AB Sea Photo
Products and ServicesUsed EquipmentRental EquipmentQuestion and AnswersUnderwater TripsAbout Us.Return Home.
Images provided by David Doubit, Chuck  Davis, Donald Tipton, Mark Strickland, Christopher Newbert


What's the proper way to determine the correct focus when using framers on my Nikonos camera for macro photography? I cannot tell any difference between focusing my 35mm lens at infinity or at 2.75 feet when I'm using extension tubes.



Can't find what you are looking for? Please contact us so we can find the housing or accessory that you need.


Questions and Answers (Q & A's)

From Alan Broder (from Ocean Realm Magazine - January 1994)


With extension tubes as with virtually any other lens setup, there’s a plane of sharpest focus, and in addition, there’s some sort of depth of field. Without getting any deeper into optical theoretics and confusing the hell out of both of us, let’s just say that the plane of focus is sharpest and that planes falling within the depth of field are kinda sharp enough. Looking at the field as a line which is on the axis of the lens, the image will appear "kinda sharp enough" at some measurable distance in front and in back of the sharpest point of focus, and this observation will be consistent for a given magnification at a given f-stop. At higher magnifications, there will be less that will be acceptably sharp in front and in back og the point of focus. At larger apertures (lower f-stop numbers), acceptable sharpness will drop off faster on either side of the sharpest point.

At high magnification in macro photography, the depth of field becomes very short. Even when selecting f22, the smallest aperture available on a Nikonos lens, you’ll have only about three-sixteenths of an inch between the nearest and farthest point of acceptable focus when using a 1:1 extension tube.

Your observation that there seems to be no difference between setting the focus at nearest or farthest distance on the lens is probably a function of the difficulty in accurately making a test. When using a Nikonos with a 1:1 tube and framer, you’re trying to measure (estimate(guess)) where, in the roughly one-eighth-inch-plus wire of the framer, at a distance of a bent arm’s length with a subject about the size of a postage stamp. Since there is detail in the subject both in front and in back of the exact part of the thickness of the wire you think you placed at the exact point on the subject where you think you placed it, you can’t be sure if what is sharpest in the photograph is what you expected to be sharpest when you snapped the shutter and probably moved the camera ever so slightly, maybe a one-sixteenth of an inch (nearly half the entire depth of field). Let’s face it, it’s a tough test.

If you were taking the same photograph with an SLR camera with through-the-lens focusing, and you were using an extension tube behind a normal lens which had similar optical characteristics to the Nikonos lens with extension, you would see the plane of sharpest focus move in and out as you focused the lens. The rest of the subject that’s within the depth of field moves in and out with it. When you look at your subject in the framer, you see it at roughly your reading distance, and the subject area fills only a few percent of the viewing area of your retina. When you view the same composition through a reflex camera with a large finder, the same image fills almost your entire viewing area and can thus be much more precisely evaluated. The framer on your Nikonos is too gross compared to the small subject within it to be precisely focused in a controlled manner.

Once the length of the extension is determined, only on point on the axis of the lens will be in focus, and that point, along with its accompanying depth of field, is fixed. The manufacturers arbitrarily position the framer as a reference to the photographer. It apparently has been the intent of makers of extension tubes, in order not to thoroughly confuse the diving public, to agree on a reference for determining the plane of focus. This intervention has been established by conventions since the invention of extensions. Most manufacturers have agreed that the back of the framer should locate the plane of focus when the lens is set at it’s nearest focusing distance. If you set the framer, frame down, on a flat surface, that surface will be at the correct focus distance.

As the magnification is decreased from life-size 1:1, the depth of field increases proportionately, until at 1:3, f22 depth of field is just about three-quarters of an inch. The placement of the framer is more precise for selectivity and much less critical when it comes to missing the subject altogether. Framers are mounted onto tubes in various ways by different manufacturers, sometimes allowing for adjustment, either intentional or inadvertent. You may wish to find the exact plane of focus for your particular (possibly "readjusted") set of tubes with a more controlled test. You can do this in the kitchen sink, bathtub, or any other porcelain or non-porcelain container which will allow you to submerge the lens and framer and still get enough light on the test subject to get a usable exposure. This test will give you a more critical result at a larger aperture.

Place a credit card or other flat object or material with surface detail on the bottom of the container with enough water in it to cover the front of the lens when the camera rests on the framer. Place a penny on the card and a stack of two pennies at another place on the card. For 1:1 or 2:1 (twice life-size—about half the size of the 1:1), rest the framer on the card and take a photo with the lens set at closest focus and wide open. You can hand-hold your flash, keeping in mind that very little flash will be required in mostly air at the large aperture. You can do another shot at f8, then another at f11, f16, and f22 to see how the depth of field changes. Depth of field at the back of the plane of focus can’t be observed unless you shim up the framer by resting it on more credit cards, stacks of quarters—anything flat and stable. You can repeat these tests with your 1:2 and 1:3 tubes. See what happens when you change your focus to infinity while you’re at this: you’ll have plenty of frames. There’ll be more room for more penny stacks and they’ll have to be higher. I have not done these particular tests myself as the bank won’t loan me the money.



AB Sea Photo 9136 Sepulveda Blvd., Los Angeles, CA 90045 (2 minutes North of LAX)
Phone: (310)645-8992 Fax (310)645-3645 | Email: info@absea.net Web: http://www.absea.net

Copyright © 2003, AB Sea Photo. All RIGHTS RESERVED.
All trademarks mentioned herein belong to their respective owners.