Overexposure Due to Stuck Aperture Lever

[Lucas]

Hi guys, just wanted to tell the tale of my experience with my 70-200mm over the last 4/5 months, in the hope that it may help someone if a similar problem ever arises for them.

I first noticed that something wasn't right about 6 months ago, of course during my first paid job covering a Christening with my girlfriend. I had been shooting the boy wide open at f/2.8 initially, using aperture priority. I then would have stopped down to something like f/5.6 when getting some shots of the boy with his parents, and then with most of the other guests. The shots that followed were over exposed, so I think I added some negative ev compensation. Of course when I opened back to 2.8 for subjects on their own, I was underexposing, so I switched to manual for the rest of the day. At the end of the day I was quite dejected, as I thought I had been at fault for not checking the meter properly before shooting, and put it down to the pressure of my first 'job'.

I should have checked things out properly back then, and I might have found the real problem, but I guess life gets busy and some things get pushed to the background. I also didn't use the 70-200 a lot, and when I did use it, I'd usually be wide open, so the metering would be fine.

A couple weeks back, I did a photosession of a little boy, and again I noticed a lot of overexposed shots, again I thought I'd faltered under the pressure. The following weekend I went to Victor Harbor with a 24mm, 50mm, and the 70-200mm. I had been using the 50mm to shoot a breakwater leading into a boat, I think at f/5.6, then decided to try a close up angle, so switched to the 70-200mm. I was in manual, so was expecting the exposure to be fine, but to my surprise it was way overexposed, about 2 stops I worked out by the time I'd adjusted the shutterspeed to get a good exposure.

I stupidly thought I'd discovered something new, that adjusting focal length adjusted the amount of light coming through the lens or something. Of course when I looked into this I found that it was a ridiculous notion. So I started googling about problems with overexposure, and came across a couple of people who had found the problem was due to a stuck 'aperture lever'. I'd never heard of such a thing before, but sure enough there was a little lever on the back of the lens, (and every other lens - at least with Nikon mounts). I gave the lever a little nudge, and the saw the aperture blades close down. A gentle push the other direction and they opened smoothly... Eureka!

I think I'm lucky, and maybe I'd gently knocked it when mounting it one day causing it to bind. It seems to move freely now, and the problem didn't occur in my tests since. However, others have reported it being completely stuck, in which case it is important to stress that it only be gently moved - forcing it could lead to further damage if it's more than just a little stuck.

Maybe this is all common knowledge to some, but I wish I had read something about this before hand, as I'm sure the problem would have been resolved much earlier. I always thought f/5.6 was pretty soft, I'll be very interested to see what results I get now that my aperture is working as it should.

Cheers

[OzzieTraveller]

G'day Lucas

An excellent discussion and explanation of a problem and its cause
Without putting anything against your experiences - we all have to have one like this [at least once] to get the message that "the world ain't perfect and things sometimes go wrong"

At least you now are aware of this issue & can pass your knowledge onto other nikon users ~ well done
Regards, Phil

[ricktas]

Which 70-200. There have been several variations of this one from Nikon alone, and then Sigma etc make them too! Maybe it relates to the specific variant you have.

[arthurking83]

Also, when you hand force the aperture lever, the action should be snappy, never 'smooth'.

They should flick closed when you push on the lever and then release very quickly (snappily) when the lever is released.

But at least you sorted your problem now!

[ameerat42]

...that adjusting focal length adjusted the amount of light coming through the lens or something. Of course when I looked into this I found that it was a ridiculous notion...
Cheers

A most interesting discussion and resolution of your problem. The excerpt above needs a revisit, though. Adjusting the focal length does change the amount of light "coming through the lens" - its intensity. It's just that the camera system is supposed to compensate. I know that this was not your main problem, but it's not such a "ridiculous notion".

Am.

[Lucas]

Rick, was the Nikkor VR1. From what I've read of other people with similar issues, it isn't linked with a single lens. Usually it's a failing lens, in my case I think I was probably just a gumby putting it on once (unusual as I'm generally most fastidious in taking care of my gear)

Arthur, you're absolutely right, of course. By 'smooth' I meant the action wasn't gritty or restricted in anyway - but thanks for clarifying.

Am, just how much are you talking? And how is the camera meant to compensate? By adjusting the shutter speed? I should probably go check, but I'm pretty sure that in aperture priority my 70-200 will get the same shutter speed at 70mm as it does at 200mm.

[ameerat42]

...And how is the camera meant to compensate? By adjusting the shutter speed? I should probably go check, but I'm pretty sure that in aperture priority my 70-200 will get the same shutter speed at 70mm as it does at 200mm...

1. Yes, Lucas, adjusting exposure time is what I would expect. 2. And yes, why not? 3. If you have a constant aperture lens then that would be right.
Ultimately, varying the focal length of a lens alone changes the intensity of the light being transmitted.

Mind you, it is not the major issue that I was addressing - that of the stuck aperture lever. Rather it was the side issue raised in your post. It is an important enough principle - and law of optics - not to dismiss too lightly.
Am.

[Lucas]

I'm confused, #3 seems to contradict, 'Ultimately, varying the focal length of a lens alone changes the intensity of the light being transmitted.' Perhaps I am missing something?

[Lucas]

'A 100 mm focal length lens with an aperture setting of f/4 will have a pupil diameter of 25 mm. A 200 mm focal length lens with a setting of f/4 will have a pupil diameter of 50 mm. The 200 mm lens's f/4 opening is larger than that of the 100 mm lens but both will produce the same illuminance in the focal plane when imaging an object of a given luminance.'

Taken from wikipedia, seems to support my original thoughts.

[gerry]

Interesting problem an dthe first i have heard of it on the nikkor 70-200, glad you got it sorted. The only thing that rings a bell for me is oily aperture blades, this is a classic problem that plagues a few nikkor lens and other branded items, easily spotted by tell tale dark pacthes on teh blades and a stiff smooth motion of the blades which often stick open.

I have a 28mm lens which had oily blades and they stuck open all the time, i did end up fixing it by pulling it apart and cleaning teh aperture blades - it was a interesting experience, google 'oily aperture blades 28mm nikon' for my experience if your interested.

[ameerat42]

OK, but 1) this is not a comment on your main issue, and 2) I hate sounding didactic, so apologies if it does.

Here's the point as a verbal illustration (I hope):

Take a very simple zoom lens with focal range f=50 - 200mm, and let's say it has only one fixed aperture, say 25mm wide, making its aperture 50/25, or f/2 at its 50mm setting. (Don't worry, it was a cheap lens.)

Now, zoom this lens to its f=200mm focal length. It still has the same 25mm aperture, but now its f-stop, or focal ratio has become 200/25, or f/8.

The intensity of the light it transmits has been reduced to 1/4 of the intensity that it transmitted at the 50mm length.

So to get the same exposure value at 200mm as you had at 50mm, you would have to increase the exposure time - by four times, actually.
End of illustration.

Now, translate that to many modern lenses.

Many have pre-set f-stops marked on them, like f/2, f/4, etc.
If you look at many zoom lenses, they will have the likes of "f=50-200mm f/2-5.6" stamped on them. These numbers reflect the fact that I was saying above, that their widest aperture will vary from f/2 to f/5.6 over the zoom range. Of course the camera will compensate for this variation by increasing the exposure time.

But some zoom lenses have a "constant aperture" system built in. The aperture blades actually vary to give the same effective aperture over the whole zoom range. Thus the exposure time can stay the same over the whole zoom range.

If you do any searches, use the terms "effective aperture" and "zoom lens".
Am.

[Lucas]

Not didactic at all, and thanks - new word for my vocabulary :P

I understand where your are coming from now, I guess I should have been more specific and mentioned the condition of the effective aperture remaining constant. Having said that, I had only been talking about f-stops which are an expression of effective aperture, so it could be assumed that I wasn't talking about the actual diameter.

Of course, given the nature of the topic brought up (that being how optics behave), it's quite important to be specific in what we talk about, so thank you for bringing it to my attention

[Lucas]

Gerry, quite an adventure! I REALLY hope that's not the case with my lens. The blades definitely are snappy, no stickiness at all once it was unstuck, and they appeared clean at the time, will have another check armed with the info from your story. Thanks

[arthurking83]

Even tho the issue is resolved and in effect there was no issue, and this thread was more of an informative notice, the idea that focal length impedes exposure in some way even for a constant aperture zoom lens is not as ridiculous as first thought!

First of all apologies for going OT again, but as it was mentioned, I thought it may make sense to touch on the topic again and the OP can look into it more for themselves if they wish.

But the wiki definition of focal length vs exposure doesn't really apply in a situation like this(where the lens referred too is a zoom lens).

I'm assuming that the wiki's definition of pupil diameter is actually referring to the entrance pupil diameter .. there is also an exit pupil diameter to be mindful of too!

While they're correct that the entrance pupil of the 200mm f/4 lens needs to be a min of 50mm(in reality they're usually larger... such as 62 or 67mm) and the 100mm f/4 lens needs an entrance pupil of 25mm (and once again this will generally be designed to be larger, or as large as possible being mindful of cost and bulk .. the wiki is referring to a prime lens designs when they speak of these factors!

What you are referring to with your issue .... or more specifically, query on ridiculous notions!... is for a zoom lens design.

The front lens(opening/entrance pupil) of the zoom lens is approximately 77mm. Take into consideration the fact that the lens has a focal length of 200mm(at infinity focus!!) this means that the front lens is pushing the boundaries of the theoretical f stop factor of the lens. That is, the lens design being 200 f/2.8 means that the minimum front lens size needs to be almost 72mm in size .. perilously close to the actual size of 77mm. There's not much leeway in the design.

Now at the shorter end of the focal length(70mm) the front opening of the lens for a 70mm f/2.8 lens design only needs to be 25mm, yet your zoom lens actually has a 77mm front lens size.
Theoretically, it wouldn't be hard for the manufacturer to design into this lens an aperture size of f/0.9, as the hard task of making the lens the required size is already done!
(FWIW the problem of actually doing this stems from the distance of the rear lens group to the required register distance).

So back to the OT topic.
What you will now get from this lens is that because the design of the lens is marginal at 200mm and f/2.8, whereas at 70mm the design is (lets say) far more accommodating, you generally get much more vignetting(and bad vignetting too!) at the longer end. This is vignetting that is generally not 'seen' so much by the camera(although some cameras may have the ability).
This vignetting phenomenon will manifest in the form of reduced exposure ... and it could be a difference of up to one stop over the majority of the frame.
That is one stop difference in exposure between the 70mm and 200mm end of the lens's focal ability .. and all at the same aperture values.

So while Am was referring to the variable aperture type lens, and wiki has the theory of lenses covered, the reality of zoom lens design is that it's a hard nut to crack and get it perfect.

Now this exposure difference is no where near the values you will see from an issue of a stuck aperture blade or whatever, but the notion is not ridiculous at all!

A quick flick through any lens testing site will reveal this info, and you can do the test for yourself to see it in action(but you need to be weary of specific issues tho.
(make sure the camera is not set to auto compensate for vignetting, and using manual mode and so on)
Shoot a static plain evenly and constantly illuminated single colour background to see what is meant here.

Another reality aspect to consider for a zoom lens is that of T-stops of the lens. Because the design of a zoom lens is generally so complicated(especially these constant aperture versions!) the setup of all the internal lenses varies wildly between the focal length extremities of the lens.
Look into the front of the lens itself as you zoom from one end to the next to see this.. the lens elements inside are all moving relative to each other is set groups, and each group will have a different distance to another as you zoom. This creates a different air-lens surface distances and may have an impact on the T-stop value through the entire lens at different focal lengths.

Thing you can see for yourself to understand that loss of exposure value by a simple zoom(whilst maintaining a constant aperture) is not such a ridiculous notion:

zoom and or focus the lens and watch the position of the rear most lens group relative to the back of the lens
watch the inner lens elements all moving relative to each other as you zoom, and to a lesser degree as you focus.
Now that the lens is working properly again, take a few test shots.
Manual setting, exposure with spot metering at the centre focus spot and shooting in manual with the same setting for both 70mm and 200mm shots.
A bright blue cloudless sky at close to midday is a good way to do this. Shoot at 70mm and again at 200mm at the same apertures. Up to about f/5.6 or f/8, the 200mm shot will always be darker than the 70mm shot.... overall an actual central spot brightness level check may reveal a very close exposure, but the entire image will look darker from the 200mm shot(vignetting).
As you stop the aperture down(eg f/8) the vignetting changes from minimised to eliminated, and the exposures can be close enough to be considered 'the same'.