TTL Flash Readout

[jmtonkin]

This might be a stupid question, but I couldn't find the answer anywhere.

If I shoot my camera on Priority mode, it tells me all of the settings (flash, shutter speed, ISO). If I fire my flash in TTL, does it tell me anywhere what power it shot at? I feel as if that would be a great way to take out a lot of the guess work with my flash, but still get consistent power.

 

[Braineack]

If it'd be anywhere it'd be EXIF, but I've never seen anything like that.

 

[jmtonkin]

That was my thoughts, too. I didn't think it was possible, but I remained hopeful.

 

[Scatterbrained]

If you want to take the guesswork out you could buy a light meter.

 

[WayneF]

You can have a very rough idea about flash power level by watching the Ready LED. Maximum power will take 2 or 3 seconds to come Ready again, a long time. Low power will become Ready almost instantly. So you can differentiate somewhat that way. Watching the Ready LED for different levels of manual mode flashes once will give you a fair idea what it is doing. This works out to be more useful than it may sound right now.

A light meter won't work for TTL. Preflash interferes.

More than we care, but you can experimentally determine it by duplicating the TTL effort with Manual flash mode. The manual level that gives the same exposure would be what TTL did. I can't think what use that might be?

The camera does not know the flash power level. I think it is a communication that does not exist. It does not always even know what model of flash it is (certainly does not know about Remotes with the Commander, and there are third party flash brands too). So it cannot set "1/2 power", it does not know what that would be on that model. Instead the camera meters the preflash, and then tells the flash "now 5.3 stops more than that". The flash does it if it can.

The Nikon Exif does have a value Exposure Difference, between what ambient meters, and what it ought to be. Flash fills in that gap.

Speaking of Nikon TTL flashes, the really important thing to know is to watch the Ready LED for additional warning blinks for three seconds after the shot. That tells you the camera was requesting more power than the flash model was able to deliver, so it is a warning that the picture may be underexposed. And the flash LCD then tells you by how much, -1/3 stop, etc (actually, it blinks for 0 EV too, it may not be a real problem, but it tells you 0 EV then). You can force this (to see it once) by bounce flash on the ceiling, at say f/16, which cannot work (at low ISO). The flash manuals describe this, normally called "When insufficient flash output for correct exposure is indicated". In your SB-900 manual, page D-4. Something we really need to be aware of. This excessive warning tells you that flash compensation could NOT deliver any more, so you have to change some setting, aperture or ISO, or step up to a closer distance.

 

[Scatterbrained]

.......

A light meter won't work for TTL. Preflash interferes.

.....

Obviously, but I get the impression the OP wants to know what power level the camera is selecting so he can then lock it there, similar to the way some people will put the camera in Auto, take a shot, and then use those settings in M rather than just use the meter in the camera to determine their settings.

 

[CameraClicker]

There is a lot of information in EXIF. Unfortunately Canon does not seem to fill in flash power.

Here is the list:
ExifTool Version Number : 8.19
File Name : 2014-08-29_19-14-06_322C9295.CR2
Directory : .
File Size : 23 MB
File Modification Date/Time : 2014:08:29 19:16:10-04:00
File Permissions : rw-rw-rw-
File Type : CR2
MIME Type : image/x-raw
Exif Byte Order : Little-endian (Intel, II)
Image Width : 5760
Image Height : 3840
Bits Per Sample : 8 8 8
Compression : JPEG (old-style)
Make : Canon
Camera Model Name : Canon EOS 5D Mark III
Preview Image Start : 90400
Orientation : Horizontal (normal)
Preview Image Length : 1440341
X Resolution : 72
Y Resolution : 72
Resolution Unit : inches
Modify Date : 2014:08:29 19:14:06
Artist : Artist
Rating : 0
Copyright : 2013 Artist
Exposure Time : 1/200
F Number : 6.3
Exposure Program : Manual
ISO : 100
Exif Version : 0230
Date/Time Original : 2014:08:29 19:14:06
Create Date : 2014:08:29 19:14:06
Components Configuration : Y, Cb, Cr, -
Shutter Speed Value : 1/197
Aperture Value : 6.4
Flash : On, Fired
Focal Length : 35.0 mm
Macro Mode : Normal
Self Timer : Off
Quality : RAW
Canon Flash Mode : External flash
Continuous Drive : Continuous, Low
Focus Mode : One-shot AF
Record Mode : CR2
Canon Image Size : Unknown (-1)
Easy Mode : Manual
Digital Zoom : None
Contrast : Normal
Saturation : Normal
Metering Mode : Center-weighted average
Focus Range : Not Known
Canon Exposure Mode : Manual
Lens Type : Canon EF 16-35mm f/2.8L II
Long Focal : 35 mm
Short Focal : 16 mm
Focal Units : 1/mm
Max Aperture : 2.8
Min Aperture : 23
Flash Activity : 150
Flash Bits : Manual, External
Zoom Source Width : 0
Zoom Target Width : 0
Manual Flash Output : n/a
Color Tone : Normal
SRAW Quality : n/a
Auto ISO : 100
Base ISO : 100
Measured EV : 6.75
Target Aperture : 6.3
Target Exposure Time : 1/203
Exposure Compensation : 0
White Balance : Flash
Slow Shutter : None
Shot Number In Continuous Burst : 0
Optical Zoom Code : n/a
Camera Temperature : 23 C
Flash Exposure Compensation : 0
Auto Exposure Bracketing : Off
AEB Bracket Value : 0
Control Mode : Camera Local Control
Measured EV 2 : 6.875
Bulb Duration : 0
Camera Type : EOS High-end
ND Filter : Unknown (-1)
Canon Image Type : Canon EOS 5D Mark III
Canon Firmware Version : Firmware Version 1.0.7
Owner Name :
Canon Model ID : Unknown (0x80000285)
Thumbnail Image Valid Area : 0 159 7 112
AF Area Mode : Single-point AF
Num AF Points : 61
Valid AF Points : 61
Canon Image Width : 5760
Canon Image Height : 3840
AF Image Width : 5760
AF Image Height : 3840
AF Area Widths : 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190 190
AF Area Heights : 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188 188
AF Area X Positions : 288 0 -288 1520 1232 944 639 288 0 -1520 -1232 -944 -639 -288 1520 1232 944 639 288 0 -1520 -1232 -944 -639 -288 1520 1232 944 639 288 0 -1520 -1232 -944 -639 -288 1520 1232 944 639 288 0 -1520 -1232 -944 -639 -288 1520 1232 944 639 288 0 -1520 -1232 -944 -639 -288 288 0 -288
AF Area Y Positions : -648 -648 -648 -433 -433 -433 -433 -433 -433 -433 -433 -433 -433 -433 -215 -215 -215 -215 -215 -215 -215 -215 -215 -215 -215 0 0 0 0 0 0 0 0 0 0 0 215 215 215 215 215 215 215 215 215 215 215 433 433 433 433 433 433 433 433 433 433 433 648 648 648
AF Points In Focus : 27
AF Points Selected : 27
Bracket Mode : Off
Bracket Value : 0
Bracket Shot Number : 0
Raw Jpg Size : Large
WB Bracket Mode : Off
WB Bracket Value AB : 0
WB Bracket Value GM : 0
Live View Shooting : Off
Flash Exposure Lock : Off
Lens Model : EF16-35mm f/2.8L II USM
Internal Serial Number :
Dust Removal Data : (Binary data 1024 bytes, use -b option to extract)
Exposure Level Increments : 1/3 Stop
ISO Speed Increments : 1/3 Stop
AEB Auto Cancel : On
AEB Sequence : 0,-,+
AEB Shot Count : Unknown (5); 2
Safety Shift : Disable
Custom Controls : 0 0 0 2 0 0 0 0 0 0 0 4 0 8 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 7 3
Dial Direction Tv Av : Normal
Add Aspect Ratio Info : Off
Tone Curve : Standard
Sharpness : 3
Sharpness Frequency : n/a
Sensor Red Level : 0
Sensor Blue Level : 0
White Balance Red : 0
White Balance Blue : 0
Color Temperature : 5200
Picture Style : User Def. 1
Digital Gain : 0
WB Shift AB : 0
WB Shift GM : 0
Measured RGGB : 536 1024 1024 458
VRD Offset : 0
Sensor Width : 5920
Sensor Height : 3950
Sensor Left Border : 140
Sensor Top Border : 96
Sensor Right Border : 5899
Sensor Bottom Border : 3935
Black Mask Left Border : 0
Black Mask Top Border : 0
Black Mask Right Border : 0
Black Mask Bottom Border : 0
AF Micro Adj Active : No
AF Micro Adj Value : 0
Peripheral Lighting : On
Peripheral Lighting Value : 70
Original Image Width : 5760
Original Image Height : 3840
Peripheral Lighting Setting : On
Auto Lighting Optimizer : Off
User Comment :
Sub Sec Time : 14
Sub Sec Time Original : 14
Sub Sec Time Digitized : 14
Flashpix Version : 0100
Color Space : sRGB
Exif Image Width : 5760
Exif Image Height : 3840
Interoperability Index : R98 - DCF basic file (sRGB)
Interoperability Version : 0100
Focal Plane X Resolution : 3942.505133
Focal Plane Y Resolution : 3950.617284
Focal Plane Resolution Unit : inches
Custom Rendered : Normal
Exposure Mode : Manual
Scene Capture Type : Standard
GPS Version ID : 2.3.0.0
Thumbnail Offset : 79984
Thumbnail Length : 10413
Photometric Interpretation : RGB
Samples Per Pixel : 3
Rows Per Strip : 395
Planar Configuration : Chunky
Strip Offsets : 3267448
Strip Byte Counts : 21261506
Raw Image Segmentation : 1 2960 2960
Aperture : 6.3
Flash Exposure Compensation : 0
Drive Mode : Continuous Shooting
Flash Type : External
Image Size : 5760x3840
Lens : 16.0 - 35.0 mm
Lens ID : Canon EF 16-35mm f/2.8L II
Preview Image : (Binary data 1440341 bytes, use -b option to extract)
Red Eye Reduction : Off
Scale Factor To 35 mm Equivalent: 1.0
Shooting Mode : Manual
Shutter Curtain Sync : 1st-curtain sync
Shutter Speed : 1/200
Create Date : 2014:08:29 19:14:06.14
Date/Time Original : 2014:08:29 19:14:06.14
Modify Date : 2014:08:29 19:14:06.14
Thumbnail Image : (Binary data 10413 bytes, use -b option to extract)
Circle Of Confusion : 0.031 mm
Field Of View : 55.8 deg
Focal Length : 35.0 mm (35 mm equivalent: 34.0 mm)
Hyperfocal Distance : 6.28 m
Lens : 16.0 - 35.0 mm (35 mm equivalent: 15.5 - 34.0 mm)
Light Value : 13.0

 

[jmtonkin]

.......

A light meter won't work for TTL. Preflash interferes.

.....

Obviously, but I get the impression the OP wants to know what power level the camera is selecting so he can then lock it there, similar to the way some people will put the camera in Auto, take a shot, and then use those settings in M rather than just use the meter in the camera to determine their settings.

That's exactly what my end-goal is.