Conditions:
- Normal indoor ambient lighting
- Subject five feet away is a small statue of a boy on a horse placed in the center of the frame to be sure the flash metering will measure it properly
- Background is a bookcase 10 feet away
- Camera in Matrix metering
- Flash in TTL mode
Note: The images in this study are directly from the D200 camera without any modification except compression to make them suitable for uploading.
Image 1: Flash OFF, Camera A mode, 1/25th, f/2.8, Camera ev=0
I shot Image 1 with the flash Off using camera A mode with f/2.8 selected. I could see in the viewfinder that the camera automatically selected 1/25 shutter. Notice that the image is properly exposed with pixels in the histogram extending all the way from black shadows on the left to bright highlights on the right and there are some shadows on the face and is approximately the same brightness as the background. The yellow color is due to the incandescent lighting and the poor Auto White Balance function on the D200. I will not try to correct or balance the color during this study.Image 2: Flash On in TTL mode, FEC=0, Camera A mode, 1/60, f/2.8, Camera ev=0 Note: FEC means Flash Exposure Compensation
In Image 2 the flash was used, and as soon as the flash was turned ON, the shutter changed to 1/60th, which is the lowest available flash shutter setting as determined by CSM e2. Note that the faster shutter darkened the background a bit compared to Image 1, which is to be expected. You can see how the dark pixels are bunched up against the left edge of the histogram, meaning that detail is being lost in the shadows. The hump of pixels on the right come from the subject. Note that the histogram shows that there are no blown out pixels (ie, climbing up the right edge), but the subject still looks too bright for the scene. This happened because the flash metering system measures a little too 'hot' on my D200/SB800 combination. I think this is a very common thing on the D200/SB800 combinations. That's why I normally dial in -.7 ev on the flash for indoor shots (but this is a sidebar and not the main subject of this study).
Notice also that the color of the bookcase in the background is still yellow since it is lighted mainly by the ambient incandescent lights while the subject is much whiter since it is lighted mainly by the flash.
Image 3: Flash ON in TTL mode, FEC=0, Camera A mode, 1/100th, f/2.8, Camera ev = -2
In Image 3 I decreased the camera EV to -2.0. You can see that both the background and the subject got darker (just like the SB800 book says they should). You can also see that the hump of pixels on the right moved left just like the darker pixels on the left, piling more of them up on the left edge. The question I am studying is exactly how did the camera & flash achieve this left shift of the pixels?
Well, I observed that the shutter automatically increased to 1/100, which darkened the background. You would have expected that when starting at 1/60th it would have increased to 1/240th to achieve two stops darker. However, I have discovered that the camera knew the ambient was actually at 1/25, and 1/100 is exactly two stops darker than 1/25. Pretty smart system.
Remember that changing shutter speed does not affect the flash portion of the exposure, because the flash occurs is so short (less than 1/1000th second).
And you can see that changing the camera ev definitely darkens the subject as well, which means it has to decrease the flash power. That indicates that the camera compensation information is sent back to the flash metering system when shooting in TTL mode.
So, when you change the camera ev, it changes the camera f/ stop or shutter speed settings to adjust the background, and it sends the ev change to the flash to change the flash ev by that same amount to adjust the subject.
Image 4: Flash OFF, Camera M mode, 1/25th, f/2.8, Camera ev = 0
To further verify exactly what the camera ev settings tell the flash, I first put the camera in Manual mode and set 1/25 and f/2.8 and shot Image 4 above without flash as a reference. Note that Image 4 and its histogram looks essentially identical to Image 1, as it should, since the same camera settings were used.Image 5. Flash ON in TTL mode, FEC=0, Camera M mode, 1/60th, f/2.8, Camera ev=0
Then in Image 5 I increased the shutter to 1/60th and fired the flash, and this image looks just Image 2, as it should.Image 6: Flash ON in TTL mode, FEC=0, Camera M mode, 1/60th, f/2.8, Camera ev = -2
Now, in Image 6, with the camera in Manual mode, 1/60th, f/2.8, you can see the effect of setting camera ev to -2. Comparing with Image 5, the subject in Image 6 definitely got darker, and the histogram hump on the right moved slightly left, but the background didn't change. This proves that ev changes on the camera are sent to the flash to the change its power, and the reason the background didn't change is because ev changes to the camera will not affect the f/ stop and aperture when in camera Manual mode. In the previous example, the camera was in A mode, and camera ev changes will change the shutter speed.Image 7: Flash OFF, Camera M mode, 1/250th, f/2.8, Camera ev= 0
I took this shot without flash as a reference image to show just how little ambient there is at 1/250th. 1/250th virtually takes the ambient light out of the exposure in normal indoor lighting.
Image 8: Flash ON in TTL mode, FEC=0, Camera M mode, 1/250th, Camera ev=0
In Image 8 I turned on the flash and took the shot. Notice the dark background, because the flash exposes the subject correctly, but only a small amount reaches the background five feet behind the subject (due to the square law - power decreases by the square of the distance).Image 9: Flash ON in TTL mode, FEC=0, Camera M mode, 1/250th, f/2.8, Camera ev = -2
Then, in Image 9 camera ev was reduced by 2 ev and the brightness of the subject (power of the flash) definitely decreased compared to Image 8. The brightness of the background barely changed, though because not much of the flash power is getting back there. This is showing that you can adjust the brightness of the subject without changing the background by changing flash power.Image 10: Flash On in TTL mode, FEC = -2, Camera M mode, 1/250th, f/2.8, Camera ev = 0
Lastly, in Image 10 I set the camera at 1/250, f/2.8, and ev 0, and I used the Flash ev on the back of the flash to -2 ev. As you can see, the image looks exactly like Image 9, and the hump of pixels on the right (representing the subject) is in the same spot on the histogram.
This study proves conclusively that camera ev is sent to the flash in TTL mode and can be used to adjust flash power, just like the FEC button on the front of the camera, and the FEC button on the flash itself.
19 comments:
Very nice study. Now I think I understand the subtle difference in behavior between the semi-automatic modes and the full manual mode when playing with the EV compensation.
Well done!
Stéphan
PS: There is a type in the heading for image #7. It should read 'flash off'.
Stephan,
Thanks for the nice comments. I'll fix the typo.
Keep this stuff coming my friend. I love it! D300 guy here who also shoots weddings. One should always be learning and trying new concepts!
Another fine piece of article!! I don't think anyone would have explain clearly how the camera EV compensations affect the flash output... well done!! Now I understand more about the CLS system!!
Raymond,
I appreciate your nice feedback.
Thanks,
Russ
For me there was another lessson -
"With camera in Manual Mode, Camera EV has no effect on Aperture/Shutter Speed". I never realised that. Thanks a lot and keep the great articles coming.
Best Regards
Russ,
Was your flash pointing directly at the statue? Was it bare flash or through an umbrella? Or, were you bouncing flash?
I'm guessing you were shooting bare flash at the subject?
Thanks,
Matt
Hi Matt,
For this study I was using the bare flash pointed directly forward, because I just wanted to study what happens when you change camera ev while using flash in TTL mode.
Bouncing or using an umbrellas doesn't change anything regarding the effects of changing camera ev (as long as you stay within the power limits of the flash).
Russ
Great article & great web site. This cleared a few things up in my mind.
I would be grateful if you could clarify one thing: When in manual, are the EV and the FEV settings combined. For example, if EV is set to +3 and FEV is set to +1 will this mean, in practice, a cumulative increase of +4, or does one cancel the other out?
Hi Tim and Jo,
Thanks for the compliments!
In TTL mode only, the camera EV and the Flash EV are cumulative on the flash setting but not on the camera exposure.
In other words, +2ev on the camera and +1 flash EV will result in the flash at +3 EV. However the Flash EV doesn't add to the camera exposure. So, in this example, the camera will stay at +2ev and the flash will be at +3 ev.
TTL-BL works differently, and somewhat unpredictably, so I never use camera ev when using TTL-BL unless I feel adventuresome :)
Hope that helps,
Russ
Hello Russ,
I'm an D80 owner and trying to follow by your steps here. My investigation is stopped at Image3 when TTL flash is ON, with camera in A-mode and EV=-2. Though the picture becomes darker (as it should be), the shutter remains the same as with EV=0 (1/60).
So in my case the negative compensations lowers the flash power only, but how about the shutter? From this one could conclude that FEV and EV do the same thing, but this is VERY, VERY strange...
Is it an D80 issue or there is something wrong with camera's settings? Or something else? Could you tell?
And again, let me acknowledge my appreciation and many thanks for the great job you are doing here.
BR,
j0t.
Hi j0t,
I think that in your case, the ambient light must have been dimmer than in my experiment.
I think that 1/60th was already 2 stops darker than the ambient, so it didn't get any faster.
Check the ambient with the flash turned off. I bet it will be much lower than my 1/25th. This also has to do with the lens. If you don't have f/ 2.8 and are using say f/3.5, then the ambient shutter could be around 1/4th. Then two stops darker would only be 1/16th. Obviously if the shutter is limited to 1/60th it can't do anything.
Hope this is clearer,
Russ
Hello Russ,
Thanks for quick response!
The tests has been taken with kit lens, so f/5.6.
My ambient shutter was 1.5" when EV=0 and 1/2.5 when EV=-2
(1.5/4=0.375 apx. 0.4=1/2.5)
Also you meant the shutter speed limitation.
Yes, it could be the reason...
But where it could be controled?
My CS24 (Flash Shutter Speed) menu has only the values from 1/60s to 30s.
Thanks again,
j0t
Hi j0t,
That explains it. The ambient shutter of 1.5 sec was way too slow to be affected by your -2 ev change on the camera because the shutter is fixed at 1/60th as a minimum when shooting flash. You can make it longer, as you saw, but I have no idea why you would want to do that.
+2 ev over 1.5 sec is about 1/4th sec (divide 1.5 in half twice). This is obviously way below the 1/60th limit, so the 2 ev camera change will not affect your shutter.
So, the reason you can't duplicate my test is that your kit lens doesn't open up wide enough to allow a high enough shutter speed. However, your lens should open up to f/3.5 at the wide end (18mm I think), and in bright ambient light you might see the effect I mentioned.
In normal use, you won't be able to make you backgrounds bright without going to very low shutter speeds, and to do that you need to switch to either camera Manual or Shutter priority mode.
Try putting your camera in Manual mode with the aperture wide open and setting the shutter to 1/4th, I think you will get nice backgrounds. However, you might also get some ghosting, so you should select Rear Sync to place the ghost trails on the correct side of the motion.
Russ
Hello Russ,
Of course! My light conditions requires 1/4, but
shutter speed limitation do not allow this. Silly me!
In addition to your advise (to use wider aperture or bright ambient light)
is it possible to reproduce the effect just increasing ISO?
And obviously, M mode (or S, with some restrictions) will give to all of us what we want :)
Thank you very much for clear explanations,
you are just the wizard of flash :)
j0t.
j0t,
Yes, ISO will do the same thing as opening the aperture. If you double the ISO, that is equivalent to opening the aperture by one stop.
I wouldn't go higher than ISO 400, however, or your images will get very noisy.
Regards,
Russ
Great blog!
Hi Russ,
about compensation of exposition and brightness of background and subject,I've a question: does the zoom of the flash influence brightness of frame?Ie,I try to fire some shot at the same subject,aproximately at the distance of 3 foot,and changin' zoom of flash with the same camera zoom (set on 70mm),I observed that Histograms were similar from flash on 24mm to flash on 85mm;so flash set power differently for same subject with different zoom of flash?It means that if I use a zoom flash different from zoom camera I have always subject exposed correctly (TTL),naturally if subject is near the camera?And so is there any vantage use a different zoom between flash and camera?
Maurizio
Maurizio,
Zooming in on a subject does not change the distance to the subject, so the flash power should theoretically stay constant.
However, if you have read my blogs, you might remember that the flash sets its power based on the reflected energy of the monitor preflashes from the center of the frame. So TTL may set a slightly different flash power when you zoom in, because the subject occupies a larger portion of the frame, which can cause the reflected energy from the monitor preflashes to change slightly.
Zooming in will sometimes give a more accurate flash setting on the subject. Sometimes I zoom in, hit FV Lock, zoom back out to take the picture. This will allow you to avoid reflections from things that are nearer to the camera than the subject.
Also remember that every time you zoom, you have to refocus. Most lenses will not hold their focus as they are zoomed.
Russ
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