Difficult exposure question

StewartR

StewartR

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Stewart
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Thought experiment.

I have a lens with an 82mm front element, and I want to fit a 10-stop ND filter onto the front of it. I have a filter but it's only 58mm.

I attach the 58mm ND filter to the centre of an 82mm filter using duct tape so that light can only get in through the ND filter, and then attach that to the lens.

By a curious coincidence 58^2 / 82^2 is almost exactly 1/2, so it seems that I'm only letting in half as much light as if I had an 82mm filter.

Q: By how much is the exposure affected when I use the filter like this? 10 stops, or 11 stops? And why?
 
Interesting question. I would think the light that goes through the centre is unaffected by the outside duct tape. So it is a 10 stop still but your shot will suffer from vignetting.
That's probably completely wrong and when someone gives the right answer I will feel a total Twit
 
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StewartR said:
Q: By how much is the exposure affected when I use the filter like this? 10 stops, or 11 stops? And why?
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There isn't one answer to the question, Stewart.....like most things, there are added complications.

If you were shooting wide open and the 58mm filter was blocking your view of the diaphragm when viewed from the front then you would have some reduction in exposure. However, if you were shooting stopped down some way and the edges of the diaphragm were visible within the circle of the 58mm then the exposure would be unaffected.

There was a similar discussion on here many moons ago with reference to the EF180/3.5 macro. The lens has a 72mm front element but the MT-24EX mounts via an adapter and has a 58mm diameter orifice. One would expect that exposure would be reduced accordingly (when the MT-24 was mounted) but it doesn't as the view of the diaphragm isn't compromised. Maybe the 180/3.5 was deliberately slugged a little given that it's very close in dimensions to the 200/2.8. Having a shorter focal length in a marginally larger body would seem to allow it to be faster, not slower.

Bob
 
Canon Bob said:
There isn't one answer to the question, Stewart.....like most things, there are added complications.

If you were shooting wide open and the 58mm filter was blocking your view of the diaphragm when viewed from the front then you would have some reduction in exposure. However, if you were shooting stopped down some way and the edges of the diaphragm were visible within the circle of the 58mm then the exposure would be unaffected.
Click to expand...
Thanks Bob. I suspected it would be something like that.

In this particular thought experiment I'm shooting at f/16 or smaller, because I want to cut down the amount of sunlight (clue!) coming into the lens by a huge amount. So I guess the exposure won't be affected.
 
If it becomes a reality, Stewart, then I suggest a little bit of matt black card around the rear of the duct tape to avoid and reflections from tangential light that hits the unused portion of the front element.

Bob
 
StewartR said:
Thanks Bob. I suspected it would be something like that.

In this particular thought experiment I'm shooting at f/16 or smaller, because I want to cut down the amount of sunlight (clue!) coming into the lens by a huge amount. So I guess the exposure won't be affected.
Click to expand...

Stewart, Lee are recommending a 20 stop filter for the event to which I suspect you are referring, but I'm going to try some experiments this week, later, (If we get some sunlight!!) and it will be interesting to see what results come out of it.

Just got the 20 stop on Saturday.
 
George said:
Lee are recommending a 20 stop filter for the event to which I suspect you are referring...
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Yes, I'm thinking about the eclipse later this month.

20 stops doesn't sound entirely unreasonable, though I'd have thought 10 stops would be OK. Thinking cynically, maybe Lee have sold a load of 10-stoppers and want to create another market?

The standard DIY solution is Baader solar film. This is described as "ND5.0", which to my mind clearly implies 5 stops, but it's claimed that it "reduces the intensity of sunlight by 99.999%" which is 16.6 stops. Hmmm. Maybe Lee are right with the 20 stops recommendation.
 
OK, a bit of research and I understand the terminology the astro guys use.

ND5.0 film attenuates the light by a factor of 10^5, which is 100,000, and in photographic terms that's 16.6 stops.

The ND5.0 film is recommended for visual observations. However Baader also make an ND3.8 film which is recommended for photography. This attenuates the light by a factor of 10^3.8 which is 6,300, or 12.6 stops in photographic terms.

So I reckon that means a 10-stopper ought to be OK for photographing the sun, but you'd still best not look at the sun through it.
 
At a guess, the reduced transmission for actual viewing is to protect the viewers' eyes while recording media are less susceptible to permanent damage from the visible light. The solar film also blocks out other damaging (to eyes) wavelengths beyond the visible spectrum which is why they're recommended for viewing rather than using welding glass or 10 stop filters.
 
On the ND scale, each stop is 0.3 (B&W and Lee denote their filters this way).

Bob
 
I'm trying to find out a bit more about Lee's thinking on this, the exposures they are suggesting to me don't make sense at the mo, will update asap.

They aren't making the 20 stop available at the mo, Stewart, it's just a bit of fun for the eclipse....don't think sales would be very big!!;),

George.
 
Actually, I apologise.........this filter will be available soon, it's a niche market one I think, designed specifically for taking shots of the sun.
The disadvantage of using 2x10stop ones is the possibility of internal reflection........the suggested exposure is 1/400@f8 and iso 400.
Will try some images (as soon as the sun comes out...it's snowing at the moment!!) and post if interesting.

George.
 
If your thinking solar eclipse then there's resonably price solar filters that fit over then end of the lens @michael23 has a couple of threads/posts worth checking about this and myself I've just received a 8inch square of solar film to make up my own filter to sit on the end of the lens
 
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