Long Exposures

[Alex1994]

Hello

I want to take a long-exposure photo during the day. Here's the question: how do I get an exposure of, say, 20 seconds at 100ISO without having a ridiculously overexposed photograph? Narrow aperture like f/22 is a good idea, anything else needed? Any general tips on long exposures?

As usual I'll be using film, I have heard about this thing called reciprocity failure, if anyone could explain it I'd be very grateful.

All help much appreciated

Alex

[postcardcv]

I reckon using a ND filter would be your best bet - depening on the strength these will cost you ~2-10 stops - http://www.warehouseexpress.com/cate...spx?cat03=3072

[miketoll]

Basically reciprocity failure is when the film stops responding to increased exposures in the normal way resulting in the wrong exposure so you end up under exposing so you have to compensate by giving a longer exposure than you think. A very rough rule of thumb is to double the exposure time for exposures much over 1 second but the effect gets more pronounced the more over one second you go (ie not constant) and also varies from film to film. Bracket fairly widely is the answer and for really long exposures you may well get a colour cast as well. I have not looked but you may well be able to find more detail on what sort of correction you might need for your Velvia on line, there used to be tables of time versus estimated correction for each film.

[Alex1994]

I'll be using Ektar for this shoot, and research tells me it has very little reciprocity failure. I won't be going longer than 2s so it won't be a problem. I will need an ND filter though.

[yelvertoft]

Originally Posted by Alex1994

Hello

I want to take a long-exposure photo during the day. Here's the question: how do I get an exposure of, say, 20 seconds at 100ISO without having a ridiculously overexposed photograph? Narrow aperture like f/22 is a good idea, anything else needed? Any general tips on long exposures?

Don't shoot on a very bright day at noon would be a tip I'd add.

[miketoll]

Colour negative film like Ektar has more exposure latitude than slide film so as you say you should not have to much trouble with 2 seconds. In your original post you did mention 20 seconds though so that would make a difference.

[Alex1994]

Sorry, revised what I want - 20 seconds is an unfeasibly long time, and I think the motion blur with 1 or 2 seconds would be quite sufficient. I'll take some average light readings someday and see what the camera thinks the correct settings are.

[ArtphotoasiA]

Interesting reading... it happened to me also to have this problem when I wanted to take a shot of a fountain during the day and have the water not frozen but silky.... hard to do... also with dslr need not to have a bright day first... iso100 f22 and luck if more then 1 or 2 sec.

[Alex1994]

The camera currently meters 1/30th or 1/15th of a second under daylight conditions, f/22 and 100 ISO. I want about 1/2 or 1 second exposure time, so I'm about 3 f stops short. I'll have a look in Jessops to see what they have.

[Don Hoey]

Alex,
If you have a polariser that will cut 2 stops so you are nearly there.

Don