Photography Advice

[HJM]

I know I could probably post on more photgraphy biased sites than this, but I know you lot better

I found yesterday that my camera struggled to focus on darker cars and of the blurred pics that I took, they were nearly all dark colours.

I shoot with a low shutter speed to get the background and wheel blur for movement and the sun was in and out yesterday like a yo-yo, but can anyone tell me what I am doing wrong?

Is it just that when the sun goes in, the shutter speed is too low to clearly focus on dark objects?

Thanks for reading and all advice will be gratefully received but please be gentle with me, for my technical knowledge of photography is self taught and pretty much zero


J

[I-S]

Canon (like all SLR manufacturers) use a phase-detection autofocus system. This relies upon focusing on a contrasted edge.

Which body is it you have, because that will alter my answer to your question?

[HJM]

Canon (like all SLR manufacturers) use a phase-detection autofocus system. This relies upon focusing on a contrasted edge.

Which body is it you have, because that will alter my answer to your question?

Thanks Isaac - it is the 20D.

J

[I-S]

Right, ok, same as mine.

The shutter speed used in the exposure has no bearing on the AF (but the incoming light level does, which can affect the shutter speed used. Point being that in a given lighting condition, AF is totally unaffected whether you're using 1/40th or 1/4000th).

The 20D employs two different types of focus sensor. The centre point and the others.

The "others" (ie the 8 that form the edges of the diamond) are vertical sensors which work down to F5.6. If you are using a lens slower than F5.6 then they will not work (or if you're using a third-party teleconverter which does not report the effective aperture change, they may attempt to work but usually fail or perform unreliably). With F5.6 or faster, they are a vertical array, which means that they will focus on a horizontal contrasted edge. In other words, where you have a light colour and a dark colour meeting at an edge, it will focus on that if it is perpendicular to the sensor orientation. If you turned the camera through 90 degrees so that the sensor orientation and the edge orientation are the same it will no longer be able to focus on it.

The centre focus point is a different kettle of fish. For starters, it is a cross-type sensor, rather than single-axis. That means that it does not matter if you have a horizontal or vertical edge, it will be able to focus on it, again at F5.6 or faster.

However, it has another trick, whereby if you have a lens of F2.8 or faster, it switches into "High Precision" mode. Normal precision will place the focus within 100% of the depth of field of the lens. High Precision will place focus within 33% of DOF.

So, what does all this mean in practice? Get a blank, textureless piece of paper and try to focus on it. No matter how much light you have, it won't be able to do it reliably. Now use a black marker pen and ruler to draw a straight line an inch long. In good light, try focusing on it at various angles with various focus points... the centre point should always be able to focus on it, but the other points will become unreliable or unable when the line is vertical in the viewfinder (assuming normal camera orientation).

Why has this affected darker cars? There's less contrast available. The windows and door frames and paint are all dark, whereas there's contrast available with lighter cars. In poor light, this becomes all the more important, and will result in the problem you're experiencing.

[HJM]

Wow - thanks again Isaac.

I will read this properly tonight.

J

[Wodjno]

Right, ok, same as mine.

The shutter speed used in the exposure has no bearing on the AF (but the incoming light level does, which can affect the shutter speed used. Point being that in a given lighting condition, AF is totally unaffected whether you're using 1/40th or 1/4000th).

The 20D employs two different types of focus sensor. The centre point and the others.

The "others" (ie the 8 that form the edges of the diamond) are vertical sensors which work down to F5.6. If you are using a lens slower than F5.6 then they will not work (or if you're using a third-party teleconverter which does not report the effective aperture change, they may attempt to work but usually fail or perform unreliably). With F5.6 or faster, they are a vertical array, which means that they will focus on a horizontal contrasted edge. In other words, where you have a light colour and a dark colour meeting at an edge, it will focus on that if it is perpendicular to the sensor orientation. If you turned the camera through 90 degrees so that the sensor orientation and the edge orientation are the same it will no longer be able to focus on it.

The centre focus point is a different kettle of fish. For starters, it is a cross-type sensor, rather than single-axis. That means that it does not matter if you have a horizontal or vertical edge, it will be able to focus on it, again at F5.6 or faster.

However, it has another trick, whereby if you have a lens of F2.8 or faster, it switches into "High Precision" mode. Normal precision will place the focus within 100% of the depth of field of the lens. High Precision will place focus within 33% of DOF.

So, what does all this mean in practice? Get a blank, textureless piece of paper and try to focus on it. No matter how much light you have, it won't be able to do it reliably. Now use a black marker pen and ruler to draw a straight line an inch long. In good light, try focusing on it at various angles with various focus points... the centre point should always be able to focus on it, but the other points will become unreliable or unable when the line is vertical in the viewfinder (assuming normal camera orientation).

Why has this affected darker cars? There's less contrast available. The windows and door frames and paint are all dark, whereas there's contrast available with lighter cars. In poor light, this becomes all the more important, and will result in the problem you're experiencing.

That is some in depth sh!t there Probably not for you Issac, but for me it is.. All very interesting and makes absolute sense Must be the way you explained it, as i've read Photography books before and they just made me All Confuddled

Smarty Pants

[I-S]

A lot of photographers belittle those taking a close interest in the equipment. The way I see it is that if you understand the technical aspects of what makes the system work and how, you can use it to your best advantage. The same is true of cars, bikes... anything really. Maybe I just think that because I'm an engineer...

[NevGroom]

A lot of photographers belittle those taking a close interest in the equipment. The way I see it is that if you understand the technical aspects of what makes the system work and how, you can use it to your best advantage. The same is true of cars, bikes... anything really. Maybe I just think that because I'm an engineer...

Isaac - Couldn't agree more - Both me and my partner are extemely keen photographers. Thing is, she hasn't a clue how to use her kit to take full advantage but has a fantastic "eye" for a picture. I on the other hand, know how to use the kit at all levels but just don't have that artistic "eye". I generally picture animals/cars/planes as there is a lot less creativity/artistic flair involved....

Nev

[HJM]

Thing is, she hasn't a clue how to use her kit to take full advantage but has a fantastic "eye" for a picture. I on the other hand, know how to use the kit at all levels but just don't have that artistic "eye".

LOL - I don't have either of those skills!! I am only interested in taking pictures of moving objects (cars, animals or birds and the occasional CVR4 footie match) and capturing a moment (other than the very occasional crop, I do no manipulation at all) - I am not in the slightest bit artistic (well, unless you include p!ss artistry ), but I will stand around for hours in all weathers if there is a chance of some good action shots.

Thanks for the info Isaac - it makes more sense now, even if there is very little I can do about it in the 'field' so to speak.

Cheers,

Jane

[Xeroid]

Another serious photog here, with a KonicaMinolta 7D and taking pix of Rally Cars whenever I can ( checkout www.rallyshotz.cjb.net )

Simple answer, spot exposure and spot focus ( for moving cars ) Prefocus/expose on the road/track just before the car arrives and half hold the button to keep it fixed ( most cameras will do this in some form ).
I use Continuous focus and multishot, take about 5-7 frames, one has got to be good and most important ...PAN !! follow the car right through while you are shooting, you may find your fuzziness is movement blur cos you stopped when you pressed the button but the car didn't. I track right around, finger held all the way, can always discard rubbish but if you miss the all critical shot cos you didn't keep pressing you'll kick yourself.
Have plenty of memory obvioulsy, I run 3 gig, get about 900 frames which I can use in a day easily at an event.

Good luck ....

[Brunty]

...an even more simple answer:

Dark cars are faster!

I left the pub late on Sunday night and there was an Aston Martin in the car park - this car is so fast it appeared to be blurred even when stationary - proof surely.