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OK so as you will guess form this post I am a litte bit of an amateur when it comes to settings

I shoot in AV mode with my 350D, I always use F8 for day time and adjust as needed for night time.

Up until now I used the standard kit lens, well now I have the 24 - 70 L f2.8 & the 70-200L f2.8 and wanted to get the full benifits of these lens, so I can do day time and night time shots ok (so I think anyways as with most things always room for improvement) my question is this , at what point will I loose focus when trying to get DOF in my images??

I get that I can use 2.8 at night and also use 2.8 at day as long as its at it greater zoom be it 70 or 200 on the zoom.

So if I take a day time photo say at 35 on the 24 to 70 at what point will it go south? I just got the lens and because of the rain I am yet to do any thing remotely serious with it.

I hope this question is not to stupid and I have explained it ok and I hope some one can offer some advise.

Thanks Matt

http://www.firecall.com.au

Matt, congrats on getting 2 great lenses.

Both these lenses will work at f2.8 right up to their minimum aperture (probably something like f16) thru their entire range.

When you open a lens up to it's maximum aperture (f2.8 in this case), you are letting more light in to strike the sensor and register the image. So you can take photos when there is less light around. Or you can take photos with a shorter exposure time, like 1/1000 sec, which will freeze action and minimise the effect of camera shake.

As well as altering the amount of light that hits the sensor, when you vary the f stop you are varying the Depth of Field (DoF). When you focus your camera on a point, some of what's in front of that point will also be in focus as will some of what's behind that point. Not as sharply in focus as at the exact point of focus but near enough to be pleasing to your eye. What's in and out of focus is determined by the distance from the camera to the subject and the size of the lens aperture (f stop). There is not an instantaneous change from in focus to out of focus. It's a gradual change and when used well can give a pleasing result. You can purposely blur the foreground and background and make the main subject stand out by being sharply in focus.

Where f stop comes into DoF is that for a given distance to the subject, the larger the aperture (the smaller the f number), the shallower will be the DoF. You need to experiment with different f stops with subjects at different distances. With a bit of practice, you'll get to know what works and what doesn't.

But, to answer your original question, both those lenses will work fine at any f stop thru their entire focal length range (that is from 70 all the way to 200 in the case of the 70-200 and 24 all the way to 70 in the case of the 24-70). It's just the amount of light and DoF that will vary.

As an example of how DoF will vary, set the 24-70 at 24mm or 35mm or 70mm (you choose) and f2.8 and focus on something that's a long way away from you, like a mountain in the distance. You will find that everything from (guessing here) 20 meters out to the mountain will be in focus - the closer stuff not quite as sharply in focus as the mountain, but pretty close. Now try the same thing at f16. You'll find that everything from just a few metres to the mountain will be in focus.

Ok, now try the same exercise but focussing on something up close like a large flower at say 2 metres away. At f2.8, the front and rear petals of the flower will soften and be slightly out of focus. Now bump the f stop up to f16 and you'll see that the whole of the flower and some stuff in front and behind it is in focus.

So with DoF and f stop, there is no right or wrong answer. It's what effect you want to achieve with your photo. To work this out, experiment and see what results you achieve. The instant feedback from your experiments is one of the great things about digital photography.

Hope this helps.

Cheers
John

matt_au_70 wrote:I hope this question is not to stupid and I have explained it ok and I hope some one can offer some advise.

Firstly, it is never stupid to ask a question if you want some information. It's probably stupid not to!
So, starting at the beginning of your questions:

matt_au_70 wrote:I shoot in AV mode with my 350D, I always use F8 for day time and adjust as needed for night time.

At somewhere around the f5.6 to f8 mark is probably the "sweet spot" for most lenses, and it will also give you a medium depth of field. The general rule is one-third in front of your focus point and two-thirds behind it.

When you want less depth of field, select a larger aperture, say around F4 to F2.8. This will reduce the depth of field and focus becomes more critical. Your camera will select the correct shutter speed for you if you are in aperture-preferred mode.

Conversely, if you want a greater depth of field, use a smaller aperture, say f11 to f16. It's a little bit counter-intuitive, but the smaller numbers are the largest openings and the larger numbers are the smallest openings.

matt_au_70 wrote:I get that I can use 2.8 at night and also use 2.8 at day as long as its at it greater zoom be it 70 or 200 on the zoom.

Yes, you are more likely to use f2.8 (larger aperture or opening) at night or at the tele end of your zoom because you need more light in these circumstances.

matt_au_70 wrote:So if I take a day time photo say at 35 on the 24 to 70 at what point will it go south?

.

I'm not quite sure what you mean here. Your photo is more likely to "go south" because of a low shutter speed (assuming you are hand holding your camera) than a large aperture. Generally photographers use a shutter speed no slower than the reciprocal of the length of the lens; so at 200mm, keep your shutter speed above 1/200 sec to avoid blur due to camera shake. At 35mm you should have no trouble at 1/60 sec or even 1/30 sec if you have good technique. Just make sure that the aperture you have selected doesn't let the shutter speed drop below those levels.

If you need any more information, just ask. There are plenty here who have been where you are and will help.

Also, have a look at this site's tutorials, which will help with some of the questions you are asking:

http://www.dslrusers.net/files/PHOTOGRA ... TSHELL.pdf

http://www.dslrusers.com/files/Exposure101.pdf

cricketfan wrote:I'm not quite sure what you mean here. Your photo is more likely to "go south" because of a low shutter speed

Thanks John & Trevor.

What I was saying with that was when I had the kit lens when I adjusted the f stop some of the item I was trying to focus on to get DOF was only partially in focus and the rest was not so it ruined the shot.

Thanks for the help I will read and as soon as the opportunity presents its self get out and have a go.

Thanks again , Matt

matt_au_70 wrote:What I was saying with that was when I had the kit lens when I adjusted the f stop some of the item I was trying to focus on to get DOF was only partially in focus and the rest was not so it ruined the shot.

OK, as John and I explained, that's probably because the lens was adjusted to a larger aperture, like f2.8.

How do you tell whether the aperture is OK for the pic you are taking? Well, it's a lot harder now. There used to be a DOF indicator on most zoom lenses which told you the range of distance that would be in focus.

These days, I think there are two options. Firstly, your camera probably has a DOF preview button somewhere near the lens mount. When you press this in, the shutter is closed to the selected aperture and you can judge the DOF. However, the viewfinder goes darker as a result of less light coming in, and sometimes this can make the process a bit difficult.

The other way, if you have time, is to take a shot and check your LCD screen. That will give you an indication of the DOF, particularly if you can magnify the image on the LCD.

Also, your lens manual will probably have a DOF table somewhere.

And finally, as you take more photos and gain experience in using the different settings, you will come to learn what aperture you will need for a certain DOF.

Good luck and keep shooting.

matt_au_70 wrote:
What I was saying with that was when I had the kit lens when I adjusted the f stop some of the item I was trying to focus on to get DOF was only partially in focus and the rest was not so it ruined the shot.

That is part of the equation

As you know, shutter speed and aperture work hand in hand to get you the right exposure. If you bump one up, then you have to bump one down. And which one you adjust will have a lot to do with the needs of each situation you find yourself in.

For example. In low light, say you want to take pics with no flash indoors for candid birthday party shots. You want to keep your shutter speed at 1/60 or faster because you have a 50mm lens. So you open up the aperture to compensate, say to f/1.8 (assuming a 50/1.. This will of course mean that you have a very thin DOF so you have to be super-careful with focusing.

Another example, you are taking a shot of a person with a landscape behind them. You want maximum DOF so you can get your subject as well as the background in focus, so you close down the aperture to f/11 and compensate with a slower shutter speed.

The beauty of the lenses that you have is that they are constant f/2.8 throughout their range, which means you can use anything from f/2.8 to whatever (generally f/16 or f/22 for most lenses) at any focal length you choose. WHAT you choose depends on the needs of that photo. Sometimes you might want a wafer-thin DOF for artistic purposes, and sometimes you will need more.

Of course, to confuse you, DOF is a complex relationship between focal length, aperture and distance to subject - not just aperture!!!! The scenarios below all have f/2.8 but have wildly different DOF.

f/2.8 @ 24mm & 10m distance has 140m of DOF.
f/2.8 @ 24mm & 2m distance has 76cm of DOF.

f/2.8 @ 200mm & 10m distance has 26cm of DOF
f/2.8 @ 200mm & 2m distance has 1cm of DOF

Therefore, it's hard for anyone to give you a recommended aperture setting because it really depends... hope that helps

Geez these brings back memory when I first started photography... Some great explanations/help there.

What helped me understand DOF was to line up a bunch of objects and shoot at different apertures to gauge how it all works.

cricketfan wrote:The other way, if you have time, is to take a shot and check your LCD screen. That will give you an indication of the DOF, particularly if you can magnify the image on the LCD.

just do this!

go outside, find a plant or some stationary object, and take several photos of it at varying aperature.

you'll soon get the hang of it!

if you're ever unsure whilst out shooting, check the LCD after you shoot the shot when possible.

cricketfan wrote:And finally, as you take more photos and gain experience in using the different settings, you will come to learn what aperture you will need for a certain DOF.

i think there was a site called dofmaster.com that would calculate the DOF you have with any lens at any aperature. that also helped me understand it in my head.

http://www.dofmaster.com/dofjs.html

Hope this helps

Bob