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11-21-2021, 08:49 AM   #1
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Flash with background exposure

I noticed when I use a flash , images are sharper than with any other exposure (mechanical shutter, electronic shutter has rolling shutter). Flash exposure gives me super sharp subjects. Unfortunately, when using a flash the inverse square law applies, when the subject is properly exposed the (bokeh) background exposure is dark or darker. How do you manage to have the subject lit with the flash , and the background rendered as if photographed without flash. I tried a few things with my K1, not very convincing, either it's a compromise with the flash only adding some contribution to subject lighting with natural lighting still playing a role, or the flash illumination is dominant but then the background is dark(er).

11-21-2021, 10:03 AM - 1 Like   #2
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Add more light to the background relative to subject either through modifiers, reducing the main (flash) light, or putting more light to the BG. when using a flash it's duration can effectively be the shutter speed giving you that added sharpness. W/o knowing what flash you have, see if you can dial the power down to 1/2, 1/4, and 1/8 and note the changes (all while maintaining proper expo for main subject).
11-21-2021, 10:15 AM - 2 Likes   #3
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You need to use a slower shutter speed to allow more ambient light in. If this is not an option, then I just raise the ISO and lower the flash output by the number of stops I increase the ISO by.
11-21-2021, 10:23 AM - 2 Likes   #4
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I usually set the camera for background exposure first, then dial the flash up/down for subject exposure.

11-21-2021, 11:00 AM - 2 Likes   #5
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Here is the basic rule
As noted above, the ambient light exposure (shutter speed + aperture) you meter to your intent. The flash augments the ambient and you control by how much. At shutter press, the two are quite separate and distinct with the only qualifiers being that the shutter speed must be at or below X-sync and that the aperture setting affects both the ambient and flash exposure.*


Steve

* Flash exposure variables = distance from flash to subject and will determine subject illuminance. Distance from subject to camera will determine both ambient and flash contribution.

Last edited by stevebrot; 11-21-2021 at 11:26 AM.
11-21-2021, 12:32 PM - 1 Like   #6
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For a given exposure, increasing ISO will increase all areas of the subject as will opening the aperture (which will also affect depth of field). However, slowing the shutter will only increase the background since the flash duration is fixed. Using the latter to adjust flash contrast can cause blurring if the shutter is slowed too much (and this can be put to intentional effect using trailing shutter sync).

Adjusting flash power will primarily affect the foreground but will have some effect on the background depending on distance from the flash.

Try to keep these rules in mind and they will help with your flash shots.
11-21-2021, 01:21 PM - 1 Like   #7
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QuoteOriginally posted by biz-engineer Quote
I noticed when I use a flash images are sharper than with any other exposure
Then you are using too slow a shutter speed for the "other" exposures and you are getting camera shake or subject movement.

QuoteOriginally posted by biz-engineer Quote
How do you manage to have the subject lit with the flash , and the background rendered as if photographed without flash


Meter for, and set your camera to expose the background as you want....then use a flash output that lights your subject and not the background.

You need to be using manual exposure on both camera and ideally flash too to get this correct. Pay attention to the flash distance indicators your flash will indicate before you take the picture.

11-21-2021, 03:12 PM - 1 Like   #8
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Just adjust your camera settings for the background first in a conservative way (not too high an ISO, Shutter Speed, or Aperture number) so that it looks the way you want, then adjust your flash power to accommodate the subject.

Your camera and flash being in Manual mode may give you the most control when performing this type of shooting.
11-21-2021, 06:42 PM - 2 Likes   #9
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Some very good flash tutorials here.

? Flash Photography Techniques - Tangents
11-21-2021, 11:26 PM - 1 Like   #10
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QuoteOriginally posted by biz-engineer Quote
I noticed when I use a flash , images are sharper than with any other exposure (mechanical shutter, electronic shutter has rolling shutter). Flash exposure gives me super sharp subjects.
Do you have SR turned on?
11-22-2021, 10:47 AM   #11
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QuoteOriginally posted by JohnMc Quote
Add more light to the background relative to subject either through modifiers, reducing the main (flash) light, or putting more light to the BG
I've thought about how to have the main subject lit by the flash and the back lit by natural light , by means of a double exposure: first exposure with flash , second exposure without flash, but haven't found any way to do it automatically with the cameras.
Now, to achieve a balanced subject / background exposure in a single capture with flash, for instance nature subject, the background is too large to be lit with artificial light. However, if the subject is back lit, then it becomes possible to have the subject mostly illuminated by the flash , with the background illuminated by natural light.

---------- Post added 22-11-21 at 18:47 ----------

QuoteOriginally posted by Paul the Sunman Quote
Do you have SR turned on?
Yes it was turned on. Can SR cause issues when using a flash?

---------- Post added 22-11-21 at 18:49 ----------

QuoteOriginally posted by pschlute Quote
Meter for, and set your camera to expose the background as you want....then use a flash output that lights your subject and not the background.
Yes, thanks for the recommendation.

---------- Post added 22-11-21 at 18:51 ----------

QuoteOriginally posted by stevebrot Quote
As noted above, the ambient light exposure (shutter speed + aperture) you meter to your intent. The flash augments the ambient and you control by how much.
I've played with manual flash and exposure compensation on the camera (camera exposing for the ambient light). It was more trial & error, I could achieve proper exposure after a few test shots.

---------- Post added 22-11-21 at 18:55 ----------

QuoteOriginally posted by Bob 256 Quote
For a given exposure, increasing ISO will increase all areas of the subject as will opening the aperture (which will also affect depth of field). However, slowing the shutter will only increase the background since the flash duration is fixed.
Thanks for the summary. I get that. I was initially trying to make a double exposures in burst mode with the flash firing at the first exposure and no firing at the second exposure so that I would blend the two exposures to create one picture. But that didn't work because the camera exposed the same way with and without flash.
11-22-2021, 11:09 AM - 2 Likes   #12
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Nature comes in all sizes. I'd suggest you set up a still life by a window that gets a good range of light during the day and experiment with the equipment you have an a more controlled manner. There won't be a one size fits all solution; the flash that does well at dusk may not perform at noon.
11-22-2021, 12:56 PM   #13
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QuoteOriginally posted by JohnMc Quote
There won't be a one size fits all solution; the flash that does well at dusk may not perform at noon.
Difference of color temperature between flash and ambient light it also a problem.
11-22-2021, 01:16 PM - 1 Like   #14
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QuoteOriginally posted by biz-engineer Quote
I noticed when I use a flash , images are sharper than with any other exposure (mechanical shutter, electronic shutter has rolling shutter). Flash exposure gives me super sharp subjects. Unfortunately, when using a flash the inverse square law applies, when the subject is properly exposed the (bokeh) background exposure is dark or darker. How do you manage to have the subject lit with the flash , and the background rendered as if photographed without flash. I tried a few things with my K1, not very convincing, either it's a compromise with the flash only adding some contribution to subject lighting with natural lighting still playing a role, or the flash illumination is dominant but then the background is dark(er).
Reduce flash exposure to about 1/2 stop over ambient. Your subject will pop without looking pasted on.
11-22-2021, 01:17 PM - 1 Like   #15
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You don't say what type of picture you are taking. With formal portraiture you can set up lighting how you like with separate appropriate lighting for subject and background.

However, for an impromptu or relatively informal indoor shot, I would use bounce flash, aiming up at a white[-ish] ceiling at a point a little nearer to you than the subject. Or even aim the flash up and behind you if there is a light-coloured wall there. In this, it is best to have a powerful flash with a head that can be tilted and swivelled in any direction. I find this creates a much more general wash of light over the scene, and also avoids any hard flash shadow line on the background. It escapes the inflexible point source inverse square law by creating multiple light paths.
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