Need some help???

[mw0dbb]

The gf bought me a sigma 10-20mm lens for Christmas for my Canon 550d and thought I would have a play at doing some night time photography. I have noticed though that the edges of the shot are out of focus, am I doing something wrong? The camera is on a steady tripod and there was no wind.

John


IMG_3287 by johndbb

 

[Dale_tem]

The edges are closer/further away than the centre so this is a depth of field issue, what were you shooting at and where were you focussing? A good start would be to focus at the far right and shoot in aperture mode at f/16, this should keep it all in focus.

When using a higher f value, you get better depth of field, but you need to focus about a third of the way into the frame to get it all in focus,


If you get chance to go back, try shooting at different f values to see the difference

 

[mw0dbb]

I was not sure about the F-stop, I used f5.6, ISO 100 and 15 second exposure. I always thought that with the longer exposure that the F-stop was not as critical?

John

 

[cw318is]

I've been looking at the exif on the image, as Dale mentioned its worth stopping down the extend the depth of field - handy little tool here: http://www.dofmaster.com/dofjs.html

Also, under f8 the sharpness of the Sigma ultrawide drops off a lot around the edges. Little test - from 10' away photograph a brick wall head on, so the focal plane will cover the entire wall and have a look at the results. You will notice a bit of barelling which is normal but also the image will be soft.

 

[cw318is]

I was not sure about the F-stop, I used f5.6, ISO 100 and 15 second exposure. I always thought that with the longer exposure that the F-stop was not as critical?

John

Not quite, long exposures come about because of low light levels - either as a night shot like this or by restricting the amount of light entering the lens - by using a smaller aperture.

As an aside you can also get some interesting effects at different aperture values, if you go for a small aperture, like f16, any lights in the image will be all nice and twinkly

 

[Dale_tem]

I was not sure about the F-stop, I used f5.6, ISO 100 and 15 second exposure. I always thought that with the longer exposure that the F-stop was not as critical?

John

F has 2 effects, will effect the shutter speed, so f/2.8 will allow faster shutter speeds, f/16 will require longer shutter speeds. The other big effect which when shooting landscape is how much of the photo do you want in focus? F/2.8 would mean just the focal point will be in focus (good for portraits or focussing attention), f/16 will mean nearly everything will be in focus (might need higher values, depending on focal length)

 

[Dale_tem]

Also, under f8 the sharpness of the Sigma ultrawide drops off a lot around the edges. Little test - from 10' away photograph a brick wall head on, so the focal plane will cover the entire wall and have a look at the results. You will notice a bit of barelling which is normal but also the image will be soft.

When you say under, do you mean f/4 or f/16, I am never sure on under or over when it comes to f values.

 

[Lynton]

When you say under, do you mean f/4 or f/16, I am never sure on under or over when it comes to f values.

under = smaller number = wider hole.

the bigger the number the smaller the hole, the more in focus..

 

[mw0dbb]

Thanks for that guys, I didn't think it applied so much for night time shots. Just need a nice evening to try again!

John