Exposure Factor Needed with Diopter Closeup Lens?

When using a supplementary closeup lens on a prime lens do I need to make any ex posure adjustment? For example, a +2 diopter on a 150mm lens (focused at infini ty,) gives .3x magnification. If I gain that magnification via extension, I kno w it will require about 2/3 stop more exposure. Do I also allow the 2/3 stop wi th the diopter and then add any extension compensation, or is it really "free" magnification? Just wondering...with 35mm I've always metered TTL, and never needed to worry too much about this.

Alan, clear diopters do not cause any light loss in achieving their magnification [I'm not an authority on them, so I consulted "Closeups in Nature", by John Shaw (my guru in all things macro), and he agrees]. Supplemental diopters do have some disadvantages, but I'm sure you are aware of them already.

Diopters basically make your lens a shorter lens. So, your 150mm lens with a diopter screwed in is a more complex lens (it has an additional air-spaced element) - it has a shorter focal length and is faster. Thus the extension you have coupled with a shorter focal length lens will allow you to focus closer i.e., achieve greater magnification. So, in that sense, you are right - the magnification is free and does not cost you any light.

Diopters are useful in 35mm and MF because the lenses themselves are designed to focus only this close - if you want to focus closer than that, you will need to either get some extension by tinkering around with extension rings, bellows etc or use diopters to shorten the focal length. With large format, if you have the bellows, I would suggest not using diopters.

Diopters complicate the lens and upset the corrections designed into the lens. They provide two additional air-glass surfaces with a resultant increase in flare. Most diopters tend to be cheap lenses without the benefits of careful corrections. So, if you can get to the magnification you want with just extending the bellows, I would suggest sticking to that. But diopters can be fun to play around with. And in conditions where speed is important, they may be the only viable alternative.

Cheers, DJ

Try two-element high quality diopters from Canon (250D) or Nikon. They are more expensive, but so much better than single element cheapos from Tiffen.