Quick question

Nickler

Nickler

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Nikki
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Could someone just explain why if I use a 35mm lens on my d3100 it's best to leave it on f/1.8 for better focus
 
Not sure what you mean.

Focus is focus. It's either right, or it's not. The lens will focus the same whatever f/number you shoot at, as it always does the focusing bit at lowest f/number.

How much depth of field you get is another question, and the shooting aperture plays a big part in that. The main reason for getting a prime is the low f/number, like f/1.8. If you not going to use that, and get the shallow depth of field that goes with it, you might as well use a zoom and benefit from the extra versatility.

DoF on a zoom set to 35mm at f/5.6 will be the same as f/5.6 on your prime. The pictures will look identical.
 
That's what I thought but have read a few people saying with the d3100 it's hRd to focus this lens but I thought it had auto focus
 
Nickler said:
That's what I thought but have read a few people saying with the d3100 it's hRd to focus this lens but I thought it had auto focus
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Ah right. Is this the AF-S Nikon lens business with your camera? I thought that lens was fully compatible with a D3100?

Does it auto focus, or do you have to do it manually? If it does AF, then it has the right gubbins inside, and it will focus just fine.

Perhaps they mean it's hard to focus because the DoF is so shallow at f/1.8, in which case you've just got to be careful. Use single-point AF and that will nail it. Just be careful that neither you or the subject moves after focusing, as when you're close DoF is very shallow. If there is movement, keep checking and re-checking it, or maybe think about using servo to track it.
 
Perhaps they mean if you're manually focussing the shot you'll get the most accurate focus with the lens wide open. Then stop down to your desired exposure and you're good to go.
 
It is said that all lenses have a focus 'Sweet Spot'.

They recon that lenses focus better around 2 stops from its largest aperture, ie f/2.8 will be at its crispest focus at around f/8.
 
Last edited by a moderator:
Stratman said:
Perhaps they mean if you're manually focussing the shot you'll get the most accurate focus with the lens wide open. Then stop down to your desired exposure and you're good to go.
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The lens is wide open until you press the shutter. It makes no difference what aperture you've set on the camera, the view through the viewfinder is always wide open. Unless you have and use a DOF preview button.
 
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