EFKE R25 Spectral Sensitivity & Red Filter
OK, if you look at the technical data for EFKE R25 film at:
My question is, what effect will a #25 red filter have with this film? A red filter will pass it's own wavelength, which this film is not very sensitive to and it will block the lower blue wavelengths. If you look at the spectral sensitivity chart for this film, this leaves just about the green wavelengths with some red and whatever blue isn't filtered out.
How would this look in landscapes in the sierras, for instance on a sunny day? Black skies, light evergreen trees, lots of contrast? Apparently only light within 500 - 600 nm will be hitting the film.
Would a #12 yellow filter be a better choice so as not to give pitch black skies?
What about close-ups of rocks and trees? I have read that shadows consists mostly of blue light, so a red filter would make the shadows thinner in the negative - no matter what the exposure, correct? After all if the red filter is removing the blue wavelengths, it doesn't matter how long one leaves the shutter open, the light from the shadows will not hit the film.
EFKE PL 100 is much more red-sensitive so light from 500 - almost 700nm will be hitting the film with a Red #25 filter on so more light will be striking the film overall - green and well into the red.
Is this correct or do I have this all wrong?
I am going out tomorrow photographing in the sierras concentrating on granite rock and I was wondering what EFKE 25 with a #25 red filter would produce if the full frame is just a well illuminated piece of granite rock. Will it enhance texture strongly or will the negative be too contrasty?
I don't have the data sheets in front of me, but #25 is a sharp cut red filter, so it should only be letting 600-700 through. Used with Efke 25 you're looking at some VERY long exposures, as you'd only be using the 600-630 band where the film isn't very sensitive in the first place.
A Yellow filter will (usually) let 500-700 through, so then you'd be basically looking at the greens. Given that Efke 25 is Orthochromatic you'd get the same effect as a green filter. An Orange would be 550-700.
I've been working on software to simulate some of this - if you'd asked in a few days I could give you some plots showing exactly the resulting spectrum.
On a related note, Ilford's spectral data is a little unclear - The data sheets show "sensitivity", but say that it's in response to 2850K light. Do the ilford sheets show sensitivity (in which case you mulitply by light to get response) or response to 2850K (in which case you'd divide by light to get the spectral sensitivity)?
I know this: I've found Efke 25 not to be a great film for landscapes with rock. Because it is orthopanchromatic, the attenuated red response made rocky landscapes kind of flat to my eye. In general, I just never got the hang of this film. YMMV.
I've never tried Efke 25 with a red filter. My instinct is that you'll be disappointed: you're filtering for a frequency band to which the film is less sensitive. I dunno. Try it and let us know. If it were me, I'd shoot some other type of film as well.
"If You Push Something Hard Enough, It Will fall over" - Fudd's First Law of Opposition
Try a #15 deep yellow filter. It renders beautiful skies with Efke orthopanchromatic emulsions. If you have any foliage in the image, local contrast there will be enhanced.
Just started using EFKE 25 & have tried both yellow & orange filters. The orange gives a high-contrast, almost surreal appearance (see image in my gallery), whereas yellow appears more natural - at least for driftwood, no rocks in Florida.
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Watch out for shadow values when using a strong yellow filter. It will push them down to the point where you may begin to drop into zone 1. Yellow is still the first choice with Efke 25 for skies and clouds. Without the yellow filter, skies can end up very bland and without substance.
In my initial testing of efke 25, I shot through #29,(Dark Red), #25,(Red) #22, (Red Orange). Given the spectral response, none produced desirable results, (unless one is looking for a black bear at midnight effect! ). I found #23, (yellow), produced similar results as a #25 on pan film.
I mostly use #21, (light yellow), adds some density to the sky, contrast on clouds, builds contrast between foliage and rock.
I love this film, but the spectral response requires some close attention.
Here is a scan of a negative which is EFKE 25 taken of a rock face with NO filter. There seems to be plenty of detail and texture in the rock. Possibly shooting without any filter would be a better idea.
Iagree, mike. I recently posted a scan of a medium format Efke 25 negative taken with no filter that shows a lot of detail and texture in the rock.
Everything is analog - even digital :D
OK - I'm home now, with data sheets, and I've run a couple of simulations!
Here's the response of efke 25, multiplied by a #25 filter.
Thats not good. The simulation doesn't take into account the lighting, or the subject (I don't have data for those), but if you measure the area of these curves, we've reduced the sensitivity by a factor of 12.
By comparason here's the same filter on hp5:
Here the sensitivity is resuced by about 6 - hp5's red resonse isn't that great, but it's still needs only half compensation that the efke.
As sugguested by others I tried a #15 yellow:
Now senisitivity is only reduced by a factor of 2.4 - pretty manable.
Of course of theres a lot of light in JUST the right bit of the spectrum, then these factors will change dramatically.
The Filter data I used for these is pretty accurate (and easy to obtain), but film data is much more rough - I had to measure the curves from the Efke and Illford data sheets. If anyone has numeric data, they can share that would be really usefull to me.