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What are the ins and outs of using a diopter close up lens on your large format camera? Do you need to compensate for exposure. I imagine it will cut down on bellows extension. I still have some from my 35mm days, though when I used them it was with through the lens metering, so am unsure with external metering.
Theories, ideas or anyone used one?
hi leonard --
i've been using various things as diopters/close up lenses on my graflex slr.
i am compensating ... mainly because i have reduced my aperture, not because i am using a close up lens ...
( the last 2 images i posted in image thread are using a diopter or lens element &C infront of my lens ... )
---- john
Hi Leonhard,
when the unsymmetrical lenses came out (decades ago) they lost the opportunity to get a longer focal length by unscrewing the front half of the lens. At that time attechemnt lenses showed up to get a longer or shorter focal lenght. I tried such a Focar on a Voigtländer Skopar and found the performance not so good. As close-up-lenses these attechments were common an 35mm cameras and quality was accepeble when stopped down.
You do not need to compensate exposure with most diopter "filters". Technically, you should recaculate the aperture for the new effective focal length, but since you should calculate by actual lens-to-film distance anyway, using the diopter to focus closer at the same bellows extension really doesn't require any compensation. Just measure your bellows extension and calculate bellows factor as you normally would, and you're good to go.
Donald,
I use one of those Camulet exposure calculators (much like the exposure disk) where you place the disc on the scene and measure the size on the ground glass to get the exposure compensation. Would that be roughly the same result too, or would the extra enlargement correct the errors?
Thanks for replies,
Len
Interesting question, Len. My first thought would be that the Calumet/disc approach might over-compensate, as the measurement ratio would be based on the conventional relationship between bellows extension and magnification. With the "close-up" lens, you'll have less bellows extension than you would otherwise, and thus require less exposure compensation. But, I've been wrong before. ;)Quote:
Originally Posted by Leonard Metcalf
Increasing magnification given extension by using a diopter lens to reduce focal length requires no exposure compensation. None. That's the good thing about using 'em. The bad thing about diopters is that most introduce substantial field curvature and on many lenses give mediocre image quality.
I've never tried 'em, but the 35 mm guys say the two element achromatic diopters that Canon and Nikon sell are much better than the cheap one element ones.
By all means try yours out.
To use that device, you'd need to calculate the corrected lens focal length and use the correct object magnification table/formula (if you have a 250 mm lens with +2 diopter attached, you have in effect a somewhat faster 165 mm lens); you'd also need to use a recalculated aperture scale in this case -- lots of complication for what ought to be a simple operation. Otherwise, you'll overexpose, because you'll measure the object as larger at a given extension and apply correction for the wrong extension.Quote:
Originally Posted by Leonard Metcalf
If, OTOH, you measure from the film plane to lens node, you'll always get the *effective* focal length of the lens and the same compensation applies regardless of diopters use to bring the object focal plane closer to the lens (well, technically, there's a small change because the aperture magnification factor changes -- the lens gets a tiny bit faster at any aperture setting, assuming the diopter itself doesn't restrict the pupil, because the aperture is further magnified by the diopter lens -- but this change is generally very small unless you're using a long lens with a very strong diopter).
I have used a +1 dioptre close-up lens screwed on the rear thread of my Fujinon-W 300mm f5.6, the standard lens for my Tachihara 810HD.
The reason for using the lens was not to do close-ups but rather to get a wider angle of view for infinity subjects. The 300mm lens becomes effectively a 230mm lens with still enough (barely) coverage to fill the 8x10 format. There is also a lens speed gain of almost one stop, again only for infinity subjects. This has got to be the cheapest wide angle converter lens in large format photography!
The downside is loss of corner sharpness and a trace of curvilinear distortion but this is eased by a f.64 ( f.90 on the lens scale) stop-down and by selecting subject matter without prominent straight lines eg forest landscape.