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PostPosted: Thu Jan 28, 2010 9:54 pm 
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Hi all,

I wonder if you could help to understand how to choose the correct aperture value for perfect sharpness.

The basics are clear. If the aperture value is low, i.e. the lens is wide open, only the subject in focus will be sharp. Everything in front or behind that will be blurred. The higher the aperture value, i.e. the more closed the lens is, the bigger the depth of field and the more on the picture is sharp.

Though, I’m still unsure how to choose the correct aperture value to achieve good depth of field and sharpness for landscape pictures. For reference: my thoughts are all based on my current kit, i.e. a Canon 450D with a 17-55mm f2.8 lens.

1. I have book from which I understand that the aperture value doesn’t matter in view of sharpness/dof if you are working in infinity. Does that mean that if lets say the lens has markings on the focussing ring for 0.5, 1, 3 meters and infinity, and if everything in the picture is in the infinity (lets say 10 meters or further away), then everything in the picture will be sharp, regardless of the aperture. Is that correct?

2. I found a “depth of field” calculator online (dofmaster). It is easy to see that the total depth of field increases, if the focussed subject is further away. I wonder to what extend the subject distance really matters. To make an extreme example: with f2.8 and 55mm (crop) it shows a difference in total depth of field depending on whether the subject is 40 or 50 meters away. Are these just theoretical values or does it really make a difference in practice. To me it seems that from a certain subject distance (lets say 10 meters and more) the camera just focuses in the infinity. It doesn’t matter whether the subject you focus on is 20, 30 or 40 meters away.

3. I believe to have read that even though the depth of field increases with higher f numbers, sharpness will actually decrease from a certain point, depending on the quality of the lens. Is that true?

I know there’s a depth-of-field preview button to check the “range of acceptable focus”. Maybe I need to get used to it, though I find it quite difficult with the small view finder and the image getting darker for the preview.

So I wonder, how do you determine the correct aperture if you want everything in the picture to be as sharp as possible?

Thanks for you help


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PostPosted: Thu Jan 28, 2010 11:54 pm 
I would also like to have answers to optiman - question - I have been struggling for some time with the same problems. I think my photographs looks good until I crop them - and find that they are not sharp. I have one photo that is crystal clear - from near to far - and I will be dog/gone if I can duplicate that action again - even knowing the settings....so someone please help us.


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PostPosted: Fri Jan 29, 2010 12:00 am 
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1: correct
2: correct
3: Almost. It doesn not depend on lens quality, more on the relationship between the sensor and the lens. It's called diffraction. The image becomes less sharp from F/8 and onwards on DX (cropped sensor), or F/11 on FX (full frame).

However, this is very theoretical and not really field relevant, especially the last question. Besides, an unsharp landscape because of focus would be worse than a landscape slightly unsharp because of diffraction. It makes sense to stop down to F/22 in some cases :)

Also, welcome to the forums :)

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PostPosted: Fri Jan 29, 2010 12:07 am 
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Quote:
The image becomes less sharp from F/8 and onwards on DX (cropped sensor)
for clarification - by onwards you mean going toward F22?


Last edited by larrysch on Fri Jan 29, 2010 12:09 am, edited 1 time in total.

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PostPosted: Fri Jan 29, 2010 12:09 am 
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Yes indeed (this can get a bit confusing, with the number getting larger and the aperture becoming physically smaller). :)

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PostPosted: Fri Jan 29, 2010 12:14 am 
I can see from that - that i have been totally wrong in my thinking of aperture. I always thought that it would be better to be at a very small aperture to have a photo very sharp. So now I understand that it would be better to be at the sweet spot say - f/8 - to bring my landscape photos in more sharp focus - understanding that infinity may be soft if you go to that depth of field? The more I read the more I think this all relates back to the concept of hyperfocal distance.
The "rule" of one or two stops down from maximum is the "sharpest aperture" is correct most of the time. This rule is especially true for less expensive lenses. However I have discovered that once you discover the sharpest aperture the lens will be equally sharp for several additional settings. For example I have a lens that has a maximum aperture of f/5.6, so applying the above rule, this lens should be sharpest starting at f/11.0. In reality it may be a little less or a little more - experimenting would be the only way to find out for sure.
Quote:
Well, technically....you do get the most sharp image at F/8....but only in the part where it is in focus!
It's a tradeoff between DOF (the area that is in focus), and overall sharpness.
It took me a long time to understand this - and you are totally correct. Now I have to make the decision on DOF or sharpness in my photographs.


Last edited by larrysch on Sat Feb 27, 2010 4:09 pm, edited 7 times in total.

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PostPosted: Fri Jan 29, 2010 11:06 am 
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Well, technically....you do get the most sharp image at F/8....but only in the part where it is in focus!
It's a tradeoff between DOF (the area that is in focus), and overall sharpness.

Anyhow, did you ever notice your F/22 shots were really soft? (I didn't). It's all theoretical ;)

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PostPosted: Fri Jan 29, 2010 12:19 pm 
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It also depends on the lens. Each has a sweet spot. I shoot at 7.1 or 8 for landscapes with my 14mm.

It is not necessarily the sensor size that matters, rather the individual pixel size, which tend to be larger on a FF sensor as they are spread out over a larger surface.

A search on Google for the sharpest aperture setting for your lens should provide you with the information you need.


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PostPosted: Fri Jan 29, 2010 8:31 pm 
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It's true there is a sweetspot for every lens. To find out, put your camera on a tripod and focus on the desired subject - then switch to manual focus to ensure the camera doesn't adjust it by mistake.

Then use aperture priority to take a photo at every f-number and examine the results. Starting at the smallest f-number, you'll see the focused area become gradually sharper as the f-number increases, then become softer again.

The sweetspot is where the subject looks the sharpest - it might be at f4, it might be at f8 or a differetn value. It depends on the lens and also the size of the sensor (or rather, the pixel density anyway).

Note this technique only applies to finding the optimum aperture when a specific depth of field is not important. Obviously if you want a certain depth of field, you may have to accept that the lens may not be performing as well as it can be. But as citruspers says, sometimes it's worth making the sacrifice of ultimate quality to ensure you're getting everything in focus.


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PostPosted: Tue Feb 02, 2010 4:33 am 
1. When focused at infinity, everything at "infinity" will be in focus. When focused at 3m, everything at 3m will be in focus etc.

3. After a certain point the edges of aperture blades start to affect IQ, this usually happens past f/11 with my lenses but it's not a fixed point.


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PostPosted: Thu Feb 04, 2010 9:37 pm 
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Hi all,

thanks a lot for your answers - very helpful!


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