2 Stops Down for Ultimate Lens Sharpness?

Zarch

Zarch

Messages
1,047
Name
Mick
Edit My Images
Yes
In trying to get the best of out my Nikon 18-70 lens (variable aperture 3.5-4.5) I've read that most lenses are at their sharpest when the aperture is 2 stops down from wide open.

This was also briefly mentioned in my previous thread "Lens Maximum Aperture Question" : http://www.talkphotography.co.uk/forums/showthread.php?t=312054

According to this review site (http://www.slrgear.com/reviews/showproduct.php/product/131/cat/13), its "blur index" seems to suggest that this is true for my lens and plenty of other lenses I looked at on the site.


So when shooting, rather than automatically going for wide-open (3.5) at 18mm on my 18-70, I'd be better shooting 4.5 (2 stops down)

And at 70mm, rather than wide-open (4.5), I need to looking at shooting at 5.6 (again, 2 stops down from the maximum available)

Is that the recommendation?

Note: Obviously 1/3 step-increments on my D3100 (as I assume it is on most cameras?)



So in "theory", the reason why constant 2.8 lens are so good is that 2 stops down is 3.5. So 2 stops down on a 2.8 is exactly what i'd wanted to use on my 18-70, but I can't if sharpness matters to me and I have to use 4.5?

Does that mean that another benefit of the 2.8 lens is that you get far more light in at its "sharpest", so you can use fast shutter speeds and/or lower down the ISO?

I hope i've got all that right? :thinking:
 
I'm sure others with far greater knowledge will come along soon but in the mean time.....

Generally speaking lenses aren't at their sharpest at their maximum f-value although the more expensive the lens, the better they are wide open.

Unless you're using really cheap glass there probably wont be too much difference in "real-World" tests by dropping 2-stops.

The main advantages of fast glass is the ability to shoot in low light conditions without having to use a flash or where you can't use a flash and the ability to choose a shallow depth of field.

Most lenses have a "sweet spot" which refers to an f-stop where they are at their sharpest. For example, if you're shooting a landscape and want as much as possible in focus, you would probably use f8-f16, the actual value to give you the sharpest image will depend on the lens. Defraction will start to take place when you reduce the size of the aperture and from what I've read on here f16+ is common for most lenses.

As with most things it's about compromise, using a lens wide open will give you the shallowest DoF and allow you to use a higher shutter speed and/or lower ISO but the cost could be a slight reduction in sharpness.

It comes down to the subject matter and light conditions at any given time but obviously the lower the f-stop available, the more options you have ;)

HTH

Russ
 
Zarch said:
In trying to get the best of out my Nikon 18-70 lens (variable aperture 3.5-4.5) I've read that most lenses are at their sharpest when the aperture is 2 stops down from wide open.

This was also briefly mentioned in my previous thread "Lens Maximum Aperture Question" : http://www.talkphotography.co.uk/forums/showthread.php?t=312054

According to this review site (http://www.slrgear.com/reviews/showproduct.php/product/131/cat/13), its "blur index" seems to suggest that this is true for my lens and plenty of other lenses I looked at on the site.


So when shooting, rather than automatically going for wide-open (3.5) at 18mm on my 18-70, I'd be better shooting 4.5 (2 stops down)

And at 70mm, rather than wide-open (4.5), I need to looking at shooting at 5.6 (again, 2 stops down from the maximum available)

Is that the recommendation?

Note: Obviously 1/3 step-increments on my D3100 (as I assume it is on most cameras?)



So in "theory", the reason why constant 2.8 lens are so good is that 2 stops down is 3.5. So 2 stops down on a 2.8 is exactly what i'd wanted to use on my 18-70, but I can't if sharpness matters to me and I have to use 4.5?

Does that mean that another benefit of the 2.8 lens is that you get far more light in at its "sharpest", so you can use fast shutter speeds and/or lower down the ISO?

I hope i've got all that right? :thinking:
Click to expand...

Reading your thread here and you other thread, it sounds like you are getting very hung up on this. I bet you are also looking at you images at 100+% in photoshop aswell. I would suggest that you get some of your images printed and look at them at normal viewing distances.

A couple of quick question.

1 are you shooting in raw or jpeg?

2 Do you do any post processing?

3 Do you sharpen the images in PP?
 
why don't you do a series of test shots on a tripod at a selection of different focal lengeths at all the different f-stops then you will know for certain where your lens is the sharpest?
 
Sharpness should not be an issue with this lens, rather sharp focusing is what is important. Stopping down the lens may improve the lens ability to resolve detail which is quite different, hence a £1,000 Nikon 24-70 2.8 will still probably produce a 'sharper image' wide open that the 18-70 stopped down to say f5.6 or f8
 
Davec223 said:
Reading your thread here and you other thread, it sounds like you are getting very hung up on this. I bet you are also looking at you images at 100+% in photoshop aswell. I would suggest that you get some of your images printed and look at them at normal viewing distances.

A couple of quick question.

1 are you shooting in raw or jpeg?

2 Do you do any post processing?

3 Do you sharpen the images in PP?
Click to expand...

You aren't wrong there Dave.... :cuckoo:

I want to understand what is going on, gain knowledge etc, but at the same time its making me more paranoid.

In answer to your questions.....

1. RAW
2. A Little in LR, but i'd probably do more harm than good.
3. Occasionally, but not got that far in my book yet, so not sure best way about it! :D
 
Zarch said:
You aren't wrong there Dave.... :cuckoo:

I want to understand what is going on, gain knowledge etc, but at the same time its making me more paranoid.

In answer to your questions.....

1. RAW
2. A Little in LR, but i'd probably do more harm than good.
3. Occasionally, but not got that far in my book yet, so not sure best way about it! :D
Click to expand...

OK. Couple of things,

All RAW images need to be processed and they will need to be sharpened, It was/is the one thing I struggle with and I have found that I was always to conservative with sharpening.

Stop pixel peeking, iirc 100% in photoshop and the like is not the best to view at does something strange and doesn't look as sharp as it really is.

noise and sharpness never seem as good on the screen as they do in print, I was supprised when I had some images printed the difference, on the computer they were not great but printed they were fine.
 
You shouldn't get too hung up on this, but a lens' sweet spot is not necessarily two stops down from max. It is the point where the loss of sharpness due to diffraction overtakes the gains made by raising the f/number (which reduces most other aberrations).

In reality, that almost always happens at around f/5.6 on a crop format camera, and f/8 on full frame (at least in the centre of the image) but you have to look damn close to see it.

And you've got your stops mixed up (are you counting clicks on the dial?). A 'stop' is any halving or doubling of the exposure and in lens aperture terms a change of one stop means the f/number is multiplied by 1.4 (square root of 2) so for two stops different you should double the f/number, ie starting at f/2.8, one stop is f/4, two stops is f/5.6.
 
Zarch said:
Does that mean that another benefit of the 2.8 lens is that you get far more light in at its "sharpest", so you can use fast shutter speeds and/or lower down the ISO?

I hope i've got all that right? :thinking:
Click to expand...

Regardless off where a lens is sharpest - you'll get more light in with a larger aperture.

Example

You select f5.6 (largest available aperture) and the camera gives you a shutter speed of say 1/200th second. That may be too slow for your subject matter so you're forced to crank up the ISO to gain back the shutter speed but depending on which camera you have that might not be great.

f2.8 on the other hand may result in a shutter speed that's suitable for the subject so you can leave that ISO where it is.

Also a lens will focus 'wide open' even if you're not using it's maximum aperture which can make a difference.
 
Zarch said:
So when shooting, rather than automatically going for wide-open (3.5) at 18mm on my 18-70, I'd be better shooting 4.5 (2 stops down)

And at 70mm, rather than wide-open (4.5), I need to looking at shooting at 5.6 (again, 2 stops down from the maximum available):
Click to expand...

3.5->4.5
4.5->5.6

neither of these are two stops, they are both 2/3 of a stop
 
HoppyUK said:
You shouldn't get too hung up on this, but a lens' sweet spot is not necessarily two stops down from max. It is the point where the loss of sharpness due to diffraction overtakes the gains made by raising the f/number (which reduces most other aberrations).

In reality, that almost always happens at around f/5.6 on a crop format camera, and f/8 on full frame (at least in the centre of the image) but you have to look damn close to see it.

And you've got your stops mixed up (are you counting clicks on the dial?). A 'stop' is any halving or doubling of the exposure and in lens aperture terms a change of one stop means the f/number is multiplied by 1.4 (square root of 2) so for two stops different you should double the f/number, ie starting at f/2.8, one stop is f/4, two stops is f/5.6.
Click to expand...

Uneducated_Rick said:
3.5->4.5
4.5->5.6

neither of these are two stops, they are both 2/3 of a stop
Click to expand...

Thanks guys, I see where i've ballsed up there. :thumbsdown:

Yes Richard, I was thinking a click of the dial was a stop, but its not, its a 1/3 (I think).
 
I didn't even realise this would make any difference, good to know I guess but I haven't noticed a difference yet so I guess not to worry.. Or maybe I should get my eyes checked out.
 
Lenses tend not to be at their sharpests wide open, and stopped right down isn't usually best either due to diffraction effects. But that shouldn't stop you using the aperture wide open.

I recently got a 35mm 1.4 and its sharper at 2.8 but that doesn't stop me shooting at 1.4, why? Because I can.

Don't sweat it at 100%, use this to judge performance and your technique but you rarely view an image at 100% and when you make a print that large or larger you don't stand a few inches in front of it to view it. Also by the time you sharpen in post it won't matter.
 
lensflair said:
Also by the time you sharpen in post it won't matter.
Click to expand...
Here's me questioning all the hardware I've got Graham, but after playing around in LightRoom last night I think its the default amount of sharpening that LR applies that could well be the root of the problem as I sharpened some shots manually and they looked more like i'm hoping for.

I think by default, LR applies only a very small amount of sharpening, which in some cases appears hardly changes the image at all and leaves them soft (to my eyes anyway).

Maybe my 18-70 lens isn't a bad one after all? :love:
(although this won't stop me thinking about a 2.8 walkabout!)
 
Zarch said:
Here's me questioning all the hardware I've got Graham, but after playing around in LightRoom last night I think its the default amount of sharpening that LR applies that could well be the root of the problem as I sharpened some shots manually and they looked more like i'm hoping for.

I think by default, LR applies only a very small amount of sharpening, which in some cases appears hardly changes the image at all and leaves them soft (to my eyes anyway).

Maybe my 18-70 lens isn't a bad one after all? :love:
(although this won't stop me thinking about a 2.8 walkabout!)
Click to expand...


Yeah, I just add a bit more when needed. Objects like buildings look good with a lot more sharpening than people too. Use the mask slider to mask off skin and sky etc so it won't be affected by sharpening. If you hold down option while moving the mask slider it will show masked areas in black, areas in white will be sharpened and grey bits varying amounts depending on how close to white the mask is.

Areas of higher contrasting edges such as eyes and lashes will be sharpened, while continuous tone like sky and skin will be masked.
 
I dont know if everyones LightRoom does this, but as soon as I click "develop" along the top row of buttons, some settings change automatically:

Sharpening always goes to 25, radius 1.0
Brightness seems to adjust itself automatically/randomly

I used to reduce the amount of sharpening to around 10-15pts (radius 1.0) but recently have increased slightly to around 30pts (radius 1.5) dependant on each individual case of course!

Some people however increase sharpening to 100+ which (for me) seems way high! If I do this my picture suddenly starts looking very strange and lots of noise/patterns appear all over the place!
 
You must log in or register to reply here.
Back
Top