Focus on 50mm f1.8

philxthomas

philxthomas

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Got a 1.8 t'other day. Everything is fine, using it in manual focus (D40) but cant seem to get focus of far away things (maybe about 100m away) when at 1.8, tried setting to infinity and slightly back.

Above f2 is fine.

Normal?
 
I wouldnt even contemplate using f1.8 for anything other than real close ups as it gives a very shallow DOF and you'll find everything away from dead centre (or a manually selected focus point) will be slightly blurred which is not ideal for anything any distance away, increasing towards the edges. I find with the 50 for anything any distance away (certainly 100m +) or landscape I used f5.6 at the very minimal, but usually f8 and above. Obviously I'm using the Canon nifty fifty, but its also a 1.8 and I should imagine its the same for both.

I'd be surprised if the whole image at f2 is in full focus from that distance?
 
When you say you can't get far away things in focus, do you mean at the same time as near objects or just far objects? The hyperfocal distance is about 70m so when focus set to this then from about 35m to infinity should in theory be in focus. However, as with many lenses the 50mm f/1.8 is not at its best wide open and gets noticebly sharper above f/2.8-f/4. So if as you it's fine above f/2 then sounds perfectly normal to me.
 
odd jim said:
I wouldnt even contemplate using f1.8 for anything other than real close ups as it gives a very shallow DOF and you'll find everything away from dead centre (or a manually selected focus point) will be slightly blurred which is not ideal for anything any distance away, increasing towards the edges. I find with the 50 for anything any distance away (certainly 100m +) or landscape I used f5.6 at the very minimal, but usually f8 and above. Obviously I'm using the Canon nifty fifty, but its also a 1.8 and I should imagine its the same for both.

I'd be surprised if the whole image at f2 is in full focus from that distance?
Click to expand...

I'm a little confused here. Usually, the closer the object to the camera at f1.8, the shallower the deepth of field would be. Therefore, if the subject is further away, there should be a greater depth of field? :thinking:
 
My understanding is that at such a low F number (1.8) anything other than the subject that is being focused on would be out of focus.

Just to be clear, is the issue that when you view the photo nothing is in focus?
Or is it that when taking the photo, the camera fails to lock on to any subject?

Hope that makes sense.
 
Big_Bill said:
My understanding is that at such a low F number (1.8) anything other than the subject that is being focused on would be out of focus.
Click to expand...

yeah thats what happens.

its not really about how i can get a bigger depth of field, its more that I cant focus on distant objects when at 1.8.

anyway, i think my question has been answered so thanks.
 
You should probably post an example. 100m with a 50mm lens is close enough to being at infinity you could probably rack the focus out to infinity. It's probably softness at f/1.8 though.
 
Its probably in-focus but just not very sharp.

I find for infinity shooting I am usually just below infinity, some lens will let you go past the infinity mark too. Look for the green dot, then just refocus until its JUST gone out. It should be OK. But it'll never be sharp wide open in this context.
 
i think i deleted the shot in question cause it was **** but ill have a look..
 
According to DOF calculators if you focus on an object @ 100m it should give you 'acceptable sharpness' from 36m up to infinity
 
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