Explain how to tell the difference between surge marks and bromide drag on negatives and what cause is. Also, explain to common solution to the problem for both.
I don't think that's a complete explanation. I have had surge marks around the perforations a few times, and I was doing nothing out of the ordinary in the way of agitation: a single simple inversion, holding the tank by the top, followed by a tap on the table as I put the tank down. It's what I always do, and I don't usually get these marks.Surge marks are caused by excessive agitation,
@Dustin McAmera You're right. Thinking from the first principles, the 35mm film perforations should always be adding some disturbance to developer flow. Agitation speed is just an amplifier.
In fact, I think that all 35mm negatives have surge marks on them. It's the degree of their severity that varies.
For example, if you take an extremely thin negative with a scene of a smooth and solid surface, scan it, and then crank up contrast to absurdly high levels, there will almost always be, no matter how faint, the same pattern. What I am saying is that the flow of liquids is never perfect, and every negative has some imprint of those flows, but sometimes they become more visible due to other factors: scene composition, over-agitation, under-exposure, perhaps film age, or al of the above.
@MattKing Yep, but I clearly remember reading somewhere that it's impossible to achieve a perfect chemistry flow in a small tank. Expose an entire 35mm roll shooting a grey card, develop and sample its edge with a densitometer. I bet that no matter what you do you'll see density variations consistent with the perforations placement. They will be small, but they should be there.
In general, proper agitation is most important for the initial stages of development. That is the time that the gelatin is swelling and development is just beginning. Unevenness in the distribution of fresh developer at this time usually causes rather severe effects.
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