Is f/4 enough for wide angle landscape astrophotography?

erotavlas · Oct 21, 2013

Hi I have Sony Nex 5N and I want to get a wider angle lens than my current one (Zeiss 24mm). I like the Zeiss because it goes all the way down to f1.8, but my other options are a 10-18mm zoom at f/4 ($899) and a 12mm at f2.8 ($1299)

Should I go with the f/2.8 or save some money and get the f/4 even though I will end up using higher ISO maybe or longer exposure.

Perhaps the difference in price isn't enough and I should just get the lens with the lower f stop (i.e f/2.8)?

What would you do?

molested_cow · Oct 21, 2013

If you are just doing long exposure, like star trails, then it doesn't matter much. However if you want still star shots, it will also depend on how good your camera perform in high ISO settings. $1299 is a lot of money, why not rent one and try it out first?

djacobox372 · Oct 22, 2013

I don't see much need for a very fast wide angle lens... youre not going to get tight dof with a wide angle anyway, and you can handhold a wide angle at very low shutter speeds and still get sharp photos.

astroNikon · Oct 22, 2013

I'm not sure on your objective.

There's a big difference in getting a Wide Angle Lens, like a 24mm

versus getting a more "up close" picture of a star cluster where you don't want that wide angle, such as using a telescope.

For instance, my primary 2,000mm telescope has a fixed aperture of f/8. But I can isolate star clusters, Jupiter, Saturn, etc with it. The dimmer the object then the higher the ISO or longer shutter speed (while trying to prevent star trails if a fixed setup).

The Wide Angle will give you a wide expanse of the sky. Nice if you know what you want to look at as there will be multiple star clusters, etc all in it.

Anytime I point to space I use long lenses. Though I have used my 24mm to just take "night sky" shots.

erotavlas · Oct 22, 2013

astroNikon said:
I'm not sure on your objective.

There's a big difference in getting a Wide Angle Lens, like a 24mm

versus getting a more "up close" picture of a star cluster where you don't want that wide angle, such as using a telescope.

For instance, my primary 2,000mm telescope has a fixed aperture of f/8. But I can isolate star clusters, Jupiter, Saturn, etc with it. The dimmer the object then the higher the ISO or longer shutter speed (while trying to prevent star trails if a fixed setup).

The Wide Angle will give you a wide expanse of the sky. Nice if you know what you want to look at as there will be multiple star clusters, etc all in it.

Anytime I point to space I use long lenses. Though I have used my 24mm to just take "night sky" shots.

My objective to get wider view of landscape and sky. I find the 24mm I have not wide enough for some applications.
My dilemma is whether or not to save money and get the lens with f/4 or just pay extra for f/2.8
Its like a 3 stop difference isn't it? So i'm guessing an exposure at 8 seconds at ISO 3200 for the f/2.8, I'd have to increase that to 15 seconds on the f/4,
Its stuff like that which I'm not sure yet how it will affect me. I guess the only way is to try it out.
I think I will do an experiment with my current lens set it to f2.8 and also to f/4 and see what I can get out of it at various settings.

astroNikon · Oct 22, 2013

My objective to get wider view of landscape and sky.

So are we also talking about night time astronomy photography too ?
I'm confused.

For regular landscape/sky pictures getting wide is a problem I have too. My widest lens is a 18mm, though part of the kit 18-105. Whereas my widest is a 24mm prime 2.8. I'm never wide enough it seems.

The wider lens you get you also have to watch out for distortions. I'm being very careful and slow on trying to find a good wide angle lens that is affordable. I'm actually tilting towards the cheap albeit good old fashioned AI-S Vivitar lenses for my Nikon.

When I think wide, I think of what width do I need. So a 10mm vs 12mm is the key there, less so I think than the 2.8 v 1.8. If you get a 12mm 1.8 and find out you really need a 10mm, then the 1.8 doesn't do you much good if it's not the focal length that you need.

I'm not at all familiar with the Sony NEX so I'm not much help on lens specifics. But try out the aperture variations on your lens to see that part of the equation.

KmH · Oct 22, 2013

F/2.8 to f/4 is just 1 stop.
Most DSLR cameras are set by default to adjust lens aperture in 1/3 stop steps. So, it takes 3 clicks to make a full 1 stop change in lens aperture.

Because it's an opening in the lens, the area of the opening and not the diameter of the opening is what matters.

F/4 lets in 1/2 as much light as F/2.8 does, and the math factor used to divide is 1.4142, the square root of 2, instead of 2.
F/2 lets in twice as much light as f/4 does by the same 1.4142 factor.

With shutter speed and ISO x2 or /2 is the factor used to determine a full stop.

The classic full stop steps of lens aperture are: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, etc.

erotavlas · Oct 22, 2013

KmH said:
F/2.8 to f/4 is just 1 stop.
Most DSLR cameras are set by default to adjust lens aperture in 1/3 stop steps. So, it takes 3 clicks to make a full 1 stop change in lens aperture.

Because it's an opening in the lens, the area of the opening and not the diameter of the opening is what matters.

F/4 lets in 1/2 as much light as F/2.8 does, and the math factor used to divide is 1.4142, the square root of 2, instead of 2.
F/2 lets in twice as much light as f/4 does by the same 1.4142 factor.

With shutter speed and ISO x2 or /2 is the factor used to determine a full stop.

The classic full stop steps of lens aperture are: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, etc.

thanks for that, i guess on my lens I see it has more gradations of f stop. I think there a couple extra for me in between f2.8 and f4 which is why I counted 3. But in cameras like mine that have extra f/stop values if I count 3, should I also increment decrement the shutter speed by that number as well to compensate? Or is the scale of shutter speed set differently than the scale for f/stops?

KmH · Oct 22, 2013

If the camera is set by default to 1/3 stop steps, that applies to shutter speed, lens aperture, and ISO.

1/100 of shutter speed lets in 2x more light than 1/200 of shutter speed does. 1/2000 of shutter speed lets in 2x more light than 1/4000 of shutter speed does.

1/3 stop step increments allows more precise control over exposure than 1/2 step or full stop steps.

If you want to maintain the same exposure, and change the lens aperture by 1 stop (or 1/3, or 2/3 of a stop), yes change the shutter speed by 1 stop (or 1/3, or 2/3 of a stop) in the opposite direction.

Or, in the case of more than a 1/3 stop change, you can split the compensation between both shutter speed and ISO. So for a full stop change in lens aperture, you can change shutter speed 2/3 of a stop and ISO by 1/3 of a stop for a total 1 stop compensation.