Just a thought about 35mm adapters and ISO ratings

You did not overestimate a 2 stop difference - 2 stops should be 2 stops, but you have seen they are not in this context. You observed correctly but didn't know what to make of it. I was completely floored when i first noticed this and it took me awhile to digest it.

Redrock does it less than others but they all do it - look at the rest of the f stops on a waveform. to check it out - though I can't recall how far off the M2 goes.

Lenny
 
I will. I hope you did not take my post as an attack. I will be sure to thoroughly look into this. I just find it odd.

This is what the forum is for after all. Perhaps we will all learn something from this.

Thanks
-ryan
 
Simple fact is that with or without a ground glass adapter in place, the f-stop difference between f/1.6 to f/2 isn't as linear as you would expect. With an adapter in place it seems to get even less linear.

It may be something to do with microlenses on the chip, but if not that, then either I don't know why it's happening, or the manufacturers just get a bit optimistic when they rate the f-stop, or the actual t-stop just isn't that high.
 
Barry, You're reffering to changing the video cameras f stop. I'm referring to lack of linearity in how the video camera with a fixed -stop responds to changes on the 35mm lens f-stops. the 35mm lens should be linear - photgraphy is based on this - but through the adapter it is not.

Prodigy - no harm taken it is hard to get your head around but just test it yourself. I keep mentioning this on the boards but few people have tested it and most opeople just seem to shake their heads and misunderstand what i said.

As I recall that the M2 was more linear. it was my first adapter & I didn't notice this until the Brevis. Then I saw they all did this to some extent. I haven't tried to be careful in testing except for the difference between 1.4 and 2,8 because that really does affect our conception of how fast an adapter is.

By the way the Brevis responds completely differently if you change the diiffusing screen.

Lenny
 
cardmaverick said:
You know, I just did some more thinking, if the GG does in fact cause issues with the f/stop on the SLR lens, then maybe the best approach is to simply pick an f/stop on the SLR lens and only use that one f/stop. Many DP's shoot whole films at only one f/stop, so this isn't uncommon.

All exposure adjustments (including initial ISO rating) would then be made using the camera aperture instead. If you want to use different f/stops on the SLR lens, then you could just create a small list of f/stops and their ISO ratings, not too unreasonable.

Sometimes you just gotta think outside the box!

I'm very interested in testing that approach, I bet it would work out just fine too.
Click to expand...

Has anyone tried my theory above? I totally understand what Lenny is talking about now regarding these diffusion screens. I think thats the big confusion, its the fact that the light on the reverse side of the screen (the image your camera sees) acts differently from the light hitting the front of it via the 35mm lens.

I can also understand beating a topic to death that others simply have not tried to understand... like using a light meter with a video camera! LOL. I really do use it all the time on my shoots for exposure, Andromeda or not, and never have any issues (despite the problem Barry talks about with the wider f/stops, I rate at 90 IRE for a reason, and I don't normally shoot that wide open anyways for focus reasons), but I've also been doing this for years, so I have a good grasp on how its done.
 
Lenilenapi said:
Barry, You're reffering to changing the video cameras f stop. I'm referring to lack of linearity in how the video camera with a fixed -stop responds to changes on the 35mm lens f-stops. the 35mm lens should be linear - photgraphy is based on this - but through the adapter it is not.

Prodigy - no harm taken it is hard to get your head around but just test it yourself. I keep mentioning this on the boards but few people have tested it and most opeople just seem to shake their heads and misunderstand what i said.

As I recall that the M2 was more linear. it was my first adapter & I didn't notice this until the Brevis. Then I saw they all did this to some extent. I haven't tried to be careful in testing except for the difference between 1.4 and 2,8 because that really does affect our conception of how fast an adapter is.

By the way the Brevis responds completely differently if you change the diiffusing screen.

Lenny
Click to expand...


whats going on here?

Prodigy, this is an issue that has been discussed thoroughly before.
If you are unclear as to why this occurs, please state so. But opinions and confusions don't really matter when you compare it to the fact of 35mm adapters and the use of certain GG.

The FACT is, with your camera aperture locked down, the change in 35mm lens is, as stated, not linear. From F2 to F2.8, the change in light loss is equivalent to a third of a stop on my CAMERA aperture. So no, there is nothing wrong with the lenses or their fstops, its simply the way the GG transmits light. For this, i do not understand the science behind it, but i do understand that this in fact happens.

point finale.
 
Papa, Whats your take on this approach... by reversing things around and locking down the SLR lens, and changing exposure only on the camera, it should work out fine. The adapter/SLR lens now act more like a fixed ND filter.

cardmaverick said:
You know, I just did some more thinking, if the GG does in fact cause issues with the f/stop on the SLR lens, then maybe the best approach is to simply pick an f/stop on the SLR lens and only use that one f/stop. Many DP's shoot whole films at only one f/stop, so this isn't uncommon.

All exposure adjustments (including initial ISO rating) would then be made using the camera aperture instead. If you want to use different f/stops on the SLR lens, then you could just create a small list of f/stops and their ISO ratings, not too unreasonable.

Sometimes you just gotta think outside the box!

I'm very interested in testing that approach, I bet it would work out just fine too.
Click to expand...
 
in that respect yes, you could may be able to use a meter there, however i don't know now, with the adapter, AND the fact that the cameras aperture is stepped down, how to rate the camera, because wide open is now 2.6... so it would get quite confusing to use a light meter to rate your iris. So like i said before, it is still very handy for contrast ratios and measuring the depth of your darks and lights.
 
PaPa said:
whats going on here?

Prodigy, this is an issue that has been discussed thoroughly before.
If you are unclear as to why this occurs, please state so. But opinions and confusions don't really matter when you compare it to the fact of 35mm adapters and the use of certain GG.\
Click to expand...

Well this is the first I have heard of it, so I apologize.

To be perfectly honest I still don't understand why the GG is the reason for this phenomena.

Anything that cuts light down has a certain percentage of light transmission. Despite which GG I use, as long as I am using the same GG the % of light transmission should be the same, therefore I don't see how the GG would cause a non linear curve.

I am not trying to beat a dead horse. I would like to know why this happens. If it is simply just what Barry was talking about and it's all happening in the lens, well that is different.
 
We'd all like to know why it happens, but I haven't been able to figure out a reasonable explanation yet and I haven't heard one from anyone else either. It is most puzzling.
 
Actually I think it is something related to vingetting but its not quite the same. Something to do with lens from the wider parts of the iris not getting picked up by the video lens. I wonder if the Brevis with flip optics will change at all.

Cardmaverick - I guess you can do that, but your causing yourself a lotof trouble for the dubious goal of being able to set exposure with a light meter. Why bother - you can still use the light meter just check a monitor.

Barry I think the mystery of why wide open video f stops don't seem linear is even more bizarre to me. I've never compared them before but certainly the HVX seemed that way and that really confused me when I started testing adapters. I had the same thought about manufacturers but that doesn't make sense either . Why should they bother, I suspect its an optical issue since f stops are mechnical measurements as opposed to say T-stops, though who knows whetehr thoase are real f stop numbers.
 
cardmaverick said:
by reversing things around and locking down the SLR lens, and changing exposure only on the camera, it should work out fine. The adapter/SLR lens now act more like a fixed ND filter.
Click to expand...


Oliver Stapleton (DP, My Beautiful Laundrette, The Cirder House Rules) discussed this tangentially in an edition of IMDb's ask a filmmaker a while back: http://us.imdb.com/indie/ask-archive?date=20051219. Sticking with the same f/stop would mean you can't change your DOF which is pretty much the main reason for using a 35mm adapter. It's a tradeoff between making things technically easier or having more creative options. With film/video these things are almost always inversely related.
 
Last edited:
Stapleton is talking about making a creative decision to stick with a uniform f stop for the look of the film which is fine and relatively common if you can afford to work that way. But he's not making that decision so he can use a light meter.
 
Ok I found the answer to my question in the first post. Does the lens on the front of the 35mm adapter change the ISO. I went back to the freshdv tutorial and in the comments, the Kendall Miller who conducted the tut said:

"Changing the 35mm lens will not change the ASA or ISO rating of the rig…"

That's all I wanted to know.
 
Lenilenapi said:
Stapleton is talking about making a creative decision to stick with a uniform f stop for the look of the film which is fine and relatively common if you can afford to work that way. But he's not making that decision so he can use a light meter.
Click to expand...

I see what you mean, but its not a big deal, just have a small list of ISO ratings for each f/stop. Simple as that, and since f/stops are a standard, it should apply to any lens, especially if you use good lenses from companies like Nikon and Zeiss

As for why I light meter, well, for one, its how I've always worked. When I first got into video I looked toward the "film guys" to see what they were doing, because there stuff looked better than typical video if you know what I mean (even when they did shoot video as well). And one thing I noticed was the close attention to reflectance via spot meters. So why not me? As for setting the exposure, again, I can't always take a waveform around. Years later, even though I'm now a full time DP, I'm still starting out and work in the lower budget production world at the moment (working my way up steadily though). Only one of my productions so far had the luxury of providing a waveform, and you know what, I didn't have time to screw around with it. I had to meter up and move, and it all worked out fine.

Another reason for doing the "meter thing" is it really forces you to think about what your doing. I literally practiced by shooting blind. Just me and the meter, no monitors allowed. I've benefited quite a bit by doing this. The more intimate you get with your camera setup prior to a shoot, the better you are, and light meters are a very good gate way to this.

But thats just my personal philosophy.
 
cinebuddy said:
Ok I found the answer to my question in the first post. Does the lens on the front of the 35mm adapter change the ISO. I went back to the freshdv tutorial and in the comments, the Kendall Miller who conducted the tut said:

"Changing the 35mm lens will not change the ASA or ISO rating of the rig…"

That's all I wanted to know.
Click to expand...

Given the GG transmission effects, I don't think thats a full answer. Its more like...

"Changing a 35 mm lens won't change the rigs ASA as long as you use the same f/stop on each lens. Any f/stop change on any 35mm lens will, to some degree, affect the ISO."

That also assumes you only change exposure via the camera aperture, and don't mess with the 35mm lens. Hope I'm not confusing you!

This has been a good thread!
 
Cardmaverick,
Well at least I understand where your coming from about not having a waveform around. I'm used to it. The meter is a discipline I've completely gotten out of the habit of and I do understand why you would enjoy working that way. I kind of miss it actually as I also miss shooting film.

Consider springing for a Panasonic 17" or even an 8" monitor with the waveform built in. You will like the tool.

Lenny Levy
 
You must log in or register to reply here.
Back
Top