My lens has a spot on it, is there anything I can do?

[Brian Drysdale]

The speed of light is a constant.

You should go through the explanations earlier in the thread again and think about the mathematics of how f stops are calculated. Bear in mind that for longer focal lengths you need a bigger diameter than short focal length lenses, so if your iris diameter is correct at the short end when wide open, the front of the lens is not going to be large enough for the long focal length in a compact lens, so the latter going to be effectively stopped down because the diameter of the lens is acting like an aperture for setting stops.

BTW Don't use zoom lenses on your night exteriors, you lose too much light with large sensor stills camera zooms because they're slow lenses. F1.4 to f2 primes is what you should be using.

[Ryan Elder]

Okay thanks, I only have two night sequences in the script and can restrict the zoom lenses to the day scenes then if that's better. It's just that is why I thought a camera with good low light vision, would be good, cause then the ISO can be high enough to shoot at say, f8 or something like that, where you can fit more actors into focus, with a deeper DOF.

As for a constant aperture, it's just that when you zoom, the lens becomes longer, so I thought it would take light longer to travel through the lens then.

As long as the aperture remains constant, if I buy a lens with a ring. If I buy a lens with a ring, will their be any hidden surprises that will make it less usable, or any hidden catches with it at all? Or will it perform perfectly with no hidden catches, as long as I am stopped down enough?

[Paul R Johnson]

Well - if the lens is longer, it does taken light longer to travel down it. If the lens extends 100mm, then it takes an extra .3335 nanoseconds longer to get to the other end - but why does that matter?

[Ryan Elder]

I thought that it matters because the longer the lens gets, the slower it will take for light to travel through and hit the sensor. Isn't that why the camera is using the auto exposure as I zoom, cause it is trying to make up for the light slowing down, as I zoom in closer?

[Paul R Johnson]

No it just gets dimmer, and that is why the camera tries to compensate.

Did you read Brian's excellent explanation up the topic? Explaining why the iris functions oddly when the focal length gets longer. Worth going back and trying to understand it. you can actually see this in action of you hold a proper lens up to your eye and close and open the iris. Because your eye is only looking through the centre rather than the full field like a sensor, you can see the change in brightness as you zoom. This is what the camera tries to compensate for. google lens ramping for more info, but Brian's post explains it quite simply.

[Ryan Elder]

Oh okay, I went back and read it, but this is also what I was not understanding before either. If zooming in causes the image to become dimmer, how does stopping down help be a constant aperture then? Wouldn't it still get dimmer as you zoom?

[Paul R Johnson]

no - the best I can explain is that of the the overall light that gets to the sensor, through the lens has a pretty strange distribution. As you close the iris, you reduce the light getting through at the edges, working to the middle. The trouble is that at the longer focal lengths, there is very little light at the widest part, so if the lens is stopped down from say f22 to it's specified largest aperture f2, it can be that once you get to f3.5 closing further has little effect, because there isn't any light at the edges to iris out! It suddenly becomes ineffective. If the lens is zoomed out, then the level can be reduced between f3.5 and f2. The practical upshot of all this is that when you have the iris closed so you are only using the centre of the glass, you can zoom through the entire range and there is no drop in light level, but when you use the iris wide open, at some point the light starts to drop off as the focal length increases. and there is no light path that the iris can uncover to recover the brightness.

You'll notice that lenses that have a larger diameter are usually 'faster' - the larger diameters of the glass lenses let more light through. They can manage to function properly in a linear fashion. My Sigma lens is a good example of compromise. It looks quite large in diameter but as you zoom in, the front extends and you can see the diameter of that section is quite small - so at full extension the light has gone down quite a bit, and the camera can compensate in stills by adjusting the shutter opening time to recover the light. Doesn't matter in stills but in video the ramping is obvious. I don't think I've done a great job explaining this to be honest but without spending much longer, it's the best I can do for the moment keeping the language and the physics simple. For the people who know lens physics, I apologise for simplifying some of the actual physics, but it's all about tube diameter, and the clusters of lenses inside the package.

[Ryan Elder]

Okay thanks. I understand parts of what you are saying, and just trying to process it.

So would a lens that is not constant aperture though, but has the ring and no servo controls, be able to remain constant aperture, or no, if longer focal lengths, means less light is getting in, at the edges?

[Brian Drysdale]

The long lens needs a larger diameter hole to let the same quantity of light through than the short focal length lens, but if the lens is too small a diameter the amount light coming through is reduced at the long end of a zoom lens compared to the short focal because of this, This is because the front of the lens itself is also acting as a stop aperture, restricting the quantity of light at the long end of the zoom.

As you stop down using the stop aperture a point is reached where the light passing though all the focal lengths becomes the same because the long focal length is no longer being affected by the restriction of the lens' because of its small diameter.

It's just the mathematics of how f stops work (which involves diameters), if you can't follow it, just accept that when you stop down to the smaller stop in the wide open aperture range, the aperture is then constant at all focal lengths. You don't need to know the reason to use the lens.

How the exposure system works doesn't come into it, so ignore that. It can't bring in more light than the lens allows by the nature of its design.

[Ryan Elder]

Oh okay, that seems to make sense, thanks.

This is what I thought, but how come if I stop down to f8, like in the video I posted before, the aperture still goes down as I zoom in though? This part I wasn't understanding before.

[Paul R Johnson]

In your example, the image content was not even, so maybe the small highlights passing through exposure measuring zones triggered it? Try it again on a plain grey background and see what happens.

[Brian Drysdale]

As Paul says, your exposure system was probably not totally on manual, so it was still making adjustments. If you had set the aperture, it was probably making adjustments to the shutter speed.

These are two separate items, the lens and the camera. Remember DSLRs are primarily designed for stills photographers, so they may not be as convenient for operation as video cameras, especially older ones. Unless you really know the exposure systems you may end up switching from aperture priority to shutter priority (or the reverse) instead of going totally manual. That doesn't include Auto ISO, which is another setting you want off.

[Ryan Elder]

Oh okay, but I thought it was the aperture that is changing because if put the lens on a camera that requires an adapter, then it won't get brighter as you zoom in. So therefore, wouldn't it be the aperture that is compensating, and not the shutter speed and ISO?

[Brian Drysdale]

We don't know how your camera is set up and given the variables you need to ensure that you're following the correct procedures.

DSLR Basics: 8 Easy Steps to Learn Manual Mode for Canon DSLR Cameras - Nature Photography Simplified

The exposure changes that your video displayed wasn't characteristic of a lens issue because the exposure was intermittently changing during the shot. This was either due the lighting changing during the shot or the exposure being adjusted by the camera or the camera operator.

A lens exposure issues would be linear, not intermittent, unless you got some flickering flare from a light hitting the lens and that cause is external to the lens.

[Ryan Elder]

Oh okay, well the lens goes from darker to brighter, to darker to brighter every time I've tried it though, which is a lot of times as I've used in my shoots so far.

So if it was the lighting, would lighting in locations do this, this commonly, that happens every time?