'f' number understanding

sworks

sworks

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Mark
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I'm having problems with sharpness but starting to think it's me not the equipment. I usually shoot in aperture priority setting the F number I want and iso and letting the camera sort the shutter speed. This results in not so sharp images due to the shuter speed being I think too long? some times I get fantastic results but maybe I'm lucky? I always thought that the lower the F number the more bokeh and the higher the number the better the focus / sharpness front to rear for a landscape but think I'm maybe going too high - If I was taking a photo of 4 people I would use maybe f8 as I thought F2.8 would give me a narrower focus band and not cover all people? I think I need to go right back to the beginning and try and understand things a bit better. Once I get the 'f' numbers sorted am I better off shooting manual and using the cameras light meter for exposure? Horrible weather at the moment is stopping me from going out and 'playing' so any advice is appreciated
 
The basics of what you've written are correct. The final part of the puzzle is to get the shutter speed high enough to avoid camera shake (if that's your problem) and you can do this by raising the Iso.....doubling the Iso value will double your shutter speed.

Bob
 
I thought I was going mad and picked up everything wrong. I had a 400d and kit lens and never seemed to have as much problems until i upgraded. So if I was approx 10 feet away from a group of 4 people and in aperture priority and f8 everybody should be in focus? would i select one person to focus on in the centre? Is manual a better option with the light meter? I generally use iso 100 for less noise but do increase it up to around 800 / 1000 if needed
thanks
 
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sworks said:
I thought I was going mad and picked up everything wrong. I had a 400d and kit lens and never seemed to have as much problems until i upgraded. So if I was approx 10 feet away from a group of 4 people and in aperture priority and f8 everybody should be in focus? would i select one person to focus on in the centre? Is manual a better option with the light meter? I generally use iso 100 for less noise but do increase it up to around 800 / 1000 if needed
thanks
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Sounds about right. Check your DoF on this calculator http://www.dofmaster.com/dofjs.html

You can set the exposure any way you like, manual, Av, P etc. With compensation or program shift or whatever, they can all be made to deliver exactly the same exposure at whatever combination of settings you want. The main feature of manual is that it that once you have set it, everything is locked.
 
Thanks for the help so far, I have some volunteers at the weekend to let me practice portrait stuff. Am I right in thinking if there is one person I can go as low as f2.8 and focus on their eye closest to me and if there are two people F5 and focusing on one of them should make the other person as sharp - providing the camera picks the correct shutter speed. Thought I had it sussed but when looking at exif on flickr it all gets confusing again and a bit random
 
I recently sold my Sigma 24-70 as I thought it was soft but looks like it might be me and the incorrect shutter speeds, I replaced it with a 24-70L but what's the chance of me being very unlucky and getting a soft one? As said earlier i 'learned' on a 400d and kit lens and never had as many problems
 
whichever aperture you choose... have a look at the shutter speed the camera is giving you first by half pressing. Then compare the shutter speed to the focal length.

eg. f/5 - iso 100 - 70mm - 1/40th. This set up is going to suffer camera shake, so you need your shutter speed to be at least equal to the focal length, in this case above 1/70th. If it isn't, increase the ISO until it is or widen the aperture slightly or a combination of both. Result!
 
sworks said:
Thanks for the help so far, I have some volunteers at the weekend to let me practice portrait stuff. Am I right in thinking if there is one person I can go as low as f2.8 and focus on their eye closest to me and if there are two people F5 and focusing on one of them should make the other person as sharp - providing the camera picks the correct shutter speed. Thought I had it sussed but when looking at exif on flickr it all gets confusing again and a bit random
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Pretty much, except for the group shot you need to focus somewhere in the middle to optimise the DoF in front and behind. For subjects like this, 'focus one third in' is a good guide.

Click on the DoFmaster link above, input some figures and see what comes out. It's all there. There is always more DoF behind the focused point than in front, but when you're very close it's almost 50/50%, at common 'people shooting' distances it's roughly 35/65 as mentioned, and with distant subjects there is massively more DoF behind, ie to infinity.

Understand the DoF parameters, which are basically image size (magnifcation) and f/number. Image size is determined by focal length and shooting distance and they pretty much cancel out, ie use a longer lens but also move back to maintain framing (image size/magnification) and DoF stays the same.

Sensor size also has an effect, and that's what is adjusted when you input the camera type, but obviously you can't change that.
 
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Am just getting learning how to use dslr's myself, and have been struggling with f numbers and shutter speeds. There is a few videos on youtube that really helped me the understand it a lot better!
I found it easier with someone telling/showing you what they do instead of just reading a load of different stuff about it. So try and search youtube for some videos about it.
Hope this helps :)
 
Will halve the shutter time :D
 
CW90 said:
I found it easier with someone telling/showing you what they do instead of just reading a load of different stuff about it. So try and search youtube for some videos about it.
Hope this helps :)
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That's a great piece of advice and one that that can be used for many things other than just for photography :clap:
 
sworks, if you post up one or two of your out of focus images with the exif data we may be able to see where you are having trouble.
 
Not much to say other than what has already been said. But having read the post about changing from the Sigma 24-70 to the Canon 24-70 2.8L, they are notorious as being soft.

To be fair, I've shot groups of 4+ at everything from f2.8 - f11 depending what I wanted from the image.

All shots can be extremely soft on the Canon L lens which is quite annoying....
 
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Canon Bob said:
.....doubling the Iso value will double your shutter speed.

Bob
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Andy_S said:
Doubling the iso will half the shutter speed :)
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No it won't. I think I understand where you're coming from but your interpretation doesn't follow my wording.
Doubling the shutter speed is increasing it by the normally used definition.

Bob

Edit....just saw Richard's post explaining the same (should have read further before replying)
 
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DaveKing said:
sworks, if you post up one or two of your out of focus images with the exif data we may be able to see where you are having trouble.
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I think this is excellent advice. There so many things that can affect softness:
-focussing technique
-lens design
-camera sharpness and contrast settings
-too large an aperture (dof issues)
-too small a aperture (diffraction)
-camera shake (shutter speed)
- user expectation
-and I am sure I have missed something.....

For what it is worth I found my images got noticeably softer when I moved from a D50 to a D300 and I was a bit frustrated for a while. The consumer orientated D50 has more sharpening and contrast by default and also tends to higher shutter speeds in program. The D300 being more pro orientated assumes you know what you are doing with settings so errs on the conservative side by default. Over 6 months on I think I am only just starting to find out what the D300 is capable of.
I suspect there is a similar difference between Canon consumer and pro DSLRs.

Toby
 
Thanks for all your help, I will try and post some up. Interestingly, I was talking to a local photographer tonight and it would seem that I have been using way too high an 'f' number in a lot of my shots. User error is easier to fix but I suppose that means that maybe the Sigma wasn't as soft as I thought. He tried the lens on his 1D mk111 and he seemed to think it ok - he will have a better look on his pc but he noticed the zoom ring problem http://www.talkphotography.co.uk/forums/showthread.php?t=338239
Unfortunately I can only find 1 of 2 receipts :(
 
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