I’m looking for advice on close wide-angle photography at night. I’m photographing fish in a river which are very light sensitive so I cannot use a focus light but an occasional strobe is ok. I’ve found that I can use back button focus to first focus on an object at a set distance and then try to get the same distance away from the fish and take the image but that doesn’t always work because the total depth of field is very small. Since the fish are very light sensitive I can only take an image ~once a minute so if my distance to the fish is off a little and it’s out of focus then the fish might move and I might miss that opportunity. Are there any other options I should try? 

According to a DOF calculator, shooting a subject that’s 1ft away at F/4 gives me .96” (24mm) of total depth of field which is not a lot of wiggle room to get the distance to my subject correct. Shooting at F/8 gives me 2 inches (50.8mm) of total depth of field which is better but I’d have to crank my ISO way up and still isn’t a lot of wiggle room to get my focus correct. It seems I have no good option and my only real path forward is to decrease the aperture and crank my ISO up while keeping my shutter speed as low as possible but would love to hear how others do it.

I guess you have already tried to use a red focus light, but your fish do not like it?

Many creatures do not recognize red and are not scared away while AF works to some extend...

Edited 5 hours ago5 hr by Architeuthis

35 minutes ago, Architeuthis said:

I guess you have already tried to use a red focus light, but your fish do not like it?

Many creatures do not recognize red and are not scared away while AF works to some extend...

Thanks for the suggestion, I tried a white focus light, haven't tried a red focus light but that's a good idea. I'm photograhing salmon and my understanding is they can probably see red but it's worth a try. I don't believe they can see IR but my camera has an IR filter that I would not want to remove/modify.

If anyone has a favorite red focus light please post a link.

Edited 2 hours ago2 hr by brightnight

I would also recommend trying a red focus light. The fish in my local river are less spooked by red compared to white.

I'm a little confused by your other issue. If you're using strobes, then I'd expect you to have enough light to shoot at f/8 at fairly low ISO. Also, lowering shutter speed will have no effect in the dark (unless you're using continuous light instead of strobes).

32 minutes ago, Isaac Szabo said:

I would also recommend trying a red focus light. The fish in my local river are less spooked by red compared to white.

I'm a little confused by your other issue. If you're using strobes, then I'd expect you to have enough light to shoot at f/8 at fairly low ISO. Also, lowering shutter speed will have no effect in the dark (unless you're using continuous light instead of strobes).

Thanks for responding. I have my strobes turned all the way down and diffusors to limit the light to the fish as much as possible because as I mentioned these fish are super shy. I haven't dialied in the strobe power except to say that the photos look reasonable and I've aired on the side of caution with light so I might be able to come up a bit on strobe power but I haven't had a limited number of chances to experiment.

My thinking was to try to keep a fast enough shutter speed to stop blurring so I can freeze a fish digging out a redd, but now that you mention it I'm thinking about shutter speed as it related to daylight but at night where the only light comes from the strobes and a little from the moon, so it doesn't matter and I can move up to 1/100th without issue.

If you have a favorite red focus light please post a link.

Strobes fire so fast (1/10,000 of a second) that fish don't typically react to them at all. It is the steady state lights that typically cause issues and scares off the fish.

Most lights sold as UW photography focus lights have a red-light mode. Some high quality and low-cost examples are the OrcaTorch 710v or Kraken Hydra 1800 WSR.