OK, right, from the beginning. Aperture Priority.

[.Markski]

I know I asked this before but didn't quite understand the answer
Can anyone explain in words of 1 syllabubblelele or less what Aperture Priority is and what difference should I see on a picture if I have it set at F2.8 and then F8.2 because I'm buggered if I can tell!

[luca brazzi]

I'm sure there are better answers...:

Aperture priority (as opposed to shutter priority) will keep a fixed aperture while adjusting the shutter speed as amount of light entering the camera changes (if you're pointing it at a lightbulb instead of a carpet)

It means your aperture takes priority over other variables.

So....
Aperture 2.8 shooting outside in sunlight may require the shutter speed to be 1/500 of a second to give a proper exposure.

Aperture 2.8 shooting indoors without flash may need 1/60 of a second to achieve good results.

Because you chose Aperture priority, the camera adjusts the shutter speed to ensure you get a good picture.

Aperture 2.8 (the light goes through a big hole): will give only a small area of depth in focus....eg shooting a portrait you may get the face in clear focus while getting a blurred background (and thereby emphasising the face)

If you used an aperture of 8.2 (ie a very small hole for the light to get to the camera), the background would come out clear as well as the face.

Its the same as squinting your eyes....Things get a little darker, but also slightly sharper.

LB

examples:
Large aperture (ie small number, eg 2.8)


Small aperture (ie big number, eg f8 or f11)


>> Edited by luca brazzi on Tuesday 3rd August 17:49

[chris.mapey]

.Markski said:

I know I asked this before but didn't quite understand the answer Can anyone explain in words of 1 syllabubblelele or less what Aperture Priority is and what difference should I see on a picture if I have it set at F2.8 and then F8.2 because I'm buggered if I can tell!

The reason for using Aperture Priority is to choose the "depth of field". This is shown in the first piccy Luca posted above. The bit focussed on is pin sharp & the background is blurred. If you want to achieve a similar effect, choose AP and a wide aperture (a low number, ie f2.8 etc).

The second piccy Luca posted has a greater depth of field, ie more of the background is in focus. For this effect choose a smaller aperture (ie higher number, f16 etc).

Shutter Priority is used when you need to choose how fast the shutter operates. Shooting a fast moving car on track, choose a fast shutter speed. If you want to blur something (say a waterfall or river) choose a slow shutter speed.

I hope that is clear enough, it reads OK to me, but then I know what I'm trying to say...

Chris

[simpo two]

Spot on.
I'd just add that a correct exposure is a balance of aperture and shutter speed. As one increases, the other decreases. The total exposure is the same, but you're setting one of the parameters manually to a certain effects (eg shallow DOF, deep DOF, motion blur, frozen action).

It's worth pointing out that DOF is also massively dependent on focal length. The longer the lens, the shallower the DOF, and the same is true of close-up lenses.

Peasy!

[.Markski]

Thanks chaps time to go try a few things out I tihnk.

To recap, F2.8 will give me a blurred background with the object in focus sharp and the F8.2 will pretty much have a sharp entire image.

I always leave mine on auto, I cannot adjust the shutter speed so I guess it makes little difference?

[simpo two]

.Markski said:

To recap, F2.8 will give me a blurred background with the object in focus sharp and the F8.2 will pretty much have a sharp entire image.

It's not quite as clear cut as that. I'm not sure why you're picking f8.2 as your 'small' aperture but most lenses go to f16 if not f22 or beyond (much smaller). So if you really want deep depth of field then head that way.
DOn't forget the focal length effect - for example if you're using a wide-angle lens then DOF is always greater for any given aperture. The extreme example of this is a fisheye lens where DOF is so great you barely need to focus at all!

.Markski said:

I always leave mine on auto, I cannot adjust the shutter speed so I guess it makes little difference?

Then the camera will choose its own combination of shutter speed and aperture for each shot, and you're out of the equation. DOF should still increase in bright light as the camera will use a smaller aperture to deal with it.
Anyway, hope this helps?

[.Markski]

simpo two said:

I'm not sure why you're picking f8.2 as your 'small' aperture but most lenses go to f16 if not f22 or beyond (much smaller).

Coz that's as far as it goes

[simpo two]

.Markski said:

Coz that's as far as it goes

Ahhh.
How's your piggy bank? It might be time to upgrade

[.Markski]

simpo two said:

Ahhh. How's your piggy bank? It might be time to upgrade

If only. It is a good camera but wish this forum was up and running before I'd got it. I went for max zoom at my budget as I had used a 10x Optical on a digital before.
In hindsight I may have chosen differently knowing what I know now. That is not to say though that the one I have is rubbish, it does in my opinion take excellent shots, it's just that I may have moved on a bit.

[fatsteve]

simpo two said:

Spot on. I'd just add that a correct exposure is a balance of aperture and shutter speed. As one increases, the other decreases. The total exposure is the same, but you're setting one of the parameters manually to a certain effects (eg shallow DOF, deep DOF, motion blur, frozen action). It's worth pointing out that DOF is also massively dependent on focal length. The longer the lens, the shallower the DOF, and the same is true of close-up lenses. Peasy!

Interesting, wasn't aware of that. I though DOF was purely down to f-stop. So f2.8 on a 28mm lens vs f2.8 on a 300mm lens, the 300mm len would have the shallowest DOF?

So its a fair assumption to make the DOF is controlled by f-stop AND focal-length.. hhmmm....

Also, isn't f-stop a factor of the focal length anyway?, ie f2.8 means 1/2.8 size of focal length.

Steve (still learn'in..., so many questions..)

[murph7355]

f-stop is indeed Steve.

I believe the 'f' may even stand for focal length...

f-stop also equals aperture size...

Somewhere on the web is an equation that lets you calculate the DOF. I think the Canon forums (http://photography-on-the.net/forum/viewforum.php?f=23) had a link to it a little while ago.

[antonyb]

murph7355 said:

I believe the 'f' may even stand for focal length... f-stop also equals aperture size...

f does indeed stand for focal. it comes from the days where to change apature you had to slide out a plate and physically change it to a smaller or larger hole. This process was called stopping down or opening up. thus the phrase we use now f-stop.

focal distance/length is effected by f-stop becasue of the way that the light travels through the lense at varying apatures. at lower f-stop values, which let in less light due to the smaller hole, the rays cast at a more accute angle and therefore hit the film (for explanations sake) closer together, therefore they are more in focus and there is a smaller circle of confusion (CoC). (which is another related topic) The eye perceives sharpness down to around 0.25mm (ever noticed that "0.28 dot pitch" on a computer monitors specifications before?) and therefore if the dots recorded on the film seperate by less than this then we percieve them as 'in focus'. If the apature is large, say f2.8, then the rays cast down the lense at a larger angle and hit the film further apart (within a greater CoC), which is exagerated by having to travel further down a longer lense, and therefore our eyes perceive these areas as 'out of focus' or blurred.

A rule of thum is that for every stop you close down (eg, moving from f8 to f11) you should double the exposure (eg, from 1/100th to 1/50th sec. The smaller the aperture, the greater the depth of field, so very much more of the background/foreground would be in focus with, say, f16 than with f2.8.

Then there is the Hyper-Focal distance.

Generally the hyper-focal distance can be set using the rule of two thirds, where you focus 2/3 of the way 'up' the image frame whilst keeping 2/3 depth in focus. To do this focus on the furthest point of focal interest and then on the closest. At each point have a look at the reading on the lense (usually the middle ring of 3 all with numbers) and then adjust your focus back to the point two thirds up the frame. Set the relative aparture and exposure so that the depth of field reading is on the number between these two points when focusing at 2/3.

i hope i explained that right

>> Edited by antonyb on Thursday 5th August 00:41

[simpo two]

Wooo - you've been reading that 'luminous light' forum thing haven't you!!! I was surprised to read that, apparently, DOF does *not* vary with focal length - though from personal experience that's exactly what happens, or it does a damn good illusion of it.

For example, this was taken at f5 - but at 165mm focal length:

[antonyb]

simpo two said:

apparently, DOF does *not* vary with focal length - though from personal experience that's exactly what happens, or it does a damn good illusion of it.

...and you are correct - it does a damn good illusion of it. DoF is very directly related to magnification factor, regardless of focal length.


excuse the dodgy diagram...

the illusion comes because a long lense will always have a higher starting f number due to amount of elements required for magnification (and therefore the light being 'absorbed' by the addition glass). so f5 on your example is a likely to be a wide apature meaning that you will get more depth of field. if you shot the same framing of the subject it would not matter what focal length you use... eg. 50mm, 135mm or 300mm the DoF would be pretty much the same.

the narrower field of view on a long lense will mean that the effect is greatly exagerated in a closely framed subject area (like corn). what you actually have is a fairly normal hyper-focal distance on a very close up subject taken at some distance... your distance from the subject will effect the DoF rather than the length of the lense.

of course i could be wrong, i'm no expert

ant


>> Edited by antonyb on Thursday 5th August 00:46

[murph7355]

The following will allow you to play til your heart's content

http://dfleming.ameranet.com/dofjs.html

[.Markski]

This is excellent chaps - thanks, keep it coming


Already forming my next question

[simpo two]

murph7355 said:

The following will allow you to play til your heart's content http://dfleming.ameranet.com/dofjs.html

Yipes. Think I'll stick to my DOF preview button, Otherwise I'll need a surveyor's tape and a laptop!

Ah, DOF preview one of the things the F70 didn't have that the D70 does

[murph7355]

Simpo - this is a boy's site with the occasional honorary female member.

What is life without figures and toys to play with. Preview buttons are all well and good, but can they tell you that anything between 97.2m and 103.4 will be sharp? No. They cannot.

Now get playing with that calculator so you know what your DOF button's showing you.

[simpo two]

murph7355 said:

Preview buttons are all well and good, but can they tell you that anything between 97.2m and 103.4 will be sharp? No. They cannot.

I'd rather get on with taking the photo. Sometimes just knowing the princple is sufficient, without rowing across a lake with a tape measure in my teeth
http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&category=13873&item=6111277196&rd=1

[fatsteve]

Simpo does the D70 DOF preview actually doing anything other than make the image darker,.. or is that just a feature on the 300D!!

Steve