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Photo Forum / Film Photography / Darkroom / July 2004Change in Enlarger Head Height corresponds to Change in Exposure Time, but by how much?
Hi, I am planning to make small B&W prints until I find my desired print and than print it on a bigger scale. Is there a formula I can convert the printing time for the new scale? It sounds logical to me that if I double the height of the enlarger head I should double the exposure time. Is it this simple? What if I additionally raise the enlarger head less (say 2/3) than the previous height of the head? Regards > Hi, > I am planning to make small B&W prints until I find my desired print and [quoted text clipped - 3 lines] > this simple? What if I additionally raise the enlarger head less (say 2/3) > than the previous height of the head? Regards My experience tells me it doubles as you double the size, yet it is approximate. In other words.....going from an 8x10 enlargement exposed at 10 seconds to a 16x20 would give you an exposure time around 20-22 seconds at the same aperture. One factor you may encounter is that a larger print seems to require a little more contrast filteration than a smaller print to look the same......never the less that could be deemed subjective. If that is the case it may make the exposure a little different >>>>towards the plus side.  SignatureLF Website @ http://members.verizon.net/~gregoryblank "To announce that there must be no criticism of the President, or that we are to stand by the President, right or wrong, is not only unpatriotic and servile, but is morally treasonable to the American public."--Theodore Roosevelt, May 7, 1918 I don't have the formula handy right now but it's not quite double the size double the time. BTW it's much more direct to measure image size on the baseboard or easel than to measure the height of the enlarger head for such changes. I have an old slide rule that GraLab used to sell that calculates such things easily but it's probably easier still to get an Ilford EM-10 ($20 US) and calculate exposures. Since what you meter affects your results I always pull my carrier and measure the intensity of the raw light on the baseboard and "null" the meter using the aperture ring exposure time is then constant. Signaturedarkroommike ---------- > Hi, > I am planning to make small B&W prints until I find my desired print and [quoted text clipped - 3 lines] > this simple? What if I additionally raise the enlarger head less (say 2/3) > than the previous height of the head? Regards > I don't have the formula handy right now but it's not quite double the size > double the time. BTW it's much more direct to measure image size on the > baseboard or easel than to measure the height of the enlarger head for such > changes. "...not quite double..." The lens becomes faster as enlargement size increases. I think the area method one I'd go with. Dan > "...not quite double..." The lens becomes faster as enlargement size > increases. I think the area method one I'd go with. Dan I think the opposite is true, the lens becomes slower as enlargement size increases. Guillermo > > "...not quite double..." The lens becomes faster as enlargement size > > increases. I think the area method one I'd go with. Dan [quoted text clipped - 3 lines] > > Guillermo I think your both wrong,....b ut - Tu' es mas correctamundo ;-). The slowing is more likely reciprocity,......or the need for a greater degree of filteration. SignatureLF Website @ http://members.verizon.net/~gregoryblank "To announce that there must be no criticism of the President, or that we are to stand by the President, right or wrong, is not only unpatriotic and servile, but is morally treasonable to the American public."--Theodore Roosevelt, May 7, 1918 > I think your both wrong,....b ut - Tu' es mas correctamundo ;-). The slowing is more > likely reciprocity,......or the need for a greater degree of filteration. Gregorio, me think tu no es correctamundo :-) Reciprocity does not affect the speed of a lens (where speed is expressed in f/stops), reciprocity does affect how long you have to expose the emulsion, though. The lens "slows" down as the enlargement increases because the effective focal length of the lens increases directly proportional with the enlargement/magnification (actually proportional to the infinity focal length of the lens multiplied by the sum of the magnification plus 1), so if you increase the enlargement/magnification you then have a larger distance lens to image (baseboard) but you still have the same lens diaphragm aperture diameter, consequently, the numeric value of the lens' effective f/stop would be greater (f/stop = distance lens to baseboard divided by the diameter of the aperture, assuming a thin lens, for the sake of simplicity), the greater the numeric value of the f/stop the slower a lens is. The lens "slows" down as the enlargement increases and that may or may not cause the exposure time fall into the realm of reciprocity. The above is not different than what happens when you use bellows extensions greater than the focal length of the lens. Comprende compadre Gregorio? Guillermo Guillermo/William :-) > Gregorio, me think tu no es correctamundo :-) Posible' mi amigo. > Reciprocity does not affect the speed of a lens (where speed is expressed in > f/stops), reciprocity does affect how long you have to expose the emulsion, > though. My point,.........which is why I stated it as such. (Maybe not too clearly however). > The lens "slows" down as the enlargement increases because the effective > focal length of the lens increases directly proportional with the [quoted text clipped - 10 lines] > different than what happens when you use bellows extensions greater than the > focal length of the lens. Comprende compadre Gregorio? Si Yo comprendo,.....lo es un funcion del fuelle elemento. As well as perhaps running into possible paper reciprocity (to a small degree). Syntax Ok? :-) SignatureLF Website @ http://members.verizon.net/~gregoryblank "To announce that there must be no criticism of the President, or that we are to stand by the President, right or wrong, is not only unpatriotic and servile, but is morally treasonable to the American public."--Theodore Roosevelt, May 7, 1918 Inverse square law applies here. 2x bigger requires 4x exposure. 1.4x bigger requires 2x exposure. Etc. Ge ahold of a Kodak Darkroom Dataguide, it has a computer wheel in it that calculates this for any change of magnification. -- > Hi, > I am planning to make small B&W prints until I find my desired print and [quoted text clipped - 3 lines] > this simple? What if I additionally raise the enlarger head less (say 2/3) > than the previous height of the head? Regards Roughly, light intensity decreases in proportion to the square of the distance. So, doubling the height quadruples the exposure. There are (1) formulas which you will find in the archives of this newsgroup (see Google groups). Also, Kodak publishes a Darkroom Guide that has an exposure wheel that enables you to quickly ascertain the new time without computation. Francis A. Miniter >Hi, >I am planning to make small B&W prints until I find my desired print and [quoted text clipped - 3 lines] >this simple? What if I additionally raise the enlarger head less (say 2/3) >than the previous height of the head? Regards Try this.... http://www.chibardun.net/~craigclu/enlargecalc.xls I did this some time back and believe it is accurate. >Hi, >I am planning to make small B&W prints until I find my desired print and [quoted text clipped - 3 lines] >this simple? What if I additionally raise the enlarger head less (say 2/3) >than the previous height of the head? Regards Craig Schroeder craig nospam craigschroeder com -Eschew Obfuscation- > Try this.... > > http://www.chibardun.net/~craigclu/enlargecalc.xls > > I did this some time back and believe it is accurate. Assuming that what you call "original and new positions" are the distances from lens stage to baseboard, I believe the formula in cell "B7" is not quite correct, I think it should be just B6*((B4)/(B3))^2 But then again, I've been found confused and incorrect a number of times!!. Care to explain how formula in cell "B7" came about? If you were dealing with magnifications instead of distances to baseboard, then, to find the exposure time factor, you would add 1 to each magnification before dividing them and squaring them: exposure factor = [(M+1)/(m+1)]^2 Guillermo >> Try this.... >> [quoted text clipped - 7 lines] >again, I've been found confused and incorrect a number of times!!. Care to >explain how formula in cell "B7" came about? I don't recall.... It was awhile back and about that time I began using a whole different meter/timer combo that worked off of maximum black and never got back to it. Sorry if I sent anyone down the wrong trail! It may have been a foggy, late night project that adapted your referenced formula below into the wrong interpretation...? >If you were dealing with magnifications instead of distances to baseboard, >then, to find the exposure time factor, you would add 1 to each >magnification before dividing them and squaring them: exposure factor = >[(M+1)/(m+1)]^2 > >Guillermo Craig Schroeder craig nospam craigschroeder com -Eschew Obfuscation- > Hi, > I am planning to make small B&W prints until I find my desired print and [quoted text clipped - 3 lines] > this simple? What if I additionally raise the enlarger head less (say 2/3) > than the previous height of the head? Regards If you've a computer handy, you can check my software, version 3 is pretty stable now: http://www.bonavolta.ch/hobby/en/photo/labsoftV3.htm Although, it drives directly the enlarger, you don't need absolutely the hardware and can use it just for the calculations. So, if electronics makes you sick, just go down to software and download the english version. Documentation is still poor and will follow later. Check also the wet side that manages the processes: http://www.bonavolta.ch/hobby/en/photo/labsoft.htm This page has also some Excel spreadsheets you can print. Regards, SignatureClaudio Bonavolta http://www.bonavolta.ch > Hi, > I am planning to make small B&W prints until I find my desired print and [quoted text clipped - 3 lines] > this simple? What if I additionally raise the enlarger head less (say 2/3) > than the previous height of the head? Regards Divide the area of the larger print by the smaller, then use that factor. 8x10 = 80 sq in. 11x14 = 154 sq in. Going from 8x10 to 11x14 would require roughly double the exposure. > Hi, > I am planning to make small B&W prints until I find my desired print and [quoted text clipped - 3 lines] > this simple? What if I additionally raise the enlarger head less (say 2/3) > than the previous height of the head? Regards Doubling the enlarger height will double the print dimensions, spreading your light over four times the area -- so you'll need to increase exposure by two stops. You can do that by opening the lens two stops, if you were already stopped down beyond optimal to get a longer exposure, but more likely you'll have to add time -- which means you'll also have to account for reciprocity failure, and will need somewhat more than four times the exposure time at the same aperture. How much more than 4x? Depends on the paper; it's been twenty-five years since I've done much printing, and in those days it would have been about double the 4x figure. Modern papers may have less effect -- but bottom line is you'll have to test the paper you're using, but once you have a ratio between (say) a 4x6 print and an 8x12, it should remain pretty constant (though a denser negative may require still more additional time on the larger print, again because of reciprocity failure). SignatureI may be a scwewy wabbit, but I'm not going to Alcatwaz! -- E. J. Fudd, 1954 Donald Qualls, aka The Silent Observer Lathe Building Pages http://silent1.home.netcom.com/HomebuiltLathe.htm Speedway 7x12 Lathe Pages http://silent1.home.netcom.com/my7x12.htm Opinions expressed are my own -- take them for what they're worth and don't expect them to be perfect. Even if there is no reciprocity effect, the inverse square law is not correct, although it works tolerably well for small adjustments. For major changes in enlargement size, do a search here for: gudzinowicz enlarger height factor and you will find the correct equation. Reciprocity is probably ignorable if the specs of the paper manufacturers are to be belived. > Even if there is no reciprocity effect, the inverse square law is not > correct, although it works tolerably well for small adjustments. For [quoted text clipped - 3 lines] > > and you will find the correct equation. I don't see why inverse square law is not correct. M.Gudzinowicz writes: new_time = old_time x (new_M +1)^2 / (old_M+1)^2 So if M = new magnification and m = old magnification, the exposure time factor would be: Factor = (M + 1)^2 / (m + 1)^2 Let's assume M = 4 and m = 2 Factor = 25 / 9 => Factor = 2.77 Now lets see what inverse square law says: if N = distance negative to lens, B = distance lens to baseboard, F = "thin" lens' focal length and E = magnification or enlargement E = B / N N = B / E 1 / N + 1 / B = 1 / F 1 / (B / E) + 1 / B = 1 / F (E + 1) / B = 1 / F Distance lens to baseboard = B = F ( E + 1) When Enlargement = m = 2 Distance lens to baseboard = B = F ( 2 + 1) = 3F When Enlargement = M = 4 Distance lens to baseboard = B = F ( 4 + 1) = 5F Inverse square law tell us that the exposure time factor would be: Factor = new_distance_lens_to_baseboard^2 / old_distance_lens_to_baseboard^2 Factor = (5F)^2 / (3F)^2 Factor = 25 / 9 Factor = 2.77 Which is same as calculated with M.Gudzinowicz formula above. Guillermo Guillermo wrote (edited): > I don't see why inverse square law is not correct. ........ > Factor = 2.77 Which is same as calculated with M.Gudzinowicz formula > above. > > Guillermo You are quite right. I should have been more careful with my description. I was referring to the common incorrect expression of the inverse square law that goes something like: "new size is 2 x old size, so light is reduced by a factor of 2^2 = 4, so new exposure is 4 x old exposure." Also, I had missed your previous post in this thread in which you referred to the use of (M+1)^2 / (m+1)^2 Interestingly, and unfortunately, the use of this correct formula makes it not possible to prepare a single table with exposure factors for old size and new size unless a fixed negative size is specified. I mean, going from 4"x5" print size to 8"x10" print size will require a different factor for 35mm and 120 size film because the magnification is different. A table would work if the parameters were old_magnification and new_magnification, and Kodak used to publish one. |
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