Sorry, an urgent flash question...

[68lbs]

...been playing with the flash 580EX ready to hopefully take some pics at the flowershow tomorrow.

So... Manual, 70mm, f/2.8, flash on ETTL, maybe 2-3 feet away from the subject. I have shot on shutterspeed 1/60 and 1/250 and can see no difference.

Why?

I've worked out how to dial the flash up/down (not difficult) so can adjust the exposure like that.

Am I doing it all right? Or am I doing something wrong?

Oh... metering is the 400D's version of spot (ie, big spot in the middle), but I ignored it because it's too dark in here anyways at the moment and I knew the flash would compensate.

I think, what I'm trying to work out is am i doing it right, and what would i have to do if i wanted to try and underexpose the background slightly to bring out the flower more. Or is that not possible cos I can't direct the flash that narrowly or precisely enough to just illuminate a flower.

 

[photon]

The background would have to be quite a way distant to not be illuminated by the flash. You could bounce from the ceiling or the wall behind you or to one side to allow less light to spill forward.

With that setup, the minimum range for my flash is about 2m and I wouldn't not bounce.

 

[tdodd]

An exposure with flash involves two exposures. One is an exposure for the ambient lighting and incldes everything in the scene. This exposure is set by your choice of aperture, shutter speed and ISO. The second part of the exposure is whatever the flash does for your subject, whether guided by your input or left to it own devices.

The duration of the flash itself is very short - 1/1000" or much less. As far as the flash is concerned it matters not one jot whether your shutter is open for 1/60" or 1/250".

So, what will vary, depending on the shutter speed, is how much ambient light is included in your exposure. By the sound of things, it is so dark that whether you use 1/60 or 1/250 makes little odds to the ambient exposure and the flash is doing all the work in both scenarios. Effectively that means that despite your different settings the exposure you end up with is essentially the same.

Tomorrow, when presumably you will be shooting in fairly good light, all you need to do is set your manual exposure so that the general scene is underexposed by a stop or however much you wish. Turn on the flash and let it bring up the exposure for your subject. If the background is far enough away then the flash will illuminate your subjct more strongly than the background - inverse square law and all that. To make sure the flash doesn't end up sending too much light onto the background you may need to get close to your subject, so that in realtive terms the background is at least as far behind the flower as the flower is from the flash. If the background is as twice the distance from the flash as the flower then it will only get 1/4 the strength of the flash illumination.

Depending on how bright the ambient conditions are you will need to consider the max sync speed for your camera, which is normally 1/200". If you need/want a faster shutter speed you will need to use High Speed Synch on the flash, which will let you shoot at any shutter speed you like.

Here are a couple of shots to show just how easy it is to make the background dim and your subject bright, if you set things up properly. The first image is the general setup, and shows how bright it was outside - normal daylight. The second shot is the picture I was aiming for, where the robin is perfectly lit and the background has almost vanished into darkness. You will note that I had a remote flash very close to the feeding table. This is the flash that illuminate the subject. Because it was close to the subject but far from the background I got a very strong contrast in lighting between the two.

 

[whiteflyer]

Well you learn something everyday. Thanks for that post Tim, I'd never thought about under exposing the ambient light to get the effect of shot you have there,but you've made it so darn obvious now.

 

[68lbs]

Aha, wondered if dark room was the problem...

Depending on how bright the ambient conditions are you will need to consider the max sync speed for your camera, which is normally 1/200".

I take it this means that if I shoot faster than 1/200 the flash wont 'flash' quick/strong enough, so the shutterspeed wont pick up all of the light that it should?

With that setup, the minimum range for my flash is about 2m and I wouldn't not bounce.

Sorry... too many negatives... does that sentence say i should bounce?

 

[tdodd]

I take it this means that if I shoot faster than 1/200 the flash wont 'flash' quick/strong enough, so the shutterspeed wont pick up all of the light that it should?

Most flash guns and studio strobe do not have a High Speed Synch mode. This means that when you use flash the fastest shutter speed you are able to use is the sync speed - either 1/200 for Rebel bodies or 1/250 for most others. In bright conditions (sunny day) this can be very restrictive and requires you to use a pretty small aperture. Under bright sun, at 100 ISO and 1/200 you would need to use an aperture of f/11 in order to avoid overexposing the scene. If you want to shoot a portrait or use some creative shallow DOF then f/11 is completely useless. When you cough up for a fancy flash like the 580EX you are blessed with the High Speed Sync facility (needs to be enabled), which allows you to shoot at any shutter speed you like. This gives you full creative freedom to shoot however you like.

If the ambient is bright then your flash will have little extra work to do to add a fraction more light and will be easily up to the task. Remember that the 580EX has a guide number of 58m with a 105mm (or longer) lens. Put another way, the flash will have a reach of over 20m at 100 ISO and f/2.8, even in darkness, when used at the sync speed. HSS will reduce that distance, maybe to around 14m, but I expect that will be ample for your needs.

If you want to see a video where more of this is explained and demonstrated then look here....

http://www.youtube.com/watch?v=Xnn5nzPvoIM

 

[photon]

Sorry... too many negatives... does that sentence say i should bounce?

Indeed ;-)

I'm not familiar with your camera, but don't you get an indication of under/over exposure, hopefully in stops (rather than just under/over)?

Set things up manually so aperture and ISO allow a shutter speed of 1/(focal length) for handheld shots e.g. 1/60 ~ 1/125 for your 70mm focal length, then tweak things so the scene is metered as underexposed by the desired amount.

The less underexposed the background is, the more that shadows from the ambient lighting will enliven the scene and the more attention you should pay to your custom white balance. Set the WB off a pale blue item if you wish to warm things up.

 

[68lbs]

Thanks for the help guys

Tim... everything makes perfect sense and I follow 100%. I shall check out how I turn on HSS just in case i need to go over 1/200

Proton... You've completely lost me regarding white balance. I always have it on auto and if necessary adjust in pp as I always shoot raw. The only thing I can think you might mean is... does the fact that I'll be combining ambient and flash mean that there will be different lighting so the WB will be all to cock?

 

[tdodd]

Colour temperature of flash should be close to normal daylight. If you try to mix flash with tungsten, for example, in roughly equal measure, then you will get some pretty funky colouring, as the background will be tungsten lit and your subject will be flash lit. It's not easy to balance when you have two distinct colour temperatures to deal with at once. The approach then would be either to dominate the scene with flash entirely, bouncing it to fill the room, or use a gel to match the temperature from the flash to the ambient light.

Assuming you're shooting outside then you probably will not need to worry about mixing light sources. Indoors, under artificial light, you might have a problem.

 

[68lbs]

Thanks for the clarification Tim. I'll be shooting inside... Great Yorkshire Showground. Permanent prefab marquis things from what I remember. I never normally pay much attention to the buildings. I am pretty certain they must have some sort of interior lighting.

If i'm going for limited DoF and blurred backgrounds it probably won't matter too much will it if the backgrounds aren't true to colour?

I dont want to complicate matters even more. This is going to be my first time using flash properly.

 

[tdodd]

I'm sure you'll be fine.

Where I've noticed the problem is when shooting people, with flash, in a large tungsten lit room. If you aren't careful, you end up with people in the background looking a horrible tungsteny yellow, while people in the foreground are lit by blueish light from the flash, and a white wedding dress can look very blue-white. Of course, that's all down to bad flash technique, and not easily fixed in post processing unless you use layers and adjust the background and foreground independently.

Here's an example where nothing was easy - daylight mixed with tungsten mixed with flash. I only use Lightroom for editing so layered adjustments was not an option. See how the people in the foreground vary from sort of normal to a bit blueish, while those in the background all look peach coloured....

 

[68lbs]

Right, well I took some pics... would really appreciate some 'lighting' feedback as to whether the shots are ok and as to be expected, or what I could do to improve them. The lighting does seem quite strong in the Carnivorous Plant Pics here, and I seem to remember trying having the flash angled at 90°, and also at something like 45°... my thinking being that I wouldn't get the main blast, so softer. And if too weak, I could increase the flash power a bit. Not sure if that's right.

The other thread of pics can be found here. These seem better in that the light seems to have illuminated the heads nicely, whilst the background is quite dark. But again, I really did feel like I was doing it a little aimlessly, just pointing, seeing what came out, dialing the flash up/down a little.