F-Stops and 35mm Adapters: An Idea?

x-angel

x-angel

Well-known member
One of the biggest things I hear about 35mm adapters is how you will lose f-stops due to the ground glass/focus screen or what have you.

I have looked inside a few lenses and have seen where even at 1.8 or 1.4 there is still a little bit of the iris in the lens protruding from the sides that could stop light from entering the lens chamber.

Now the key to my idea is this: Is the lowest f-stop on a lenses supposed to be based off of the iris being WIDE open, as in almost an f/0? Or is there supposedly still some of the Iris affecting the imaging?

If it is the latter...why not just remove the Iris from the lens, or find a way to open it completely without having to use a f-stop ring of the lens?

It's seems that most of these adapters HAVE to shoot at the lowest f-stop (1.4, 1.8, etc) in order to have enough light to make a decent looking picture.

I have never heard of anyone shooting lower that the lowest settings (most open) on the Lens on the 35mm adapter...and the dvx's and other 3-ccd cameras have ways of compensating for light flood...

Please someone correct me if I am completely ignorant on how lenses work, but it just seems odd to me that no one has mentioned this before.

Because using this logic, I could theoretically get some sort of busted lens that goes down no further that f4 or so, and remove that iris, and have a wide open 35mm lens for an adapter, and then the GG/Focus screen would block some of the light, and then you have the internal f-stops of the camera...

I may be all wet, but I jsut thought I would throw that out there.
 
You actually don't want to open the SLR lens wide open as most perform at their best 1 or 2 stops down. E.g., I have a Nikkor f/1.8 that hits its sweet spot (sharpness, color aberration, etc.) around 2.8 or 4. So, really, if you want the best shot from your 35 adapter, you need more light and not necessarily a wide-open lens. If only the opposite were true!

Others who actually know what they're talking about are invited to chime in. :)

Fox
 
I get what you are saying. but the limiting factor isnt the iris not opening as much as it can, its the rear element of the lens. the size of the iris wide open matches the rear element of the glass. the rear element is what limits the light, and on most lenses, the iris already opens past that size. so its not a limiting factor.
 
Jason makes a good point. DP's do not shoot films necessarily with iris settings at full wide for the reasons stated. If you disable the lens iris you will not be able to control DOF and GG exposure levels. A GG that is overexposed will introduce some haze and a variety of other image degradations. Further, keeping sharp focus on a 35mm lens at f1.4 is very difficult due to the very shallow DOF, making higher f stops desirable under many conditions. Depending on which adapter you chose, you may not however be able to use f4 without visible grain. This should be a factor in choosing :-)
 
what qualifies the sweet spot for teh f-stop on a lens? I mean what are some of the unwanted things you will see if you open your iris on your lens to its most open f-stop? Also does teh sweet spot idea hold up for the stock lenses on the dvx or xl2?
 
With 35mm adapters, the image is being projected from the 35mm lens onto the focusing screen. Adjusting the f/stop of the 35m lens is the method for changing the DOF of the image. The f/stop of the camera (with a 35mm adapter) does not change the DOF of the image since the subject of the camera is a single plane (the GG surface).

Most moving GG adapters still maintain quality images at high F/stops. The Snod35 has made great images at F/16. The M2 also has test footage showing F/16 being used. So the point is that using higher F/stops is not always a problem. Of course the scene needs to be lit well (but it should be anyway in most cases for DV).

The sweet spot for most 35mm still lenses is 1 or 2 stops below full open. If I remember correctly it is around f/5 for the DVX. By staying around the sweet spot of lenses, you will gain sharpness. So for example, f/2 or f/2.8 on a given 1.4 or 1.8 35mm lens (50mm FL) should be nice and sharp. F/16 on the same lens may still look good, but will not be as sharp.

So shooting with the 35mm lens fully open can cause a slight loss in sharpness when compared to its sweet spot. Images shot outside of the sweet spot are of course usable, but it all depends on what you are looking for.

Also, shooting with the 35mm lens wide open (ie, f/1.4 or f/1.8) will create a very shallow DOF. It may be fitting for certain shots, but it would be very difficult to shot an entire film in this way and maintain focus.

The iris is an essential tool and is the primary way that is used to adjust the DOF. Focal length and subject to camera distance will also change the DOF, but aren't always as practical.

P.S. the Snod35 DIY DVD will cover these topics :)
 
It could be off-topic but I'm having a problem up here.

I'm using a small camcorder (not yet a DVX, in fact it's a sony but I'm saving money!) and about a month ago I built my own 35mm adapter.
But my problem is that when I use f2.8 or lower, I can see a big circle around my 'projection'. I suppose I'm seeing the opening from my lens only because my camcorder does everything automatic. While normally I suppose I should see my image getting darker when I go from f1.4 to f2.8? So while I'm doing this, my image isn't getting darker, however I'm getting bigger and massive vignetting!

I think that -what causes the problem- the lens opening shines trough my GG, whiso I see a brighter image in the centre (so no problem while on f1.4) and that makes my camcorder automatically adjust his settings so on f2.8 I can only see an image as wide as my lens opening.

It sounds complicated! But is there anybody who could solve this??

Thanks,

you can always mail me: piet.deyaert@telenet.be
 
As Mtech said, the amount of diffussion your GG gives affects the overal vignetting. The other 2 factors that will effect this, is distance from camcorder to GG (increasing distance will reduce vignetting, and using a condenser lens, usually a PCX will reduce this effect.
 
Thx

I'm using an AOL DVD case as my GG :-s

I've also used some types of glass but never found the right one.
Do you know where I can find the 'perfect' ground glass?
 
When ever i shoot i over light the scene, then stop the 35mm lens down to appropriate DOF, then adjust Exposure by the DVX iris. IT works perfectly everytime.

Anyone who comes to the Manhole: IV screening will see the SGpro and a full kit of CAnon FD lenses in action.

Ithink its just that some people who arent too familar with the nature of the optics and Exposure are having trouble grasping the concept. Especially if you havebnt purchased an adapter yet, and arent familiar with it s properties. For example, on Jim's Go35, if you "over expose the GG" and stop down the DVX, you get the best performance because its static and neeeds the extra light to get rid of the Grain. The SGpro, on the other hand which is a spinning GG, doesnt care in the least how much light you get to it. it just trandsmits the image. But the best performance comes from Giving the GG way more than enough light and adjusting exposure with the DVX iris. Thsi si practically universal. However, in bright situations, like out doors, ive had the CAnon lenses at F 16. and still needed to turn down the iris to get proper exposure. Dont for get how light sensitive your DVX is. Even if you cut out 8 stops of light with the 35mm lens iris, its still more than enough to resolvea crisp image with the DVX CCD block.

This is Purely experiance talking. Dont worry so mcuh about Light loss with these adapters. you should be lighting your scenes with ample light anyway to get clean noise free images. the 1 or 2 stops you lose with the adpater shouldnt hurt you unless youre shooting all natural light at twilight or night.

All my best
 
You must log in or register to reply here.
Back
Top