Which “photo equations” do you carry in your head? Do they interfere?

[Andrew O'Neill]

Bellows extension. Easy to remember the formula, but forget about calculating in your head. Having a chart in my exposure record form helps big time.

[Emmanuel BIGLER]

Bellows extension.

Ext = M x F
Dist_from_lens_to_subject = (1 + 1/M) x F

but this more useful to chat on various LF forums and be the first to answer the F.A.Q : how much bellows draw do I need with my ...

[Bill_1856]

Well of course I always remember that the luminance of the moon is 250 c/square ft (or whatever).
It frequently helps when I'm drunk as a skunk and can't find my light meter.

[MumbleyJoe]

While I think I can remember the equations I opt to make cheatsheets that I keep in my little exposure notebook instead.

I only own 3 lenses (90, 150, 210mm) so I make tables that are more practical to carry around based on my own gear instead.

I just put together a table for 1/u+1/v=1/f. Basically, since my bellows is always somewhere between 90mm and 300mm (full extension of my bellows), I have one table that shows the focal distance for each of those 3 lenses in 1cm increments from 90-300mm. For as often as that's helpful in the field, I figure that's good enough.

Bellows | 90mm | 150mm | 210mm |
90mm | infinity | NA | NA |
100mm | 900mm | NA | NA |
110mm | 495mm | NA | NA |
...
150mm | 225mm | infinity | NA |
etc...

I also put together a table for bellows extension and effective f/stop in 1/3 stop increments, along with their corresponding exposure times (this was I always have a handy table that shows how 'stops' and 'exposure times' correlate). This was I can measure the bellows and figure out, within 1/3 of a stop, how to determine the effective f/stop (or exposure time).

f/stop | exposure | 90mm | 150mm | 210mm |
0 | 1x | @90mm | @150mm | @210mm |
1/3 | 1.3x | @101mm | @168mm | @236mm |
2/3 ....
1...
...

I also carry reciprocity tables for the films I use and some other cheat sheets.

I guess I could have just said I don't try to remember the equations in the field, but rather make sure I get it right when I've got the time and use cheat sheets when it counts.

[jnantz]

i don't carry any equations in my head
i just take the photograph ...

[aduncanson]

Sunny 16 is probably the one I most use. Others (like 1/f = 1/So + 1/Si) are firmly fixed in my head and I can't forget them even though I rarely use them while photographing. The two image conjugates, Si = f*(1+m) and So = f*(1 + 1/m), are highly for useful for closeup photography.

I often find myself re-deriving the relationship between aperture, subject distance, CoC, and depth of field so I cannot claim to carry that in my head, but Emanuel Bigluer's rule: "CoC = format diagonal/1780" is key.

One formula that I have recently developed with the intent to memorize is the following approximation for Ilford's published correction for HP5+ reciprocity failure:
t' = t^2/10 +2*t . It is accurate to within about 4% from 5 through 35 seconds.

[bob carnie]

Sunny f16
Fstop scale
shutter scale

[Heroique]

For reciprocity correction, it’s noteworthy that AA recommends the following corrections (from Kodak) for all b/w films, until you’re good enough to test the films you actually use. For intermediate times – such as “5 sec.” or “50 sec.” – simply interpolate, he says. (Note the “either/or” pairs for aperture/actual exposure are not always equivalent.)

Very simple – easy enough to memorize in a pinch :

Metered time / (either) new aperture / (or) actual exposure / development time
1 sec. / 1 stop more / 2 sec. / -10%
10 sec. / 2 stops more / 50 sec. / -20%
100 sec. / 3 stops more / 1,200 sec. / -30%

[Doremus Scudder]

No equations in my head. Not one. I don't want to be figuring mentally when photographing.

I carry a light meter to calculate exposure. I have made a guide book for myself with reciprocity tables for various films, bellows extension factors for all the lenses I own, exposure compensation for different developing schemes, filter "fudge-factors" (more later) etc. In short, if I need to calculate something in the field, I've done it already at home, with the calculator with significantly greater precision than I can in my head, and I just look it up (which likely takes less time than figuring in my head as well).

For filters, I read through them with my spot meter and then apply "fudge-factors" which I have determined through testing, and which compensate for small changes in contrast and exposure. These are only needed for the strongest filters and I usually have them in my head (#25 = +2/3 stop and N-1, for example). I also know my exposure compensation for N-2 through N+2. These can hardly be considered photo equations, however.

I'm fairly confident in calculating exposure, since I've tested everything. For that reason, I don't have to do a lot of figuring in the field. All the testing just so I can concentrate on the image and the adjustments to the focal plane and related camera movements (which requires some 3-D thinking) instead of doing the numbers on-site.

About reciprocity correction: every film is different. The Adam's "recommendations" are really the Kodak data for their older films. Howard Bond and others have tested the newer versions and have arrived at significantly different results (I posted just recently about this very thing). And, if your working E.I. is significantly different than the rated ISO, your reciprocity results may be way off. Get the data for the film you are using (Google is your friend here) and use that as a starting point, adjusting compensation time and developing time adjustments to get results you are happy with. Applying the old Kodak data is better than guessing, but not precise for many films today.

Best,

Doremus Scudder

[Heroique]

I don’t carry any equations in my head...

No equations in my head. Not one...

Perhaps a case of not meaning what one says? No one will convince me that you (you, of all people, Doremus ) might come upon a situation where, say, “sunny-16” is best, then have to pull-out charts, tables or notes to recall the principle and use it.

Another “bellows correction” example (simple for the head, similar to John’s post #6) occurred to me, which some people recommend if your subject is closer than 10x your focal length (e.g., w/ a 240mm lens: 10 x 240mm = 2400mm, or closer than 2.4 meters).

If you’re closer, simply add ½ stop for every 25% increase in your bellows extension (beyond infinity).