Focussing advice needed!

[ajax_andy]

I tried out my new 50mm prime lens last night (Canon f/1.4) and came across some problems focussing on the object I was shooting.

I know it's against the rules to post more than one pic, so Mods please excuse me for posting 2, but I need advice on why I had problems despite trying different distances to the subject.

My first effort the top left and top right of the front page seem quite soft (yet the pages after that seem ok)



Exif data
Camera Canon EOS 450D
Exposure 0.013 sec (1/80)
Aperture f/3.5
Focal Length 50 mm
ISO Speed 200
Flash On, Fired


So I thought... hmmm maybe i'm too close and that is what is causing the top right to be out of focus? So I took a step back, but then what happened is that the left hand side became very soft?!



Exif data
Camera Canon EOS 450D
Exposure 0.013 sec (1/80)
Aperture f/3.5
Focal Length 50 mm
ISO Speed 200
Flash On, Fired


I'm VERY confussed... I know there is a shallow "sweet spot" when shooting wide open with a prime, but these were shot at f/3.5 so I would have expected the difference between a couple inches (if that!) between how close the front right in the 2nd version is and the top left to not make such a big difference?!

What am I doing wrong?

I'm also very disappointed with the clarity of the images, they look pretty grainy I think, yet I only shot with an ISO of 200. Again is this because of something I have done wrong? This is my 1st prime lens, and have only shot with a kit zoom before, so I'm guessing I'm doing something seriously wrong?

The pics are a maybe bit small here to see exactly what I mean, so please feel free to view them full size here if it helps: http://www.flickr.com/photos/andyhud...7627198920410/

[zona5101]

Depth of Field:
On a 450 at f3.6 with a 50mm lens and a camera to subject distance of 1.5 feet your depth of field is .02feet. what is that like a quarter of an inch? That's what's going on.


Online Depth of Field Calculator

[autofocus]

Originally Posted by zona5101

Depth of Field:
On a 450 at f3.6 with a 50mm lens and a camera to subject distance of 1.5 feet your depth of field is .02feet. what is that like a quarter of an inch? That's what's going on.


Online Depth of Field Calculator

+1 on Bruce's comments. Remember, there are 3 factors that effect DOF. Lens focal length, aperture, and it this case focus distance to the subject

[ajax_andy]

Originally Posted by zona5101

Depth of Field:
On a 450 at f3.6 with a 50mm lens and a camera to subject distance of 1.5 feet your depth of field is .02feet. what is that like a quarter of an inch? That's what's going on.


Online Depth of Field Calculator

Wow! Ok that really is a narrow focus area!

Thank you for your comment... I need to look more at how to work out the DOF on a prime lens. I knew the factors you mentioned above played a part in it, but I hadn't realised it would be so extreme at an aperture of f/3.6.

In this situation I would have been better selecting a narrower aperture by the looks of things and will bare that in mind for the future.

If you dont mind me asking, how did you work out the DOF? I've seen DOF calculators but i'd prefer to understand the maths so can do it on the spot if shooting on location

[ajax_andy]

I just had a look at a DOF calculator and played around with different distances from a subject.

It would seem that the further away from a subject I am then greater the DOF becomes. So shooting a head shot from 5 feet away, with an aperture of only f/2.2 would leave me with a DOF of just 1/4 of a foot, but at about 10 feet away it'd give me a DOF of about 1 foot.

So using both as an example (sorry i know this will probably sound dumb!):

At 5 feet i'd focus on the area of the subject I desired (say someones eyes) and that would give me roughly 0.12 feet of focus in front of the focus point, and 0.13 behind. Meaning anything with more depth than 0.25 of a foot (i.e. 3 inches) would be soft or out of focus.

At 10 feet i'd focus on the area of the subject I desired and that would give me roughly 0.5 feet of focus in front of the focus point, and 0.5 behind. Meaning anything less than a foot deep (i.e. 12 inches) should be in sharp focus?


Actually i've just doubled it to 20 feet away and it came out at roughly 4 feet of DOF... have I just worked out a formula... i.e. distance x 2 = DOF x 4?

[zona5101]

Originally Posted by ajax_andy

Actually i've just doubled it to 20 feet away and it came out at roughly 4 feet of DOF... have I just worked out a formula... i.e. distance x 2 = DOF x 4?

It's a bit more complicated than that... If you have a palm device or itouch/iphone you can download applications to calculate the DOF. Short of that I would create a chart on paper with common fstops/distances for each of your lens.
If your camera has a depth of field preview button that would make all but the most critical DOF calculation moot.

[ajax_andy]

Originally Posted by zona5101

It's a bit more complicated than that... If you have a palm device or itouch/iphone you can download applications to calculate the DOF. Short of that I would create a chart on paper with common fstops/distances for each of your lens.
If your camera has a depth of field preview button that would make all but the most critical DOF calculation moot.

Yeah I sat and created a chart after posting my last message... I think that's definately the way forward. For the several f/stops I worked out it does seem that (roughly) when you double the distance it increases the DOF by about 4 times. Its not a perfect rule but helps me get my head around how DOF works. It does mean you need to know the original DOF however to work out x 4 of the DOF, so probably not a very useful rule in all honestly lol!

I wasn't aware of apps on phones for that but I have an iPhone so am sure there'll be something on there that i can download.

I haven't heard of a DOF preview button before, i'll consult my manual and see if the 450d has one

[devildogx2]

Originally Posted by ajax_andy

I wasn't aware of apps on phones for that but I have an iPhone so am sure there'll be something on there that i can download.

I haven't heard of a DOF preview button before, i'll consult my manual and see if the 450d has one

You might want to check this site for it's online calculator.
DOF master
Online Depth of Field Calculator

They do have an I-phone app too. I just downloaded it myself (just got the phone).
The good thing about your new 50mm lens is you now have sensor size and focal length fixed. Now you can practice varying your distance or aperture and get a feel for what the 50mm lens will give you until it becomes intuitive.
Have fun.

[spookie]

HaHa, this comes back to bite me far more than I want to admit lol. I forget to envision in mind what the DOF will be at the setting I'm using. One piece of advice someone gave me if you're rushed thru the shoot was, take a few shots at one fstop, then change the stop 2-3 clicks (usually up) and take a few more lol. I miss the old 35mm lenses that had the guidelines on the lenses lol.