In Camera Flash and Aperture question....

rvrkids

TPF Noob!
Joined
Sep 6, 2014
Messages
18
Reaction score
1
Can others edit my Photos
Photos OK to edit
Hello -

I am a hobbyist photographer. Have been using my DSLR Nikon D50 for several years now and used it mostly for photos of my kids and sports....but I found out I really love the intricacies of photography.

My question is....I usually try not to use flash at all but when I do I only have/use the in camera flash. I just took 2 identical photos with in camera flash on and aperture of f/8 and one at f/2.8 with a Tamron 28-75 1:2.8 lens. The image with the smaller aperture (f/8) was brighter....why...both had the same shutter speed of 1/60. Does the in camera flash change based on the metering, settings, etc.? I would have expected the smaller aperture to be darker with everything else exactly the same.

This was an indoor picture at night. I took pics in raw and exported to jpg for attachment using PSE12.

Thanks.




$lexi-1-2.jpg$lexi-2.jpg
 
I'd imagine that some of what you are seeing is the natural vignetting of the lens at f/2.8 and the effect it has on metering, coupled with the randomness of TTL flash metering.
 
Wouldn't vignetting just be around the perimeter of the image? You can tell that the exposure in the center of the image is different which seems like it would not be due to vignetting.

So, you mentioned randomness of TTL flash metering...does that mean that the in-camera flash does behave / flash differently based on metering/settings, etc ? If yes, why didn't it compensate for the smaller aperture and expose the photo correctly?

Basically, if the flash is on, how does it determine what to do?

Thanks for any help.
 
In TTL mode the camera fires very short pre-flashes the light metering sensor sees that reflect off the scene.
Using that reflected light information the camera CPU processes the info to set the power of the flash unit when the shutter is released.

By stopping down the lens the camera tells the flash unit to use more power to compensate for the smaller lens aperture.

When using flash shutter speed controls the ambient light exposure.
Flash unit power and lens aperture control the flash exposure.

The D50 has a max flash x-sync speed of 1/500. Most other DSLRs only have a max x-synce speed of 1/200 or 1/250.
Actually, with non-dedicated flash units and using manual mode, the D50 (D40, D40x, D70, and D70x too) can x-sync flash all the up to it's max shutter speed 1/4000.
 
.I usually try not to use flash at all but when I do I only have/use the in camera flash.

The hobby will become more interesting and rewarding when you obtain a speedlight. A speedlight is naturally a few more inches off the axis of the lens, which in itself will improve the looks of a flash. Speedlights can also be aimed at nearby surfaces to "bounce" light toward your subject and therefore be modified to be a larger light source than the speedlight head itself.

Then, if you really want to explore what they can do, you can fire it off camera in conjunction with modifiers.
 
I just took 2 identical photos with in camera flash on and aperture of f/8 and one at f/2.8 with a Tamron 28-75 1:2.8 lens. The image with the smaller aperture (f/8) was brighter....why...both had the same shutter speed of 1/60. Does the in camera flash change based on the metering, settings, etc.? I would have expected the smaller aperture to be darker with everything else exactly the same.


Hard to say specifically without knowing lots more details. ISO is a big one to know, and also the indoor light level. But the big picture is pretty clear.

f/8 would adjust the TTL flash power level stronger, to maintain same flash exposure, to be sufficient flash for f/8. f/2.8 would use 3 stops less flash power (only 1/8 as much flash power) to match the aperture, to maintain same flash exposure. Both should be the same flash exposure, enough to be correct for the aperture.

But all flash pictures are possibly/probably a double exposure, two exposures, flash and ambient continuous light. (one of them could be insignificantly low sometimes, but in general, a double exposure).

The shutter speed does not affect the flash exposure. The flash is faster, more instantaneous, it just has to fire sometime while the shutter is open.

Shutter speed does affect the ambient exposure, but it was a constant 1/60 here. However, the f/2.8 lets in three stops more ambient (8x more), but f/2.8 instead turns the flash down 3 stops (to still be same flash exposure). This 3 stops is a big difference, which affects the sum of the total of the two exposure sources.

And if at high ISO, f/2.8 might have been closer to a correct exposure for ambient, so that ambient can be seen much better to make much more difference. But at f/8, ambient would have been underexposed 3 stops more (at low ISO, f/8 would be a dark ambient picture). A "Balanced" flash system (which D50 is) could react differently to the relatively bright ambient offered by high ISO, so we still have missing details.

So, what would be interesting to see, is the same picture repeated twice more, at same 1/60 and same ISO, at both f/2.8 and f/8, but without any flash. These would be dark pictures without the flash ,but would show the contribution of the ambient to the total illumination (good clues). The total exposure is the sum of the flash and the ambient. If the ambient is brighter, the total illumination will be brighter. Flash pictures are double exposures, flash and ambient (a very necessary thing to know).

Four Flash Photography Basics we must know - Flash pictures are Double Exposures
 
Last edited:
Thank you all for the explanations. I think I get it now, especially after the description of the ambient + flash illumination.

One other question though.....when using a flash, no matter what kind (although my case specifically is in camera flash), since the flash compensates for the missing light anyway how do you know what aperture/shutter speed to use (assuming it's too dark to get a properly exposed photo without flash). Basically, no matter what my settings are, the flash will read and compensate so do I just choose something based on other variables like do I need a fast shutter or what do I want my aperture to be to affect my depth of field?
 
Thank you all for the explanations. I think I get it now, especially after the description of the ambient + flash illumination.

One other question though.....when using a flash, no matter what kind (although my case specifically is in camera flash), since the flash compensates for the missing light anyway how do you know what aperture/shutter speed to use (assuming it's too dark to get a properly exposed photo without flash). Basically, no matter what my settings are, the flash will read and compensate so do I just choose something based on other variables like do I need a fast shutter or what do I want my aperture to be to affect my depth of field?

Compared to most external flash units, the in-camera flash does not have a lot of power. When using ETTL, usually I set an aperture for DOF, set shutter speed to sync speed and let the flash work out how much power to use. As long as the flash can deliver enough light, all is good. Flash is pretty brief, the less power, the briefer the flash, so you can set a slow shutter speed and still stop motion if there is not enough ambient light to cause blur. There are lots of more complicated things you can do with flash, but this is simple and usually delivers.

TTL/ETTL is not random, but it sees the scene differently than we do and that can result in unanticipated results. Setting manual flash might make it easier to see what is going on.
 
Thank you all for the explanations. I think I get it now, especially after the description of the ambient + flash illumination.

One other question though.....when using a flash, no matter what kind (although my case specifically is in camera flash), since the flash compensates for the missing light anyway how do you know what aperture/shutter speed to use (assuming it's too dark to get a properly exposed photo without flash). Basically, no matter what my settings are, the flash will read and compensate so do I just choose something based on other variables like do I need a fast shutter or what do I want my aperture to be to affect my depth of field?

Right, the camera automation tries to set up for the ambient. That's what cameras do. Then the TTL flash metering tries to set a correct flash power level for whatever the situation it discovers it is in.

But there can be exceptions, and it can depend somewhat on which camera model.

For example, Auto ISO. Your D50 will not increase Auto ISO if it recognizes flash is being used (which is definitely the best plan). However, newer models mostly set Auto ISO for the dim ambient, and the flash indoors always discovers it is working into high ISO. But we don't have to use Auto ISO with flash.

And normally, it is best to give the plan a slight bit of thought.

Normally we set camera A or M mode, and set an aperture for the flash power. Like maybe f/5 for bounce flash which needs a lot of flash power. Or if enough power is not the issue, we set whatever aperture we want to use. Again, shutter speed does not affect flash, but does affect ambient.

And indoors, where it is dim, where we need flash, the ambient meters pretty low (shutter speed). But reach up and turn the flash on, and camera A mode will jump to a Minimum Shutter Speed With Flash, typically 1/60 second. Every indoor flash picture will be 1/60 (which only affects the ambient).

But we can use camera M mode, and then set any shutter speed we want, maybe 1/60, or maybe faster 1/200 second to keep more ambient out (incandescent lights are orange, a white balance problem with flash).

The TTL flash still automatic flash exposure in any camera mode, including camera M mode.

When the TTL automation is off a bit, we use Flash Compensation to make it perfect. Just do what you see is needed to make it perfect. This quickly becomes very easy.

There is more at Four Flash Photography Basics we must know - Flash pictures are Double Exposures
 
OP, your general logic of expecting the f/2.8 shot to be brighter than the f/8 shot is sound, but only if the flash pulse was of equal power in both cases, which it probably wasn’t if you relied upon the camera to calculate it.

It’s been so long since I have used my pop-up flash that I can’t even remember if you’re allowed to set its power manually, but if so, give that a try and take the two shots again while forcing the flash output to remain at a certain value. In that case, you will definitely see the f/8 shot being darker than the f/2.8, just as you’d expect.

Your question about choosing apertures and shutter speeds for flash photography has no catch-all answer and is a very broad topic with much to be learned. As a very simple example of a decision you might need to make, sometimes you will want the flash to be the sole source of light (which may make the background pitch dark in the result), and sometimes you will want to use the “available light” to make the background appear in the image for context, while the flash simply throws additional light on a foreground subject.

The grossly over-simplified, two-sentence way of describing how to do this is as follows: Treat the background and the foreground subject as two separate exposures. Select an aperture and ISO, set the exposure of the background by varying your shutter speed, and then set the exposure of your subject by varying your flash power.
 

Most reactions

Back
Top