How does exposure comp. effect flash

[steven_roberts2]

<p>In his book, "The Hot Shoe Diaries", Joe McNally states the following:</p>

<blockquote>

<p>The camera EV is an exposure-wide adjustments. A global input, if you will. If you program underexposure into the scene, then you're programing underexposure into the flash, as well</p>

</blockquote>

<p>To put this in context, he is discussing Nikon in Aperature Priority Mode.<br>

Am I correct in interpreting this by saying, for example, if I am in Av and set my exposure compensation to -2, my flash power will also be reduced to compensate for the negative exposure compensation? The kicker being that if I reduce camera exposure for background ambient light, I need to then increase flash exposure if I want the subject to remain properly exposed.<br>

If this is true with Nikon, is it also true with Canon?<br>

By the way, I'm really enjoying the book.... some good "real situational" examples with detailed explanations.<br>

Thanks,<br>

Steven</p>

[frank_skomial]

<p>Expposure compensation and Flash compensation are separately programmed values. Depending of flash mode, e.g. iTTL/BL, one affects the other, and Joe is correct in his statement depending on mode, otherwise your concerns are valid ...</p>

[jean_marc_liotier]

<p>That is why you want to use "M" mode for flash photography when you want to control ambient and flash lighting separately.</p>

[picturesque]

<p>As far as I know, what you say above is true. Nikon offers the ability to combine ambient and flash comp--in exactly what way, I don't know, as I use Canon systems. With Canon, one cannot do this. The flash and ambient exposure controls are separate and are controlled separately.</p>

[pankaj purohit]

<p>I usually don't use flash so I also don't have much knowledge about flash so<strong><em> I would also like to add my question here :</em> </strong></p>

<p>I don't' have flash comp. in my camera so if I use M mode and compensate EV, than how the flash will behave...?</p>

[aperryproductions]

<p><a href="http://www.photo.net/photodb/user?user_id=2378894">Pankaj</a>, if you're shooting in manual mode then there wouldn't be any EC. EC is to alter the balance between properly exposed shutter and aperture. If you are shooting Av and a properly exposed f/2.8 image is at 1/200 moving EC down will cause the shutter to go faster and vice versa. If you're in complete Manual mode, than there would be no EC. You can still have FC (but you don't have flash compensation). More information like your camera and flash models would be useful.</p>

[picturesque]

<p>Pankaj--what camera do you have? Alex, I think Pankaj was referring to FLASH comp while using the camera in manual mode. I don't know about Nikon, but with a Canon camera and flash, M or manual camera mode will give you what you input re the ambient exposure. The flash exposure should be compensated as you instruct. If you input -1 for the flash exposure comp, it should be ramped down by one stop (the FLASH exposure only).</p>

[mike_bisom]

<p>OK- it's late and maybe I just can't keep up ;)</p>

<p>Nadine- I don't think the Nikon system differs that much from Canon. I can control FEC on the camera and I don't remember being able to do this on the Canon- but FEC would still be separate from EC. </p>

<p>Alex- you can dial in EC using manual. If you dial in -1, your in-camera meter will give you a -0- with the -1 EC factored in. As example, if the in camera meter is -0- at 1/200th f/4.0 and you dial in -1 EC, the meter will now read -0- at 1/400th f/4 or 1/200 at f/5.6. This then has the same effect on the flash- whether in manual or AV mode: the camera metering the scene one way and dialing in EC changes the parameters. Now this is a "global" adjustment in so much as the flash will consider what the camera is metering but the effect of this, whether using iTTL or eTTL, is difficult to judge because both the Nikon and the Canon system fire a pre-flash in order for the flash to determine how it will fire. While I realize this pre-flash will "combine" with the camera metering to determine correct flash exposure, I believe that more weight is put on the pre-flash than any EC. Of course you could still dial in +/- FEC. To further complicate things, using the original example, I may dial in - EC but NOT + FEC since I find that with my Nikon flash I am usually dialing in - FEC (whereas with the Canon I was dialing in + FEC almost all the time).</p>

<p>I guess in summary, I don't see that much technical difference between Nikon and Canon (though I believe Nikon's evaluative metering to be more accurate). EC controlling EC, FEC controlling FEC with the pre-flash providing at LEAST some sort of information. Since we can't determine what information the pre-flash is gathering in any given situation, it is a variable making it difficult to conclusively say how much effect EC has on overall flash exposure. Which is a good reason to consider shooting manual in the flash as well!!</p>

[pankaj purohit]

<p>Thanks Alex and Nadine, I have a Nikon F75 film SLR and its maximum flash sync is 1/90, It doesn't has flash compensation, EV can be compensated only.</p>