Nielsen · Aug 25, 2012

Interesting thread. The only zooms I have ever had are modern Nikkors and have CPUs in them so 5.6 is always 5.6 but 4 is not always 4. However if I get a variable aperture zoom for my OM2 I will be finding the same problem as the OP. Good to know if I ever put a variable aperture zoom on a meterless camera!

Diapositivo · Aug 25, 2012

I had already read the quoted excerpt from the quoted link before writing my post.

I wrote it because I am not convinced and wanted to inquiry the matter further.

It could be that the quoted author infers what he writes because while zooming he sees a variation in the shutter speed chosen by the camera.

This is natural, because the TTL light metering is normally performed at full aperture, where this behaviour cannot help manifesting itself.
This is compensated by the TTL metering itself, as explained by Leigh B in post #15 above.

The problem with mechanical-coupled diaphragm simulators possibly arises only at full aperture where, let's say, the simulator says f/4 but the lens only transmits f/5.6, but this is compensated by the TTL measuring which observes a fall in light and compensates with a slower shutter speed.

If what I suspect is true, not just the zoom lens maintains f/8 at all focal length, but also there is no problem in mechanical diaphragm simulation as the mechanical simulator always says f/8. So the "variable aperture" could only manifest itself at full aperture, and not for other apertures, and the TTL metering works OK in all cases.

The answer to my doubt can be given by performing the same kind of exposure reading while zooming activity, but with the lens stopped-down.

I will try to do some test tomorrow (now it's night, and I have a headache as well). But I only have a "not so variable aperture" zoom.

So I ask anybody reading this thread and having a variable aperture zoom to perform the following test:

Put the zoom at its minimum focal length.
Put the diaphragm ring to f/11.
Stop-down the diaphragm.
Read the metered exposure.
Now, while keeping the diaphragm stopped-down, zoom to maximum tele position and observe if the exposure changes.

I suspect that the suggested shutter time does not change (meaningfully).

LJSLATER · Aug 25, 2012

Chris Nielsen said:
Interesting thread. The only zooms I have ever had are modern Nikkors and have CPUs in them so 5.6 is always 5.6 but 4 is not always 4.

Same here, except the minimum aperture is variable too. Try it; the further you zoom in, the further you can stop down. I don't understand the physics, but I think Chan Tran is right regarding electronic cameras.

At any rate nwilkins, I'd humbly suggest you use the TTL meter on the camera for best results. Regarding AI prime lenses, there are tons to be had for dirt cheap. And you can even use autofocus lenses on your camera as long as they have aperture rings.

Some of the most inexpensive (yet optically fine) Nikkors include the 28mm f/3.5, 35mm f/2.8, and 135mm f/3.5. Add the 50mm of your choice and you are set for anything.

Variable aperture lens question

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