Exposure settings of two different lenses

[Chefgage]

Sorry for the vague title but here goes.

Let's say I have two different DSLR lenses used at night. One is a Canon 18mm f/3.5 and the other is a Samyang 135mm f/2 lens. I have imaged with both. With the Samyang 135mm at f/2 I can take an image with an exposure of 2 minutes and the image is not over exposed. But with the Canon 18mm at f/3.5 I can only take an image with an exposure of 15 seconds before the image becomes over exposed or washed out. This was when the moon was out at about 60%.

So the question is (without me doing some experiments) when using a wider angle lens does this decrease the possible exposure time?  It is my understanding that the Samyang at f/2 will let in more light than my canon at f/3.5 but the Samyang lets me image at a far longer exposure time?

[scotty38]

All things being equal f2 is f2 regardless of the lens but with 18mm and 135mm the field of view is massively different. Point them both at the same plain grey wall and they’ll need the same exposure but under normal circumstances they will differ based on the scene they’re capturing. Try spot metering on the 18mm vs the normal matrix type exposure and watch the reading change as you point at different stuff. That’s sort of what the 135 is doing compared with the 18. 
 

Hope that all makes sense as it did to me 🤣

[Chefgage]

48 minutes ago, scotty38 said:

All things being equal f2 is f2 regardless of the lens but with 18mm and 135mm the field of view is massively different. Point them both at the same plain grey wall and they’ll need the same exposure but under normal circumstances they will differ based on the scene they’re capturing. Try spot metering on the 18mm vs the normal matrix type exposure and watch the reading change as you point at different stuff. That’s sort of what the 135 is doing compared with the 18. 
 

Hope that all makes sense as it did to me 🤣

I think I get what you are saying. I assume as the 18mm is likely to take in more light pollution being as it is looking at a larger field of view. Hence the lower expire time I was experiencing. That's what you are saying isn't it 😁

[scotty38]

Yep exactly

[Alien 13]

F ratio is a funny thing, useful for determining exp settings but its never a good idea to assume that everything is equal. I could have a f/2.8 DSLR lens or a Phone camera with a f/2.8 lens and it should be obvious which one actually collects more light..

Alan

[wulfrun]

10 minutes ago, Alien 13 said:

F ratio is a funny thing, useful for determining exp settings but its never a good idea to assume that everything is equal. I could have a f/2.8 DSLR lens or a Phone camera with a f/2.8 lens and it should be obvious which one actually collects more light..

Alan

Correct, but the tiny smartphone lens is a very short focal length and focusing on a tiny sensor, whereas the DSLR lens will be much longer F/L imaged onto a vastly larger sensor. Given equal pixel sensitivities on the sensor (not a given in reality) they do still both need the same exposure for the same scene & lighting.

Edited April 23, 2021 by wulfrun

[ollypenrice]

I don't think the usual 'rules' from regular photography get us very far in these conversations.

What we are interested in is how much light arrives on each pixel. This depends on the ratio between the pixel size (its area) and the area of the aperture.  F ratio does not define this ratio, nor does focal length.

Now, each pixel receives light which includes both light from the object and light from polluting sources. On very very widefield images the LP will be worse in one part of the image than another. (It will always be worse near the horizon and may be worse in the direction of a city, etc.) But, other than that, a lens with a wide field of view is no more or less susceptible to light pollution than one with a narrow field of view.

If you want a rule of thumb from readily available information, forget F ratio and forget focal length. Both are likely to lead you up a gum tree. Just look at arcseconds per pixel. This tells you how much sky illuminates each pixel. Obviously, the more sky illuminating each pixel, the more light each pixel gets - and that will define your exposure time.

Olly

[Chefgage]

On 23/04/2021 at 21:04, ollypenrice said:

I don't think the usual 'rules' from regular photography get us very far in these conversations.

What we are interested in is how much light arrives on each pixel. This depends on the ratio between the pixel size (its area) and the area of the aperture.  F ratio does not define this ratio, nor does focal length.

Now, each pixel receives light which includes both light from the object and light from polluting sources. On very very widefield images the LP will be worse in one part of the image than another. (It will always be worse near the horizon and may be worse in the direction of a city, etc.) But, other than that, a lens with a wide field of view is no more or less susceptible to light pollution than one with a narrow field of view.

If you want a rule of thumb from readily available information, forget F ratio and forget focal length. Both are likely to lead you up a gum tree. Just look at arcseconds per pixel. This tells you how much sky illuminates each pixel. Obviously, the more sky illuminating each pixel, the more light each pixel gets - and that will define your exposure time.

Olly

Sorry for the late reply. Thank you for explaining that. It's starting to make sense to me know.