Spread of light through lenses

[swmcl]

I have a question that I hope to explain clearly so please bear with me.

IF I were to shoot at an evenly lit target at less than the focal length of my lens what would happen on the film ? Would I get an uneven exposure ?

OK so picture this. I shoot at my lightbox with my lens set at infinity (large format) and the subject distance of say 100mm from the 180mm lens.

OK so take it to the real world. Can I shoot through something like an Expodisk which is attached to my LF lens to get an even exposure across the film ? Why ? Pre-exposure film flashing.

I think I am seeing a variation in exposure doing this.

I've a feeling that if I want to get an evenly exposed sheet of film I should shoot an evenly lit target and maybe be only slightly out of focus to try and blur any inconsistencies in the target ? Obviously I can't focus on an Expodisk that is attached to the lens ... so if I focus on a lightbox at a couple of feet ?

The question comes down to the behaviour of the lens due to its design I think. I don't know enough about lens design to answer my own question.

The issue doesn't arise of course for those SLR digital types who may only be using the information the Expodisk conveys at the very centre of their viewfinder on spot metering I guess. Can one use an Expodisk to evenly expose a sensor for dead-pixel testing or something ? I don't have a digital SLR ...

Does the same thing apply for those white lens caps ?

Hmmm.

Cheers,

Steve

 

[Prof_Pixel]

All lenses suffer from inverse cosine law light falloff - the center of the image gets more light than the edges. If you REALLY want to get an evenly flashed sheet of film, why do it in a camera?

 

[bernard_L]

A uniform target against the front of the lens operates just like a uniform target at infinity. As concerns the remark of Prof_Pixel, the amount of cosine faloff depends on the location of the exit pupil of the lens. Regular (non-retrofocus) wideangle will be worst, plain long focus (non-telephoto) will be best.

 

[Xmas]

Try the preflash with the smallest aperture or one stop up from smallest.

As well as cosine style illumination fall off most lenses use deliberate vignetting to control off axis aberrations but at smaller apertures this is normally less apparent.

Good luck

 

[Nodda Duma]

A uniform target against the front of the lens operates just like a uniform target at infinity. As concerns the remark of Prof_Pixel, the amount of cosine faloff depends on the location of the exit pupil of the lens. Regular (non-retrofocus) wideangle will be worst, plain long focus (non-telephoto) will be best.

This is best reply. Except last sentence is not a hard and fast rule since exit pupil position is not necessarily dependent on the lens focal length. I'm currently working on a wide FOV design with a requirement to be image-space telecentric and also have no vignetting. This will result in even illumination throughout the image plane. It's not even a hard problem to solve in and of itself.

As another example, the Four Thirds System standard requires telecentric lens designs, so those lenses do not exhibit cosine law illumination roll off.

 

[RalphLambrecht]

All lenses suffer from inverse cosine law light falloff - the center of the image gets more light than the edges. If you REALLY want to get an evenly flashed sheet of film, why do it in a camera?

You've got a point there.

 

[RobC]

as you focus closer the lens moves away from the film and the size of the projected image circle becomes bigger. So if you use a set lens extension from infinity of the square root of 2 ((1.414 * focal length)-focal length), then by adding that amount of lens extension your effective aperture gets 1 stop smaller. And this means you are using only the central portion of what you are looking through which if its giving uneven light will help. But not if the uneveness is in the centre but it should reduce any light fall off.

However, I think the real answer is that you need to use a sufficiently diffusing diffusion sheet to get really even lighting and not some thin diffusion sheet that was NOT designed specifically for purpose.

Yes using an evenly lit subject to point it at will help too.

 

[RobC]

you may want to try some of this put into a filter holder.

http://www.leefilters.com/lighting/colour-details.html#400&filter=dc

 

[Sirius Glass]

All lenses suffer from inverse cosine law light falloff - the center of the image gets more light than the edges. If you REALLY want to get an evenly flashed sheet of film, why do it in a camera?

The falloff is on the order of

________1_______
cos4(off axis angle)
​

Which becomes steep.

 

[swmcl]

Thanks guys,

I'm encouraged that if I have a diffusing device it wont matter if it is close to the lens or farther from it. I'm just liking the idea of staying off the lens a little.

I've been using one of those Chinese white lenscaps. They screw on to 86 or 82 mm and are not anything like a hemisphere (lumisphere). I'm using a 180 Sironar S as the lens & shutter. There is a distinct variance in densities across the film.

So far, my take away in this is that I could perhaps use a greatly diffused material perhaps a little removed from the lens front with the lens set at say 1:4 ? I like the idea of the Lee diffusion material but I now wonder how different that might be to a white sheer material from the local haberdashery. Although I am accused of going over-the-top quite often, I'm not sure that the Lee product will justify the cost.

I might add that it might be a good idea to have several layers of diffusing material with fair separations.

Thanks guys.

Cheers,

Steve

 

[RobC]

How tight are you?

http://www.pnta.com/lighting/gels/lee-400-leelux/

 

[swmcl]

Ha ! Tight and getting tighter ...

I may have it solved. A 36" octobox (softbox) with an internal diffuser and a reflective section at the back. The Bowens mount is way bigger than the lens so I just need to hold / mount the softbox on a separate tripod in front of the camera and point it at the sun. Today is raining so I don't think I can try it out.

Should be diffuse light and lots of it ...

I have a number of softboxes for my Bowens flashes.

 

[swmcl]

Just thought I'd reply back to say that a white lenscap does not distribute light evenly. I used it at infinity on a 180mm LF lens - maybe I could've used it at a mid-focus distance.

The octobox worked brilliantly. As linear as !! Somewhat awkward though !! (Really, really awkward ... )

Cheers,

Steve