Problem exposing snow with 120 film when overcast

[andre_lefort1]

<p>Hi everyone, <br /> I get a correct exposure when shooting snow on a clear day. When the sky is overcast I get this problem with the exposure - I get a darker spot on the rough center of the frame.<br /> 120 film, processed the same way compared to correct exposure when sky is clear. Changed lens without any improvement.<br /> any idea?.<br /> Thanks for your input,<br /> <img src="file:///Users/andrelefort/Desktop/H-%20201364.jpg" alt="" /><br /> <img src="file:///Users/andrelefort/Desktop/H-201364.jpg" alt="" /><img src="/Users/andrelefort/Desktop/H-201364.jpg" alt="" /> Camera: Bronica SQA 6x6<br /> *Lens: Zenzanon=PS 80mm<br /> Film: Ilford HP5-Plus<br /> Speed: 1/60 sec.<br /> Diaphragm: F-8<br /> Developer: Kodak D-76<br /> Processing: 7 min. à 20°C</p><div></div>

[paul_wheatland]

<p>Take a meter reading, then open up one stop, keep shutter speed as before!</p>

[Alan Marcus]

<p>All camera lenses project an image, however only the central portion, called the circle of good definition. is acceptable. Unless a lens is designed to have an expanded circle of good definition, we stick to lenses with a focal length about equal to the diagonal measure of the format or longer. <br>

<br>

Why is the central part of the projected image more faithful? The lens works by refracting light rays inward. Image rays that fall on the boundaries are refracted the most. Their path is longer and they arrive obliquely. The iris diaphragm as seen by the film from the borders has an oval shape (not seen as a circle). The oval has less surface area -- thus the image it projects is dimmer. <br>

<br>

Additionally, the imaging rays that arrive at the boundaries arrive at an angle as opposed to central rays that arrives straight on. These oblique rays are more spread out -- thus the image is vignette. This is called cosign error. All images will display a vignette. Most times we do not see the vignette because the image is busy. The vignette is most notable if the image is mundane and uniform like your snow vista. </p>

[Bill C]

Hi, the effect that Alan describes would actually darken the outer edges of the image, so I don't think this is your explanation.

 

If you are using minimal agitation when you develop, this is the sort of result I'd expect - full development near the edges of the film, but

falling off toward the center. The heavy exposure due to the white snow would aggravate things. But I don't have any explanation for why it

doesn't happen on clear days.

[James G. Dainis]

I assume the negative was fixed to completion i.e. there is not a milky looking area in the center of the film. Does that cloudy area run through the clear space between frames?

[steven_clark]

<p>That looks a little like some light leaking to me.</p>

[bethe_fisher]

<p>If the lighter edges are the edges of the film, then the agitation was not quite right. If you use a tank with a bar to swirl the reel, do not use it. Inverting the tank will give better agitation. Yes, BTDT. Basically, the edges are more developed (darker on the neg) than the middle because they got more fresh developer from the agitation than the middle. </p>

[dave_s]

<p>Right on, what Bill and Bethe said. This pretty much has to be development. <br>

(i) It's too uniformly gradated to be a light leak.<br>

(ii) An arthritic focal plane shutter might cause uneven exposure, but your camera has leaf shutter lenses.</p>

[larrydressler]

<p> Refix and see. </p>

[randy_cooprider1]

<p>The lighter area looks to me if it is fairly uniform from top to bottom near the edges of the negative. A roll film developer should have no idea where the edges are to cause this regular exposure difference. Since your camera has a leaf shutter it also would not do this. Are the internal surfaces of the camera body (vertical sides) uniformly blackened? As one poster suggested overexposing will minimize the effect.</p>

[Aoresteen]

<p>As others have said - agitation & fixer. Do as Larry says - refix with FRESH fixer, hypo clear, and wash.</p>

[andre_lefort1]

<p>Thanks guys,<br>

I'll pay extra attention to the mentioned sources of the problem.</p>

[dave_s]

<p><em>>>A roll film developer should have no idea where the edges are to cause this regular exposure difference.</em></p>

<p>Au contraire. Like most 6 x 6 cameras, the SQA has vertical film transport. So the sides of the frame are up against the flat sides of the reel.</p>

[randy_cooprider1]

<p>Dave, the last roll film camera I used was a Koni Omega Rapid in which the film goes from side to side, sorry for the memory lapse and thanks for correcting me.</p>

[dave_s]

<p>Never apologize for anything! I was thinking the same thing as you at first, trying to figure how the developer knew it was the side of the frame. (I have a Mamiya RZ, you see.)</p>