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Flash Tests

This page covers some test with the Canon EOS 20D and were prompted by reported issues with flash exposure metering, particularly at higher ambient light levels.

All files are RAW files and any adjustments for colour bias (only) will be visible if opened in DPP. Save by right clicking and selecting "save target" or double left click and select "download" from the pop-up, unless you have an application that can open RAW files as a double click.

The shots all consist of an average gray card or in some cases an average gray card and white paper to look for bias due to the selected autofocus point (fp). The white paper is always on the left of the centre fp with the line between gray and white on the centre fp.

Measured tone is derived by converting using DPP's default tone curve to JPG compression 6, 0 sharpening, and noting the gray section mean histogram level in Photoshop 7. The gray area was cut and pasted into a new document in the event of the white paper being used in the shot.

The change in histogram level was also calibrated using the digital brightness control in PDD to be typically 49 steps per stop.

This permitted the change in tone to be reported in stops, relative to the first tone in each of the tables below.

Note the 18-55mm does not report focus distance to the camera body so the effects with a lens that has this capability can be expected to vary.

Ambient Level 1

This is odd as an earlier set of experiments that I did not keep seemed to show about a 1-stop bias with left or right fp under the 50% white and 50% gray condition. 

It is probably worth repeating this part of the experiment to check this.

Ambient Level 2

An interesting point is when the shutter speed is slow in aperture or shutter priority there is a tendency for about half a stop over exposure relative to the reference ambient only exposure.

Ambient Level 3

Ambient Level 4

Bright Indoor Daylight (Evaluative Metering Using internal flash and the 18-55mm kit lens shot range ~ 1m)

NB As this is daylight there may be some small variation in light levels from frame to frame although all the exposure are automatic exposure.

We see again the fill in flash has a tendency to cause over exposure by about half a stop.

For ambient light with a half gray half white frame undexposure can occur between 0.4 and 1.2 stops depending on the selected auto focus point.

For ambient plus flash light with a half gray half white frame undexposure can occur between 0.25 and 0.6 stops depending on the selected auto focus point, for some reason fill in flash seems to be affected less.

General Observations

A general observation is that for 50% gray and 50% white frames the white part of the card has a tone level of about 202 which is about 1 stop above the frame average and 2.2 stops above average gray, ie the camera has metered correctly. 

However, this tone level looks quite dull, a brightness adjustment for a comfortable level in photoshop on the screen is needed of about +46 (1 stop), resulting in a white area tone level of 250.

This is why so many say exposure is under on flash shots, the under exposure just represents the headroom in the 20D's dynamic range. 

This is less noticeable in outdoor photography as there will often be sources of light such as the sky filling up the top end of the dynamic range.

This will not be a problem as long as one shoots RAW, and makes the correct adjustments to ensure a balance between dynamic range and perceived tonality.

Last Updated 05/06/2008

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