Q: In a recent column you made several glaring errors. For instance, you responded to a question about an enlarging lens with an answer directed to a camera lens. And your response, even so, was inaccurate.
There exists in photogaphy a definite distinction between sharpness and depth of field. Sharpness refers to a quality of the image as a whole, whereas depth of field is actual range (measured as a distance) of acceptable focus. They are by no means identical. An image can be very sharp and yet have very little depth of field.
Depth of sharpness, then, is a misnomer. An image is either sharp or not sharp. On enlarging lenses of the same focal length, say 50-mm, an f/3.5 lens will be sharper at f/11 than an 5.6 lens. This is because all lenses achieve their maximum sharpness at three stops closed down from wide open.
A: Thank you for eliminating some of the glare. Actually the question raised was ambiguous in that it didn't specify if the two lenses to be compared were also comparable in quality -- which may make a greater difference in resolution than how far they are stopped down.
Another concern with enlarging lenses is their flatness of field -- the edge-to-edge sharpness of the projected image, which can vary.
Although there is no such thing as depth of sharpness, there is such a thing as depth of focus.
In the interest of accuracy, let's define depth of field, depth of focus and sharpness.
Depth of field is the distance between the nearest and farthest points in the subject that are acceptably sharp on a common image plane at a given f/stop.
Depth of focus is the distance the film plane can be moved and still retain an acceptable sharp focus.
Sharpness is the definition of detail, especially around the borders or edges of the elements in the picture.
Depth of field extends approximately one-third in front and two-thirds in back of the point focused on.
Depth of focus extends equidistant in front and behind the plane of optimum focus.
Sharpness is the one plane only where the definition of the image is best.
Q: When using a 2X or 3X multiplier, how can an automatic strobe be adjusted for the exposure? We have a Vivitar 2500 flash unit and have heard that this is possible.
A: Yes, an automatic strobe can be adjusted to synch with a 2X or 3X multiplier by adjusting the auto f/stop setting to the equivalent normal lens adjustment. That is: One stop is lost with each magnification so that a 2X loses two clicks on the lens setting; if the strobe is set for f/8 with a normal lens, then it should be set at f/4 with a 2X multiplier.
But there is an easy way to convert your Vivitar 2500 for this use. Vivitar makes a macro flash sensor accessory that mounts on the barrel of the lens and plugs into the sensor of the flash unit. In this way only the area taken in by the close-up lens is metered (something like a spot meter) and thus you should get a perfect exposure every time.
The attachment is called MFSI, macro flash sensor, and its catalogue number is 0236506. The device can be ordered from Vivitar through your dealer at a recommended retail price of $39.95.
Q: I have a problem with my negatives when I want to send for prints. The numbers on the edge of the negative strip are often illegible and sometimes completely black.
I use a Keystone everflash. Could the flash have something to do with this losing of the numbers? This has caused me a lot of problems because I have to put down the right numbers when I want to order reprints.
A: The margin frame numbers are printed on the film at the time of manufacture. They are out of the picture area and should not be affected by the exposure in the camera. The framing inside the camera or the frame of the cartridge of 110 size films cuts out this margin section.