Fast Lenses

[jamesev]

Its seems that if there were no constraints then everyone would go for fast lenses.

Obviously these allow wider aperture, thats lets more light onto the sensor but a shallower depth of field.

My question is why is a shallower depth of field seen as a good thing?

Is it that when doing things like portrait photography a softened background does not detract the focus from the subject and therefore makes a "better" photo.

But for sports photography where subjects are moving in 3 dimentions quickly, whilst a fast shutter speed will prevent motion blur, a shallow depth of field can be a pain as it means that focusing at the subject at the time of "click" is sometimes hit and miss

Or like Wedding Photography is Sports photography considered and "art"

Please can I have views from experts

[RichardTaylor]

I am not an expert but do like shooting wide open for lots of subjects.

For sports it is because you may be able to isolate your subject from the background.

Sports photography, when done properly can be considered an art form.

I shoot motor racing and because the subjects can be a fair distance away it is difficult to isolate the cars when racing by using just shallow DOF (there are other methods) . I normally only shoot wide open in poor light when shooting the racing.
You do not want to shoot at very high shutter speeds because you may as well be shooting parked cars. It is good to have blurred wheels, at the very minimum.

Example: from a drag racing meeting.



Here are a couple of examples where it can be more than jus a "record shot"

(1)


(2)


(3)


Hope this helps.

[gturner]

getting a fast moving action shot in focus is about 2 things - the AF tracking speed of your camera and your shutter speed.
A freeze frame baseball/ cricket/ motorsport short requires at least 1/1000 shutter speed. If you have perfect sunshine you can play with your aperture, but in anything less than great light or under spotlights, you need the 2.8 speed.

Just look at the lenses on the side of the fields at all World Cup games/ 200 to 400 f2.8.

Jim Bryant will tell you how many times he shots wide open for baseball

[Swisstony10]

Also, don't forget that the further away from the subject you are, the greater the depth of field actually becomes.

For example, using a Canon 50D with a 50mm lens at f2, with a subject 1 metre away from you, you'll have a depth of field of 3cm (that's about an inch and a bit, for you Imperial folks).

Same lens, same camera, same aperture, but with a subject 50 metres away, your depth of field will have extended to just under 180 metres.

Even using a dirty big 400mm telephoto lens (on a x1.6 crop camera), the depth of field at f2.8 is still over a metre and a half (on a 5D Mk II it's over two and a half).

When shooting some sports, the photographer can, for obvious reasons, be a fair distance from the action (hence the huge expensive telephoto lenses), so that widening of the depth-of field gives a better margin for AF-tracking.

As to the "art" question, I think all genres of photography can be considered an art in their own right. There's an art to taking good portraits, just as there's an art to getting great wedding shots, killer sporting moments or close-up shots of spiders' nostrils.

Russ.

[gturner]

Yep, Russ makes a great point about the AF's need to have the light available to track effectively.

[Elmo]

For me the issue is isolation. The faster the lens the more the subject can be isolated from distracting foreground and background. I shoot in the f:2.8 range 90 percent of the time. My only reason for changing is when I need to include a near or far object.

[daft_biker]

Other than being big, heavy and expensive there are other reasons why one wouldn't want "fast" lenses - they can't draw straight lines so are not desirable for archiecture and often don't stop down past f/16 so aren't particularly desirable for macro photography either. If you want accurate drawing and extensive DoF fast lenses can be a disadvantage.

In terms of AF speed I don't know of any DSLR that needs anything faster than f/2.8 to get full functionality out of all focus points. Having faster lenses might help lock onto something in low light but the faster the lens the more glass will need shifted to focus so tracking might be very slow. (ever tried a Canon 85mm f/1.2? even the new version is really slow to focus because of the amount of glass being shifted....the f/1.8 is much faster to focus). That said you don't need AF to pre-focus

[Swisstony10]

Actually, the point I was trying to make was about the depth of field at distances. If you're shooting Formula 1 (for example), there's a good chance that you're a fair distance away from a car which is travelling towards you, past you, or away from you at a very high speed.

If you're using a 400mm lens in the situation I described above, then you have a 1.8m depth of field, meaning that if you're using tracking AF, it has more margin for error if the focus is playing catch-up with the car a bit than if that depth of field was reduced to, say, 6 inches...

Andrew's points are all also VERY valid though - you're not kidding about the time it can take those big L lenses to hurl their glass around. Mind you, that focus-distance switch that Canon put on the pretty white lenses helps a bit with that..

Russ.

[daft_biker]

As a bit of an aside has anybody worked out the average speed of the plane of focus for a given lens?

For example my 50mm can get from 45cm to infinity (say 50m - 1000x focal length?) in a couple of seconds. Am pretty sure if I could run 50m in a couple of seconds I'd be moving pretty fast!

If I measured the time it took my lens between, say, 5m and 15m I could probably find out how fast a bike I could photograph with it (on APS-C as I'd be closer with FF so it would be slower).

Anybody ever been bored enough to work it out?

[jamesev]

thanks for the responses