dslr normal photo focusing

[chemtom24]

hi all,

I guess im not really a newcomer as such anymore, but in my mind i am still a rank amateur!

basically i took about 20 shots of this white moth, using fully automatic setting, and the auto depth of field setting.

I focused on the body of the moth with the centre red dot using autofocus.

alot if not all of the images came out like this one.

I have 2 questions -

1, why does my camera and autofocus tell me its in focus if it isnt.

2, how do i get my dslr to act like a high quality compact digital, ie have everythgin in focus (or almost everything)

Is there a fault with the camera itself?

Its clear that focus IS acheivable, just autofocus does not use it correctly.

Thanks,

Tom

CANON 400D, 50-250 ef-s IS lens

[chemtom24]

ps - through the finder it looked like the moth was in focus, and as already said, the central red dot was palced on the moth body and it flashed red, showing me (supposedly...) that the moth was in focus.

[Photosbykev]

at this magnification the DoF is measured in millimeters so accurate focussing is critical. I normally focus on the head or even the eye if I can and use a decent aperture of f16-f22 to maximise the DoF. Were you shooting handheld? if so then any camera movement could easily move the point of focus, on this image the point of focus is beyond the moth

Small p&s cameras have a much smaller imaging chip which effectively gives more DoF for an equivalent image.

What were the camera settings used? ideally you should of been thinking about either manual or AV mode with an exposure compensation to get the correct exposure.

To prove the AF is correctly focussing you need to securely fix the camera in place and shoot something like a newpaper with a lot of fine detail

[Naz]

It looks as though you had a wide aperture set, as the depth of field is very shallow. You need a small aperture, f8 or f11 at least.

You would also need to use a tripod or some form of support, also ditch the auto settings and set the aperture to a small number and adjust the shutter accordingly.

[Digz]

It looks like there is a shallow depth of field on this shot. Part is in focus, but it is such a small part and the rest of the frame is out of focus it can look like there is nothing in focus. If you look at the background you can see a strip that is in focus starting from the lefthand side just above the halfwaty mark and slanted upwards. As such the focus point appears to have missed the moth (or maybe just clipped the wing)

Do you know what the shutter speed, ISO, aperture and focal length were? This will allow me to comment further on this.

In relation to question 2 I wouldnt dream of wanting my DSLR to perform like a compact camera! Have a google search of Depth of Field or DoF and that will shed some light on the situation. Compact cameras, due to chip size and the small optics often use narrow apertures (small holes) therefore the depth of field is greater thus getting more in focus front to back. One thing I love about using my DSLR is the ability to control the DoF and get creative with my shots.

I wouldnt say it was a fault with the camera just a product of narrow DoF (due to large aperture)and probably a tight focus (i.e. a macro type shot)

HTH

[chemtom24]

thanks for replies

ive kind've read about aperture and small aperture giving more DoF - however, all pics taken with a small aperture have to have significatnyl larger EXP times otherwise they come out too dark! is it ok to use 1600 ISO to try and sort this out? I dont know what im doing. long exp makes handheld shooting worthless. ARGH its so annoying.

I appreciate that a tripod is better, and i do have one but im not always gonna have it with me and sometimes i WILL want to P+S, so how do i do this?

Only brought this damn thing for astro work but I want to be able to use it properly now!

[Digz]

ive kind've read about aperture and small aperture giving more DoF - however, all pics taken with a small aperture have to have significatnyl larger EXP times, so handheld shooting is worthless. ARGH its so annoying.

This is a rather simplistic explanation and not strictly always true.

The exposure required for a photograph is broadly made of four things:

1. Shutter Speed

2. Aperture

3. ISO

4. Available Light

These four items are balanced to give the desired shot you are after which is informed by the cameras inbuilt lightmeter.

That is the camera will meter the available light on your scene and advise on what settings you need. Here we are ignoring the fact that every camera will exposed for 18% grey - but thats another discussion.

It therefore follows that you can adjust any of the four items above to suit.

For example if you want a wider DoF and therefore a narrow aperture you will need to offset this by either adjusting Shutter Speed or ISO. If you adjust ISO then the shutter speed will remain the same, if you adjust the Shutter speed then the ISO can remain the same.

You could also, of course, adjust the available light to suit.

What Im trying to say is that a wider DoF doesnt always have to result in longer shutter speeds. You can compensate for this by increasing the ISO or available light. Also, as Photosbykev alludes to, macro photography often results in razor thin DoF which can exacerbate the situation.

Generally in everyday photography you wont find so narrow DoF, unless using an ultra fast lens like an f1.4.

HTH

[chemtom24]

If you look at the background you can see a strip that is in focus starting from the lefthand side just above the halfwaty mark and slanted upwards. As such the focus point appears to have missed the moth (or maybe just clipped the wing)

exactly - i can forgive a ciouple of errors in autofocus, but 20/30 shots liek this is very annoying.

settings were as follows ( from properties of file)

ISO 400

s/s 1/60

F/5.6

FL 250mm

metering mode = pattern

thanks

[Digz]

PS - adjustments are measured in steps.

Therefore a step increase in aperture will need to compensated by a step decrease in shutter speed to maintain the same exposure values.

For example, your camera is telling you that for an ISO of 200 on a given scene will need a shutter speed of 1/60th and aperture of f4. You decide, however that you want more DoF so want to use f8.

If you look here: F-number - Wikipedia, the free encyclopedia , you will see that f8 is 2 full stops from f4 (i.e. you have f4, f5.6, f8) therefore you need to adjust your shutter speed by two full stops equating to 1/15th.

This may result in a wobbly shot without a tripod, so you could increase your ISO by two stops to keep the shutter speed the same.

[Digz]

exactly - i can forgive a ciouple of errors in autofocus, but 20/30 shots liek this is very annoying.

settings were as follows ( from properties of file)

ISO 400

s/s 1/60

F/5.6

FL 250mm

metering mode = pattern

thanks

It will be difficult to see if its an error without some scientific tests.

How big was the AF field in comparison to the subject? Are you certain it was on the spot you mention? Could it have moved as your body sways back and forth?

The easiest way to tell will be to mount on a tripod and take some controlled shots.

HTH

[chemtom24]

It will be difficult to see if its an error without some scientific tests.

How big was the AF field in comparison to the subject? Are you certain it was on the spot you mention? Could it have moved as your body sways back and forth?

The easiest way to tell will be to mount on a tripod and take some controlled shots.

HTH

i cnt be sure it wasnt movement - but there's always going to be some movement with handheld and alot of shots will be handheld spare of the moment.

the camera can perform extremely well - i have some excellent robin pictures, here are a couple of good ones

so i know the camera CAN perform, i just cant always get it to

[Photosbykev]

1/60 sec @ f5.6 with 250mm FL handheld is close to impossible to get sharp because of camera shake and the odds of hitting the correct focus point @ f5.6 is also very marginal. If I was taking the shot I would expect it to be little more than a record shot and would probably trash it.

Any support you can get will help you improve the overall quality of the image. Something as simple as a bamboo cane supporting the camera from the floor can significantly improve the percentage of good images, maybe supporting the camera on your knee or against a tree or wall, anything like that will improve your results. Also try shooting in burst mode I normally find the 2nd image is sharper than the first one. If you can't use or don't have a separate flash gun then a hand held torch might improve the image quality by allowing you to use a smaller aperture and faster shutter speed.

Pushing the iso up towards 1600 will help but the noise introduced into the image will kill the quality, it is always better to try to keep the iso to the lowest usable value which you can only really do by supporting the camera or increasing the lighting.

[Digz]

A good rule of thumb for handheld shots is to ensure a shutter speed equal to or better than 1/focal length. Therefore a focal length of 250mm should require a shutter speed of 1/250th s depending on how steady your hands are.

Another thing to remember in your original shot it only takes a small movement to shift your focus by quite a bit.

[chemtom24]

thanks kev, its nice to know its something fixable

trouble is is that there is no half auto settings that still allow customisation over all the features. Im not ready to take the plunge into full blown manual, adjusting for every shot, but at the same time fully auto just isnt cutting it.

is this a common dilemma in the photography world?

pain the backside

[Photosbykev]

You can use Av mode (where you adjust the aperture and the camera controls the shutter) or Tv mode (where you set the shutter speed and the camera controls the aperture) but ideally take the iso off of the auto setting and manually set this yourself to suit the lighting conditions.

The full auto mode can be useful IF you taking random scenery style pictures but you will get better results by using the camera metering as a guide to the correct exposure and then tweaking it in manual, Av or Tv to get a better image.

Stepping up to DSLR cameras is a steep learning curve and you really do need to learn the basics and that takes time and plenty of mistakes

The best piece of advice I can give you is use the tripod for every shot. It will slow you down and make you think about what is important to get the best out of the subject.

[Brinders]

Depth of field is a product of focal length and aperture. Long focal length and wide aperture = small depth of field. Small focal length and small aperture = large depth of field.

To illustrate: the first pic is of a local church taken this spring using a wide angle zoom lens at 18mm with a small aperture (f11 at 1/60th sec) which shows apparent sharpness from front to back, while the second pic of a wild flower is at a focal length of 300mm at 1/125 sec at ISO 400 in bright (not sunny) daylight taken yesterday. Depth of field is very shallow giving a pleasing out of focus background.

Focusing for the first image was taken across the frame. The second image was taken using the centre focus point only and spot metering. Both images used shake reduction on a Pentax K20D DSLR.

Brinders

[chemtom24]

thanks eveyrone for the help its put me at ease witht eh original problem - in that i now know it was my errors, not the cameras, that caused it.

kev you wernt joking mate, its a blumming steep learning curve, i had no idea, i stupidly thgouth dslr was just a more expensive but sharper camera which could be used fully auto AS WELL as manual if you knew what you were doing

thanks again everyone

tom