How TTL works?

[hikerpix]

I had 3 shots with TTL mode. For each shot, I only change 2 pairs among Aperture, ISO, and Speed, to make the expose meter stay in the middle. I want to see how the TTL flash works. I use D850+50/1.8+SB600.

1st: F8+ISO4000+1/60

2nd: F5.6+ISO2000+1/60

3rd: F4+ISO2000+1/125

From the histogram, you can see the 1st and 2nd are almost exactly same, but the 2nd and 3rd are different. I was expecting the histogram for the 3 shots are same, but unfortunately, they are not, seems the 3rd one (speed+aperture) didn't work the other two. So I want to know why the 3rd one is different? how the TTL control the outputs?

another thought is maybe there is an issue with D850 when it evaluates the output of TTL for the shutter speed changes.

 

[bclaff]

There are a lot of different Nikon "TTL" flash modes and they also interact with metering.

You might want to start by reading Guide to SLR TTL flash modes

 

[tomnorth]

With flash, the shutter speed determines how much the ambient light affects the exposure. If you shorten the shutter speed the flash will put out more of the light. You can see that in the shadow on the wall. It is much more distinct on the third photo. I suspect that drove the slight change in the histograms.

 

[romfordbluenose]

The first two, the EV value is exactly the same. the third one is slightly different due to the difference in speed from 1/60 to 1/125 is not exactly one stop, so I would expect it to be slightly different.

 

[mrjpack]

I had 3 shots with TTL mode. For each shot, I only change 2 pairs among Aperture, ISO, and Speed, to make the expose meter stay in the middle. I want to see how the TTL flash works. I use D850+50/1.8+SB600.

1st: F8+ISO4000+1/60

2nd: F5.6+ISO2000+1/60

3rd: F4+ISO2000+1/125

From the histogram, you can see the 1st and 2nd are almost exactly same, but the 2nd and 3rd are different. I was expecting the histogram for the 3 shots are same, but unfortunately, they are not, seems the 3rd one (speed+aperture) didn't work the other two. So I want to know why the 3rd one is different? how the TTL control the outputs?

another thought is maybe there is an issue with D850 when it evaluates the output of TTL for the shutter speed changes.

If you are using TTL flash, the flash puts out a pre flash that controls the output. It doesn’t always get it exactly the same. If you want consistency , a external light meter and manual settings is the way to go. You can go trial and error in manual but that is very inefficient way to do it...

 

[Bill Janes]

With flash, the shutter speed determines how much the ambient light affects the exposure. If you shorten the shutter speed the flash will put out more of the light. You can see that in the shadow on the wall. It is much more distinct on the third photo. I suspect that drove the slight change in the histograms.

Tom's explanation makes sense. With flash there are always two exposures. The first exposure is from the flash. Since the flash duration is very short, the flash exposure is not affected by shutter speed, and the aperture controls exposure.

The other exposure is from ambient light, and both shutter speed and aperture affect this part of the total exposure.

Hand held light meters still have some utility with flash. My Seckonic Litemaster Pro L-487D gives the ratio of the flash illumination to ambient. Lacking such a meter the OP could redo the experiment and make an exposure with the flash turned off. This would indicate the ambient light exposure.

 

[mrjpack]

With flash, the shutter speed determines how much the ambient light affects the exposure. If you shorten the shutter speed the flash will put out more of the light. You can see that in the shadow on the wall. It is much more distinct on the third photo. I suspect that drove the slight change in the histograms.

Tom's explanation makes sense. With flash there are always two exposures. The first exposure is from the flash. Since the flash duration is very short, the flash exposure is not affected by shutter speed, and the aperture controls exposure.

The other exposure is from ambient light, and both shutter speed and aperture affect this part of the total exposure.

Hand held light meters still have some utility with flash. My Seckonic Litemaster Pro L-487D gives the ratio of the flash illumination to ambient. Lacking such a meter the OP could redo the experiment and make an exposure with the flash turned off. This would indicate the ambient light exposure.

 

[James809]

If you are using TTL flash, the flash puts out a pre flash that controls the output. It doesn’t always get it exactly the same. If you want consistency , a external light meter and manual settings is the way to go. You can go trial and error in manual but that is very inefficient way to do it...

(piggybacking on your post, not directing my reply to you)

Also consider that regular lights cycle multiple times/second. The odds of you catching the same point in the cycle for every photo is, essentially, 1:60 per second (presuming you're in the USA or anywhere that uses 60 hertz electricity).

So that would feed into the histogram, which is a graph of the capture at the time the photo was taken, not the when the light was metered. Shifting to 1/125 means your shutter speed was half the length of the cycle, with 1/60 being the full length of the cycle (again, presuming you're in the USA). The ability of the light to fill the room can also be a factor (ie, older bulbs mixed with newer bulbs).

 

[Bill Janes]

Also consider that regular lights cycle multiple times/second. The odds of you catching the same point in the cycle for every photo is, essentially, 1:60 per second (presuming you're in the USA or anywhere that uses 60 hertz electricity).

So that would feed into the histogram, which is a graph of the capture at the time the photo was taken, not the when the light was metered. Shifting to 1/125 means your shutter speed was half the length of the cycle, with 1/60 being the full length of the cycle (again, presuming you're in the USA). The ability of the light to fill the room can also be a factor (ie, older bulbs mixed with newer bulbs).

An excellent point. Flicker will not occur with incandescent lights, but does occur with most LCDs (which are increasingly popular). CFL bulbs are usually driven with electronic ballast in the megahertz range, as are regular florescent bulbs driven with electronic ballast and these should not have a problem with flicker. The florescents driven with the older magnetic ballasts do flicker.

My D850 does have a flicker warning which comes on when I shoot under household LCD illumination. LCDs used for professional photography are direct current driven and do not flicker.

Bill Janes