topic Re: Long exposure limit in EOS DSLR & Mirrorless Cameras

Long exposure limit

Tsleel2811 — Tue, 24 Jan 2017 16:04:02 GMT

Rather stupid question, I think. I own a 60d and I want to get into astrophotography. I know the effective iso range is about 1000. Maybe 1600 if I'm good with lightroom noise reduction. the maximum exposure time is 30 sec. But in bulb mode I can leave it open indefinitely so long as I keep the button pressed. Most photography tutorials say that exposure times can reach upwards of 3 minutes if I want visible streaks. My question is as follows: can the sensor be damaged if I leave the Shutter open too long in nighttime conditions. I'm thinking overheating or too much light hitting the sensor.

Thanks for your input....

Re: Long exposure limit

Bryston3bsst — Tue, 24 Jan 2017 16:37:54 GMT

Doing astro you should use a remote shutter release so you don't touch the camera with the shutter open. And you shouldn't have to hold the release in bulb. I use a Vello remote and the shutter stays open without holding after a few seconds and a second press will close it.

My understanding too is the best star trails are the result of stacking many 30 second exposure as oppsed to leaving the shutter open for 3 minutes. Plus that time duration would require a tracking device to compensate for the earth's rotation.

And please note...I am not an astro guy. I'm sure a more experienced astro person will probably chime in here.

Re: Long exposure limit

Tsleel2811 — Tue, 24 Jan 2017 17:20:59 GMT

I should've specified. Im using a remote as well. I guess stacking would be the best. But my question is can the sensor get ruined of left open for too long at night time

Re: Long exposure limit

Tsleel2811 — Tue, 24 Jan 2017 17:22:13 GMT

And I don't need to use a smart tracker because me desired result would be the earth standing still and the stars moving. So a regular tripod would do. I'm using a heavy one (Manfrotto) so it stands firm anyway

Re: Long exposure limit

kvbarkley — Tue, 24 Jan 2017 17:43:28 GMT

No, the sensor will be fine. But if it does heat up you might get more noise and "hot" pixels.

I think that video overheats because of all the digic processing and readout, not the actual light gathering. For a still image, nothing much is happening when the shutter is open.

Re: Long exposure limit

Waddizzle — Wed, 25 Jan 2017 18:01:11 GMT

@Tsleel2811 wrote:
And I don't need to use a smart tracker because me desired result would be the earth standing still and the stars moving. So a regular tripod would do. I'm using a heavy one (Manfrotto) so it stands firm anyway

Actually, you may want to use a tracker, if you want to have circular star trails around the North or South pole stars.

Re: Long exposure limit

RobertTheFat — Wed, 25 Jan 2017 18:23:47 GMT

Waddizzle wrote:
Tsleel2811 wrote:
And I don't need to use a smart tracker because me desired result would be the earth standing still and the stars moving. So a regular tripod would do. I'm using a heavy one (Manfrotto) so it stands firm anyway
Actually, you may want to use a tracker, if you want to have circular star trails around the North or South pole stars.

In astronomy class in college we were taught that there is no South pole star. Have they found one in the meantime?

Re: Long exposure limit

Waddizzle — Wed, 25 Jan 2017 19:12:57 GMT

@RobertTheFat wrote:
@Waddizzle wrote:
@Tsleel2811 wrote:
And I don't need to use a smart tracker because me desired result would be the earth standing still and the stars moving. So a regular tripod would do. I'm using a heavy one (Manfrotto) so it stands firm anyway
Actually, you may want to use a tracker, if you want to have circular star trails around the North or South pole stars.
In astronomy class in college we were taught that there is no South pole star. Have they found one in the meantime?

https://en.wikipedia.org/wiki/Pole_star

There is a star, Sigma Octantis, that is used for aligning telescopes in the Southern Hemisphere. Like Polaris, it is not precisely aligned with the Earth's polar axis, and is visible to the naked eye. It is not as closely aligned with the Earth's axis of rotation as Polaris.

Re: Long exposure limit

TCampbell — Wed, 25 Jan 2017 23:48:01 GMT

I have a 60Da which is Canon's camera specifically designed especially for astrophotography (the standard 60D filter is replaced with a different filter which allows more full-spectrum light through and dramatically increases sensitivity in the "reds" (camera's designed for normal photography actually trim the visible light spectrum to mimick the sensitivity of the human eye and the human eye isn't actually particularly sensitive to reds.)

It turns out the ideal ISO setting for a 60D is ISO 800.

But there are a few things you need to know to go along with this. I'm doing astrophotography of "deep sky objects" (DSOs). I'm not shots of the Milky Way over landscapes at night. Because of this difference... I either attach the camera to a telescope like I did for this shot:

Or I put the telescope on a "tracking" head (apparently I haven't uploaded a prior example and that shot is on my other computer). But the "tracking head" just means you're using a normal camera + lens combination but the "head" is rotating at the same rate that the earth is rotating -- but in the opposite direction (it cancels out the movement of the sky so you can take very long exposures without the stars "smearing" due to the rotation of the Earth.)

BTW, the shot above is not a single image... I shot many 8-minute long images and those are then combined using a stacking process. The color and contrast details are "stretched" to exaggerate them (otherwise the image would mostly just appear as a black & white). So to be clear... this NOT what you get straight out of the camera. The adjustments above cause the younger & bluer stars to look more "blue"... and the older and more yellow stars appear more "yellow". (So the color differences aren't made up - but they are exaggerated).

I do have some examples of images that where the colors are not exaggerated -- they really do come out of the camera looking pretty vivid. It all depends on the object.

The reasons ISO 800 turns out to be ideal is because this is the magic ISO for a 60D which maximizes the "upstream" signal amplification and minimizes the "downstream" signal amplification. This tends to produce the strongest "signal to noise ratio" (SNR) and astrophotography is all about maximizing the SNR. The reason we shoot a LOT of images of the same object is to improve the SNR even more.

The image-aquisition steps to astrophotography are somewhat nit-picky... and the image processing process is even more nit-picky.

There are numerous web-tutorials that can help... I have found the "photographingspace.com" website and "DSLR-astrophotography.com" website seem to have quite a bit of good info.

Your camera is very capable of capturing fantastic images... but it will require some work on your end (it require work no matter which camera you use.)

Re: Long exposure limit

ebiggs1 — Thu, 26 Jan 2017 14:19:27 GMT

Now for the correct answer.

With long duration of exposure more total and random pixels heat up. This is because power is applied as long as the sensor is open. Keeping the sensor powered longer means more heat will be generated.

Duration related noise effects thermal noise. Increasing the ISO requires amplification, which requires more power.

Basically in a given shooting environment, increasing ISO has the greatest adverse effect.

The camera knows when it gets too hot. It will shut down until temps normalize.

Re: Long exposure limit

Tsleel2811 — Thu, 26 Jan 2017 14:39:13 GMT

Thank you very much for your help. I guess stacking is the way to go

Re: Long exposure limit

TCampbell — Thu, 26 Jan 2017 14:52:27 GMT

I routinely take exposures with my 60Da in the 5-8 minute time range. But what's more... I'm really trying to grab a few hours worth of data. So the shutter closes, writes the file, and a second or two later the shutter re-opens to start capturing the next file. It's not a problem for the camera.

If I do this with Backyard EOS (astrophotography image-acquisition software) then it also reports the camera's internal temperature (there is a temperature probe inside the camera body). On cold winter nights it doesn't even manage to get warm. But on warm summer nights it might make it up to around 109ºF, but this isn't a problem.

As Ernie says... the camera will warn you or even shut down if it really gets too hot.

The best information I've ever read on the topic of noise was in the first chapter of the book "Lessons from the Masters" (edited by Robert Gendler but the chapters are all written by different authors where each author is considered *the* expert/master on that particular field of astrophotography). But since "noise" is a general problem in almost every type of astrophotography they dedicated quite a bit of time on the topic and explained the quantum principles that cause noise to exist in the first place and how to use stacking to reduce noise.

The noise isn't just a simple matter of boosting the ISO. It matters whether the boost is coming from analog (upstream) signal amplification within the imaging sensor or whether it's coming from digital (downstream) signal amplification by the camera or computer after the imaging sensor has already collected the data. While any boost in ISO tends to also boost noise, it turns out the upstream amplification within the chip can boost the good signal significantly more than the noise and you end up with images that only have a tiny increase in noise for the ISO boost. But each sensor model has some point where it can't boost anymore and has to resort to downstream (digital) amplification. For the 60D that point occurs at ISO 800.

Most Canon models get to ISO 800... some models get to ISO 1600... and some models appear to get above 1600 -- nearer to 3200.

Re: Long exposure limit

ebiggs1 — Thu, 26 Jan 2017 16:02:07 GMT

" It matters whether the boost is coming from analog (upstream) signal amplification within the imaging sensor or whether it's coming from digital (downstream) signal amplification by the camera or computer after the imaging sensor has already collected the data."

This is a point either ignored or unknown by most photograhers.

Re: Long exposure limit

Tsleel2811 — Thu, 26 Jan 2017 16:10:13 GMT

This is the first time I'm hearing about this. Do I have any control over it?

Re: Long exposure limit

ebiggs1 — Thu, 26 Jan 2017 16:37:19 GMT

You always have options. Photography is mostly a trade off. You give to get and there is no free lunch.

Test your camera don't just rely on reviews you read on the ole inner web.

Re: Long exposure limit

Bryston3bsst — Thu, 26 Jan 2017 21:44:54 GMT

@ebiggs1 wrote:
Now for the correct answer.
With long duration of exposure more total and random pixels heat up. This is because power is applied as long as the sensor is open. Keeping the sensor powered longer means more heat will be generated.

Now for the correction to the 'correct' answer.

The sensor doesn't 'open'.

Re: Long exposure limit

ebiggs1 — Thu, 26 Jan 2017 21:50:39 GMT

The intent was and as it is usually stated in this manner "open" if the shutter is opened. The mirror is up. Meaning the sensor is "open" to the world. Clear?

Re: Long exposure limit

TCampbell — Thu, 26 Jan 2017 22:16:29 GMT

@Bryston3bsst wrote:
@ebiggs1 wrote:
Now for the correct answer.
With long duration of exposure more total and random pixels heat up. This is because power is applied as long as the sensor is open. Keeping the sensor powered longer means more heat will be generated.

Now for the correction to the 'correct' answer.

The sensor doesn't 'open'.

Technically "true" but Ernie's point (which is correct and valid) is the difference between the "active" vs. "inactive" sensor.

When the shutter is closed the sensor is literally "off" (it's cold and if it was hot it's in the process of cooling down). When the shutter is open the sensor is active and it's heating up.

It's really the heat issue... there's a direct correlation between the physical temperature of the sensor and the level of noise as one factor that contributes to noise. Dedicated astrophotography imaging cameras often have active cooling systems to drop the temp by 40-50º below whatever the ambient temperatures are just to drive down the noise levels. That's usually not an issue for traditional photography becuse usually the sensor is only active for a fraction of a second.

Re: Long exposure limit

RobertTheFat — Thu, 26 Jan 2017 22:28:37 GMT

ebiggs1 wrote:
The intent was and as it is usually stated in this manner "open" if the shutter is opened. The mirror is up. Meaning the sensor is "open" to the world. Clear?

Perfectly. But I do think we have to credit you with inventing that usage of the word. A Google search I just did betrays no hint of it. Until it catches on, you may want to reconsider your inclusion of the word "usually".

Re: Long exposure limit

Tsleel2811 — Thu, 26 Jan 2017 23:39:00 GMT

Wow. Thank you very much. The next I go out, iso 800 it is.

Did you make the adjustments and stacking in lightroom?

And what is the 60d filter?