Reciprocity Timer App

I don't know the answer to that, Ian, but I suspect you are right. If the film has an extended red sensitivity, it will be faster in tungsten than daylight. Have you checked the Foma technical information for this film? It may give an indication of whether or not speed is affected by tungsten light. One other thing that occurs to me, and perhaps someone else can shed some light on this, is that I think you should routinely cut development time with long exposures such as this. That would, of course, result in reduced density in your highlights. I cannot recall where I saw that advice, but it was in relation to night photography. The reasoning, however, was connected to the longer than normal exposures. I will try to locate the book where I read that.
Alex

 

I had a look at the timer to see if I could work out why the corrected exposure would be so long. First, is it definitely 100 classic you're using, rather than Fomapan 100? There is a difference in the reciprocity calculation. The 100 needs less correction, according to the App.
The second question is whether or not the bellows extension correction should be added before, or after the reciprocity correction. It makes quite a difference. The App seems to do reciprocity first, then bellows extension. I'm not sure that's right. I had thought you should do reciprocity last, which gives a shorter time.
The biggest difference, however, seems to be between the two versions of the film.
Alex.

 

Ian, what lens were you using? I ask because I made up a chart for bellows extension factors many years ago and I'm curious how my chart + the Reciprocity chart posted here compares to what the app provided.

I agree with Alex that the reciprocity correction should be applied after the bellows extension correction. Very odd that the app seems to work it the other way 'round.

 

I had a look at a couple of books on view camera technique last night. It seems that reciprocity corrections should be applied last after bellows, filter factor etc. In the book 'Using the View Camera' by S. Simmons, there is quite detailed info about cutting development time for long exposures. It covers Kodak, Agfa and Ilford films, but not FOMA. Ilford is said to need least correction of 18% at the sort of exposure you are working with, but Agfa needs something like 48%!! You should try to get a look at the book if you can find a copy. I would also try to sort out whether your film is 'Classic' or not. The box of 4x5 FOMA 100 I have doesn't say 'Classic', so I would assume it's the other entry on the App.
Alex.


Sent from my iPhone using Tapatalk

 

I can't see the logic of applying the correction anywhere other than last; after all (pun intended) the correction is to the required exposure, which can only be determined after all the factors have been allowed for. These factors may be very significant - a Lee big stopper and a double extension on the bellows would be quite a hit on the exposure time!

So far as film development goes, the older books always stated that reciprocity failure meant both a reduced sensitivity to light, and also contrast changes. I can't recall off the top of my head which way round it was, but I think that my way of remembering was that night scenes were high contrast anyway, and reciprocity increased that contrast even more - hence a need to cut development. That would imply that very short exposures would reduce the contrast. The usual definition of very short and very long was exposures less than 1/1000th second or more than 1 second.

On the other hand, I've consulted the data sheets for FP4, HP5 and TMax films and although exposure increases are given, nothing is said about development adjustment. Unless I've missed it (and the Kodak sheet is more a small booklet of 17 pages) this can't be because the makers assume that the exposures will be processed as part of a roll with uncorrected exposures on, as Kodak cover sheet film by explicitly saying not to use TMax developer with sheet film.

 

It's just showing how sensitivity varies with wavelength, in this case, the highest sensitivity is at the red end; rather like FP4. HP5 has a more even distribution of sensitivity, and Kodak make the point explicitly that TMax is relatively less sensitive to the blue end of the spectrum than their other panchromatic films, and this means that blue comes out slightly darker, more in accord with how we see. (Our eyes are 4.6 times more sensitive to green than to red, and 76 times more
sensitive to green than to blue.)

I suppose, as the curves are specified for a particular colour temperature source, we should also be considering the spectral curve of the light, and how it differs from daylight.

 

Only indirectly; the lower the colour temperature, the yellower the light, or putting that another way less of the total light is made up of the shorter (blue) wavelengths. If a film had a (horizontal) straight line "curve", then assuming that a light meter was colour blind, the exposure metered exposure would be same for daylight and artifical light and equally correct in both cases.

If the film is more sensitive at the red end, that implies that in warmer light the exposure should be less than the meter indicates. This is why you can get two EI ratings for a film (from some manufacturers) depending on whether it's exposed in daylight or artifical light. This also assume artificial lights as we used to have - hot tungsten filiaments.

The assumption about exposure meters is just that - CdS cells are overly sensitive to red, giving rise to the advice that if you meter through a coloured filter (to save adding in a filter factor) then you should add an extra stop if the filter is red. Silicon blue cells are more even in their response. I don't actually know where selenium cells lie.

Hopefully that's clear - although havinf re-read it, I'm not sure I couldn't/shouldn't do better.

 

Ian, I found a short write-up in an old newsletter that my photo mentor used to write concerning how to calculate bellows extension; my chart is based on this simple principle:

Convert the focal length of the lens to the same number f/stop. The progression in inches of extension is the same as the progression of f/stops.

You may want to convert that to cm or mm, of course. Is it exact? No, but certainly close enough to get a good negative. I don't use much bellows extension very often in my LF work, but this is the way I've always done it.

Therefore, the lens used here is approximately a 6" lens which was extended to approximately 10 1/2". Therefore, converting your lens to the closet f/stop (5.6) and the extension to the closet f/stop (f/11), the bellows extension factor is 2 stops. Given that your lens is 5.6+ and your extension is 11-, I'd say a bellows extension of 1 2/3 stops, as stated by the app, is pretty much dead on. Per the Reciprocity chart posted here for Foma 100 (btw, I just bought a box of it and nothing anywhere says anything about 'classic'), a metered 8 secs exposure requires an adjusted 59 secs; add on 1 2/3 stops more for bellows extension and we're looking at about 157 secs adjusted exposure time; roughly 2 mins 50 secs. If we apply the bellows extension first, then go the the Reciprocity chart you get an adjusted exposure of about 218 secs; or, about 1 min longer than the prior way.

I would check my math, but even if I'm a little off my calculations are quite a bit shy of the near 6 mins you got from the app! All I can say is that there must be a world of difference between 'Classic' and not.

Bottom line...I would do a little testing under controlled conditions, write everything down, then evaluate the processed negs.

Good luck!

 

Thanks for the video, Ian.

 

Ian,

My 'reading' of the bellows extension... lens FL = 150mm. Distance from film plane to the rear nodal point of your lens = 270 mm. Therefore your extension is 'only' 120mm.... not 270mm. At least.... that is 'my reading' of the information you have provided.... The 120mm extension means that you are not even at the 1:1 magnification ratio on your film.

If my 'reasoning' is correct you are obviously well over the 'correct' exposure time on which you must/maybe have to 'add additional time' to allow for 'failure of the law of reciprocity'.

But again... I now have a somewhat 'weary/fuddled' brain having spent too many hours trying to learn to scan my negatives 'properly'... and I may not be 'reading' your information correctly.

Ken

 

I don't know this app but the standard way of calculating bellows draw compensation is as follows:

Adjusted Time = Indicated Time x (Total bellows extension in cm squared) divided by (Lens focal length in cm squared)

For your settings:

Adjusted Time = 8 secs x (27 x 27 = 729)/(15 x 15 = 225)

Adjusted Time = 8 secs x 3.24

Adjusted Time = 25.92 seconds

For this film the reciprocity failure chart for 26 seconds indicates the adjusted time to be 288 seconds or 4 minutes 48 seconds.

For practical purposes the formula takes as it's baseline the assumption that for a lens focused at infinity the bellows extension will equal the focal length of the lens (This is not true for retro focus lenses or other specialised optics).

 

Damn... You are correct. Its been too long since I did that much photo-macrography (on a daily basis when) 'working'.

I did so much on a regular basis that I benefited so much by relying on software that was 'loaded' in my Palm Pilot... a lot faster and more accurate than the ever-present 'stubby' pencil and small spiral pocket notebook. If my memory serves me better (now that I am not quite so tired of 'fighting' with my new Epson), the freeware was made available by a Bob Wheeler'.. and extremely dependable for the calculations I usually required on a daily basis. I even took it out 'into the field'. I'm now going to have to hunt for 'the pocket machine', power it up and get it going again... rather than trying to rely on my 'memory'.. or my cellphone calculator.

Ken

 

Sometimes I think we rely too much on technology Ken. If we remember the basics, when the batteries fail or the pencil breaks, we can always scratch the calculations on patch of bare earth or the nearest wall.