close up lenses

[edawn]

I just got a 50mm 1.7 for my Minolta 5D, and I inherited a set of close up lenses (+1, +2, +4) that fit on it. I've been messing around with them a little bit, but I was wondering if there were some basic "rules" about close up lenses? I mean, what exactly do the various magnification numbers refer to? It looks like they provide a very shallow depth of focus. Can anyone help me understand how to get the best out of these?

Thanks!

Elaine

[Mr. Pickles]

Closeup lenses are a quick and cheap way to get "closer" to whatever you want to shoot. It is based on a "kinda crazy" theory, but it goes something like this...

When your camera lens is focused at infinity, putting a No. 1 close-up lens lets you focus one meter away, no matter what the focal length of the camera lens. Using a No. 4 lets you focus 1/4 meter away, and a No. 1 plus a No. 4 equal a No. 5 and let you focus 1/5 meter away from the subject.

Hows that

[JohnC]

The numbers are diopters (like an eyeglass prescription or an eyepiece adjustment).

The limited depth of field is due to the magnification. You would get the same thing using extension tubes and/or a macro lens.

The idea with macro photography is that you set the lens focus distance to give you the magnification you want and then you focus by moving the entire rig (camera and lens) back and forth (set the camera to manual focus and don't turn the focus ring on the lens).

John

[cameraguy21773]

Whatever you decide, stick with multi-element diopters. They provide the best the image quality. I don't know any third party makers with 2-element design, so you might be limited to Canon and Nikon. Nikon's diopters are only available in sizes up to 62mm I believe. Canon make a number of larger sizes. Both are quite good.

[dougjgreen]

Generally speaking, those close-up diopter lenses will produce quite good sharpness in the center of the image, and much less so on the edges. As a result, they will usually be quite good for shooting things like flowers, but much less satisfactory for things like flat-field copy work. For something like flat-field work, I'd strongly recommend a dedicated macro lens, but, for casual use with things like flowers and insects, the add-on diopter lenses should be satisfactory.

[edawn]

Thank you, ALL, for your input! I hope to be getting some practice with these soon. I think I have a better grasp on how to make the most of them.

Thanks!

Elaine

[scoundrel1728]

The numbers are the focal length of the lenses, in reciprocal meters. That is, your +1 diopter lens has a focal length of 1/1 meter; your +2 lens, 1/2 meter, and your +4 lens, 1/4 meter. As long as the lenses are mounted close together, the effect of a lens combination is about the same as a single lens with the same total diopter rating.

If you change the camera's focusing distance to a diopter reading also, you can approximate the focusing range of your lenses your lenses will have with the close-up lenses. A lens that focuses down to an object distance of 1/2 meter from the lens' nodal point (+2 diopters), for example, will focus down to 0 + 2 and 2 + 2 diopters (1/2 to 1/4 meter) when the main lens is focused between infinity (zero diopters) and 1/2 meter (2 diopters).

Before you do the conversion to diopters, you will need to make a correction to your lens' focusing distance because the convention for 35 mm camera focusing distances is generally distance from the subject to the film plane. Some other cameras, like my Coolpix 995, use the convention from the subject to the front of the lens mount instead. For these calculations with close-up lenses, you need to use the distance from the subject to the lens' object nodal point. Except for wide-angle lenses, which you wouldn't usually use for close-up shooting anyway, or very long lenses, subtracting the focal length of the lens from the distance between film plane and subject will usually get you within a couple centimeters of the correct distance for conversion. This still leaves no correction for the fact that the nodal point of the lens is often several centimeters from the stack of close-up lenses, as well as the fact that the object and image nodal point are not the same in a complicated lens, so such corrections are still only approximations.

The optical rules for using bellows or extension rings are also a bit different than for using close-up lenses. At normal focusing distances, i.e., distances that are very long compared to the lens' focal length, the lens' image distance can adequately be approximated by using the focal length of the lens. However, when focusing especially close, this approximation starts breaking down. For proper calculations for magnification and f/stop, this change needs to be taken into account. In practice, the dividing point occurs about at the point when the object-to-nodal point distance gets down to about 8 times the focal length of the lens. At this distance, the magnification error becomes about 12 percent and the exposure error becomes about 1/3 stop when ignoring this effect in your calculations. This effect does not occur when using close-up lenses instead of extension rings or bellows because the lens-to-image plane distance does not change much. Instead, the close-up lenses effectively change the focal length of the lens system just enough to compensate and keep the lens-to-image distance the same.

A couple of tips to remember about using close-up lenses: the inexpensive kind are simple lenses subject to the same kinds of aberrations as any other simple lens. The fancier ones, such as the Canon 250D (+4 diopter) and Canon 500D (+2 diopter) but not the Canon 500 have a more complicated lens design that compensates for some of these aberrations. The effect of many of these aberrations can be lessened by shooting with small apertures, which you may want to do anyway to get adequate depth of field. Stronger lenses will tend to interfere more with image quality than weaker ones of similar design and stacking several lenses will usually give poorer image quality than using a single strong lens. If you must stack more than one close-up lens, put the strongest one nearest the primary lens.