Why is it so? Or, am I missing something?

<p>Take an EOS 40D, ISO on 800, Canon 100 f2.8 L IS Macro lens. Evening, almost dark but you can see the texture of a large moth (size of moth about 75mm - 3 inches), tripod. Select zero exposure compensation, use Canon 580 EX II speedlite. Distance from front element to moth approx 450mm or 1ft 6in.<br>

On program mode "P" the offered exposure is 60th Sec @ f2.8. If I select AV and f16 to get some depth of field so all of moth is in focus, then time given is about3 min. If I select TV and try for some realistic time of 1/2 sec, then I am offered f2.8 with no depth of field.<br>

I have had similar experiences with the EOS 630, the EOS 3, the EOS 33, EOS 450 and prior to that the EOS400. For some reason that I am unaware, and am hopefull you will be able to assit, I do not seem to be able to get a flash exposure on a selective mode (AV, TV) that seems to correspond in any way to that on "P".<br>

Can you please advise what I am doing wrong or what am I missing, and how do I get a decent depth of field with the 40D, 580 EX II and the 100 f2.8 macro from a distance of 450mm.<br>

Many thanks,<br>

Mark</p>

<p>Mark,</p>

 

<p>The best explanation for what’s going on would come from a bit of studying of the

manual.</p>

 

<p>Your best bet for on-camera flash is to shoot manual. Set your aperture for the desired DoF. Set

your shutter to the maximum sync speed (probably 1/200). Set your ISO for your desired exposure of

ambient / background light (usually a couple stops too dark, but that depends on what kind of effect you’re

going for). Set the flash in ETTL mode and trip the shutter. If you don’t like the exposure from

the flash, adjust it with Flash Exposure Compensation. If you can’t get enough power out of your

flash, boost the ISO and / or open the aperture.</p>

 

<p>See Planet Neil for all you need to know about getting a good exposure with flash as well as how to

effectively bounce flash to eliminate the dreaded hard, flat, straight-on look that on-camera flash

gives.</p>

 

<p>And then see Strobist to learn how to get the flash off the camera for even better results.</p>

 

<p>Cheers,</p>

 

<p>b&</p>

<p>And that's why the best shot you will get is when your on M mode. The other modes, the camera thinks for you. Try using M mode and remember flash will not really be affected by the shutter speed. So set your aperture to accommodate your DOF. Your shutter speed will allow the ambient light to come in if slow, if fast, the flash will do all the lighting for you. I hope I made some sense. Good luck v/r Buffdr </p>

<p>Av always exposes for the available light, regardsless of flash...</p>

<p>This may be helpful:<br /> <a href="http://photonotes.org/articles/eos-flash/#faq10">http://photonotes.org/articles/eos-flash/#faq10</a></p>

<p>In P mode, with a flash on, the camera assumes you need a handholdable and/or slightly ambient oriented shutter speed, so it sets one for you--usually 1/60th or 1/125th.</p>

<p>In AV mode, with a flash on, ambient light is always favored, but some cameras have NEVEC, which adds slight underexposure by making the shutter speed slightly faster, depending upon the ambient light level.</p>

<p>In TV mode, if you set a shutter speed that doesn't allow a realistic aperture for the ambient light level, the camera sets the widest aperture your lens will allow and then blinks it's aperture value at you in the viewfinder to alert you of the situation. If a flash is on the camera, turned on, the flash makes up for the rest of the exposure.</p>

<p>As everyone above said, use manual camera mode. Read the photonotes article and the Planet Neil article. I would think you'd want to bounce the flash or at least, modify it shooting at that small a distance.</p>

<blockquote>

<p>Av always exposes for the available light, regardsless of flash...</p>

</blockquote>

<p>I didn't realize that. That's interesting! It's definitely not the same with Nikons.</p>

<p>Yes, Nikons generally set 1/60th with aperture priority. There are a few more options in the latest Canon cameras, settable by CF.</p>

<p>shutter speed has no meaning when you're using a flash. the speed of your flash is 1 over several thousands of a second. so if you expose your picture for 3 mintues, the picture is already taken in the first fraction of a second; in fact, once you click the shutter release button, you can immediately put the cap back on your lens and put the camera back in your bag and the picture will still be correctly developped.<br>

a problem with metering in low light conditions is that it's not always accurate.<br>

what's happening with you is that your camera is metering the scene as if there's no flash attached to it. if you check the manual of your camera, you'll find where to tell the camera that whenever there's a flash just put the shutter speed to 1/200 sec. if you can't find it, just shoot on full manual mode: select 1/200s and any aperture you want, your 580 EX II will adjust its light output accordingly, and expose your scene correctly.</p>

<p> </p>

<blockquote>

<p>shutter speed has no meaning when you're using a flash. the speed of your flash is 1 over several thousands of a second. so if you expose your picture for 3 mintues, the picture is already taken in the first fraction of a second;</p>

</blockquote>

<p>Not quite correct. If the camera is suggesting a 3 minute exposure then the flash exposure will be done at hand holdable speeds. However the ambient exposure will require 3 minutes. Since flash has a limited range (it depends on flash power and aperture setting), it will not light up distant objects. For distant objects in very low light a longer exposure would be needed to get them to show up. </p>

<p>Hi Steven,<br>

"Distance from front element to moth approx 450mm or 1ft 6in". the only reason "a 580 EX II" (with a guide number of 58) won't be able to expose the scene, is that if it's turned off.<br>

second, if ambient light requires 3 minutes to properly expose a scene despite using a flash. it means you're subject is so far away, that you don't need a flash anyway. like taking a picture of niagara falls or Eiffel tower at night, using a flash.<br>

in certain circumstances, for aesthetic reasons, you sometime want both lights to expose a scene.<br>

Samer </p>

 

<p>Check the manual for the correct Custom Function. I have the 20D and in the custom functions you can have two choices for shooting with flash in AV mode. One is where the camera will set the exposure for the scene and fire the flash. Your 40D is set in this mode. That is why you get a 3 minute exposure in a dark area when setting f/16; the camera is setting exposure for the ambient lighting.</p>

<p>The other choice is that the camera will set the shutter speed at 1/125 (I think), accept your choice of aperture and use the flash for the correct exposure. In a dark area, this will underexpose for the ambient light and you will get black backgrounds.</p>

<p>I am quite sure that the 40D has the same custom function available, I just don't know the number.</p>

<p>Or as others have suggested, use manual and set both shutter speed and aperture.</p>

<p>I agree with most of the posts above, shoot all manual, plus or minus ETTL. If it's three minutes to get ambient exposure, it won't matter a lot how long your shutter stays open, as long as it is longer than your sync speed, usually 1/200s. However in other (brighter ambient) settings, ambient does contribute somewhat to exposure even at fast shutter speeds. In that case I like to have a little ambient softening the otherwise hard shadows coming from your bare flash. I usually shoot at about 1/60sec to get a more general lighting without having to add a lot of lighting gear. The place you can get into trouble is if you have roughly equal contribution of ambient:flash and a slow shutter speed, resulting in flash ghosting from motion of the subject or the camera. Although that can give you cool effects at times, flash ghosting is usually undesirable.</p>

<p>Many thanks to all. I fess up that I did not read the manual about flash settings because I wrongly assumed the camera would detect the flash, and set the correct exposure aperture to suit the setting I had selected: if I had set the aperture then it would set the speed, if I had set the speed it would set the aperture. I had better stop assuming and get back to reading.<br>

Thanks for the usefull links also.<br>

Kind regards, Mark</p>

<p>Mark, it's more complicated than that, and more versatile as well. Of course the camera can adjust flash output, and that's the main parameter the camera adjusts to create fill flash. If you are using one of the automatic or semiautomatic exposure settings, the algorithms for exposure adjustment are fairly complex, incorporating the requirement that you be shooting at or slower than the camera's sync speed. Because not all the scene will be within reach of flash, the semiautomatic camera settings are set to give roughly correct exposure even for distant objects and even when using flash. Adding flash to that, the ETTL flash pulse fires just before the camera exposure and the camera meter detects the result. Based on the result of that, flash output is adjusted to try and fill shadows without blowing out highlights. It doesn't always work the way it is supposed to, and I find that backlit subjects are notoriously underexposed using Canon's automatic settings. So, most of us shoot with manual exposure and ETTL for flash output, or sometimes all manual.<br>

Think of this as two separate exposures, one for ambient light, and the second for flash. Ambient exposure is sensitive to shutter speed, ISO, and aperture, and flash exposure is sensitive to ISO and aperture, not shutter speed. Because flash falls off to the third power of subject distance, beyond about 10 feet your little speedlight offers very little contribution. Understanding those two facts, you can do your own all-manual adjustments and get exposure correct with minimal trial and error.</p>

<p>Dave</p>