Does changing the aperture on my enlarger make any difference to the final print? If not, why is this available? Is it just about making workflow quicker / dodging and burning easier? Sorry if this si a daft question! jon
No such thing as a daft (stupid) question. The purpose is to allow for more 'wiggle room' in the actual print process. There are literally hundreds if not thousands of books written on how to use an enlarger, and it would be best to look on Amazon.com for a few ideas. However, like having two safety valves on a water line, having a separate aperture control on an enlarger helps maintain control over the print quality. There are literally thousands of points on this, and reading on it all through a few books is recommended.
1. Remember good ol' depth of field? Well, if you focus with the enlarger lens wide open and then stop down a couple of clicks, you've increased the depth of field and nicely corrected for any inaccuracy in focussing. 2. Stopping down increases the total required exposure time. This does two things: A. Ensures that at low degrees of enlargement, the exposure time will not become unmanageably short, and B. Enables you to have ample time for dodging, etc. With time, you'll probably find that you're most comfortable with exposure times of 15 - 30 seconds and you'll stop down as required to achieve them. Speaking of degrees of enlargement -- light obeys an inverse square law. This means that if you record the correct exposure time and head height for a negative enlarged to, say, 4 x 5, you can easily calculate the exposure for any size enlargement. Just set up a way to determine the distance from the film plane in the enlarger to the easel surface and record the distance. I use an old yardstick, though a scale taped to the column will also work. Make up a set of tables with your basic enlargement size [8x10?] as 1.00 and then compute the multipliers for other film plane to easel surface distances. By the by, f-stops on an enlarger work the same as those on a camera lens. If you change from f8 to f11, double the exposure time.
Many times, the enlarger lens is at it's sharpest at the lower f-stops. It is using the sweet spot of the lens. My standard procedure is to focus wide open then stop down to f 8 and make a test strip (print) and then adjust from there.
Of course. Closing it down one stop halves the amount of light, and roughly doubles your exposure time. Opening it up a stop doubles the amount of light, and halves the exposure time. Beyond having negs that may be thin or dense, and require different amounts of exposure to get a good print, you may want to reduce or increase exposure times for other reasons. You may want lots of time for complicated burning and dodging. You may want quick exposure times because the enlarger, table, room, or building may not be completely stable, and shorter times mean less chance of enlarger shake.
Alan Marcus 50+ years of darkroom experience as a photo engineer: The enlarger lens is generally most sharp at about 2 f-stops closed down from maximum. Generally this is about f-8 or f-11 but varies slightly from lens-to-lens. It is common practice to open the lens wide for focusing and then stop down for exposure. Two reasons: 1. Wide open the image on the easel is brightest so you can see well. 2. When you stop down, optically the system gains more leeway for focus error, both depth of field (at the easel plane) and depth of focus (at the negative plane). Simply stated the optical system now has greater focusing tolerance. However: It is a fact that when you stop down, a slight shift in focus occurs. This shift is most noticeable to you when using a good grain magnifier focusing aid. Every darkroom should have one. Best technique is to compose and focus wide open, stop down to the exposing aperture and re-focus for maximum sharpness at the actual aperture you will use. This technique assures maximum sharpness when it becomes truly important. It is good technique to set the lens at the mid point for your first test exposure, let me tell you why. Most of the time we will need to reprint making an exposure adjustment. So when we adjust, we have three means. a. change time b. change lens aperture c. some of both. It is best to use lens aperture as the controlling factor. Now let me tell you why: For color paper this technique will avoid a color shift that often occurs when time is used as the controlling factor. The shift is due to a phenomenon known a reciprocity departure. This is an induced change in the sensitivity of the paper caused when exposure energy is increased or decreased. The effect is prevalent and devastating when making a time change, also present when making an aperture change but with less effect. You see, color paper has multiple emulsion layers and each is affected differently by this phenomenon thus a color change is induced. Again best to get in the habit of making exposure changes with lens aperture, when possible. Alan Marcus ammarcus@earthlink.net
Thanks everyone, confirmed what I thought, I should stop using the widest aperture all the time! I guess I need to get more patient! Welcome to the forum Alan!
