SFX exposure question (again, sorry...)

sineadw

So I'm hoping to head out this weekend with the SFX. I have two filters - a red 25 and an IR 680nm. I've been reading about exposure compensation and I'm confused. Again. Most places seem to suggest overexposing by about a half a stop with the red, and more so for the IR, but I think the resulting images look totally washed out. I found this comparison post, and although the filters he's using are darker I'm loving the exposure he's getting with -1 1/3 Exp. on the Hoya R72. I've tried to find some exact info on how many stops 720 loses as opposed to 680, but I'm encountering NIR Transmission Spectra (eep..). Ilford suggests an overexposure of 1.5 stops on red, and three stops on deep red (I'm assuming that's what I'm hitting with the 680 as they class the R72 as 'very deep red').

Bearing in mind that my film camera is totally manual, and the light meter has a green dot for correct exp, and a red plus and minus for over and under (although they flicker a bit when it's borderline) should I just wing it? Bracket? Suggestions would be very appreciated.

DaireQuinlan

sineadw wrote: »

So I'm hoping to head out this weekend with the SFX. I have two filters - a red 25 and an IR 680nm. I've been reading about exposure compensation and I'm confused. Again. Most places seem to suggest overexposing by about a half a stop with the red, and more so for the IR, but I think the resulting images look totally washed out. I found this comparison post, and although the filters he's using are darker I'm loving the exposure he's getting with -1 1/3 Exp. on the Hoya R72. I've tried to find some exact info on how many stops 720 loses as opposed to 680, but I'm encountering NIR Transmission Spectra (eep..). Ilford suggests an overexposure of 1.5 stops on red, and three stops on deep red (I'm assuming that's what I'm hitting with the 680 as they class the R72 as 'very deep red').

Bearing in mind that my film camera is totally manual, and the light meter has a green dot for correct exp, and a red plus and minus for over and under (although they flicker a bit when it's borderline) should I just wing it? Bracket? Suggestions would be very appreciated.

Are you metering through the filter ? And THEN applying the filter compensation factor ? That compensation factor is generally meant to be applied to an unfiltered meter value. So yoi measure (say) 1/250 at f/5.6 using an external meter or TTL without the filter in place, then you shoot at 1/90 (or whatever +1.5 stops is, it's early !) with the filter applied.
In general, using the 25a I'd just rely on the TTL metering on the camera, and shoot at asa200, using the SFX filter with sfx I just go with ilfords recpmended exposure times and work around them, I don't meter. I wouldn't trust the TTL meter with an IR filter on the lens.

sineadw

See that's where I'm getting confused - a lot of the sites I'm coming across are suggesting massive overexposures when metering ttl. They're all digital sites, although I can't for the life of me figure out why that'd make a difference. I was gonna forget about the ttl on camera (as it's pretty ****e anyway) and use the little russian meter that I have. So you go over by stops rather than under, as suggested by Ilford Daire? And you don't have an issue with your shadows being very washed out?

I was gonna stick a CPL on top too, to help blacken the sky. Or maybe just to confuse myself more

Edit: now that caffeine has had a chance to hit my brain, I'm thinking the site I quoted above where the guy was underexposing by 1.5 stops was probably metering through the lens with the r72. So it would actually be an overexposure of 2.5 stops if he was metering externally? So I should overexpose by about a stop and a half to get the same effect with the 680nm.

I think a little bit of my brain just gave up and died there...

DaireQuinlan

Ah right. Maybe fenster can chime in here as I'm not really up on digital IR, but it's a different ballgame. Digital sensors actually have a filter attached to block IR, so I'd imagine that even metering through the IR filter you'd want to add a whole whack load of stops. But as I said I'm not really familiar with that.

What I'd do if you want to meter, is rate at box speed, meter externally, and add the filter factor for the filter you're using so if the filter factor is 3 and the metered exposure is 1/200 then shoot at 1/50 for example. And bracket bracket bracket. I'd bracket a stop in either direction. The thing about IR is that you can't actually really tell how much of it there is around. Apparently there's more IR later in the afternoon for example.

If you don't want to meter then grab the SFX datasheets from the ilford site and use their suggested EV, compensating as neccessary (and still bracket )

sineadw

Actually, that makes some sort of weird sense - maybe it is an issue of totally different f stop differences for digital over film. Right. Grand. I feel slightly saner now.

DaireQuinlan

http://www.ilfordphoto.com/download.asp?n=713&f=2007651134552223.pdf

"As an example, the correct exposure in bright sunlight with the
ILFORD SFX filter is around 1/30 second at f5.6."

That's normally what I use if I want to handhold, 24mm lens and shoot at the above 1/30 at f/5.6. If you let your eyes adjust you can very very dimly see through the SFX filter, enough to compose anyway. I focus by distance and hope the f/5.6 will cover up any errors :-)

I have a roll of 120 sitting in the fridge aswell, I must try it in my YashicaMat. For obvious reasons shooting IR film in rangefinders or TLRs is probably considerably easier than shooting it in SLRs

sineadw

I just hope the sun stays around. It's starting to grey out over here

thefizz

If you are looking for an infrared effect, forget the #25 red filter. The 680nm filter may not even be dark enough, as the R72 (695nm) filter is usually recommended but I'd give it a try anyway.

Just meter the scene without the filter and then add 4 stops and bracket around that.

Peter