[leadbelly]

I'm using a Vivitar 28mm/2.8 lens as my new walkaround. It is capable of close focus up to 1:5. What close-up diopter should I use to make it 1:1? I have a +2, but it doesn't cut it. I usually just hold the filter over the lens to take a quick macro shot. Thanks!

[stewart_photo]

Most simply purchase a kit containing the most commonly used close-up/diopter lenses (typically +1, +2,and +4). This option is not all that much more expensive than a single lens alone and allows one to select from a range of magnification, perhaps even the 1:1 you're seeking. The most popular brands are Tiffen and Hoya, but many others are certainly available.

By the way, since they are optical devices, it is probably better to actually mount the lens rather than just holding it in place. However, if that system works for you, no big deal.

stewart

[Lowell Goudge]

to answer the OP question, the issue is that a 28mm lens is the wrong one to try to go 1:1

all 1:1 means is that you have the subject to lens distance equal to the lens to film plane.

this means that you will need to be very very close to the subject, and have an impractical working distance.

you are much better to use a longer lens.

In order to evaluate the impact of a close up lens, you do the following.

with a close up lens attachged, you effectively change the focal length of your prime lens (i.e. you shorten it.

the effective focal length of the combination is

New focal length = (FL_old x FL_close_up) / (FL_old + FL_close_up)

FL_Close_up = 1 / diopter

Maximum Working distance becomes the focal length of the close up lens, since a 1 diopter lens has a focal length of 1000 mm (1 meter) the maximum working distance with a 1 dipoter lens is 1 meter.

so if you consider a 28 mm lens with a +1 diopter close up lens, at 1 meter distance you will have a magnification ration of 28/1000 or 1:40, but the same close up lens on a 100MM lens, at 1 meter gives you 1:10.

If you add a 5 diopter close up, your maximum focus distance with your 28mm lens will be 200mm therefore the magnification ratio will be 28/200 or about 1:7, but on the 100MM lens a +5 diopter at maximum distance will give you 1:2. (this is getting close to the 1:1 you want, and youo now are working at 1 meter. you will have some ability to come inside this distance as a function of how close you rlens focuses.

To get to 1:1 on your 28mm you would need about a 35 diopter close up lens. (they simply don't exist.)

Minimum working distance is lens dependant upon the minimum focusing distance of the lens you have.