topic Re: T8i ISO too high in EOS DSLR & Mirrorless Cameras

T8i ISO too high

stevet1 — Sun, 28 Dec 2025 19:46:34 GMT

I seem to have developed a problem with my T8i, and I'm not sure what to do about it.

It seems to involve Auto ISO.

When I first turn on my camera and go to take a picture, I half-press the shutter to meter the scene, and set my settings to zero out the exposure. I then fully press the shutter to take the picture. The camera, however, takes a picture that is overexposed.

If I half-press the shutter and take another meter reading a second time with the exact same settings, the camera will take a picture that is more properly exposed.

Here is an example. I used a tripod, so that I didn't inadvertently shift my framing and composition. In both pictures, the settings were 1/500, f/8, ISO 1250.

At this point, I don't know what to except take either two pictures of everything, or stop using Auto ISO. I don't know if I can trust it anymore.

Sometimes that first shot is completely whited out.

Steve Thomas

Re: T8i ISO too high

deebatman316 — Sun, 28 Dec 2025 19:54:27 GMT

If your shots are completely white I'm wondering is the lens aperture always stopping down. What lens are you using when this happens. Is it happening across multiple lenses or just one. What lenses are you using please use full names of the lenses being used. Can you share a file where the picture is completely white. That sound like a classic sign of the aperture blades not stopping down. Could it be the light meter being fooled by the background.

Re: T8i ISO too high

stevet1 — Sun, 28 Dec 2025 20:56:58 GMT

Demetrius,

I'll have to some experimenting.

So far, I've only noticed it on my 18-135 f/3.5-5.6 .IS USM lens, because that stays on my camera about 90% of the time. I have a 55-250 and a 50mm prime. I'll have to see if that behavior repeats itself across lenses.

If I get another one that's completely whited out, I'll pass it along.

If it is a case of the aperture not stopping down, is there a fix for that?

Steve Thomas

Re: T8i ISO too high

deebatman316 — Sun, 28 Dec 2025 20:58:55 GMT

The fix is to send it to Canon for repair. I’m not sure Canon still services that lens. Though last generation EF lenses are still supported but always check with Canon.

Re: T8i ISO too high

stevet1 — Sun, 28 Dec 2025 22:19:04 GMT

Demetrius,

I tested it out on my 55-250, and it did the same thing. The first shot is over exposed, but subsequent shots are OK.

It's odd, but on repeated cycles of turning the camera off and on, it's only that first shot when I take that first picture, that it's overexposing. From then on, it behaves.

I tried switching over to Av Mode with Auto ISO and didn't notice any variation in exposure, but I was in the middle of several off and on cycles, so that may not have been a legitimate test.

I guess I'll just either have to take one throwaway picture, or manually select an ISO using Live View and then fine tune the ISO to my liking.

Steve Thomas

Re: T8i ISO too high

deebatman316 — Sun, 28 Dec 2025 22:32:32 GMT

Compare what settings that VF and LV give you before you take the first picture. This sounds like a camera problem and not a lens issue.

Re: T8i ISO too high

Waddizzle — Mon, 29 Dec 2025 01:51:39 GMT

“When I first turn on my camera and go to take a picture, I half-press the shutter to meter the scene, and set my settings to zero out the exposure. I then fully press the shutter to take the picture. The camera, however, takes a picture that is overexposed.”

I have a few questions. Most of which might be answered with a sample RAW file. Metering mode? AF mode and active AF point(s), shooting mode, etc.

I don’t understand the highlighted part. What setting are you adjusting? Why are you adjusting exposure if you’re using ISO Auto?

Re: T8i ISO too high

stevet1 — Mon, 29 Dec 2025 02:20:07 GMT

Waddizzle,

I was clumsy in my wording. I'm not changing my settings. I'm just making sure the meter is zeroed out.

Most of the time, I'm using Evaluative Metering, AI Servo with BBF, Single Point Autofocus in Manual Mode.

Steve Thomas

Re: T8i ISO too high

SignifDigits — Wed, 31 Dec 2025 16:43:05 GMT

Unless you guys (who are already wizards at the settings, etc.) figure this out, I'm guessing it's a degraded electronic component that's causing either a) the Digic 8 processor to compute and perform a slightly extended physical shutter release or b) a slight miscalculation in ISO. A sticky shutter seems like it would happen every release. I'm sure the shutter draws from a capacitor and they degrade over time, so it could be reporting an inaccurate state of charge to the processor.

I don't know what would cause the processor to perform an incorrect ISO/gain calculation, but some component in the chain could do that. And, whilst I'm grasping at straws here, I have little in the way of clues why the camera would perform improperly after power on then ok from then on. I'm sure there are some functions that are performed only at that time by the camera RTOS (real-time operating system), and suspect that one of those checks is sending some bad info to the Digic 8 processor.

From the list that I have it appears no longer repaired by Canon, so hopefully you can find a setting to fix it.

Re: T8i ISO too high

stevet1 — Wed, 31 Dec 2025 18:50:28 GMT

SignifDigits,

In researching this issue, I discovered that it is not an uncommon phenomenon.

If you do a Google search for Overexposure and Auto ISO, you see entry after entry of people who experience the same thing. I even read one person who said that with Auto anything, Canon lands on the side of caution and has a tendency to lighten things up as opposed to underexposing.

I think I'll just stick with ISO100, and raise it manually as I need to. That's more time consuming, but it's better than the alternative.

Steve Thomas

Re: T8i ISO too high

SignifDigits — Wed, 31 Dec 2025 18:53:09 GMT

LOL - just a "feature" then, not a "bug". That's actually good news-ish, I suppose!

Re: T8i ISO too high

Waddizzle — Wed, 31 Dec 2025 22:03:31 GMT

“ Most of the time, I'm using Evaluative Metering, AI Servo with BBF, Single Point Autofocus in Manual Mode. “

Are those photos still inside the canera? Do they display a locked AF point?

I ask because Evaluative Metering mode biases the exposure to wherever the active and locked AF point is located. If there isn’t an active and locked SF point, then the exposure is biased to center AF point.

Note , this means when you release the BBF button the AF system goes idle!

One Shot AF locks exposure when focus is locked by a half press. Exposure remains locked until you either fully press the shutter or release the half press.

Servo AF is not as subtle. It locks exposure when the shutter is fired. But the same caveats regarding the need for an active and locked AF point still applies.