exposure difference calculation for selecting ND filter

[mike in london]

I have two shots taken during the day, one was 1/80th second at f12 and the other (with a IR filter) was 80 seconds at f12.

How do I calculate the difference in stops between the two shots?

I would like to get a ND filter to obtain the same result in colour and not be restricted to IR B/W. The strongest ND filter seems to be around 4-5stops though, can you get stronger?

I was never good at maths!

Mike

ps I realise I could shoot at night but this is often not practical

--
http://www.mikecurryphotography.com

 

[GideonW]

The f-stop was the same and i assume that the ISO was too (though you didnt explicitly state it). That leaves the only difference: the shutterspeed. The IR shot used a shutterspeed that was 1000x slower than the normal shot. 1000 is approximately 1024, which is 2^10. So there is the answer, there's a 10 stops difference.

 

[Oly Canikon]

The f-stop was the same and i assume that the ISO was too (though you didnt explicitly state it). That leaves the only difference: the shutterspeed. The IR shot used a shutterspeed that was 1000x slower than the normal shot. 1000 is approximately 1024, which is 2^10. So there is the answer, there's a 10 stops difference.

Its not 1000x though. 1/80 is .0125s so 1000x would be 12.5s not 80. The difference is more than 12 stops.

 

[GideonW]

Its not 1000x though. 1/80 is .0125s so 1000x would be 12.5s not 80. The difference is more than 12 stops.

Right, temporary brain shutdown, thanks for the correction.

The proper answer is then that the difference is light is a factor 6400 (80 * 80) and that this is between 12 (4096) and 13 (8192) stops.

 

[mucksyabc123]

I may be wrong but I believe its more than 10 stops. I think its 14 stops.

If I was to write out the all the stops from 1/80 to 80 then wouldnt it look like this?

1/80, 1/40, 1/20, 1/10, 1/5, 1/2.5 (or 2/5), 1/1.3 (or 4/5), 1.6 (or 5/4), 2.5, 5, 10, 20, 40, 80

This list outines the the increments needed inorder to increase the shutter speed by 1 stop. For example - If I'm using a shutter speed of 1/80 and need to step down (over expose) by 1 stop th correct shutter speed would need to be 1/40. If I need to stop down 1 stop from 5secs, the corect setting would be 10 secs. This is assuming that the aperture and ISO stays constant.

Therefore if we count all the stops between 1/80 and 80secs, the answer is 14 stops.

 

[mike in london]

Therefore if we count all the stops between 1/80 and 80secs, the answer is 14 stops.

wow.... dont think i can get ND filters that strong........ strongest i have seen is 1.2 ND 100mm Resin Lee filters which are 4 stops (approx £85), mind you I could stop down to f22 but then i get diffraction.

I jusr really liked the effect on movement it had

Mike

--
http://www.mikecurryphotography.com

 

[mucksyabc123]

Therefore if we count all the stops between 1/80 and 80secs, the answer is 14 stops.

Sorry slight correction. It is actually 13 stops not 14.

 

[mottjr]

Singh-Ray has exactly the solution you need, albeit at a significant price.

http://www.singh-ray.com/morslo.html

 

[GideonW]

Therefore if we count all the stops between 1/80 and 80secs, the answer is 14 stops.

wow.... dont think i can get ND filters that strong........ strongest i have seen is 1.2 ND 100mm Resin Lee filters which are 4 stops (approx £85), mind you I could stop down to f22 but then i get diffraction.

I jusr really liked the effect on movement it had

Mike

--
http://www.mikecurryphotography.com

Some ND filters can be stacked, so you can reach 12-13 stops by putting 2 or 3 on top of eachother. You'll need high quality filters though, otherwise image quality will degrade considerably due to the extra layers of glass.

edit: also, depending on the scene, you may be able to underexpose somewhat. If there are too many shadowy areas that need high detail, you can perhaps underexpose by 1-2 stops and pull it back up in post processing. It'll cost you some noise, but depending on the circumstances that might not be a big issue.