Exposure

[Cesare]

I've been reading some darkroom books, and have come across an idea that i've never corrected for in the past and wondered if this is news to anyone else.

Basically as a lens focusses closer, there is more light loss (since the image circle becomes larger than the film), and this certainly affects macro photography. I hadn't realised though how quickly you can get a 1/2 stop drop in light. My understanding is that something like a Hasselblad 80mm focused to 4feet looses you around 2/3rds of a stop of light.

Does anyone worry about these sorts of things? I usually meter to the nearest stop, and since my meter produces stops and 1/10ths of a stop I round down, so i'm usually overexposing somewhat by default. Maybe this cancells out with the loss of light from close focussing?

Of course none of this applies to TTL metering

Cesare

 

[cliffpatte]

I think you're talking about foot candles or "Lux" here. If a given light radiates in all directions equally then the intensity (candles per square foot) varies by the inverse of the distance squared which should compensate more than enough for your image circle problems.

Incidentally the Blad 80mm lens has half stops too.

 

[Anex]

My Mamiya C330 has a correction scale in the finder for this, doesn't yours Cesare?

 

[Patrick Dixon]

On view cameras the compensation for this is called Bellows Factor.

 

[Cesare]

Yes, my C3 has a correction scale - but i've only ever used 2x, 3x when doing really close up stuff. I just don't think i'd considered how easy it is to get a 1/2 stop difference in exposure from a camera system. I'll do some TTL metering with the prism on my RZ67 against a plain subject and see what focus distance gives 1/2 a stop, 1 stop.

It's just another thing to add to my lax technique - I tend to meter to the nearest stop, focus quickly, shoot hand held, develop with room temp chemicals (20 ish is close enough). It's amazing I get images at all

Cesare

 

[Anex]

TBH I think a lot of us on here take that approach.

 

[Paulicus]

I'm with Anex, minus the chemicals.

 

[johnfromnorwich]

Ditto. Just getting the right shot is enough of a challenge for me without worrying about 1/2 a stop either way or how this might differ across different wavelengths with a coloured filter. B/W film has so much lattitude anyway. Judging by your pictures, you don't have much to worry about!